INTERNATIONAL CONFERENCE

ON

MATRIX ISOLATION SPECTROSCOPY

DISKUSSIONSTAGUNC,

DER DEUTSCHEN BUNSEN-GESELLSCHAFT

FOR PHYSIKALISCHE CHEMIE

EXTENDED ABSTRACTS

WEST-BERLIN. GERMANY

JUNE 21-24, 1977

PI

OF THIK 1 DCUMENT IS BNUMTEED DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. INTERNATIONAL CONFERENCE

ON

MATRIX ISOLATION SPECTROSCOPY

DISKUSSIONSTAGUNG

DER DEUTSCHEN BUNSEN-GESELLSCHAFT

FOR PHYSIKALISCHE CHEMIE

EXTENDED ABSTRACTS

WEST-BERLIN, GERMANY

' JUNE 21-24, 1977

DISTRIBUTION OF THIS DOCUMENT IS UNLBpTED The conference is sponsored by:

Deutsche Forschungsgemeinschaft I V

z» | Freie Universitdt Berlin 3

9 1 j^Fritz-Haber-Institut der Max-Planck-Cesel1 schaf t -— 1 ^

The National Science Foundation, Washington. ^

Organizing committee:

F.W. Froben, Frele UnivcrsitHt Berlin Boltzmannstr.20, 1000 Berlin 33, W-Germany

H. Gerischer, Fritz-Haber-Institut der Max-Planck-Gesellschaft Faiedaywcg 4 6, 1000 Devlin 33, W nevmeay

G.C. Pimentel, University of California Dept, of Chemistry, Berkeley Ca. 94720, USA

fli te3E$t93?3jonnA to? oaesloft tioyitcM fl3TS5scofl-^2i9nS IIT

CONTENTS

p a g e . .. I. GENERATION OF REACTIVE SPECIES AND THEIR ISOLATION IN MATRICES.

1.1 G..C. P i m e n t e l . Generation of reactive species and their isolation in matrices. A Summation. I

1.2 A. Dendramls, J.F. Harrison and G.E. Lerol. Matrix Isolation spectroscopy and an ab-lnltlo study of the free radical HCCN. 2

1.3 F.W. Froben. Stabilisation of active nitrogen reaction Intermediates in matrices. 6

1.4 R. Pong, Th.D. Goldfarb and A. Krantz. Kinetic studies in various matrices - probing the host- guest interaction. ■ 9

1.5 P. Huber-Waichli. Trapping of unstable conformations from thermal molecular-beams in argon matrices. 12

1.6 M.E. Jacox. Stabilization of reaction intermediates in an argon matrix in discharge sampling experiments. 16

1.7 A. Krantz and J. Laureni. Preparation of matrix-isolated thilrene. 21

1.8 H. KUhne. Linear reactor-infrared matrix spectroscopy of the olefin-ozone reaction. 25

1.9 D. Mlhelclc, D.H. Ehhalt, G.F. Kulessa, J. Klomfass and M. Trainer. Measurements of free radicals in the atmosphere by matrix isolation and electron paramagnetic resonance. 29

1.10 J. Pacansky, G.P. Gardlnl and J. Bargon. The infrared spectra of alkyl radicals: the low temperature photochemical decomposition of dlacyl peroxides. 30

1.11 A.J. Rest and J.R. Sodeau. Photoprocesses in low temperature matrices. 33

1.12 H.M. Rojhantalab and J.W. Nibler. Structural studies of carbon halide free radicals in matrices. 37

1.13 A. Haas and H. W1liner. Infrared matrixspectra of lower sulfur- and selenlum- fluorides. 41 XV

SPECTRA OF METAL ATOMS AND CLUSTER FORMATION

B. Mey e r . Spectra of metal atoms and clusters. 43

2.2 P. B a r r e t t a n d P. M o n t a n o . MUssbauet and liifi.di.ed speuLLusuuulu studies uf an iron-nitrogen molecule produced in a nitrogen matrix. 47

2.3 F. Forstmann. On the theory of the matrix influence on the spectra of isolated atoms. 49

2.4 M. Jakob, H. Micklitz and K. Luchner. Observation of pseudolocalized vibrational modes in the optical spectra of rare earth atoms isolated in rare gas matrices. 53

2.5 P.M. Kolb and D. Leutloff. A'n'alysis "of matrix induced changes in the spectra of isolated metal atoms. 57

2.6 t . p . M a r t i n . Infrared absorption in LiF polymers and microcrystals. 60

2.7 P.A. Montano. P.H. Barrett and H. Micklitz. Applications of matrix isolation techniques to MBssbauer spectroscopy. 64

2.8 D. Nagel and B. Sonntag. 3s-excitation of Na atoms trapped in Xe-matrices. 68

2.9 T. W e l k e r . Optical absorption of matrix isolated silver aggregates and microcrystals. 73 V

3. STABLE MOLECULES IN MATRICES

3.1 A.J. Barnes. Stable molecules In matrices. 77

3.2 M. Allavena, H. Chakroun and D. White. Theoretical Interpretation of Infrared line broadening of molecules trapped In rare gas matrices. 81

3.3 A.J. Barnes. ' Studies of lntermolecular Interactions by matrix Isolation vibrational spectroscopy. 84

3.4 A. Behrens, W.A.P. Luck and B. Mann. Self-assoclatlon of oxlmes In argon matrices studied by IR-spectroscopy. 88

3.5 V. Chandrasekharan and E. Boursev. Cage size analysis with diatomic molecules trapped In rare gas matrices. 92

3.6 H. Forster and M. Schuldt. Induced Infrared spectra of homonuclear diatomic molecules In zeolltlc matrices. 96

3.7 L. Le Gall and A.J. Barnes. Vibrational spectra of malelmlde and barbituric acid in low-temperature matrices. 100

3.8 C. Glrardet, D. Robert, D. Maillard, J.P. Perchard and A. Schriver. Distorsion of doped nitrogen and rare gas matrices studied by lntermolecular potential development. Discussion from HC1 and HBr Infrared spectra. 104

3.9 J. Heldberg, R.D. Singh and H. Stein. External field induced vibrational splitting in trapped species on alkali halide crystals. 105

3.10 H. Hollenstein. Matrix spectrum of pyruvic acid and lsotoplc modifications. 109

3.11 B. Kndzinger and m .e . Jacox. Matrix Isolation spectroscopy of stable organic molecules In the far infrared region. I 12

3.12 D. Maillard, J.P. Perchard, A. Schriver, B. Silvl and C. Glrardet. Discussion of the change with lntermolecular distance of the HX dipole moment derivative In HX aggregates from IR intensities and spectral shifts. Comparison with MIND0/3 semi-emplrlcal calculations. 116

3.13 B. Nelander. A complex between water and formaldehyde. 118 VI

3.14 O.P. Ayers and A.D.E. Pullln. Nuclear spin conversion of l^O and D 2O in argon matrices. 121

3.15 M. Rdsanen, J. Murto and A. Klvinen. A matrix infrared study of the NHj vibrations of amides. 125 VII

4. RAMAN- AND IR-SPECTROSCOPY

4.1 L. Andrews. Spectroscopy of transient species in matrices:

Cs+Cl2” , XeF, CF2C1+ and Ca2> 129

4.2 M. Dubs. High resolution IR matrix spectroscopy with tunable diode lasers. 135

4.3 A. Glvan and A. Loewenschuss. Matrix isolation, infrared and raman soectra of binary and mixed zinc dihalides. I 39

4.4 H.J. Jodi. High pressure raman matrix isolation spectroscopy at low temperature. 141

4.5 F.J. Lltterst, A. Schichl, E. Baggio-Saitovitch, H. Micklitz and J.M. Frledt. MSssbauer studies of rare-gas matrix-isolated halide

molecules containing ^Fe, ' *^Sn and *^*Eu. 145

4.6 A. Loewenschuss and A. Givan. Matrix isolation raman spectra of FeCl^ and Fe^Clg. 149

4.7 A., Loewenschuss and A. Givan. The Infrared and raman matrix isolation spectra of some MFj and MXF molecules (M ” Zn,Cd,Hg; X ° Cl,Br,1). 154

4.8 J.S. Ogden. Some recent applications of I.R. isotope frequency and intensity patterns to matrix isolated molecules. 156

4.9 R.G.S. Pong, A.E. Shirk and J.S. Shirk. The spectrum and structure of aluminum trihalides. 161 VIII

5. HIGH TEMPERATURE MOLECULES

5.1 W. Meitner, Jr. High-temperature and Interstellar molecules.. 164

5.2 F.W. Froben. Can the production of diatomic high temperature molecules be optimized by MIS? 170

5.3 D.W. Green and G.T. Reedy. Matrix Isolation studies with Fourier transform IR. 173

5.4 K. Jansson and R. Scullman. • ' ‘ A study of the ground states of PtO and IrO. 176 IX

6. REACTIVE MATRICES

6.1 G.A. Ozln. Metal atom chemistry and surface chemistry: dlnlckel monoethylene, Nl^fC-H^); a localized bonding model for ethylene chemlsorbed on bulk nickel. 180

6.2 H.D. Breuer and J. Kriiger. PhotoreactlonS In condensed NO^ and NOj-acetone mixtures. 184

6.3 B.R. Carr, B.M. Chadwick, D.G. Cobbold, J.M. Grzybowskl, D.A. Long and D.A.M. Marcus-Hanks. Infrared and raman spectra of matrix-isolated cyanogen Iodide and Iodine lsocyanlde and other related systems obtained from the self-same matrix. 188

6.4 M. Creuzburg, J. Duschl and R. Heumtiller. Formation and luminescence of CH In an Ar matrix. 190

6.5 J. Fournier, J. Deson, C. Lalo and C. Vermeil. Thermolumlnescence following UV Irradiation of molecules trapped In rigid matrix at 6 K. 194

6.6 L.A. Hanlan and G.A. Ozln. Rhodium atom chemistry. 199

6.7’ R.H. Hauge, P.F. Meier and J.L. Margrave. Matrix Isolation I.R. and EPR studies of reaction Intermediates: lithium metal reactions with H-0, CH3OH, and NH^. 203

6.8 P.H. Kasai and D. McLeod. Generation and ESR study of lntermetalllc molecules Ag-M. 204

6.9 R.H. Hauge, J. Wang and J.L. Margrave. Matrix Isolation IR studies of hydrocarbons In fluorine matrices. 205

6.10 J.F. Ogllvle, V.R. Salares and M.J. Newlands. Photochemical oxidation of lodosllane In solid argon. 206

6.11 G.A. Ozln and W. Power. Metal atom olefin chemistry; Interaction of group VIII metal atomo with ethylene. 207

6.12 H. Rojhantalab, L. Allamandola and J. Nlbler. Fluorescence studies of CuO In Ar matrix. 21 I

6.13 R.R. Smardzewskl. Chemlluminescent matrix reactions. 215

6.14 E.E. Koch, R. Ntirnberger and N. Schwentner. 218 RELAXATION PHENOMENA, STUDIED IN MATRICES

H. Du b o s t . Vibrational relaxation in matrices. 223

M. Poliakoff, B. Davies, A. McNeish and J.J. Turner Infra-red laser-induced photochemistry in matrices. 229

L.' Abouaf-Marguln,' B. Gauthler-Roy and F. Legay. Vibrational relaxation of CH^F in a kryoton matrix at low temperatures. Influence of the rotation .(I). 235

L. Allamandola,,H. Rojhantalab and J. Nibler. Vibrational_energy transfer studies of matrix isolated . 239

G. Zumofen. Vibrational energy transfer and relaxation in pure and CO-doped solid N^. 243

G. Zumofen. Calculation of matrix shifts and splittings in the vibrational 1R and raman spectra of solid and matrix isolated CO. 247

* XI

8. PHYSICAL PROPERTIES OP MATRICES

8.1 T.J. Barton, R. Grinter and A.J. Thomson. The application of magnetic circular dichrolsm in matrix isolation studies. 251

8.2 H. Coufal, U. Nagel and E. Luscher. a q 4 1 Ar : K A model system for matrix isolation. 255

8.3 E.R. Krausz, R.L. Mowery and P.N. Schatz. MCD and MCPE studies of transient and stable matrix isolated species. 260

8.4 B. Mann and A. Behrens. Demixing effects during matrix preparation. 266

8.5 W. Schulze. H.U. Becker and H. Abe. The isolation of metal atoms in solid noble gas matrices. 270 THIS PAGE

WAS INTENTIONALLY

LEFT BLANK XIII

AUTHOR INDEX p a g e

AB E , H. 2 7 0

ABOUAF-MARGUIN, L. 235

ALLAMANDOLA, L.J. 239,

ALLA VENA, M. 81

A N D R E W S , L. 129

AYERS, G.P. 121

BAGGIO-SAITOVITCH, E. 145

B A R G O N , J. 30

BARNES, A.J. 77, 100

BARRETT, P. 47,

BARTON, T.J. 251

BECKER, H.U. 270

BEHRENS, A. 88,

BOURSEY, E. 9 2

BREUER, H.D. 184

CAR R , B.R. 188

CHADWICK, B.M. 188

CHAKROUN, M. 81

CHANDRASEKHARAN, V. 92

COBBOLD, D.G. 188

C O U F A L , H. 2 55

CREUZBURG, M. 190

D A V I E S , B. 229

DENDRAMIS, A. 2

D E S O N , J. 194

D U B O S T , H. 223

D U B S , M. 135

D U S C H L , J. 190

EHHALT, D.H. 29

FORSTER, H. 96

FORSTMANN, F. 49

FOURNIER, J. 194

FRIEDT, J.M. 145

FROBEN, F.W. 6, XIV

US G A L L , L. 100

GARDINI, G.P. 30

G A U T H I E R - R O Y , B. 235

GIRARDET, C. 104,

G I V A N , A. 139,

GOLDFARB, Th.D. 9

GREEN, D.W. 173

G R I N T E R , R. 251

GRZYBOWSKI, J.M. 188

HA A S , A. 41

HANLAN, L.A. 199

HARRISON, J.F. 2

HAUGE, R.H. 203,

HEIDBERG, J. 105

HEUMULLER, R. 190

IIOLLENSTEIN, 11. 109

HUBER-WSLCHLI, P. 12

JACOX, M.E. . 16

J A K O B , M. 53

JANSSON, K. 176

JODI., H..T. 141

KASAI, P.H. 204

K I V I N E N , A. 125

KLOMFASS, J. 29

KN03INCEIU E. 112 w KOCIl, E.Ei 00

K O L B , D.M. 57

K R A N T Z , A. 21

KRAUSZ, E.R. 2 6 0

KRllCER, J. 194

V.UHfiS, H, 75

KULE3SA, G.F. 29

LA L O , C. 194

L A U R E N I , J. 21

L E G A Y , F. 235

LEROI, G.E. 2 XV

LEUTLOFF, D. 57

LITTERST, F.J. 145

LOEWENSCHUSS, A. 149, 154, 139

L O N G , D.A. 188

LUCHNER, K. 53

LUCK, W.A.P. 88

L O S C H E R , e . 255 m a i l l a r d , d . 116, 104

M A N N , B. 2 66, 88

MARCUS-HANKS, D.A.M. 188

MARGRAVE, J.L. 2 05, 203

MARTIN, T.P. 60

McLEOD, D., Jr. 204

McNEISH, A. 229

MEIER, P.P. 203

M E Y E R , B. 43

MICKLITZ, H. 53, 64, 145

MIHELCIC, D. 29

MONTANO, P.A. 64, 47

MOWERY, R.L. 260

M U R T O , J. 125

N A G E L , D. 68

N A G E L , U. 255

NELANDER, B. 118

NEWLANDS, M.J. 206

NIBLER, J.W. 37, 211, 239

NtiRNBERGER, R. 218

OGDEN, J.S. 156

OGILVIE, J.P. 206

O Z I N , G.A. 180, 199, 207

PACANSKY, J. 30

PERCHARD, J.P. 104, 116

PIMENTEL, G.C. 1

POLIAKOFF, M. 229

P O N G , R. 9

PONG, R.G.S. 161 XVI

POWER, W.J. 207

PULLIN, A.D.E. 121

R X S X N E N , M. 125 REEDY, G.T. 173

R E S T , A.J. JJ

R O B E R T , D. 104

ROJHANTALAB, H.M. 37 , 211, 239

SALARES, V.R. 206

SCULLMAN, R. 176

S H I R K , A.ti. I 6 1

SHIRK, J.S. 161

STI.VI, B. 116 SINGH, R.D. 105

SMARDZEWSKI, R.R, 215

SODEAU, J.R, 33

S O N N T A G , B. 68

SCHATZ, P.N. 260

SCHICHL, A. 145

S C H R I V E R , A. 104, 116 SCHULDT, M. 96 SCHULZE, W. 270

SCHWENTNER, N. 218

ST E I N , H. 105

THOMSON, A.J. 251

TRAINER, M. 29

TURNER, J.J. 229

VERMEIL, C. 194

W A N G , J. 205

W E L K E R , T. 71

WELTNER, H . , Jr. 164

W H I T E , D. 81

WILLNER, H. 41

ZUMOFEN, G. 243, 247 Generation of reactive species and their isolation in matrices.

A Summation.

G.C. Pimentel

University of California, Department of Chemistry,

Berkeley Ca. 94720, USA 2

MATRIX ISOLATION SPECTROSCOPY AND AN AB-INITIO STUDY OF THE FREE RADICAL HCCN A. Dendramis. J. F. Harrison and G. E. Leroi, Department of Chemistry, Michigan State University, East Lansing, MI 48824, U.S.A.

The geometry and electronic structure of HCCN are uncertain, 1-4 despite attempts to characterize the free radical by matrix ESR, gas phase electronic spectroscopy® and chemical kinetics® experiments, and by semi-empirical molecular orbital calculations.^ HCCN has been called cyanocarbene or cyanomethylene, implying a carbene-type electron distribution for its triplet ground state, although neither experimental nor theoretical data conclusively support this designation. The ESR zero-field splitting parameters are consistent with a linear geometry, but the possibility of long-axis rotation (of. CH^) and the unavailability of hyperfine structure data prevent definitive specification of the molecular symmetry. We report in this work: (1) the vibrational spectra of HCCN and three of its isotopic modifications, obtained in Ar matrices at 1SK, coupled with complete normal coordinate analyses assuming either linear or bent geometry, and (2) an ab-initio study incorporating configu­ ration interaction, focused on geometry optimization, electronic charge distribution, and some key force constants of HCCN. The free radical was produced by in situ photolysis of the matrix-isolated precursor, diazoacetonitrile (NNHCCN), with radiation of X > 3500 A. (Successful generation of HCCN was confirmed by ESR spectro­ scopy.) The IR absorptions of HCCN in an Ar matrix are shown in Fig." 1, spectrum B. IR spectra of DCCN, HC^CN, and H C C ^ N were obtained by photolvting matrices of the appropriately-labeled parent molecule. 17 observed frequencies were used to fit 8 valence force constants; the Shimanouchi normal coordinate analysis programs® were employed. Both * 9 linear (C^) and bent (Cs) geometries were tested, with the frequency fit clearly favoring the former structure. The observed vibrational

3250 3 2 0 0 1730 1720 1130 1170 47 0 4 50 Fig. 1. IR spectrum of A 1 1 ^ | J l i li HCCN in an Ar matrix. A. Before photolysis; 8 Y~ T T v 1178.5 cm 1 4 58cm 1 After photolysis; B. 3229 cm1 C. After annealing. F (uu1) 3 frequencies for linear HCCN are compared in Table I to those calculated with the refined force constants listed. The isotopic frequency pattern and the numerical values of the valence force constants suggest allene- like bonding in the CCN group, and extensive delocalization of the two unpaired electrons. Preliminary ab-initio calculations,^ performed with a minimal basis set at the SCF level, had indicated that the HCCN ground state (triplet) potential surface possessed two minima of approximately equal energy: a linear nitrene (H-C=C-N), and a bent carbene (H-(J-C=N) . The allene-like form (H-C=C=N) was located near an energy maximum. Since the conclusions from our matrix experiments were inconsistent with these predictions, we decided to recompute the potential surface at the SCF level using the Huzinaga^ 9s,5p basis for C and N (contracted to 4s,2p) and the 4s expansion on H (contracted to 2s). The results, shown in Fig.'2a, reproduce the qualitative features of the earlier calculation, with the carbene-like geometry being slightly favored. To allow for possible correlation H-C-C-N energy differences among a) SCF the different structures, we constructed a modest config­ uration interaction wavefunc- tion by including those R=2.55 A = const. configurations which = 62.9 Kcal/mol arise by. allowing the 6 w-electrons of HCCN to be distributed in all possible combinations of the 3 lowest b)CI TT-orbitals. As Fig. 2 illus­ trates, the inclusion of Cl gives striking results. Only a single minimum is predicted in the potential r (A) energy surface, correspond­ ing to a linear geometry Fig. 2. Total energy vs. C-C bond [rcc - 1.28±0.01A; rCN = length of HCCN. a) SCF; b) Cl. 1.25±0.01A; rCH = 1.08±0.01A], -1

A population analysis performed at the equilibrium configuration (using the natural orbitals of the configuration interaction wavefunction) shows an almost perfect allene-type electron distribution, in agreement with the results of the matrix isolation experiments. We are now calculating the stretching force constants associated with the CCN group, assuming that the curvature near the potential minimum is quadratic. The theoretical and experimental values of these potential constants will be compared.

References

1. R. A. Bernheim, R. J. Kempf, P. W. Humer and P. S. Skell, J. Chem. Phys. 41_, 1156 (1964). 2. R. A. Bernheim, R. J. Kempf, J, V. Gramas, and P, S, Skell, J. Chem. Phys. 43, 196 (1965). 3. R. Bernheim, R. J. Kempf, J. V. Gramas, and P. S. Skell, J. Magn. Resonance 2- 5 (1970). 4. E. IVasserman, W. A. Yager, and V. J. Kuck, Chem. Phys. Letters 7_, 409 (1970). 5. A. J. Merer and D. N. Travis, Can. J. Phys. 4£, 353 (1966). 6. P. S. Skell, S. J. Valenty, and P. W. Humer, J. Amer. Chem. Soc. 95, 5041 (1973). 7. R. Hoffman, G. D. Zeiss, and G. V. VanDine, J. Amer. Chem. Soc. 9£, 1485 (1968). 8. T. Shimanouchi, Computer Programs for Normal Coordinate Treatment of Polyatomic Molecules, Tokyo, University of Tokyo (1968). 9. A. Dendramis, Ph.D. Thesis, Michigan State University (1976). 10. J. C. Weisshaar and J. F. Harrison (private communication). 11. S. Huzinaga, J. Chem. Phys. 42., 1793 (1965). 12. T, H. Dunning, Jr., J. Chem. Phys. ^3, 2823 (1970). 5

Table I. Normal Coordinate Analysis of HCCN. Linear Form (C ) ; Allene-like Molecular Parameters

Frequencies (cm F) Primary Contributors to V Obs. Calc. Av Potential Energy Distn. HCCN 3229.0 3228.9 (-0.1) vi CH(96) 1735.0 1737.3 (*2.3) CN(103) CC(28) E* V2 1178.5 1178.7 (*0.2) CC(80) V3

v4 458.0 458.5 (*0.5) CCH(103) CCN(17) n (369.5)* 371.0 (*1.5) CCN(89) V5

DCCN 2424.0 2424.9 (+0.9) CD(90) V1 1729.5 1730.8 (+1.3) CN(104) CC(24) V2 1127.0 1128.8 (+1.8) CC(79) CD(8) i+ V3 v4 405.0 405.0 (0.0) CCN(81) CCD(49) n v s 317.5 317.7 (+0.2) CCD(56) CCN(24) h c 13c n 3229.0 3228.6 (-0.4) CH (96) V1 1698.0 1695.7 (-2.3) CN(104) CC(26) V2 x + 1176.5 1175.1 (-1.4) CC (82) V3 v4 458.0 456.9 (-1.1) CCH(104) CCN(14) n (364.5)* 362.2 (-2.3) CCN(92) V5 h c c 15n 3229.0 3228.9 (-0.1) CH(96) V1 1718.0 1716.5 (-1.5) CN(102) CC(51) z+ V2 1168.0 1167.3 (-0.7) CC(77) V3 v4 458.0 458.5 (+0.5) CCH(103) CCN(16) n -- 368.8 -- CCN(89) V5

‘uncertain band, not used in force constant calculations

K__ = 7.533 (0.050)**; K„u = 5.567 (0.008); K„. = 12.241 (0.170); C L L it Ll'» o Fcc CN = 2.842 (0. 112); F ^ cc = -0.456 (0.033) [mdyn/A] h c c h = °-113 (°-001^ h c c n = ’0 -307 (°-004^ FCCH,CCN = ° - ° « C°s°01> [mdyn‘A]

**Numbers in parentheses represent uncertainties. 6

Stabilisation of active nitrogen reaction intermediates in matrices

F.W. Froben, Institut fur Molektilphysik Freie Universitat Berlin Boltzmannstr. 20, 1000 Berlin 33, Germany

The reaction of active nitrogen with many organic and inorgan- 1-3 ic molecules has been studied by spectroscopical methods

Active nitrogen produced in a high frequency discharge and flowing along a tube where other molecules can be added is an easy tool for the investigation of these reactions. In many cases the reaction path is very well understood, but unstable intermediates - required by the results - have been only pro­ posed in most of the experiments.

By condensation of the reacting fast flowing molecules at low temperature at a. certain point, the reaction can be interrupted and the momentary concentration of the stable molecules and radicals determined. If the moment of condensation can be changed a complete reaction mechanism can be established and the lifetime of some of the radicals measured. This is done by moving the inlet for the molecules under study '.in respect to the cold surface.

Very few experiments of this type have been reported, measured by BPR ^and by IR

For the experiments to'be reported here, two different setups have been used. In the first the discharge is burning in a mixture of nitrogen and the added molecules and in the second setup the substrate can be added at different distances from the discharge. The linear flow velocity is in the order of 7

mo

10 m/sec and the distance between the start of! the reaction and the termination by condensation can be varied between 10 cni and • 1 m so that unstable intermediates within a lifetime . . -2 -1 limit of 10 to 10 sec.can be detected. In most cases the variation of the inlet position has only a minor effect in- - 1 dieating that the lifetime of the molecules is 10 sec or longer. The concentration of N~/M has been varied between 102 to 103.

The discharge system is coupled to a variable temperature cryostat (!) - 100 K) built into a Fourier-IR-spectrometer (^000 to 10 cm-1 resolution 0.1 cm 1) and the cryostat can 8 be moved into the magnet of an ESR instrument+.

Some of the molecules measured besides ammonia and NI ^ are various ammines, acetonitril, CCl^ and phosphorus.

1 A,N; Wright and C.A. Winkler, Active Nitrogen, Academic Press, N.Y . 1968

2 D.R. Safrany, Prog, in Reaction Kinetics 6, 1 (1971)

■3 J.J. Havel, P.S. Skell, J .Amer.Chem.Soc. 9jt 1792 (1972)

4 A. Fordioni and C, Chachaty, C.R.Acad.Sci. Paris 26AC, C 37 (-1967)

5 F.W. Froben, Ber. Bunsenges. phys.Chem. ]_8, 184 (1974)

6 F.W. Froben, R. Minkwitz, Molecular Spectroscopy of Dense Phases , p. 4t>3, Elsevier Pub. Com. 1976

t The valuable advice of Dr.P.Kasai , Terry town, N.Y., arid Prof. W. Weltner, Gainesville, Florida, during the construct­ ion Of this equipment is gratefully acknowledged 9

KINETIC STUDIES IN VARIOUS MATRICES - PROBING THE HOST-GUEST INTERACTION

Richard Pong, Theodore D. Goldfarb and Allen Krantz Contribution from the Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 1179** •

The potential of matrix isolation (Ml) spectroscopy for the study of kinetic phenomena at low temperatures has received very little at- 1 2 3 It tention. Recent books ’ and review articles ’ on MI make no mention of this application. In principle, the MI study of the rates of such low activation energy unimolecular processes as rotational and confor­ mational isomerism should be possible. Such studies would be useful not only for learning about the dynamics of the isomerizations them­ selves but, when compared to kinetic measurements in other states or environments (including different matrices), they should be a sensi­ tive probe for examining the nature of the perturbation of the guest species by the host matrix. The fact that, despite the growing list of applications of the MI technique, kinetic experiments have been ignored is best explained by the difficulties that must be overcome in order to perform such experi­ ments. In addition to the usual requirements that the matrix be chemi­ cally inert toward the species being studied and spectroscopically transparent in the region of interest, kinetic measurements require that the matrix be rigid enough to prevent diffusion and aggregation in the temperature region over which the process proceeds at a measurable rate. The most commonly used matrices, N and A r , can thus be used only up to 5 30 and 35K, respectively. Of the other infrared transparent matrix materials, Xe is useful up to about 65K. Kinetic studies generally re­ quire spectral measurements over only a narrow spectral region which in­ cludes at least one absorption characteristic of each species being mea­ sured. Once the spectra of the initial and final species have been ob­ tained using an infrared transparent matrix, it is possible to select another relatively inert material such as a hydrocarbon or a halocarbon which has an infrared spectral window in an appropriate frequency inter­ val and has a high enough melting point to prevent diffusion up to the required temperature range. We wish to report results we have obtained by using this technique to study the rotational isomerization of but-3- en-2-one (lj (methyl vinyl ketone). 10

We have previously reported the photolytic conversion of the more stable isomer of 1_ to its rotamer in an argon matrix at

i 1

20 K. Prolonged photolysis produces a photostationary equilibrium in which the two rota&ers are present in roughly equal concentrations. The system is stable up to 35 K . Evaporation of the sample, warming to room temperature, followed by redeposition at 20 K results in the reversion to the original rotamer ratio. In our present experiments, we have used Xe and CCl^ matrices and have observed similar photolytic behavior for 1^. We have found that the isomer is thermally stable with respect to reversion to the form up to about 55 K . At 60 K , in either Xe or CCl^, at M/R=200, the ^£^=5 to isomerization proceeds until about 50% of the *so~ mer produced by photolysis has been converted to is measured by their respective carbonyl stretching absorptions at 1710 and 1690 cm"1 . If the temperature is raised to 70 K (only possible in the CCl^ experi­ ments), the remaining reverts to over a period of about an hour. If a photolyzed sample of 1 in CCl^ is warmed directly to 70 K, without equilibration at 60 K , we again observe about 50% of the de­ caying rapidly followed by a much slower decay for the remainder. In­ creasing the M/R to 500 has no effect on the relative amounts of the that revert to by the slow and fast processes, A kinetic analysis of the data yielded two first order decay pro­ cesses With rate constants at bU K of 7 • 3x10 1 sec and 1.6x10 1 sen 1 for the to conversion in the CCl^ matrix. An Arrhenius plot of the data obtained over the 60 -70 K range yielded EA=3.3 kcal/ mule and lug A-10.6 fur i.ne slow process and kcal/mole and log A=10.7 for the fast process. The value of the rate constant for the fast process in Xe is roughly 6.2xl0-^ sec-1 at 60 K . These results seem to be best interpreted by assuming two trap­ ping sights for 1, in both Xe and CCl^ which differ somewhat in the degree to which they inhibit the rotational isomerization. The possi­ bility that the fast rate is due to the isomerization of monomeric 1, whereas the slow rate is that of a dipolar pair or some other aggregate 11 is ruled out by our failure to observe any change in the relative amounts of the two decaying species as a function of M/R. Further studies of this system in other matrices and of other low temperature kinetic processes are planned. The authors wish to thank the national Science Foundation for support of this work under Grant Number CHF74-04594 A01.

References (1) Vibrational Spectroscopy of Trapped Species, H. E. Hallam, ed., Wiley and Sons, London, 1973. (2) Matrix Isolation. S. Craddock and A. J. Hinchcliffe, Cambridge Univ. Press, Cambridge, 1975. (3) A. J. Downes and S. C. Flakes in Molecular Spectroscopy, The Chemical Society, London, 1973 Vol. 1, p 523. (4) B. M. Chadwick in Molecular Spectroscopy, The Chemical Society, London, 1975, Vol. 3, p. 28l. (5) Ref (1), p. 39. (6) A. Krantz, T. D. Goldfarb and C. Y. Lin, ^jn. > 94, 4022 (1972). 1 2

TRAPPING OF UNSTABLE CONFORMATIONS FROM THERMAL MOLECULAR - BEAMS IN ARGONMATRICES.

P. Huber-Walchli Physical ChemisLiy Laboratory Swiss Federal Institute of Technology Universitatsstr. 22, CH-8092 Zurich, Switzerland

We report IR-matrix experiments- in which Argon beams from room temperature orifices and a substance beam from a heatable knudsen cell (T^ 300 K up to 1000 K) are codeposited on a Csl-window on LHe temperature. With thise technique unstable conformations may be enriched in the matrix. The matrix effect on the potential of internal rotation, and the conformational equilibrium on trapped molecules (1,2-Difluoroethane and 1,3-But.adiene) will be discussed.

As a typical example, fig.l shows the FIR-matrix spectra of 1,2-Difluoroethane (DFE) for T, = 300 K and 600 K. Compared -1 -1 with the band at 161 cm , the band at 145 cm increases the intensity with increasing T^ . The same behaviour has been observed at 8 bands in the midinfrared region [l]. Therefore the 145 cm ■*" absorption belongs to trans-DFE, the 161 cm ^ absorption to gauche-DFE (dihedral angle 75°). The intensity ratio of the 145/161 cm ^ pair at two T^ allows the calculation of the energy difference (trans-gauche Van t'Hoff plot). Together with the bands in the midinfrared region a value of 200 ± 50 cm ^/mol is obtained. The same value for the gas phase has been estimated iil a NlviR study [2], This agreement leads to the conclusion, that the ratio of gauche/trans-DFE present in the knudsen cell will not be changed noticeably during the deposition process. Probably the trapping of DFE-molecules in Argon-atoms proceeds in time inter­ vals which are too short for equilibration. 13

Furthermore, no changes in the spectra indicating a relaxation (trans-»gauche) are observed after matrix deposition, even under IR-illumination. ' However annealing experiments (warming up the matrix to 35 - 40 K) disappearence of the trans spectrum indicates such a relaxation. The FIR gas phase spectra [3] allow the determination of the following potentialfunction

V8 (cp) = S V8 (1 - cos ncp)/2 1) with V8 = 1000 cm'1, V8 = -1090 cm"1, V8 = 1240 cm"1

In the matrix a further potential acts on the molecule

Vm (cp) = Vs (tp) + AV(cp) 2)

For AV(cp) one obtains

AV3 = 250 cm'1 , AV4 = 110 cm'1

Fig. 2 represents these potential. The potential AV(cp) may arrlse from replacements of vacencies In an Argon cristal the heat of formation for vacencies has been found to be 450 cm 1/mol vac. [4]. Both FIR absorptions show unusual linewidth of 7 cm 1 (spectral resolution 0.6 cm 1 , M/A 1000), these may be caused by different surroundings (and therefore different AV(cp)) of DFE molecules in the Argon matrix.

The torsional potential function of 1,3-Butadiene has been the subject of many quantum mechanical calculations [5],which predict a second stable conformer, 2.5 kcal/mol higher in energy than trans and with cis or slightly gauche geometry. A 1t'ough the formulation of the Diels-Alder-reaction presumes such a conformation [6], only few experimental facts prove its existence [7], probably because of its low mol fraction of 1.5 % at room temperature. Fig. 3 shows a spectral part, further spectroscopic results will be presented at the conference. 1 4

The energy difference (cis-trans) is calculated from the spectrum to 2.7 ± 0.6 kcal/mol. The agreement with the values of quantum mechanical calculations indicates a planar cis structure. The statistical weight of 2 for gauche would lead to a value of 3.7 kcal/mol. Furthermore the observed spectra corresponding more closly to an expected cis spectrum than to a gauche spectrum.

[1] P.Huber-Walchli et.al., Phys.Chem.Le11.30, 347 (1975) [2] S.S.Butcher et.al., J.Chem.Phys.54, 4123 (1971) r3] P.Huber-Walchli, will be published [4] G.Pollack, Rev.Mod.Phys.36, 748 (1964) [5] J.A.Altmann et.al., J.Mol.Struct.36, 149 (1977) [6] S.W.Benson et.al., J.Chem.Phys.46, 4920 (1967) [7] L.A.Carrcira, J.Chem.Phys.62, 3851 (1975) J.R.Durig et.al., Can.J.Phys.53, 1832 (1975)

Fig. 1 FIR-matrixspectrum of 1,2-Difluoroethane Argonmatrix, 1.2 mm, M/A 800

Fig. 2 Tosional Potentials mo1/. of 1, 2-Dif luoroethane

TKc 3 0 0 K 2200 . •Matrix •Oar,

V. 600 K

trnnn p.auche 15

Fig. 3 Matrixspectra of 1,3-Butadiene Argonmatrix, 500um, M/A 800

100* 80 ft r— IjT 6 0 „

20 _

300 K

HOD 1300 1 ' U 00 * 13*00 1 1 HOO 1 13*00 1 ' u S T * 13S 0 1 r m n

Trans ,V(c<)

1680 1660 1620 1680 ' 1560 16

STABILIZATION OF REACTION INTERMEDIATES IN AN ARGON MATRIX IN DISCHARGE SAMPLING EXPERIMENTS

Marilyn E. Jacox National Bureau of Standards Washington, DC 20234, U.S.A.

In contrast to earlier experiments in which the total sample was passed through the discharge region, leading to extensive fragmentation, recent studies in which the molecule is introduced in an excess of argon’ at the periphery of a microwave discharge through argon and the sample is frozen onto a surface at 14 K have yielded simple fragmentation products. In these experiments, pure argon is passed through a discharge tube and exits into the cryogenic cell through a pinhole having an area of approxi- 2 mately 2 mm . This pinhole controls the argon flow rate to optimize the pressure in the discharge region yet to provide a suitable deposition pressure and minimized backstreaming of the molecule being studied into the discharge. In the first studies using this configuration"*", acetylene was found to interact with the discharged argon to give a high yield of HC2 • The C-H stretching fundamental of this radical was observed at the anomalously high frequency of 3612 cm More recently, application of this sampling 2 technique to CH^ has led to the stabilization of a high concentration of CH^. When a partially deuterated methane was used, only the CH^-d^ radicals which would be formed by the stripping of a single H or D atom from the parent molecule were formed. It has been possible to analyze the rotational structure in the vibrational transition of all of the CH~-d 7 3 n species trapped in an argon matrix, providing further support for a planar structure. In experiments in which the ultraviolet spectra of the products of the interaction with were studied, absorptions of were present, but the prominent band system characteristic of the 1216 A photolysis studies did nnr appear. When the discharge products were allowed to inter­ act with CH-CN, the most prominent absorptions were those which had pre- 3 viously been tentatively assigned to H2C=C=NH. Although in all of the earlier photolysis studies^ on matrix-isolated CH^SH the absorptions of €$2 were extremely strong, when the discharge sampling configuration was used the yield of C$2 was almost completely suppressed, and the principal product absorptions were those of t^CS and those tentatively assigned to CHyS. 17

The emission spectrum of the argon atoms emerging from the discharge region** shows prominent lines in the 4000-4700 A spectral region which originate in levels 14.4 to 14.6 eV^ above the ground state of the neutral argon atom, demonstrating the presence of highly excited argon atoms. Only weak Ar"*" emissions were observed. Because the lifetime^ of metastable argon atoms (11.5 and 11.7 eV) is greater than 1 sec, it is presumed that these species are important reactants at the periphery of the discharge. Recent studies** have indicated .that radiation trapping should also result in the 3 1 persistance of a significant concentration of and argon atoms (11.6 and 11.8 eV), in this region. These excited argon atoms can be deactivated by collisions with molecules or by wall collisions; at typical discharge * pressures the formation of A ^ , which requires three-body collisions, is negligible. Wall deactivation can account for the failure to observe decomposition products when the pinhole was in the side of the discharge 9 tube , since under laminar flow conditions there should be a gradient in the radial distribution of excited argon atoms in the discharge tube, with deactivated argon atoms nearest the walls. In this laboratory, the product yield has been found to be significantly enhanced when the pinhole is in a blunt end tube, compared to the yield characteristic of discharge tubes with a conical end profile and a similar pinhole area, again consistent with the importance of wall deactivation. Studies on the discharge interaction with HCCl^"* are of especial interest in fixing the energy range of the discharge products, since earlier photolysis experiments'*"^ have demonstrated that the most prominent product when 1216 A radiation was used was CCl^, with CCl^j andHCC10 resulting from secondary photoprocesses. On the other hand, when the 1067 and 1048 A (11.6 and 11.8 eV) argon resonance lines were used for photolysis, the principal produc-ts were HCCl^ and HCC^- The appearance potential for HCC1« produced from HCC1- in the gas phase"*""*" is 11.5 eV. In the discharge 5 + - • experiments on HCCl^, extremely prominent H C C ^ and H C C ^ absorptions were present, consistent with an important role for collisions of HCCl^ with excited argon atoms. Because very little CC1~ was present, it could * J also be inferred that the formation of A ^ on the surface of the deposit and the consequent emission of radiation of wavelength near 1250 A into the sample does not play a significant role in product formation. In a recent attempt in this laboratory to probe the relative impor­ tance of product formation in the gas phase and in the solid state, a 18 series of experiments has been conducted on CF^NNCFy Because two CF^ radicals are produced in close proximity when this molecule is photodecom­ posed in the solid, an<* N2 are exPecte^ to be the major photolysis products. When a deposit of this substance isolated in an argon matrix was exposed to 1216 A radiation for two hours, quantitative photodecom­ position occurred, and prominent abosrptions of C2F6* ^F2* anc* CF^ appeared. No CF^ was observed. Supplementary observations of the photo­ decomposition of C2F6 an argon matrix confirmed that CF£ and CF^ were formed by this process. On the other hand, in discharge sampling experi­ ments very prominent CF^ absorptions were also present, indicating that a significant fraction of the sample decomposition occurred in the gas phase or on the surface of the deposit, where extra degrees of freedom may permit separation of the CF^ photofragments. Although the product yield was constant over a wide range of argon pressure, microwave power, and cavity position, when the cavity was moved to within about 2 cm of the pinhole a substantially higher product yield resulted. The proportionately greater yield of CF£ and CF^ suggests an enhanced importance for secondary processes when the close-coupled discharge is used. A comparative study was also conducted on a sample subjected to photolysis during deposition using an 12 argon resonance lamp of advanced design , with a thin lithium fluoride window estimated to pass 50% of the argon resonance radiation. The stability of this lamp during the experiment was assured by a subsequent check on its radiation output. Although the cross-sectional area of the lamp window was 75 times as great as that of the pinhole used in the dis­ charge experiments, the yields of CF^, CjFg, an<* CF^ were lower than those using either of the discharge configurations. The presence of CF^ when argon resonance photolysis was conducted concurrently with the deposition indicated that some of the photodecomposition also occurred in the gas phase or on the surface of the deposit. In contrast to the results of 1216 A photolysis of the total ‘deposit, irradiation of a previously deposited sample using the argon resonance lamp produced no photodecom­ position, indicating that 1067 and 1048 A photons cannot penetrate the deposit. No decomposition products were observed when argon atoms from the discharge were used to bombard a previously deposited Ar:CF^NNCF^ sample, further indicating that collisions of excited argon atoms do not transfer a significant amount of energy to the argon lattice. (Since the first exciton band of solid argon is at 12.1 eV, only argon atoms excited to 19 higher energies should be effective in transferring electronic energy to the lattice.) It is concluded that gas-phase and surface processes are predominant in the discharge experiments and that collisions with excited argon atoms play an important role. In a recent study in this laboratory of the interaction of discharged argon with CC1,, very prominent absorption by CClt resulted. The gas- 11 phase appearance potential of this ion from CC1, is 11.3 eV. When the -1 microwave cavity was moved close to the pinhole, the 927 cm absorption of CCl^ was greatly enhanced. Since the appearance potential of this ion from CCl^ is above 16 eV, exceeding the ionization potential of Ar, it is again suggested that under close coupling conditions products of secondary collisions with excited argon atoms become important. In studies of the interaction of discharged argon with HCCl^F, two 13 anion products resulted. Detailed isotopic studies identified these - 14 products as HCC1F and C ^ C ’^ H F . The analogous experiments on HCCIF2 led to the stabilization of HCF2 and of (C1CF)***HF . Because the ioniza­ tion potential of HCCIF2 is near 12.5 eV, collisions with argon atoms in their first excited states, between 11.5 and 11.8 eV, cannot produce cation products. Since argon atoms in these states have an ionization potential between 4.0 and 4.25 eV, they may undergo charge transfer reactions with electron acceptors, just as alkali metal atoms do. However, because these anions were formed by charge transfer with alkali metal atoms only upon photoexcitation of the alkali metal, collisional charge transfer between HCCIF2 and excited argon would appear to require the excitation of the argon atom to a level above 11.8 eV. The Ar+ formed by the charge transfer interaction would react with another Ar atom to form Ar^, which both theory and experiment have indicated has a ground-state dissociation energy somewhat greater than 1 eV. Although weak absorptions due to CCIF* have also been identified in this system, it is likely that A ^ , which is inactive in the infrared, is the predominant cation, providing the requisite overall charge neutrality of the sample. Absorptions due to CCIF*, as well as to several other charged products, were prominent in recent studies in this laboratory of the interaction between discharged argon atoms and CCIF^. Since the appearance potential of CC1F~ from CC1F„ is 12.45 eV and that of CCIF* is 14.25 eV, from recent 15 gas-phase studies using monochromatized synchrotron radiation and mass spectrometric detection, it is likely that collision of CCIF^ with argon atoms excited to states between 14.4 and 14.6 eV results in dissociative ionization in this system. 20

References 1. M. E . Jacox, Chem. Phys. 7_t 424 (1975). 2. M. E. Jacox, J. Mol. Spectrosc. (in review). 3. M. E. Jacox and D. E. Milligan, J. Amer. Chem. Soc. 85^ 278 (1963). 4. M. E. Jacox and D. E. Milligan, J. Mol. Spectrosc. 58, 142 (1975). 5. M. E. Jacox, Chem. Phys. 12, 51 (1976). 6. Throughout this paper 1 eV = 806547.9 m *. 7. R. S. Van Dyke, Jr., C. E. Johnson, and H. A. Shugart, Phys. Rev. A 5, 991 (19/2). 8. G. S. Hurst and C. E. Klots, Advan. Radiation Chem. 5_, 1 (1976). 9i C . A. Wight, B . S. Ault, and L. Andrews, J. Chem. Phys. 1244 (1976). 10. M. E . Jacox and D. E. Milligan, J. Chem. Phys. J54^ 3935 (1971). 11. A. S. Werner, B. P. Tsai, and T. Baer, J. Chem. Phys. 60, 3650 (1974). 12. R. Gorden, Jr., R. E. Rebbert, and P. Ausloos, Natl. Bur. Std. (U.S.) Tech. Note 496 (1969). 13. M. E. Jacox and D, E. Milligan, Chem, Phys. 1£> 195 (1976). 14. M. E . Jacox and T). E* Milligan, Chem, Phys. IjS, 381 (1976). 15. H. W. Jochims, W. Lohr, and H. Baumgartel, Ber. Bunsenges. physik. Chem. 80, 130 (1976). Preparation of Matrix-Isolated Thiirene A. Krantz and J. Laureni, Department of Chemistry State University of New York, Stony Brook, NY 1.1.794 Because of their strain and putative electronic destabilization, three- membered heterocycles* possessing a cyclic array of 4u electrons offer a considerable challenge to synthesis. Such molecules are expected to be both unimolecularly and bimolecular'ly reactive, if they exist at all as energy 2 minima.. Thiirene (1) and its kindred systems, ox.irene (2), azirene (3), and selenirene (4) are of interest because of their theoretical significance ^ 3 as prototypes of antiaromatic species. To date, neither the parent nor a single derivative of these heterocyclic molecules has heen prepared and

T 7 T 7 W T 7 S O N Se I R

1 2 3 4 'V % % -v characterized. We wish to report the preparation and characterization of thiirene (1) . We have previously noted that irradiation of matrix-isolated 1,2,3-thiadiazole (5) at 8 K produces thioketene (6) and ethynyl mercaptan f7). Studies with isotopical.lv labelled (5) show that the hydrogens and carbon atoms are equilibrated in ethynyl mercaptan (7). Preliminary evidence for thiirene Q.) in the form of infrared data has been presented by us.

Irradiation of argon matrix-isolated 1,2,3-thiadiazole (£) with light of 1 = 2350-2800 A produces ethynyl mercaptan (,£) and thioketene (6) along with a substance which possesses bands at 3207, 3169, .3166, 1663, 91.2, and 563 cm *. The positions of these bands are reminiscent of 22 bands in the spectrum of cyclopropene*’’^ (8). X is converted by light of .3300-3700 A wavelength to 6 and the latter being the major product. Isothiazole (£) is an independent precursor to the species Irradiation® (X = 2350-2800 A) of either 4-13C-l,2,3-thiadiazole Q$) or 5-13C-l,2,3-thiadiazole (lp gives 13C-£ Q^) with bands shifted to 3198, 3163, 3158, 1634, 910, and 558 cm \ (]$) is transformed with light of X = 3300-3700 A to ethynyl mercaptan and thioketene, with equi- jibrAterl label. H 13/ 13 \C = C, / „ H-C3C-SH u—T-C-' Hx ,3 Hx i 3 \ / Km > + 'Crcts + ;cn:=s \ c / .13 H' H-C=C-SH

12

The observation that irradiation of 4- or 5-13C-thiadiazole gives the same species Q^), which is then photochemically converted to thioketene and ethynyl mercaptan with equilibrated label, strongly suggests that £ must be thiirene or species derived from thiirene Q.). The simplicity of the spectrum of £ and the fact that isotopic labelling does not lead to "apparent splittings" of these bands, but to shifts in the frequencies, are in favor of a single species. If £ is a single species it must be thiirene based on the spectral properties which follow. Irradiation (x = 2350-2800 A) of either 4-d-l,2,3-thiadiazole (1)5) or its 5-d-isomer Q£) produces d-£ (15) which exhibits bands at 3219, 3181, 3175, 2423, 2420, 2415, 1611, 892, and 467 cm"'*'. Note that the three species X (,L ), 13C-X (12), and d-£ (1£), do not have bands in common. The methyl and dimethyl derivatives of £ when photolyzed as above, also give intermediates, which possess spectra indicative of a thiirene and, which are photoisomerizible to thjoketenes. Hence, the production of thiirenes during the photodecomposition of 1,2,3-thiadiazoles is quite probably a general process. The bands that we have been able to observe for thiirene are collected in Table I. ■ Very marked shifts (v50 'em 3) to low frequency of the "double bond 7 stretch" have been noted for cyclopropene upon deuterium substitution. We have observed the same pattern for )C with dp, d p and d^-^ Qj£) adsorbing at 1663, 1611, and 1567 cm"1 respectively. 23

Table 1 Infrared Bands for Thiirenes (in cm "h

I 15 to 12 19 17 ■ 3207 (w) 3219 (vw) 2485 (w) 3198 (w) 3203 (w) 2970 (w) 3169 (m) 3181 (w) 1567 (w) 3163 (w) 2930 (vw) 2921 (m) 3166 (m) 3175 (w) 873 (w) 3158 (w) 1440 (m) 2865 (w) 1663 (w) 2423 (vw) 423 (m) 1634 (w) 1429 (m) 1923 (w) 912 (m) 2420 (w) 910 (m) 1036 (m) 1440 (m) 563 (m) 2415 (w) 558 (s) 897 (m) 1427 (m) 1611 (w) 650 (w) 1041 (s) 892 (w) 586 (w) ' 467 (m) 471 (w) Relative band intensities as observed in the spectrum of the mixture: vw = very weak w = weak m = medium S = Strong

The cyclopropenoid character of the ring is further supported by the fact that dimethylthiirene (,17) (Table 1) shows absorption at 1923 cm \ paralleling the very marked high frequency shift of the "double bond stretching frequency" from 1641 (parent) to 1885 cm (1,2-dimethyl derivative) in the case of cyclopropene. Thus, our argument for the assignment of a thiirene structure to )( rests on (1) the fact that the same monolabelled species ^C-X is derived from distinctly labelled precursors ,1(3 or (2) the convertibility of labelled £ to ethynyl mercaptan and thioketene both with equilibrated label, and (3) on the likelihood of the observed band positions and their behavior on isotopic substitution being due to a single species of cyclopropenoid character. The latter point rules out structure 18 whose sole qualification for consideration is its symmetry.

HCeS-CH -m - HC-SeCH to Thus, thiirene Q.) represents the first example of a heterocyclic 4ir-electron system to be stabilized and characterized. Further efforts, using the above approach to prepare other members in this series and to 24 determine how the "antiaromatic quartet" influences their chemical and physical properties are underway.

Acknowledgment Financial support from the National Science Foundation, the donors of the Petroleum Research Fund administered by The American Chemical Society, The Research Foundation, SUNY and a NATO Senior Fellowship to A. Krantz are gratefully acknowledged.

References 1. For a lead reference see W. A. Lathan, L. Radom, P. C. Hariharan, W. J.

Hehre, and J. A. Pople, Fortschritte der Chemischen Forschung, 4(3, 1 (1973), Springer-Verlag, New York. 2. For the predictions of theory see ref (1) and (a) R. Zahradnik, Advan. Heterocyclic Chem., £, 14 (1965); (b) R. Zahradnik and J. Koutecky, Coll. Czech. Chem. Commun., 26, 156 (1961); (c) D. T. Clark, "Inter­ national Symposium on the Quantum Aspects of Heterocyclic Compounds in Chemistry and Biochemistry," p. 238, Jerusalem: The Israel Academy of Sciences and Humanities, 1970; (d) B. A. Hess, Jr. and L. J. Schaad, J. Am. Chem. Soc., 95, 3907 (1973); (e) M. J. S. Dewar and C. A. Ramsden, J. C. S. Chem. Commun., 688 (1973); (f) I. G. Csizmadia, H. E. Gunning, R. K. Gosavi, and-0. P. Strausz, J. Am. Chem. Soc., 133 (1973). 3. (a) R. A. Breslow, Chem. Eng. News, 90 (June 28, 1965); (b) M. J. S. Dewar, Advan. Chem. Phys., 121 (1965); (c) R. Breslow, J. Brown, and J. J. Gajewsky, J. Am. Chem. Soc., $■), 4383 (1967); (d) M. J. S. Dewar, "The Molecular Orbital Theory of Organic Chemistry," McGraw-Hill, New York, NY, 1969, pp. 180, 212; (e) R. Breslow, Accts. Chem. Res., £, i'Ji'(1973). 4. A. Krantz and J. Laureni, J. Am.' Chem.’ Soc,, ££, 6768 (1974). 5. J. Laureni, A. Krantz, and R. A. Hajdu, ibid., 913, 7872 (1976). 6. K. B. Wiberg and B. J. Nist, J. Am. Chem. Soc. , 13^, 1226 (1961). 7. G. L. Closs in "Advances in Alicyclic Chemistry," Vol. 1, ed. H. Hart and G. J. Rarabatsos, Academic Press, New York, NY, 1966, p. 53. 8. In all cases where conditions are unspecified in text, irradiation refers to molecules matrix-isolated in argon M/R = 500-1000, at 8K. 25

Linear Reactor-Infrared Matrix Spectroscopy of the Olefin- Ozone Reaction.

H„ Kiihne Physical Chemistry Laboratory, ETHZ Zentrum 8092 Zurich, Switzerland

In the last years extensive studies about the ozonolysis of small olefins in the gas phase have been made, especially when it was recognized that ozone is formed in photochemical smog. Today it is estimated that olefin-ozone reactions constitute close to 50 % of the observed olefin degradation and oxidation taking place in the lower troposphere. This work presents the results on gas phase ozonolysis of ethene and vinylchloride gained by a novel technique using a linear reactor in combination with infrared spectroscopy. The experimental set up is schematically sketched in fig. 1. Through metering valves diluted gas mixtures of olefin and ozone with argon are introduced into the mixture section of the reactor. The whole reactor is connected with a liquid helium cryostat of our own construction. The end of the reactor is closed up either by a capillary or Knudsen orifice depending upon the experiments to be carried out.The length L of the reactor is kept variable allowing a relatively wide span in reaction time ( 0.1 s to few minutes ). The major part of the reaction mixture is pumped away whereas a small portion goes through the Knudsen orifice K condensing on a Csl window kept on LHe-temperature. In addition, the reactor set up is equipped with some options.like a cooling jacket which permits thermo- stating between 25° to -35°C, a quadrupole mass spectrometer and a small He-Ne laser used to measure the layer thickness growth rate of the matrix during deposition. The advantages of this technique should be briefly discussed: First due to the small linewidth in matrix isolated spectra even complex reaction mixture may be analyzed. The relatively high sensitivity ( ~10~''mol )allows identification of reaction products formed in small quantities. Second this set up permits a wide variation of operating conditions, freezing of unstable molecules. Third by variation of the reaction times qualitative 26 kinetic characteristics may be derived.'

To get an idea of the time resolution with such a combined experiment some model calculations for a one dimensional re­ actor involving two chemical reactions were carried out.

A1 + A2 > A3 h = kl [Al][A2]

A^ ---- > products [A^]

This gives the following relation for an intermediate A7 with lifetime to be detectable

k p I2T

kT pTToT ~ 1 • w h e r e 1 1. PA = PA denots partial pressures of the particles A^ ^ and A 0 at the entrance to the reactor.

Since one is forced to limit the partial pressure of ozone (~30 Torr ), owing to its explosive nature, particles with a liftlme greater than 0.1 s in the case of ethene and 1 s in the case of vinylchloride should be observable, taking into account the following values of k^:

For ethene k^ = 2.10^ 1 mol s-"^ 2 -1 For vinylchloride = 2.10 1 mol c

Results:

1.. Ozonolysis of ethene.

In table 1 a survey is given of the detected reaction products. Besides the well known reaction products like CO, COg, t^O, H^CO and HCOOH there are a number of particles with more than 5 atoms. Part of them have been observed for the first time. These are:

0 = C'-^C -H formic anhydride (1), (2) H 0 .0 C — C^jj glycole aldehyde (1) OH 27

CHjOH methanol

CH20CH2 ethylene oxide a)

Formic anhydride itself is an unstable molecule with a lifetime of only few minutes at room temperature . Secondary ozonide which is the major product in liquid phase ozonolysis is not formed in the gas phase (3). Furthermore neither the primary ozonide nor an intermediate of the Criegee's type zwitterion (>C'®X0, ( k ) ) have been detected with our set up. This may be a direct consequence of our time resolution.

2. Ozonolysis of vinylchloride.

First it seems that the gas phase as well as the liquid phase ozonolysis of vinylchloride is even more complex than that of ethene. Up to now we could identify from matrix infrared ex­ periments the following reaction products: i ) Products from gas phase ozonolysis:

CO, C02 , HC1, H20

CH20, CHC10 (formylchloride)

HCOOH and some spectral features in the carbonyl stretching region indicate the presence of at least three further compounds. ii) Products from liquid phase ozonolysis:

Infrared matrix and microwave spectra were taken from the reaction products of the ozonolysis of vinylchloride pro­ duced in liquid phase at -70°C using Freon 11 as solvent. Tho products observed are:

CO, C02 , HC1, H20 CH20, CHC10 HCOOH CHO-O—CHO (formic anhydride) and some unknowns. a) ' There are molecules which have only been detected in analogous microwave experiments. 28

The major product, however, was a highly explosive white . compound with a very low vapor pressure (below 10 mTorr) whose structure is not yet kriown.

Fig. 1. Linear reactor infrared matrix spectroscopy sefc up;

Table 2. Typical spectral features of species observed in the ozonolysis reaction.

-1 jMW(MHz) MS(m/e) IR (cm ) MW(MHz) M S (m /e ) i r (cm 1765 2 4 5 6 9 .0 0 2107 23860 4 8 , 32, 16 03 C H 3O H a ’ c > 25018.14 1033 25511 2 5 1 2 4 .9 0 702 25650 258 7 8 .1 7 c 2 h 4 3112 C H 3CHO 1349 1 9225.6 1441 1727 19262.2 948 1748 19265.2 CO 2138 28 CH2OCH2c> 2 3134.2 C 0 2 662 44 2 3610.4 664 2 4 8 3 4 .3 h 2o 3757 (CHO)2Oa) 1812 1 8 8 4 5 .5 0 74 1624 1 760 2 4 2 3 1 .6 3 1609 1090 25119.00 01,0 3863 19595.23 30, 29 CH3OHCHOa> 3671 22 1 4 3 .0 2 60 1743 22965.71 35 75 2275U .60 1498 26358.80 1747 24 7 8 8 .1 0 H C 0 0 H a) 3551 2 0 2 9 7 .9 0 4 6 ,4 5 ,4 4 Sn/b) 19103:7 1/68 2 2 4 7 1.20 22 1 3 6 .7 a ) ; Not found in Waveguide reaction of MW eitpi Hut fuuuu in IK and MW exp. with external linear eactor . c) Mot feund in in a.*!1.

(1) H. Kuhne,S. Vaccani,T.K, Ha, A. Bauder and Hs.H. Gunthard, Chem. Phys. Lett. , 3jB, 449 (1976) (2) S. Vaccani, U. Roos, A. Bauder, Hs. H. Gunthard, Chem. Phys . , _19,51 (1977)

(3) H. Kuhne and Hs.H. Gunthard, J . Phys .Chem . ,80/, 1238 (1976) (4) T .K . Ha, H. Kuhne, S. Vaccani and Hs.H. Gunthard 24, 172 (1974) 29

MEASUREMENTS OF FREE RADICALS IN THE ATMOSPHERE BY MATRIX ISOLATION AND ELECTRON PARAMAGNETIC RESONANCE.

D. Mihelcic, D.H. Ehhalt, G.F. Kulessa, J. Klomfass, and M. Trainer.

Kernforschungsanlage Jiilich, Institut fur Atmosph. Chemie, D-5170 Jiilich.

With some special adaptations the technique of matrix isolation followed by detection through electron paramagnetic resonance can also be used for the measurement of atmospheric radical concentrations. A light weigth cryogenic sampling device has been constructed. I t uses condensation of atmospheric

CO^ or H2 O at 77 K for matrix formation and trapping of the radicals. The sampler can be flown on a balloon for stratospheric sampling. First data on stratospheric HO 2 and NO 2 at 32 km altitude have been obtained on a flight on August 8 ^ , 1976 and will be reported.

Extension of this method to the measurement of other radicals will be dis­ cussed. 30

THE INFRARED SPECTRA OF ALKYL RADICALS: THE LOW TEMPERATURE PHOTOCHEMICAL DECOMPOSITION OF DIACYL PEROXIDES

Jacob Pacansky, G. P. Gardini and Joachim Bargon IBM Research Laboratory, San Jose, CA. 95193, U.S.A.

Attempts to photochemically produce radicals in rare gas matrices are usually frustrated by their tendency to recombine. This severely limits the spectroscopic techniques that can be used to detect the presence of a radical. In particular, relatively large concentrations of radicals are required for a reliable infrared identification. As shown below absorp­ tion , , hu j A.r R - R R . • R of a photon by the molecule R-R isolated in a rare gas matrix leads to formation of a radical pair. However, there is no net change in the system because the activation energy for recombination is zero. The radical recombination may be retarded by incorporating an obstruction M

hv, Ar R-M-R R ■ M . R into a suitable precursor. The obstruction is usually a small stable molecule which is photochemically cleaved from the radical precursor.

As an initial system diacyl peroxides (I) have been selected because they decompose

hv, Ar 30K R-C' V k R-R + 2C0„ U-U" *•

(i) to radical pairs and carbon dioxide when irradiated with ultraviolet light. The carbon dioxide isolates the two simultaneously generated radicalo and aupprcoaca their recombination. Warming the matrix from liquid helium temperatures to approximately 30K is expected to soften the environment enough to remove the obstruction created by the CO^ molecules thus allowing the radicals to combine and act as their own trapping reagents.

Acetyl benzoyl peroxide* (II) was isolated in an argon matrix and irradiated with light X > 3000A. 31

0\ hv,Ar,— . • 30K ,— , o ) -c x c-ch3 — “( O ) : +2C02+-ch3 --- ~ \ 0 ) -CH3+2C02 0— 0 (II) (III) (IV) Two intense bands at 710 and 612 cm ^ (and those for C02) appeared in the infrared spectrum. These synchronously disappeared with the appearance of the infrared spectrum of toluene (IV), the combination product of the phenyl and methyl radical, when the matrix was warmed to 30K for several seconds. The band at 710 cm ^ was assigned to the out of plane bending mode of the phenyl radical (III) while the 612 cm ^ feature is 2 due to the well known out of plane deformation of the methyl radical.

3 In a similar manner complete infrared spectra of the ethyl, n-propyl and n-butyl radicals have been obtained by photolysis of prop- ionyl (V), butyryl (VI) and pentyryl (VII) peroxides in argon matrices.

CH (CH C \C -(CH ) CH 3 2 n ''O-O^ 2 n 3 n = 1, (V); n = 2, (VI); n = 3, (VII)

The existence of each of these primary radicals was conclusively shown by the disappearance of the radical infrared spectrum with the appearance of the radical combination and disproportionation products respectively, when the matrix was warmed to 30K.

The most intense and characteristic feature of these primary alkyl radicals is the out of plane deformation mode at 541 cm ^ (ethyl), 530 cm ^ (n-propyl) and 527 cm ^ for the n-butyl radical. In addition the two CH stretches associated with the radical center are at 3113, 3033 cm ^ (ethyl), 3100, 3018 cm ^ (n-propyl), and 3105, 3018 cm ^ for the n-butyl radical.

Qualitatively, the infrared spectra of the primary alkyl radicals studied thus far indicate that the hybridization about the radical center 2 is sp . This conclusion was arrived at by comparing the radical spectrum with CH2CD2 which exhibits two CH stretches at 3095 and 3016 cm 3‘. How­ ever, the much lower out of plane bending frequency for the primary radi­ cals compared to CH2CD2 (943 cm *) is a direct reflection of the abscence of the 7i bond and the relative ease with which the radical center may dis­ tort. .32

The infrared spectrum of the isopropyl radical, a secondary radical, was observed by irradiation of isobutyryl peroxide (VIII) in an argon matrix. After a brief warm up to 35K the portion of the spectrum due to the isopropyl radical synchronously disappears with the appearance of bands due to 2, 3 dimethylbutane, propane and propene. For the assignment of the isopropyl radical spectrum we followed the concepts developed pre- 1 3 viously. ’ For this assignment it is essential that the combination and disproportionation products, 2, 3 dimethylbutane, propane and propene are observed upon warm up of the matrix. The reaction scheme is summariz­ ed below:

(ch 3 ) 2 chco2o 2 c c h (c h 3) 2 ( ch 3 ) 2ch + 2C02 + ( ch 3 ) 2ch

(VIII) A to V35K

(CH3)2CHCH(CH3)2 + CH3CH2CH3 + CH3CHCH2

The characteristic features of the infrared spectrum for this sec­ ondary radical are a CH stretch at 3045 cm ^ and an out of plane bending frequency at 375 cm

As in the case of the primary alkyl radicals a qualitative descrip­ tion of the structure of the isopropyl radical may be obtained by compar­ ison of its vibrational spectrum to those of olefins. The high frequency CH band at 3045 cm ^ leads us to predict that the isopropyl radical is 2 planar, or nearly planar, with considerable sp character while the low frequency out of plane bending frequency at 375 cm ^ again reflects an easy distortable radical center.

Theoretical calculations and normal coordinate analyses are in pro- grooo on thooo eyetome.

REFERENCES 1. J. Pacansky and Joachim Bargon, J. Amer. Chem. Soc., 97, 6896 (1975). 2. D. E. Milligan and M. E. Jacox, J. Chem. Phys., 47, 5146 (1967). A. Snelson, J. Phys. Chem. 74, 537 (1970). 3. J. Pacansky, G. P. Gardini and Joachim Bargon, J. Amer. Chem. Soc. 98, 2665 (1976). 33

■ PHOTOPROCESSES IN LOW TEMPERATURE MATRICES

Antony J. Rest and John R. Sodeau Department of Chemistry, The University, Southampton S09 5NH (UK).

The validity of relating results from studies in gas matrices at 4-20 K to studies of photoprocesses•in solution and vapour phases has often been 1 2 disputed. * Some examples are described to show that photochemical and photophysical phenomena studied in solution and vapour phases can now be observed routinely with advantage in matrices at 10 K. In the photolysis of W(C0)sL (L = pyridine, 3-bromopyridine) in argon 3 matrices at 10 K we have demonstrated for the first time, using I.R. spectroscopy that bulky ligands of high molecular weight can be detached

W(C0)5L % hV--» W(C0)5 + L hv' and that this reaction, which is in agreement with the quantum yield measurements in solution ($ » ), is photochromic. Similar reactions 4' are not observed in organic glasses at 77 K, which may arise from an unfavourable 'cage effect1 for glassy matrices or from the fact that the available thermal energy at 77 K is more than enough to produce back and secondary reactions, so that studies in glasses may be a poor test of photochemical mechanisms. The scattering nature of low temperature matrices produced by the slow spray-on method has limited the usefulness of the matrix technique for obtaining high quality uv spectra in the region below 300 nm. We have found that a pulsed deposition method,taking account of the pressure x volume product for a given pumping-speed system, has consistently given optically clear, non-scattering matrices for a wide variety of gases,

Matrix Gas Matrix Gas P x V (torr x cm3) P x v (torr x cm3)

n2 ca. 2000 cf4 ca. 500 ch4 ca. 2000 Xe ca. , 200 Ar ca. 1000 02 ca. 200

Kr ca. 500 (c h 3)4c ca. 50 enabling well resolved absorption and emission spectra to be obtained. Luminescence spectra obtained even under crude excitation and detection conditions were of better resolution than those obtainable in organic glasses at 77 K using a prime experimental set-up, e.g. the fluorescence spectra of indene (a) in N2 (MR 1; 10,000) at 10 K and (b) in a methylcyclo- 34

hexane glass (0.002 M indene concentration) at 77 K with excitation at 280 nm in both experiments. This work shows the scope that exists for

90

N) m a trix a t 10 K Methylcyclohsxar.e glaag at 77, 70

50

30

350 350 300 no

Pluoreeoence Spectra o f Indcno

development of the matrix technique and for utilisation to obtain high resolution spectra below 300 nm. In oxygen matrices aromatic molecules show two types of UV absorption bands which are not seen rare gas matrices: (i) intense bands due to contact charge-transfer interactions between the aromatic molecules and O2 and which can be correlated with the first ionisation potentials of these molecules;*’ (ii) weak bands due to Sq-T; absorptions of the aromatic molecules.^ Typical spectra are shown for benzene: (i) contact charge-transfer; (ii) Sq-Tj. The new matrix technique circumvents a major difficulty of classical high pressure (150-200 atm.) oxygen perturbation technique. It also overcomes the problem of estimating an accurate profile for the contact charge-transfer spectrum in the subtraction procedure by its actual measurement during the experiment. Vibrational progressions are observed for Sq-T; spectra in matrices whereas these are often unresolved in high pressure experiments. Aryl-xenon matrix experiments indicated that the 'heavy atom1 effect of xenon was insufficient to produce perturbed Sq-T; spectra at 10 K. However the 'heavy atom' effect of matrix gases was shown through the variation of the phosphorescence lifetime of benzene, in confirmation of earlier studies. Energy-migration and energy-transfer studies have been carried out in gas matrices for the first time. The data obtained for the self-quenching of benzene luminescence in argon matrices follows a relationship consistent with triplet-triplet energy-transfer. A critical separation between two o interacting benzene molecules was determined as 7.2 +_ 1.7 A, which agrees well with values obtained for the distance associated with energy-transfer 35

I:I Actual Spectrum o.i*

Corrected Background

Subtraction Specimen Subtraction Spectrum

320

230 Singlet-triplet Spcctrun

Contact Charge-Tranefar Spectrum - proceeding via an electron-exchange mechanism. Direct irradiation experiments failed to produce phosphorescence from a number of styrenes and so energy-transfer studies, i.e. sensitisation, were carried out"*"1"1 using the donor-acceptor system of toluene-phenylcyclobutene in argon matrices at 10 K. Even with such a good triplet energy donor as toluene, it was found that, within the sensitivity limits of the experiments, 1-phenylcyclobutene triplets do not emit but the intensity of the toluene phosphorescence and the phosphorescence lifetime were both reduced. These results have been explained as arising from inefficient intersystem crossing (direct irradiation failing to populate the Tj state) and to the presence of an efficient non-radiative relaxation route from the triplet state which is presumably not rotation about the olefinic styrene double bond. Attempts to obtain luminescence spectra from a variety of organometallic compounds in gas matrices at 10 K by direct excitation methods were unsuccess­ ful. However, energy-transfer experiments using the toluene-Cr(C0)g donor- acceptor system in argon matrices showed a drastic reduction in the toluene phosphorescence lifetime and a featureless emission which was ascribed"*'"*' to the 3T ^ -*• phosphorescence of a Cr(C0)g. This technique can clearly be applied to other organometallic compounds and may at last provide a 36

general method for obtaining luminescence data to facilitate the inter­ pretations of the photochemical reactions of these compounds.

CtSs<»,.'CW

UniM ioxiei Spectra o f ToltMnf.CrtCO)^

1. J. P. Simons, 'Photochemistry and Spectroscopy', Wiley, 1971. 2. J. A. Barltrop and J. D. Coyle, 'Excited States in Organic Chemistry', Wiley, 1975. 3. A. J. Rest and J. R . Sodeau, J.C.S. Chem. Comm., 1975, 696; J.C.S. Dalton, submitted for publication. It. J. D. Black, M. J. Boylan, P. S. Braterman and W. J. Wallace, J . Organomet. Chem.. 1973, 63, C21. 5. M. M. Rochkind, Science, 1968. l60. 197. 6 . A. J. Rest, K. Salisbury and J. R. Sodeau, J.C.S. Faraday II. 1977, 13, 265 . 7 . A. J. Root, K. yaliobury and J. R; Dodcaa, J.C.O. Faraday II. accepted. 8 . G. W. Robinson and R. P. Frosch, J. Chem. Phys., .1.963, 38, 1187 .

9 . R. G. Bennett and R. E. Kellogg in 'Prog, in Reaction Kinetics', 1967, ii, 215. 10. P. M. Crosby, J. M. Dyke, J. Metcalfe, A. J. Rest, K. Salisbury and J. R. Sodeau. J.C.S. Perkin II. 1977, 182. 11. A. J. Rest and J. R. Sodeau, J.C.S. Faraday II, submitted. 37

Structural Studies of Carbon Halide Free Radicals in Matrices

HOSSEIN M.ROJHANTALAB Department of Chemistry College of Science Jundi Shahpur University Ahwaz ,. Iran

and .

JOSEPH W. NIBLER Department of Chemistry Oregon state University • '■ Corvallis , Oregon , U.S.A.

In a series of electric discharge and pyrolysis experiments .in different oven assemblies carbon halides were generated and trapped in argon and nitrogen matrices at U K and the species were identified bv infrared spectroscopy. In each case the experimental conditions were optimized to maximize the CX, product (X=Br,Cl). -1 In CBr^ experiments , the band at 773 cm assigned earlier(1-3) to J? in CBr, radical was observed in every instance but an intriguing 871 cm absorption, not previously reported, was observed only in the electric discharge experiments. Upon repeated annealing the intensties of both these bands decreased enormously(90%) while the other bands changed only slightly (10%). This result is consistent with a small species of extreme reactivity and , by analogy with earlier .work on CClj and CC1* (3), we believe 871 cm * is the }Jj mode of CBr* species.. The resultant spectrum(U) observed tor the thermal decomposition products of CBr^ near 800 °C indicated that a very small amount of th.e parent compound survived at this temperature and only a small • . • . concentration of CBr, radical was trapped. On the other hand the very -I strong ' features between 460-340 cm indicated the alarming presence of new species in the matrix. The origin of these features is believed to be iron bromide compounds (FeBr^ , FeBr^, Fe^Br^, Fe^Br^) formed by the reaction of Br atom with the steel nozzle at the experimental condition. The formation of iron bromide molecules was further supported 38 by the absence of infrared absorption in this region for the pyrolysis products of CBr^ in the quartz oven for the same temperature range. The infrared spectrum of CC1^ and its thermal decomposition products in a Ta Knudsen cell and a steel nozzle oven in a nitrogen matrix are con­ trasted in Figure 1. In diffusion experiments the intensity of the absorp­ tion bands at 900, 848, and 225 cm diminished enormously (70%), the bands at 925, 915, 865, 825, and 583 cm 1 grew markedly (50 %), while the remain­ ing bands, below 1000 cm ' remained the same or changed only slightly (15 %), In all experiments, the j/j band of CC1 ^ was observed at 900 cm ' , in agreement with earlier work (1,2). In no experiment was a feature observed at 674 cm ', the frequency assigned to U^ of CC1 ^ (1). Another fragment produced by pyrolysis is CC1 (848 cm ' in N^) which has previously been identified at 870 cm ' in an argon matrix (3, 5). Presumably CCl^ is also produced but the region in which it absorbs (J2j = 741 cm ') (3 - 6) was

Obscured Ifl our experiments by the very intense CC11, absorptions. Unfortu­ nately Raman spectra of these radicals were obscured by strong fluorescence and also by burning of the matrix due to the presence of highly colored halogens, and metal halides. Of particular interest are the features observed in the 300-500 cm region which covers the FeCl and FeBr stretching frequency range. These bands were also prominent in CBrCl- pyrolysis experiments (4). Bands near _ 1 ^ 493 and 463 cm are comparable in intensity to the CC1^ feature and compare favorably with the strongest bands reported for FeCl (488-499 ') -1 and FeClj (465 cm ). Thus these species are clearly produced in the pyrolysis reaction. In their analysis of the electron diffraction data, Legget and Kohl (7) noted a major peak at 2. 2 °A (2.16 A when corrected for anharmoni- city) which they attributed in part to an "abnormally long" CBr distance (normally 1.94 A) produced by a "long lived excited state of CBrCl^ and/or CBrCiy. Since our results indicate the presence of some FeCI^ and FeCi ^ (even at lower temperature than they used) it is reveal inq to observe that the bond lengths in these two compounds occur at 2.17 and 2.14 ’A (6) almost exactly at the value of the "abnormal" CBr bond length reported. This suggests that a reanalysis of.the diffraction data is in order and warns that the structural conclusions reached for CC1^ , CBrCl^, and CBr^ (pyramidal with nearly tetrahedral angles) are suspect. REFERENCES: (1) L. Andrews, J. Chem. Rhys. 48, 972 (1971); L. Andrews and T.G. 39

1000 500

428 CC1 ./N CC1 325'

900 446 CC1

CC1

1000 500 FREQUENCY Ccm ) Figure 1. Infrared spectra of matrix isolated CCl^ (a), and its pyrolysis products formed after passing through: b) a Ta Knudsen cell, c) a steel nozzle oven. Matrices were formed at N 2 :C C 1 4* 1 00 in (a,b) and 1000 in (c) during a deposition period and an oven tem­ perature of: a ) 4 HR, 2 5 ’C ; b) 40 HR, 7 00 °C; c ) 25 HR 65 0 ’C; at a flow rate (mmole/HR), of: a) 1-2; b,c) 0 .1-0 .5. 40

Carver, ibid, 49, 896 (1967); 50, 4235 (1968). (2) E.E. Rogers, S. Abramowitz, M.E. Jacox, and D.E. Milligan, J. Chem. Phys. 52, 2198 (1969). (3) M.E. Jacox and D.E. Milligan, J. Chem. Phys. 53, 2688 (1970); 54,'

3935 (1 9 7 0 ). (A) H.M. Rojhantalab, Ph.D. Thesis, Oregon State University (1976). (5) L. Andrews and D.W. Smith, J. Chem. Phys. 53^ 2956 (1970).

(6) R.0.; Allen, J.M. Grzybowski and L. Andrews, J. Chem. Phys. J2.< 898 (1975). (7) T.L. Legget, R.E. Kennerly, and D.A. Kohl, J. Chem. Phys. 60,3264 (1974); T.L,. Legget and D.A. Kohl, J. Chem. Phys. 59, 611 (1973). 41

infrared Matrixspectra of lower sulfur- and seleniumfluorides

A. Haas and H. Willner Leiirstuhl fur Anorganische Chemie der Ruhr-Universitat Bochum

Binary sulfur- and seleniumfluorides in lower oxidation states re very reactive, therefore matrixtechnique should be exspecially suitable for vibrational investigations. Until now there are published matrix spectra of SF,, (1) and SeF,, (2), which are rela­ tively easy to handle.

Our .own investigations (3) showed that IR-matrixspectroscopy is an useful method for observing the transformation from FSSF into SSF2 . In addition to this work we recorded the IR-spectra of a new •seleniumf luoride and for the first time of SF2 . Since 40 years (4) SF2 is searched for and til now only identified by microwave- (5) and mass-spectroscopy (6, 7).

The formation of our SF2-matrix succeeded in exchanging the halogene with HgF2 in a highly diluted SC12/rare gas mixture. SF2 was 3 ^ S-substituted and the observed 3 11 S-shift allowed to calculate, the bond angle and the General Valence Force Field. The bond angle observed by Powell (8) and his estimated frequency values are in good agreement with our results.

A new seleniumfluoride was formed by fluorination (F2 :Ar 1:400) of elementary selenium at 200°C.' Further informations will be expected by use of different Se-isotopes. 42

(1) K.O. Christe, E.C. Curtis, Schack, S.J. Cyvin, J. Brunvoll arc V/. SawoAny SpectrochiK. Acta ? 2 A , 1141 (1976)

(2) C.J. Adams and A . J . Downs Spectrochim. Acta 28A, 1841 (1972)

(7) A. Haas and H. Willner, unpublished

(4) 0. Rail Angew. Chem. 46, 739 (1933)

(5) D.3. Johnson and I'.X, Powell Science 164, 950 (1969)

(6) 0. Glemser, W . D. Heussner and A. Haas A’aturwiss. 50, 402 (1965)

(7) P. Seel, E. Heinrich, W. Gombler ana R. Budenz Chimia 25, 75 (1969)

(8) W.H. Kirchhoff, D.R. Johnson and F.X. Powell J. Mol. Spectry. 48, 157 (1975) 43

SPECTRA OF METAL ATOMS AND CLUSTERS

B. Meyer

Chemistry Department, University of Washington Seattle, WA 98195, and MMRD, Lawrence Be.rkeley Lab, University of California Berkeley, CA 94720

This lecture will touch three subjects which will be elaborated in more detail in the printed paper. In the first part the present status of knowledge of metal spectra in low temperature matrices will be reviewed; in the second trends in the field will be discussed, and the development of interpreta­ tion and understanding of spectra will be summarized by select examples from recent work. The third part will deal with possible goals of future work and will contain suggestions for potentially rewarding experiments, as well as speculation about future developments. Supporting references to work in the matrix field and other fields related to clusters will be available in the form of an appendix. 1. Review of Current Status During the last twenty years some 40 different metal species have been deposited in atomic form in low temperature matrices. Mercury, sodium, and iron, to name only three, have been studied by at least a dozen different groups' under a variety of conditions. Recent work on systems such as copper, silver, and gold has greatly helped distinguish between the various effects which influence spectra. In several cases, we now recognize effects due to the host, those due to guest properties, and those due to other interactions, among which inadvertant impurities are no longer an unrecognized factor. VJe now know what type of spectra we can expect if we study an unknown system, but we cannot reliably predict spectra nor do we understand why the spectra of different species exhibit different sensitivity to the various parameters. Most researchers are not interested in such solid-state work, but use spectra mainly as an analytical tool to identify inter­ mediates in cryochemical reactions which aim at producing heteronuclear species. Only very few researchers have 44 attempted thorough work on homonuclear species, and all have been hampered by difficulties which are greater than expected and which make the work hard and not immediately rewarding. Thus, much work in this area has not been published. However, the long-range benefit of. such work warrants a full-range effort using modern ultra-high purity techniques and modern, high sensitive and high resolution spectroscopy of all kinds. So far we know that ESR, Mtissbauer, uv, and vacuum uv spectra can be easily recorded. Emission work which will yield most valuable data on energy transfer has been difficult due to impurities and poorly understood quenching processes. Absorption spectra are usually broad, exhibit fine structure, and are susceptible to energy shifts which depend on matrix and guest-guest interactions,site geometry, and temperature. It has recently become possible to correlate quite complex spectra of transition metals in matrices to those in the gas phase, but in all cases only very few manifolds are known. Part of the difficulty in identifying atomic spectra of atoms is still due to the problems correlated to the sample prepara­ tion. Many metals are not available in ultra-pure form, and it is hard to obtain monatomic vapors free from impurities, oxides, and hydroxides originating from the sample or its container. Several samples will be discussed, using mass- spectroscopic data to support matrix information. The interpretation of atomic data, including crystal field, spin- orbit, and others, including AMCOR, will be discussed. The matrix spectra of diatomic metal species are easier to interpret despite the fact that vibrational frequencies are normally small, and, in the visible, often comparable to the line width or hand width. Diatomics lend themselves to emission studies, resonance fluorescence, phosphorescence, and Raman work, including intensity and lifetime work. Diatomics can be produced in the vapor, in situ, by diffusion and by photolysis and other methods. Isotopes further aid identification. Triatomics and polyatomics are harder to study, because they often occur in mixture. However, poly­ atomics and metal clusters form readily and are present in many matrices, and, therefore, in many matrix spectra. 45

Clusters of alkali, alkaline earth, Group lb and lib, and transition metals have been widely studied, but little has been published. This is due to the difficulty of producing these species under reproducible conditions, experimental difficulties in identifying these species, and difficulties in obtaining individual species of uniform size and structure. However, the properties of these species are of widely recog­ nized importance in many fields, including catalysis, photog­ raphy, etc., and much proprietory work in matrices and similar systems is under way. Since success in this field could benefit from transfer of information from fields outside matrix work, some key references to work in other media and under other conditions will be quoted. This will also serve to stimulate awareness of the importance of matrix work to other fields. 2. Current Trends Most currently available data refers to systems which have been studied under very narrow preparation conditions and with only one or two detection techniques. Often, only the strongest allowed visible transition is reported, and often the spectra contain indications that the main species is in the presence of a variety of other guests which strongly interact. We now know that the dominant transitions in gas phase species can be found in and correlated to matrix spectra, but neither the intensity, nor the transition energy, line width, nor fine structure can be reliably predicted. No systematic trends have been established for members of families of isoelectronic species, such as the alkali metals in rare gases, and the influence of polarizeability, heavy atom effect, dipole moments, and other well known and established effects are not even qualitatively known. Such correlations, as for example the correlation between dipole moment of the matrix, are important for using the knowledge which is available in the area of doped ion crystals and the like. The recent extensive work on ions in rare gas matrices demonstrates the viability of such work. The recent trend of conducting Raman, Mflssbauer and ESR studies on matrices constitutes an encouraging trend. 40 but only half a dozen samples have been studied with more than one technique. Since sample reproducibility is difficult, it' is important that simultaneous study of samples be undertaken. This is especially important if annealing and site effects are involved. 3. Potential for Future Work Vegard, Pimentel, Broida, Robinson, and many others, including many participants at this meeting, have initiated and completed work which will have influence upon many fields of industry and research. At this conference the substantial potential of chemical work, both synthetic and analytical, is thoroughly represented, and reflects very well the dominant present trends. It would be desirable, and help this current work, if solid-state physics work of matrices doped with metal atoms and clusters would be pursued, to establish interactions, energy transfer, diffusion, and many other effects' which are basic to all low temperature activities. In this third part of the lecture three systems will be described as examples of the type of work which would be immediately rewarding, not only to the project involved, but also to much of the matrix work. In all these examples, a combination of complementary preparation techniques is em­ ployed, and the characteristics of periodic relations are invoked to produce data which is significant and for- which all working parameters are characterized. Such work, in­ cluding experiments to selectively prepare intermediates in the formation of metal clusters, can be enlarged and com­ plemented with other similar examples which any experienced matrix spectroscopist can propose. A detailed series of references will be supplied to document these examples and tu aid workers in 1,1 ih field.' Those who hold unpublished data in this field will be encouraged to make slides of their data' available, either before the lecture, for inclusion in the presentation, or after the lecture for aiding in the discussion. 47

Mossbauer and Infrared Spectroscopic Studies • of an Iron-Nitrogen Molecule■Produced in a Nitrogen Matrix Paul Barrett. University of California, Santa Barbara and Pedro Montano, West Virginia University

Mossbauer and infrared absorption spectra have been ob­ tained of matricies consisting of iron and nitrogen codepos­ ited on a cold substrate. At low concentrations of iron where the probability of an iron atom having another iron atom as a nearest neighbor is very small the Mossbauer spectra ^ of "”^Fe monomer has the same isomer shift -1 57 (-0.78mm s ) as the Fe monomer isolated in Ar, Kr, and Xe (2 ) matricies . The non-cubic site symmetry of the nitrogen matrix produces a quadrupole splitting of the monomer line. At higher concentrations (0.3 to 1.2% at.) of iron we observe in the Mossbauer spectra in addition to the monomer a doublet whose isomer shift and quadrupole splitting (+0.71 and 1.17mm s *) differ from those of the iron dimer isolated in rare gas matricies (-0.14 and 4.05mm s *). A positive shift of the isomer shift relative to the isolated monomer is associated with a decrease in the electron density at the nucleus and with bonding to the iron atom. The data suggests that an iron-nitrogen molecule is formed in the nitrogen matrix and that the iron dimer is the initiating structure in the bonding of nitrogen to iron in this situation. Mea­ surements of the internal magnetic field at the iron nucleus are consistent with the proposed iron-nitrogen molecule being diamagnetic. The value of the quadrupole splitting and the probable diamagnetism are not unexpected for a co­ valent iron compound. However, the isomer shift is more positive than that observed in stable covalent iron compounds and indicates less o-donation and/or n-backbonding in the iron-nitrogen ligand. Infrared absorption spectra were als.o obtained for iron-nitrogen matricies produced in a manner similar to that used for the Mossbauer measurements. Absorption lines in the range 2017 to 2261cm-1 were observed. If the spectral lines are due to the N2 stretching frequency becoming 48 allowed and shifted due to bonding to iron dimers the absorp­ tion coefficients of the lines should be proportional to the square of the iron concentrations. This quadratic relation­ ship is observed for iron concentrations below 3%. Argon matricies with low concentrations of iron and nitrogen pro­ duce Mossbauer and infrared spectra that differ from those obtained in an iron-nitrogen matrix. Experiments of iron in a methane matrix indicate that there is iron dimer-methane bonding and not iron monomer-methane bonding in the formation of low temperature matricies.

1. H. Micklitz and P.H. Barrett, Appl. Phys. Letters, 20, 387 (1972). 2. T.K. McNab, H. Micklitz and P.H. Barrett, Phys. Rev. B4, 3787 (1971) . 49

On the Theory of the Matrix Influence on the Spectra of Isolated Atoms

F . Forstmann , Insti tut fUr theor. Phys i k der Freien Uni versi tat Berlin, D-l Berlin

Metal atoms isolated in rare gas matrices have optical ab­ sorption spectra which are d iffe re n t from those of the free atoms. The absorption lines are shifted and often split, but it is generally easy to recognize the close relationship to the gas phase spectra. This analysis is based on the careful measurements on the 1-3 nine systems Cu, Ag, Au in Ar, Kr, Xe . The general observa­ tion is an increase of the excitation energy, of the energy distance between the groundstate s level and the p level of the excited outer electron, and a threefold split of the P*—S tra n ­ sition, which is only twofold split by spi n-orbi t interaction in the free atom. These principle facts have been observed earlier^. Several authors 55 / have discussed the b lu e -sh ift of . the absorption line by fi tti ng. parameters of a Leonard-Jones potential.to the shift value. This fit is based on the assump­ tions that the shifts of the electronic levels in the ground- state have the same dependence on the i ntermolecular distance as the binding energy, that this dependence can be parametrized by a Lennard-Jones (6-12) potential, that the same form of the dependence applies to the excited electron states, and that the parameters for the ground state of the heteorogeneous molecule can be gained by averaging the values for the components, assum­ ing a Lennard-Jones type interaction also between metal atoms. This parametri sa ti on is at best a description of the fact that the distance of the levels is increased when the matrix atoms approach the embedded metal atom. The explanation of this level s h if t has to be saught in the in teractio n of the metal electrons with the electrons and nuclei of the matrix atoms. The main point in rare gas matrix iso latio n is that this in te r ­ action is weak and should be treatable by perturbation theory. The wavefunction on the metal atom should be approximately 50

+ with oCi« r -ir 7 Vai atom l'C, f.i ,matri x a i and the electron energy

ET ' Eatom + ^ matri x

The first correction to the energy is the crystal field influ­ ence while the second is due to molecular orbital interaction. The crystal field perturbation can split the P*—S transition at most threefold. In one exceptional c a s e , Ag in Xe, a fo u r­ fold split could be explained by molecular orbital interaction 2 due to an accidental degeneracy of an atomic and a matrix level , In that case we derived a value of V .=0.008 eV. The shifts of a i the transitions in the here studied systems are larger by a fac­ tor of 10 to 30. Therefore it is concluded that the crystal field approximation should be a good model for analysing the ma­ trix perturbation and that molecular orbital interaction can be neglected except for cases of degeneracy. The parameters of the crystal fie ld model have also been derived by other authors for A1 and Ga® and for Au^ in rare gas m atrices. The three optical tra n sitio n s seen in the isolated atoms allow the determination of the following parameters:

1.) The difference D of the shift of the s and the p level by the spherically symmetrical part of the matrix potential at the metal:

D - ilfp l2 V0fr) r2dr - J | f / V0fr) r2 dr

2.) The parameter 8 responsible for the difference of the p- level sh ifts due to an axial symmetry of the surrounding of

the me la 1 alum:

2 . r dr

3.) The spi n-orbi t interaction of the p-level with the spheri­ cally symmetrical part of the potential at the metal atom: 51

Also the temperature dependence of these parameters was measured*’■*. From the trends of the parameters we conclude the follow­ ing characteristic features of the crystal field: ne The increase of A compared to atom value means a positive gra­ dient of the spherically symmetrical part of the crystal field. Because 'f is more extended than 'fs, the positive gradient agrees with the blue shift (D >0) of the absorption lines. The sequence A( Ar) > A( Kr) >. A (Xe) found for all metals is in accord with the sequence D(Ar) > D(Kr) > D(Xe). We suppose that the larger gradient for the crystal field introduced by the argon matrix is related to a stronger confinement and a higher rigidity of the charge distribution in an argen atom as shown by a sharper decay of the outermost wavefuncti ons and a smaller pola ri zab i 1 i ty . There are two contributions to the crystal field: the near nuclei are attractive, but the increase of the negative charge density in a solid compared to the atom furnishes a repulsive potential. The net result .of these contributions is generally repulsive, as can be deduced from comparison of work functions and atomic ionization potentials. In the case of a xenon atom isolated in Ar a decrease of the binding energy by 2 eV has been reported*1*. These arguments lead to a potential at the me­ tal atom as sketched in Fig 1. Further information can be derived from the temperature de­ pendence of the parameters. The interaction changes because of increased vibrational motion with larger mean displacements for higher temperatures and because of thermal expansion of the ma­ trix cage. In previous investigations the decrease of the level shift D for increasing temperature has been interpreted as a de­ crease of the interaction with the matrix due to an enlargement of the cage, a trend to a more free atom like behavior. The temperature dependence of the spin-orbit parameter A shows no tendency to the free atom value. Because thermal expansion is an effect due to the anharmoni city, it is of higher order than the 52 increase of the mean displacements of the atoms, which would make an increase of the interaction between metal and matrix plausible. Although the perturbation of the s and p level is increased, the difference D of the level shifts can decrease. The potential model in Fig. 1 has the consequence that less localized metal electrons will finally see the attractive potential of the neighbours, especially a smaller gradient of the potential. This fact would reverse the temperature change for A and could even lead to a red s h if t (0 0). We suggest, that this situation holds for gold, especially in Xenon.

o — \fr e e atom potential

Metal Noble gas Atom Atom Fig. 1: Schematic diagram of the potential deformation at the metal atom due to the neighbouring matrix atoms.

1) D.M. Kolb, D. Leutloff, this conference 2) F. Forstmann, D.M. Kolb, W. Schulze, J. Chem. Phys. b4, 2552 (1976) 3) f . FOrStmann, u.m. koid, D. teutl or t" , W. Schulze, J. Chem. Phys. 6 6 , 0000 (1977) 4) B. Meyer, Low Temperature Spectroscopy (Elsevier, N.Y., 1971) 5 M. McCarty, Jr., G.W. Robinson, Mol. Phys. 2 , 41b (19by) fi) J . Y. Knnr.in. Chem. Phys. L etters d . 4Uti ( l?b9 I 7) O.Y. Roncin, J. Quant. Spectrosc. Kadiat. Transf. 11, 1151 (1971) 8 ) d.H. Ammeter, D.C. S chlosnagle, J. Chem. Phys. 5£, 4784 (1973) 9) D.M. Gruen, S.L. Gandioso, R.L. Beth, J.L. L erner, J. Chem. Phys. 60, 89 (1974) 10) V. Saile et al. in: Proc. IV Int. Conf. on Vacuum Ultra­ violet Radiation Physics, 1974 Hamburg, p. 352 53

Observation of Pseudolocalized Vibrational Modes in the Optical Spectra of Rare Earth Atoms Isolated in Rare Gas Matrices

. M. Jakob, ' H. Micklitz and K. Luchner

Lehrstuhl fUr Didaktik der Physik, Universitat MUnchen, 8046 Garching, W. Germany

We have studied the optical absorption and emission spectra due to 4 fn6s^ <— ► 4fn"* 5d6s^ tra n sitio n s in Eu- and Sm-atoms iso ­ lated in the rare-gas matrices Ne, Ar, Kr and Xe. Typical sample thickness were ~ 5 /< for the absorption spectra and ~50yU for the emission spectra with a rare-earth atomic con­ centration of -10 The samples have been deposited at 4.2 K and annealed at —0.3 T^ (T^ = melting temperature of matrix). The spectra were taken with a resolution of -10 cm"*.

The absorption spectra at a matrix temperature of 4.2 K show zero-phonon lines with a half-width = 1 0 cm"* and vibrational sidebands (Stokes lines) due to pseudolocalized modes in the rare gas : rare earth phonon spectra. The observed vibrational modes have frequencies in the range "12 cm'* to 40 cm"*, i.e . they are well below the c u t-o ff frequency of the phonon spectra of the undopcd-rare gas solids.

As an example fig. 1 shows the absorption spectrum of Eu atoms 7 P Q A p o * in an argon matrix due to the 4f 6 s S7 / 2 — ► 4f 5d6s P 5 / 2 transition. The excited ^ 5 / 7 state is split by a non-cubic crystalline field into three Kramer d o u b .l e t s .. Each of the three transitions shows the above mentioned structure: zero-phonon lines (ZP) accompanied by vibrational sidebands due to the modes = ( 1 2 t 2 ) cm"*, = (26.2) cm * and = (46^4) cm"* [l]. The vibrational sideband structure of the ab­ sorption lines is only determined by the rare-gas matrix, but independent of the kind of rare earth atoms used. This can be seen in fig. 2 , which shows a comparison between the

4f^6s^ ^^ 7 / 2 -----► 4f®5d6s^ ^ 5 / 2 tra n sitio n in Eu and the

I 54

[orr'l Fig. 1. Optical absorption 33100 29900 spectrum of Eu atoms isolated in an argon matrix at 4.2 K 7 ? o due to 4 f '6 s ‘ S 7 / 2

4f®5d6s2 ®Pgy 2 transition ZP ZP ZP for two different samples, upper spectrum: sample thickness-6^* , lower spectrum: sample thickness~70yu . [l] .

•Oa <

3330 3350 [A]’

4f66s2 7F, 4f55d6s2 70. transition in Sm for the different rare-gas matrices.

Excitation of 4fn"* 5d6s2 states due to resonance absorption is followed by emission from lower 4fn"* 5d6s2-states (non­ resonance transitions). These emission lines show the same vibrational sidebands (Stokes lines) as the absorption lines, but now of course at the low-energy side of the zero-phonon lines. The vibrational sideband structure of the zero-phonon lines is also only determined by the rare-gas matrix, i.e. Eu- and Sm-lines have the identical sideband structure in the same t*ai*e-gas hiatrik (see fig. J).

With-increasing matrix temperature (4.2 K & T * 20 K) we see, as to be expected, a decrease of the intensity of the zero-phonon line relative to the intensity of the vibrational sideband and the appearance of anti-Stokes vibrational sidebands. This can be seen in fig. 4, which shows the 3655 A emission line due to

the 4f^5d6s2 ^^7 / 2 — * 4f76s2 ^ 7 / 2 transi t i 'o n o f Eu atom in an argon matrix. The excitation occured in the 4f®5d6s2 ®Dg^ i 55

[cm -1] [cm -1] 27200 26800 30200 29800

NeiSm Ne:Eu

Ar:Sm Ar:Eu

Kr:Sm

Xe:Sm Xe:Eu

3680 3720 3760 3380

Fig. 2. Optical absorption spectra of Eu and Sm atoms isolated in rare-gas matrices ( Ne , Ar , Kr , Xe) at 4.2 K due to 4f^6s^ 7/2 —»• 4f 5d6s P r,, tra n sitio n in Eu and 4f 6 s F —» 5 2 / 5 / 2 o 4f 5d6s Dj tra n sitio n in Sm. Absorption lines in the gaseous state are indicated.

state with the Ar+-laser line at X = 3511 S. With increasing matrix temperature the anti-Stokes vibrational sideband of mode Uj ( tiuj = 12± 2 cm'1) appears due to thermal population of this vibrational mode ( ~ 17 K). The additional feature of the anti-Stokes side at a distance of = 26±2 cm 1 from the .ZP-line also present at matrix temperatur of 4.2 K might be due to "hot luminescence" [ 2]. 56

[cm-'] Fig. 3. Structure of 28800 28500 25100 24800 optical emission lines for Eu and Sm atoms isolated in Ne: Eu Ne:Sm rare-gas matrices (Ne.Ar) at 4.2 K

(4 fn_ 1 5d6s 2 —

4 fn 6 s 2 transiti ons).

Ar-Fu Ar:Sm

3470 3510 3980

Fig. 4. Temperature ♦ 100 crri'-o ZP dependence of 3655 A emission line of Eu atoms isolated in an 10 K 11 K argon matrix due to

4f 6 5d6s 2 8D 7/2 4 f 7 6s 2 ®S7 / 2 tran- s i ti on after excita­ tion of Ar: Eu 4f 6 bdbs 2 8 ti,,„V 2 state wi th the Ar - i aser

1 ine at. X = .3511 A. 18 K 24 K [2]-

[1 ] M. Jakob, H. Micklitz and K. Luchner, Phys. Letters 57A (1970) 67.

[2] M. Jakob, H. Micklitz and K. Luchner, to be published in Phys. Letters (1977). 57

ANALYSIS OF MATRIX INDUCED CHANGES IN THE SPECTRA OF ISOLATED METAL ATOMS

D.M . Kolb and D . Leutloff Fritz-Haber-Institut der Max-Planck-Gesellachaft, . Faradayweg 4-6, 1 Berlin 33

It is well known, that matrix isolated atoms suffer a marked change in their electronic properties as compared to their unperturbed atomic counter parts, due to the interaction with the matrix. This has been studied by various groups for metal atoms using absorption spectroscopy, where the matrix influence manifests itself in the spectra by line broadening, level shifts and additional level eplit- 1 2 ting ’ . In order to obtain a detailed information on this matrix influence we have measured the spectra of Cu, Ag and Au in Ne, Ar, Kr, Xe and Ng matrices as a function of matrix temperature over a wide temperature range. These matrices were carefully annealed to eliminate any influence of the condensation conditions on the spectra and to obtain uniform trapping sites for the isolated atoms. The ■ optical transition under consideration is P«-S, the P level being threefold split by spin-orbit and crystal field interaction.

The peak positions of the three absorption lines and their temperature dependence were evaluated on the basis of a crystal field modeF* ® to yield the relative level shift D(T), that is the difference in the shifts for P and S levels, the spin-orbit splitting A(T) and the crystal field parameter B (T). These three parameters are shown in Fig. 1 and 2 for the system s Cu, Ag and Au-in Ar, Kr and Xe, as a function of matrix temperature T. They yield a more direct information about changes in the electronic properties of isolated atoms than the absorption spectra alone. E.g. A(T) allows us to draw conclusions about the gradient of the perturbing potential. It is a quantity which reflects only the behaviour of the P-level and not . of level differences as peak positions in the spectra or even D(T) values do. A qualitative model for the matrix influence as derived from the data shown in Fig. 1 and 2 is briefly discussed. One important conclusion is that the matrix influence obviously does not decrease with increasing temperature as was common­ ly believed, because of the temperature dilatation in the matrix. 58

320 310

29 0 Ar A u - Ar Ar 310 300

260 180 300 ItiU 200

170 Kr o 170 190

160

160 180

Xe

Xe

-7 0

0 20 40 60

6U 11U

100

<

20 60 20 40 60 T/K Fig. 1: Temperature dependence of the difference D in level shifts and the spin-orbit splitting A for Cu, Ag und Au in Ar, Kr and Xe. (-----); A -values for the free atom.

The crystal field calculation in general gives solutions for A and B only, when it is assumed that B is negative and hence the + *ies on e n e r gy

scale above the Pgy2 + 3 / 2 * ThuS dn unambiguous assignment of all three absorption peaks in the spectrum is possible. In connection with a model for the interaction the sign of B gives a first information on the direction of the distortion of the matrix cage, which causes the crystal field asymmetry. 59

80

Ar Ar Xe,

m 50 Xe

Au

0 20 60 TXK Fig. 2:Temperature dependence of the crystal field parameter B for Cu, Ag and Au in Ar, Kr and Xe.

From the somewhat different peak heights in the spectra recorded at normal incidence we concluded that the symmetry axis of the distorted matrix cage is to some extent preferentially alligned perpendicular to the surface. Measurements with polarized light at oblique angles of incidence seem to support this assumption. Finally preliminary results on fluorescence experiments are reported.

References:

1) see e.g. B. Meyer, Low Temperature Spectroscopy (Elsevier, New York, 1971).

2) W. Schulze, D.M . Kolb and H. Gerischer, J .C .S .. Faraday n, 71 (1975) 1763.

3) J.H . Ammeter and D .C . Schlosnagle, J. Chem. Phys. 59 (1973 ) 4 784.

4) D.M. Gruen, S.L. Gaudioso, R.L. McBeth and J.L. Lerner, J. Chem. Phys. 60 (1974) 89.

5) F. Forstmann, D.M. Kolb, D. Leutloff and W. Schulze, J. Chem. Phys. j>6 (1977). 60

INFRARED ABSORPTION IN LiF POLYMERS AND MICROCRYSTALS

T.P. Martin Max-Planck-Institut ftir Festkorperforschung, Stuttgart

Infrared absorption spectra are reported for LiF in various stages of aggregation. Starting with monomers and dimers the degree of aggregation is increased to include more complex po­ lymers until the transition is made to microcrystals and fi­ nally to bulk. The samples were prepared at helium temperature by the simultaneous condensation of argon and LiF vapor onto a transparent substrate. The argon matrix served the purpose of separating the LiF aggregates without strongly interacting with them. In the present work aggregates have been formed by using a relatively high concentration of LiF in the Ar matrix. This promotes cluster formation by surface diffusion during the growth of the matrix.

Figure 1 gives an overview of what happens when the LiF concentration in the argon matrix is varied over a large range. For very small concentrations, 1 part LiF to 1000 parts argon, only a few absorption lines are observed. Linevsky^ has shown that in an argon matrix the monomer absorption is a doublet with peak frequencies 843 and 838 cm . In Fig. 1 it can be seen that the absorption at this monomer doublet domi­ nates the spectrum for very low concentrations of LiF. However, even when the concentration of LiF to argon is 1 to 1000, se­ veral other lines are also present. This is due to the fact that the vapor from an oven at 7b0°C contains about b0% dimers.^ When the concentration of LiF in argon is increased above 0.1%, new lines appear in the infrared absorption spectrum. Two of these lines, at 274 and 540 cm \ disappear when the sample is annealed to 40°K, seeming to indicate that they are associated with an unstable form of a LiF polymer. These two absorption peaks are represented by a dotted line in Fig.1. Most of the remaining lines are due to aggregates which are formed after the LiF has reached the matrix. This conclusion was reached 61

after studying the concen­ MONOMER DIMER. tration dependence shown in _u’,F^ IOOO Fig. 2. Here we have plot­ ted the integrated absorp­ tion of monomer lines, I. 1 ' AGGREGATE and dimers lines, I2, di- vided by the total absorp­ tion" olf. the sample, I For low concentrations I 1 ' V) I_ and I are all propor­ t • 2 o z 3 tional to concentration, OS o: that is, I.j/1 and I2/Iq < z will be approximately in­ o i/i MICROCRYSTAL dependent of concentration. m z A relatively weak concen­ z(/) SURFACE PHONON o:< v eiui,l»-2Em . tration dependence is, how­ ever, observed in the ex­ periment. The monomer in- tcnoity goes down wi th in­ 3T° 0 creasing. concentration and BULK the dimer intensity up. PHONON This is due to a process which is proportional to the square of the concen­ 1000 800 600 <00 200 tration, i.e., the aggrega­ WAVE NUMBER (cm"1) tion of two monomers in the matrix forming a dimer. Such an aggregation is also possible between a monomer and a dimer or between two dimers. The frac­ tion of the total absorption due to aggregates made by two va­ por components -t2c/-t0 should vary linearly with concentration. Since the log of the fractional absorption of the. lines at 330, 400, 582 and 692 cm 1 have a positive slope of about one when plotted against the log of concentration, we must conclude that this absorption is due to 6 and 8 atom aggregates formed in this manner.

If we assume that each component of the LiF vapor, whether it be a monomer of dimer, occupies only one lattice site in.the argon crystal, then the probability that two vapor components 62

! MOLECULE AGGREGATE MICROCRYSTAL Z o t— £L OC 255 cm-' s .$38 CD < 723 — l 2

2 330. LL O oZ *— < 582 nr LL o92

0.01 100 CONCENTRATION OF LIF IN ARGON (PERCENT)

occupy adjacent sites is Pj,. = 12 c^, this is the probability, c , of a vapor component occupying any site multiplied by the probability, 12c, that a vapor component occupies one of the twelve neighboring sites. When c is about .08, the probability is very high that all vapor components will have aggregates and that these near by aggregates will have joined to form lar­ ger aggregates having so many atoms that they can be .considered microcrystals. This sudden growth of aggregate size above a critical concentration is typical of many problems that can be described with percolation theory.

For concentrations between 7 and 60% the infrared absorption spectrum is dominated by one broad band With a peak ranging from 480 to 32b cm-1. This behaviour indicates that microcry­ stals have formed within the matrix. For crystals small com­ pared to the wavelength of the incident light, the frequency of peak absorption is increased by the polarization charge at the surface of the crystal. A mode with uniform polarization and therefore with a large dipole moment is responsible for this absorption. The frequency, u>g, of this mode for spheri­ cally shaped particles with dielectric function e(w) embedded 63 in a nonabsorbihg medium with dielectric constant em is given

The real and imaginary part of the dielectric function of LiF has been determined by a Kramers-Kronig analysis of low tem- 4 perature reflectivity data. Using this data we find that . E q . 1 is satisfied when u>g is equal to 480 cm 1 . This value is in good agreement with the measured minimum in microcrystal transmission, Fig.1. Actually, our sample consists of aggre­ gates which are so close together that they interact with one another by means of their dipolar fields.5'® This inter­ action tends to decrease the frequency of the peak absorption as the concentration of"the LiF increases until it coincides with the bulk absorption peak at the long wavelength trans­ verse optical phonon frequency.

References

1. M.J. Linevsky, J. Chem. Phys. 3j5, 658 (1963) . 2. J. Berkowitz, H.A. Tasman and W.A. Chupka, J . Chem. Phys. 36, 2170 (1962) . 3. H . Frohlich, Theory of Dielectrics (Oxford Univ. Press, O x f o r d , 1948) . 4. J.R. Jasperse, A. Kahan, J.N. Plendl and S.S. Mitra, Phys. Rev. , M 6 , 526 (1966) . 5. S.C. Maxwell-Garnett, Phil.trans R. Soc. Lond. 203, 385 (1904) . 6. L. Genzel and T.P. Martin, Surf. Sci., 33 (1973). 64

"Applications of Matrix Isolation Techniques

To Mossbauer Spectroscopy"*

P. A. Montano. Dept. of Physics West Virginia University, Morgantown, WV 26506

P. H. Barrett, Dept, of Physics University of California, Santa Barbara, CA 93106

H. Micklitz Sektion Physik der Universitat Munchen, 8046 Garching, BRD

Various experimental techniques have been used to show that atoms and molecules isolated in a rare-gas-matrix have properties very similar to those of free species . 1 The use of the matrix isolation

technique in Mossbauer spectroscopy offers a unique opportunity for studying "almost free" atoms and molecules. This is extremely useful in the interpretation and understanding of isomer shifts (IS) and hyperfine fields .2 ’ 3 This arises from the uncertainty in the application of free atom or free ion wave functions to solids and in the choice of electron configurations for particular reference compounds.

The Mossbauer effect in 57Fe has been used to measure the magnetic

hyperfine interaction of isolated iron monomers in xenon 3 a n d a r g o n . 4

For an applied external field of 30 kOe an internal magnetic field at

the 57Fe nucleus for the iron monomer of 830 ± 15 kOe was observed. This

technique was also used to measure the magnetic hyperfine interaction

of 1zFe atoms in solid xenon. In this case a strong field dependent

Mossbauer absorption spectrum is observed, Indicating strong relaxation

effects. The ground state of these iron atoms in xenon is a triplet, which is split in the external field. The measured internal magnetic

field at the 57Fe nucleus in xenon is 700 ± 15 kOe. This value is reduced

from the free atom 5 case of 1100 kOe by the crystal field. By contrast 65

the internal field at the 57Fe nucleus in nitrogen is 900 kOe. The 1 above results indicate a strong correlation between the'internal field at the 57Fe nucleus and the:crystal field'strength.

One can also use•the Mossbauer effect in conjunction with the matrix isolation technique to measure hyperfine field parameters for simple molecules.' Using this technique the internal field at the 57Fe nucleus in iron dimers was measured, a value of 660 ± 5"kOe was obtained.

The large internal field at the 57Fe nucleus in the iron dimer indicates that the ground state of the Fe 2 molecule has a large angular momentum. This•technique has also been used to study iron- • ■ multimers in xenon 6 and -argon and:the onset of metallic conductivity in the frozen mixtures.

The internal field at the 57Fe nucleus for monovalent•iron (3d 6 4s) isolated in xenon was measured. The monovalent iron was produced using

K a s a i 's 7 method. This procedure enabled to produce Fe+ in- the matrix.

A systematic study of the:temperature dependence of the Mossbauer spectrum of Fe+ (3d 64s) was carried out. A study of all possible compounds that can be produced in the matrix between the iron and the iodine was performed. From the' temperature dependence of the quadrupole splitting and the magnitude of the internal magnetic field it was possible to identify the ground state Kramer's doublet. The effective.

Hamiltonian for the ground state doublet in the presence of an external magnetic field H, is given by

H - E Ai Si-Ii - gllBS-H - glPNT-H + Hq

where s = 1/2; I is the nuclear spin; and A^ are the components of the hyperfine constant for the ground state Kramer's doublet. The value of the free ion hyperfine constant was calculated by Freeman and Mellow using spin polarized Hartree Fock calculations; H q takes into account the 66

quadrupole interaction. In the above given Hamiltonian one can define A an effective internal field, H = „ . From the analysis of the g l U l

Mossbauer spectrum the following experimental parameters are obtained,

H z “ 3 5 0 t 10 kOe and Hx - 700 1 10 kOe. From the theoretical calculations of Freeman and Mellow the expected values are Kg ■ 368 kOe and Hx - 736 kOe; this is a- surprisingly good agreement. The above study permitted the complete identification of the iron ion and its electronic ground state in the matrix.

The Mossbauer matrix isolation technique has been applied to other

Mossbauer isotopes.a *9 The Mossbauer absorption spectra of several matrix isolated tellurium ( 12 5 Te) compounds was carried out in order to obtain a better calibration for the isomer shift. The matrix isolated

TeF6 , TeCli, and Te 2 molecules have isomer shifts of -(1.54 ± 0.05),

(1.0 ± 0.1), and +(0.34 ± 0.07) mm/sec relative to 1 2 5 Sb/Cu source respectively. There is a small difference in the IS between matrix isolated TeCl* and solid TeCl,, which probably arises from the difference in the structure between gaseous and crystalline TeCl,,. The spectra

f o r T e 2 in argon and krypton matrices are equal within the experimental errors. The Te 2 molecule shows a spectrum with a quadrupole splitting of (9.60 i 0.07) mm/sec. The Te monomer was obtained by photodissociation of H2Te in argon. The Te monomer has an IS of (0.0 i 0.2) mm/sec.

Combining this value with that of 1 2 5 TeCl,, in argon and using Dirac- rock electron density calculations a value for the mean square radius difference nf 2.8 ± .5 x 10“ 3fmv was found at 12sTe nucleus

•Supported by the National Science Foundation.

References:

1. D. M. Mann and H/ P. Broida, J. Chem. Phys. 55 84 (1971) and references

rrmralnpd therein.

2. T. K. McNab, H. Micklitz, and P. H. Barrett, Phys. Rev. B4^ 3787 (1971). 67

3. P. A. Montano, P. H. Barrett, and Z. Shanfield, Solid State Commun.

15 1675 1975 (1974).

4. P. A. Montano, P. H. Barrett, and Z. Shanfield, J. of Chem. Phys.

64 2896 (1975).

5. W. J. Childs and L. S. Goodman, Phys. Rev. 48^, 74 (1966).

6. Z. Shanfield, P. A. Montano, and P. H. Barrett, Phys. Rev. Letters

35 1789 (1975).

7. Paul H. Kasai, Phys. Rev. Letters 21, 67 (1968). /

H. Micklitz and F. J. Litterst, Phys. Rev. Letters J33, 480 (1974) .

8. H. Micklitz, and P. H. Barrett, Phys. Rev. B5 1704 (1972) .

9. P. H. Barrett, P. A. Montano, H. Micklitz, and J. B. Mann, Phys. Rev.

B12, 1676 (1975). 68

3s-excitation of Na atoms trapped in Xe-matrices D. Nagel and B. Sonntag II. Institut fur Experimentalphysik, Universitat Hamburg, Hamburg, Germany

The optical absorption of Na atoms isolated in Xe-matrices has been inves­ tigated in the wavelengths range from 2000 X to 6400 8. The experimental arrangement is shown in Fig. 1. The samples were prepared by coevaporation of Xe and Na onto a sapphire window kept below 10 K by a He-cryostat. The

Fig. I Experimental arrangement: A lamp, B lenses, C photomultiplier, D spectrometer, E vacuum chamber, F furnace, G straight-through-valve, H matrix gas delivery tube, I variable leak valve, K quartz oscillator, L sapphire sub­ strate, M He-Ne-laser, N mirror (removable), 0 photodiode base pressure of the vacuum system was 10”7 Torr. The spectra of matrix isolated atoms critically depend on the relative concentration of the metal and the rare gas condensing on the substrate.1 Therefore the thickness Of the film was determined directly by measuring the intensity of laser light (X = 6328 8) reflected from the sample. Due to the interference of the light reflected at the surface of the film and at the substrate condensate inter­ face the intensity periodically alternates with increasing thickness. For normal incidence the change in thickness Ad corresponding to adjacent maxi­ ma ic given by ^ Ad 7 Z 69 where n is the refractive index of the film. In determining the thickness of the.samples, which varied between 100 ym and 400 .ym, the dependence of n on the condensation temperature2 was .taken into account. The interfero- metric method also allowed an exact control of the condensation rate, which varied between 28 R/sec and 440 R/sec. The amount of Na incorporated in the matrix was monitored by a quartz oscillator. The Na-deposition rate was proportional to p(T)/v^, where p(T) is the Na vapour pressure at the temperature T of the furnace. Extrapolating this dependence towards lower temperatures T Na-deposition rates too low to be monitored by the quartz oscillator were determined. The ratio M/R of the number of Xe atoms to the number of Na atoms varied between 200:1 and 2000:1. The error of the M/R ratio is estimated to be less than ±50 %. Errors mainly arise from differences in the sticking coefficients of Na-atoms on the rare gas film and on the quartz cooled to liquid temperature and from uncertainties in the density p of the rare gas film, though' the effects of p and n partly compensate.

Fig. 2 AbsutpLiuu speclia of Na in Xe at M/R = 200:1 different M/R-ratios

Broad absorption bands show up at 2250 R, 4250 R and 4900 R. The relative intensi­ 1000:1 ties of these bands strongly decrease with increasing M/R: . This indicates that these bands are due to the formation of Na clusters. No finestruc- ture could be detected for wavelengths below 3500 R. The 2000:1 absorption spectra between 4500 R and 6400 R are shown in Fig. 2 for three different

4000 4500 5000 5500 6000 M/R ratios. For ratios larger WAVELENGTH (4) than 1000:1 and for tempera- 70 tures below 20 K the Na 3s 2S , "l"3p 2P transition splits into two 1/2 r 1/2,3/2 well resolved triplets, a red triplet centered at 6000 A and a blue tri­ plet centered at 5600 8. The wavelength of the absorption lines are list­ ed in table 1. Varying the deposition speed and the thickness within the

Table 1 assignment energy position cm crystal field JTE A B C 16363 red 16691 triplet 17149

17449 17790 IR745 S..

limits stated above affected the spectra 6 0 K only slightly. The temperature dependence

5 5 K of the spectra is given in fig. 3. Raising the temperature of the sample above 30 K leads to the disappearence of the red tri­ plet, whereas the blue triplet is still detectable at 60 K. Upon recooling to 10 K 35 K only the red triplet is restored.

C0 Our data are in fair agreement with the results reported by Meyer,3 The red tri­ plet seems to be better resolved in our 2 S -3 0 K o: data and there clearly is a third compo­ nent at 6116 8. The M/R ratios given by Meyer seem to be too low. The existence 201< of the additional structures reported bY Blount1* could not be verified.

Fig. 3 Warm-up sequence of a sodium-xenon film with M/P. = 2000:1 5600 5600 6000 6200 71

A number of possible explanations have been advanced for the splitting of the Na 3s-3p transition. The interaction of Na pairs proposed by Blount is in contradiction to the results of Brewer and King5 which indicate that solute-solute interactions do not account for the splitting. The insensi­ tivity of the splitting of the blue triplet fenders the explanation of the triplet structure by multiple sites for atomic trapping very unlikely. Duley's5 assignment of lines in the red and the blue triplet to the same trapping site is at variance with the temperature dependence of the spectra.

We assign the red and the blue triplets to two different trapping sites. The splitting is ascribed to a lowering of the symmetry around the impu­ rity which partially or completely removes the degeneracy of the excited p state. In case of an axial symmetry crystal field theory yields the foil-* owing energies.7

E. = E + D + 1/2 A + 1/2 £

E2 = E + D - 1/4 A - 1/4 s + 3/4 /S2+A2-2/3£A

E3 = E + D - 1/4 A - 1/4 £ - 3/4 /S2+A2-2/3£A

E is the excitation energy for the free atom, D the energy shift, A is the crystal field parameter and £ the spin-orbit parameter. For £ and A to be real there are only the two assignments given in columns A and B of Table 1. Both assignments result in the two following sets of parameters:

red triplet D (mev) A (mev) £ (mev)

-29 -45 34 -29 34 -45

blue triplet 107 -45 36 107 36 -45

The value for A and £ for both triplets are very similar.

The spin orbit parameter of the Na 2p63p state of atomic sodium amounts to 1.4 meV. An increase of £ to "v-SS meV for Na embedded in solid Xe implies a considerable compression of the final state wave function by the surround­ ing atoms. Negative spin-orbit parameters are consistent with the results of ESR measurements on A1 trapped in rare gas matrices7 . Negative spin- orbit parameters have been invoked.. for the explanation of the triplet 72 structure found in the absorption spectra of F-centers in alkali-halides.8 The negative spin-orbit parameter arises from the orthogonalization of the impurity wave function to the wave functions of the surrounding atoms.9 The close analogy of the system under study with the F-centers in Cs-hali- des and the startling similarity of the absorption spectra led us to re­ visit the interpretation of the splitting in terms of spin-orbit coupling and Jahn-Teller coupling. In case of the F-centers the energies of the lines arc described by

E = E + D + 1/2c - A E2 = E + D + 1/21, + A

E3 = E + D - c where A stands for the Jahn-Teller energy.8 The analogy to the F-centers implies the assignment presented in column C of Table 1. This results in A - 20 meV and t, = -51 me.V for both triplets. This spin-orbit parameter is in good agreement with the results obtained for the F-centers in Cs- halides. Aslong as there is no detailed information on the nature of the trapping sites a definite answer on the origin of the splitting cannot be given.

References

1. B. Meyer, Low Temperature Spectroscopy, American Elsevier Publishing Co. New York (1971 ) 2. W. Ocl.uUe, D.M. Kolb, J. of the Chem.Coc. Faraday Trano. II, 70, 1090 (1974) 3. B. Meyer, J.Chem.Phys. 4^3, 2986 (1965) 4. C.E, Blount, Technical Report No. 1, Contract No. ONR N OOOI4-66-CO 195, Texas Christian Univ. Fort Worth, Texas (1969) 5. L. Brewer and B. King, J.Chom.Phyc. 53^, 3981 (1970) 6. W.W. Duley, Can.J. of Phys. 4^, 477 (19/0) 7. J.H. Ammeter and D.C. Schlosnagle, J.Chem.Phys. 4784 (1973) 8. M.D. Sturge, Solid State Physics ^0, 92 (1967) 9. D.Y. Smith, Phys.Rev. J_37, A574 (1965) 73

OPTICAL ABSORPTION OF MATRIX ISOLATED SILVER AGGREGATES AND MICROCRYSTALS

Max-Planck-Institut fur Festkorperforschung, 7 Stuttgart 80, Federal Republic of Germany

T. Welker.

Introduction. In contrast to the discrete absorption lines of silver aggregates containing a small number of atoms, silver particles contai­ ning 103 106 atoms, called microcrystals, demonstrate one broad absoption band. This absorption is caused by a collective exication of all free electrons in the particle and can be described by a free electron gas model. In bulk silver, collectively displayed free electrons see no repulsive force and therefore the resonant frequency is zero. This results in a strong absorption at zero frequency which decreases with decreasing wave­ length . In microcrystals the surface polarization produces field opposite to the electric field of the incident light and causes an absorption maximum at finite frequencies. For spherical particles which are small compared to the wavelength of the incident light and which are embedded in a nonabsorbing medium with die­ lectric constant e , the surface plasmon frequency is defined as follows:*

(“s> = - 2 E m e ^ is the real part of the dielectric function of the particle. It would be of basic physical interest to investigate the absorption of particles with 1-103 atoms in order to see the transition from one-electrcn-exica- tions to the collective exication. The technique of isolation in frozen rare gas is very suitable for stabilizing and isolating such aggregates and microcrystals. •

Experiment. In the present work the microcrystals and aggregates were produced by a standard matrix isolation technique. Atomic silver vapour from a resistance heated Ta-furnace (1150 C) and argon were simultane­ ously condensed on a saphire window. The aggregates and microcrystals were formed by using a relatively high concentration of silver in the Ar matrix. This promotes cluster formation by surface diffusion during the growth of the matrix. The argon condensation rate was determined by observing the interference fringes of the sample in the visible and near 74 infrared. The silver condensation rate was controlled by means of a quartz oscillator

Results. In the figure, the measured optical transmission of several Ag- argon samples are plotted as a function of wavelength. For clarity, the curves are displaced vertically with respect to one another. For 0.8% there are three sharp absorption lines which can be attributed to atomic 2 ) silver. With increasing concentration up to 3% several absorption lines appear superimposed on a broad absorption band. Above 12% all discrete absorption lines disappear and only the surface plasmon absorption could be seen.

0.0 n _ . 0.8% 2.5%

3.57, - 1.0

-Z0 127.

o -3.0

65%

-6 0 CD

917. -5.0

- 0.0

300 nm 500 nm 700nm WAVELENGTH

Logarithms of the optical transmission of Ag-argon films. The mol% of sliver Is ind-l.CdL.ed In i.he figure. 17 2 Silver condensation rates: 91%-sample 5.8.10 atoms/cm min. 65%-sample 17 2 16 2 2.3.10 atoms/cm min. all other samples 7.2.10 atoms/cm min. 75

Aggregates. The triplet structure of atomic silver at 299 nm, 304 nm and 315 nm could be observed up to 5%. Two sharp but weak linesat 323 nm and 332 nm also appear at very low concentrations. They were observed even in a sample with only 0,2% silver. As the silver vapour contains only monomers^ and the halfwidth of these weak lines is very small, they must also.be assigned to atomic silver. When the sample is warmed to 40K these two lines disappear but a weak peak at 230 nm remains. This peak could be caused by atomic silver embedded in an imperfect matrix region, while the two sharp lines belong to unstable interstitial atomic silver. For silver concentration near 3% two strong lines at 265 nm and 386 nm could be observed which have been assigned to 2) B •+• X and A X transitions of Ag^ . Additionally the 386 nm peak has a shoulder at 404 nm. Two weak absorption lines at 283 nm and 503 nm appear in this concentration region. Because of the strong surface plasmon abs- sorption, it is impossible to determine the integrated absorption of the lines and so obtain information by comparing spectra of different silver concentration. However, the line at 283 nm has nearly the same strength as the 265 nm peak for a 3.5 %-sample, while for a 2.5 %-sample it is much weaker. So the 283 line must be caused by another aggregate.

Microcrystals. The surface plasmon absorption maximum for samples up to 5% is at 345 nm and shifts to 413 nm for a concentration of 91%. Using 4) equation (1) and the dielectric function of silver , one expects an ab­ sorption at 380 nm. For higher concentration the interaction between the microcrystals causes a red shift of the absorption maximum. For a layer of nonabsorbing material which contains a volume fraction f of small spherical particles equation (I) becomes^*

e , (a) ) = - e (2 + f ) / ( 1- f ) (2) 1 s m

For neglible particle density (2) becomes identical to (1). For 10vol% silver the absorption is at 391 nm. The measured maxima are shifted to shorter wavelength. This could be explained by: (i) non spherical par­ ticles, (ii) shape distributation, (iii) for very small particles the bulk dielectric function^ will be modified. The 91%-spectrum demon­ strates an increasing absorption above 700 nm. For this high concentra­ tion the absorption of the sample is qualitatively similar to that of bulk silver, i.e. the absorption increases with decreasing frequency. 76

References.

1) G. Mie, Ann. Physik 25^, 377 (1908)- 2) Schulze, Becker, Leutl, Fritz-Haber-Institut Berlin

private communication

3) Gmelins Handbuch d . anorg. Chemie 61 A2 (1970) and references in i t 4) U . Kreibig, J . Phys. F : Metal Phys. £, 999 (1974) and ref. in it 5) L. Genzel, T.P. Martin, Surface Science 34_, 33 (1973) 6) L. Genzel, T.P. Martin, U . Kreibig, Z . Physik B21, 339 (1975) and Ref. 4) 77

STABLE MOLECULES IN MATRICES

A.J. Barnes

(DeparCment of Chemistry and Applied Chemistry, University of Salford, Salford M5 AWT, U.K.)

Introduction Although the matrix isolation technique was originally devised with a view to trapping unstable species, and has since been used extensively for that purpose, it also gives many advantages over more conventional spectroscopic techniques in the study of stable molecules. The isolation of monomeric solute molecules in an inert environment reduces inter- molecular interactions, resulting in a sharpening of the solute bands compared with other condensed phases - this effect being particularly dramatic, of course, for hydrogen bonding substances. With the exception of a fex>7 small molecules, rotation does not occur in matrices thus giving much narrower'bands than are obtained in the vapour phase. Also the low temperature itself results in smaller bandwidths. Consequently, near­ degenerate bands« x-zhich overlap completely even in the vapour phase or in dilute solution at room temperature, may often be resolved in matrix spectra. Applications to vibrational assignment, conformational analysis, and quantitative analysis, arising from this advantage of matrix isolation spectroscopy, will.be discussed. The effects of intermolecular interactions are not entirely absent from matrix spectra and it is particularly important that these so-called matrix effects are properly taken into account xjhen considering the spectra of previously unknovm species. Stable molecules are a useful probe of the interaction betx^een the matrix and the solute: the effect of the matrix on the knox-m spectrum of the solute can easily be examined. Alternatively it is possible to increase the concentration of the solute species so that isolation is not .achieved (or to add a "dopant", i.e. a second solute) in order to study hydrogen bonding and other intermolecular interactions between solute molecules in the inert matrix environment.

Matrix effects The most obvious matrix effect is that the vibrational levels of the solute molecule xvill be perturbed by the matrix and thus the vibrational frequencies will be shifted from their gas phase values. Additionally, multiplet band structure may arise from rotation of the solute molecule 78 in its trapping site (distinguishable by reversible temperature dependence of the band intensities), alternative trapping sites in the matrix, aggregation of the solute, or from lifting of the degeneracy of vibrational transitions (e.g. the bending mode of HCN in a matrix such as nitrogen). Perturbation by the matrix environment may induce inactive modes (e.g. the hydrogen fundamental vibration is observed in the infrared spectrum in argon matrices) and similarly inactive matrix lattice modes (e.g. in argon) may be induced by the presence of the solute.

Study of intermolecular interactions The matrix isolation technique has been used extensively in the study of self-association of hydrogen bonding substances: hydrogen halides, water, alcohols, ammonia, amines, carboxylic acids, etc. Increasing interest is being shown in the use of matrix isolation to study hetero-association: Pimentel [l] has examined a number of hydrogen chloride and bromide complexes (Including Strongly nyarogetl-bdnded pairs such as ammonia - hydrogen chloride) and Barnes et al [2] hydrogen iodide complexes by infrared spectroscopy, Frediii [3] has used infrared and ultraviolet/visible spectroscopy to study a range of weak molecular complexes, while we have recently reported [_4j the use of Raman spectroscopy to study alkene-halogen complexes.

Vibrational assignment The small bandwidths obtained and, more particularly, the trapping of monomer species make matrix isolation a valuable tool in carrying out a vibrational assignment. The possibility of varying the concentration so as to observe the Increase in tne intensities of dimer bands at the expense of monomer bands enables assignments to be made with greater certainty than by, for example, comparing gas phase and liquid or solid phase spectra, especially when large shifts occur from monomer to dimer. This can be illustrated by reference to methanol, where the in-plane methyl rocking mode is coupled to the in-plane OH bending and thus shifts ca. 30 cm ^ on aggregation [5] , and by maleimide, where the out-of-plane NH bending mode shifts over 200 cm * from monomer to dimer [jij . Another example of the advantages of the matrix technique is provided by methylamine [7] : a complete assignment of the CH^NHD spectrum could be made from the matrix data, allowing the force field of methylamine to be refined and the position of the twisting mode (which had been the subject of controversy) predicted. 79

Conformational isomerism Conformational isomers can often be identified from small splittings of vibrational bands in matrix spectra, whereas in spectra from other phases the bandwidths may be greater than the separation of the bands due to the two conformers. This is well illustrated by ethanol , where the monomer OH stretching band in carbon tetrachloride solution is symmetrical while in matrices two bands are observed due to the trans and gauche conformers:

A r

Detailed consideration of other modes which show splitting enabled the trans conformer to be identified as the more stable. It is usually assumed that the conformational equilibrium existing in the gas phase prior to deposition is trapped out, but a number of examples have been reported where this is clearly not true and these systems appear to be in equilibrium at the temperature of the matrix. The barrier to internal rotation in the matrix should be an important parameter in determining which situation applies to a particular molecule. Some years ago, Hall and Pimentel [9] reported the infrared induced cis-trans isomerisation of nitrous acid in a matrix. The advent of tunable infrared lasers suggests the possibility of carrying out such interconversions with great selectivity.

Quantitative analysis Rochkind [ lo ] proposed the use of "pseudo matrix isolation" (pulsed deposition in a nitrogen matrix at M/S 100) as a method of analysing gas mixtures, and reported data for a wide range of small molecules. More recently Mamantov et al £llj have applied matrix isolation Fourier transform infrared spectroscopy to the analysis of polycyclic aromatic hydrocarbons. 80

References 1. B. S. Ault, E. Steinbeck and G. C.' Pimentel, J. Phys. Chem. , 79 (1975) 615. 2. A. J. Barnes, J. B. Davies, H. E. Hallam and J. D. R. Howells, J. Chem. Soc. Faraday II, 69 (1973) 246. 3. • e.g. L. Frediii and B. Nelander, Moll Phys., 27 (1974) 805. ’ . 4. A. J. Barnes, D. Cowieson and S. Suzuki, Proc. 5th Int. Conf. Raman Spectroscopy, Verlag, Freiburg, 1976. 5. A. J. Barnes and H. E. Hallam, Trans. Faraday Soc., 66 (1970) 1920. 6. A, J, Barnes, L", Le Gall., C. Madec and J. Lauransan, J. Mol. Struct., in press. 7. C. J. Purnell, A. J. Barnes, S. Suzuki, D. F. Ball and W. J. Orville- Thomas, Chem. Phys., 12 (1976) 77. 8. A. J. Barnes and H. E. Hallam, Trans. Faraday Soc., 66 (1970) 1932. 9. R. T. Hall and G. C. Pimentel, J. Chem. Phys., 38 (1963) 1889. 10. M. M. Rochkind, Spectrochim. Acta A, 27 (1971) 547. 11. G. Mamantov, E. L. Wehry, R. R. Kemmerer and E. R. Hinton, Anal. Chem., 49 (1977) 86. 81

Theoretical interpretation of infrared line broadening of mole­ cules trapped in rare gas matrices.

M. ALLAVENA. H. CHAKROUN. Centre de M6canique Ondulatoire AppliquSe (C.N.R.S.) . 23, rue du Maroc - 75019 PARIS - FRANCE.

D. WHITE Department of. Chemistry, university of Pennsylvania Philadelphia, pa 19174 - U.S.A.

INTRODUCTION- Some features of the interaction between guest molecules and tfeir host lattice may be revealed by analysis of the band shape of trapped species in matrices. Nowadays high resolution spectros­ copy of isolated species ^provides reliable data on infrared band shape and is stimulating theoretical interpretation. Re­ cent developments have focussed attention on the relaxation of simple vibrational systems embedded in rigid media such as the one-dimensional harmonic oscillator coupled to a phonon bath .' In this perspective we have developed a method to cal­ culate the absorption coefficient of an isolated molecule in­ teracting with lattice vibrations. Application to one-dimen­ sional models has confirmed the role of phonon coupling in the broadening of libron'^' or rotational lines. We present here an extention of this work to the three-dimensional model in the case of rotational-lattice mode coupling, and interpret the broadening of the R (o) line of HCl embedded in an Ar matrix as being due essentially to phonon broadening.

Model and Method- A non-rigid rotator is assumed to occupy a substitutional site in a f.c.c host crystal, in the zero phonon state, the center of mass of the molecular impurity is at the center of the Brillouin zone. The interaction potential is taken to be the sum of twelve Lennard-Jones potentials binding the molecule to each of its nearest neighbours. This potential is expressed in a basis of spherical harmonics and then expanded in a Taylor serie with respect to rare gas atom displacements . V(ecf Q Sp) = Z A*1 A)Y o m ^ r Ll*v in LN\ 82

are the spherical harmonics depending on the orienta­ tion (6,

q(t-t') S ),?]>

M(t) is the permanent dipole operator in the Heisenberg re­ presentation and e is the polarization vector. The average is taken over the canonical ensemble: for any (^operator = SpCPe-^H with JS=1/KT (K: Boltzmann's constant, T absolute temperature). 0 (t-t) is the step function and H = H„(e.

W n is the imperturbed rovibrational frequency.The band shape approaches the Lorentzian profile as a limiting case. Within these limits N (W) , is a constant and the absorption frequen­ cy is given by Uj' = ) and the width at half-height is 2 I ((•»') 83

The frequency shift is essentially due to the Rq contribution (static effect). The R^ and R2 contributions represent less than 10% of the total shift; the last contribution is tempe­ rature dependant and tends to lower the barrier to rotation for increasing temperature. In 1 ( , the first contribution is temperature-independant and temperature effects originate in the. second term (two-phonon processes) which predominates.

Comparison with Experiment. As in our previous work, we consider the case of HCl em­ bedded in an Ar matrix. The spectrum was recorded between 4 and 20°K with a resolution of .05 cm-* in the vibrational region. Theoretical results are compared with the tempera­ ture dependence of the R(o) line. A reasonable set of parame­ ters permits reproduction of the line broadening which increa­ ses from 1 to 3 cm in the temperature range considered.

References (1) H. Dubost. Chem. Phys. 1_2 139 (1976) H. Dubost, L. Abouaf-Marguin. Chem. Phys. Lett. 1_7 269 (1972) (2) A. Nitzan, R.J. Silbey. J. Chem. Phys. 60 4070 (1974) (3) J. Jortner. Mol. Phys. _32 379 (1976) (4) D. J. Diestler. Mol. P h y s . .32 1091 (1976) (5) S.H. Lin, H.P. Lin, D. Knittel. J. Chem. Phys. 64 441 (1976) (6) E. Blaisten-Barojas, M. Allavena. J. Phys. C 9. 3121 (1976) (7) M. Allavena, H. Chakroun, D. White. Mol. Spectres, of Dense Phase. Elsevier ed. 365 (1976) (8) D.F. Devonshire. Proc. Roy. Soc. A153 601 (1936) (9) R. Kubo. J. Phys. Soc. Japan 12 570 (1957) (10) D.N. Zubarev. Sov. phys. Usp. 3. 320 (1960) (11) N. Neto, G. Taddei, S. Califano, S.H. Walmsley Mol. Phys. 21 457 (1976) 84

STUDIES OF INTERMOLECULAR INTERACTIONS BY MATRIX ISOLATION VIBRATIONAL SPECTROSCOPY '

A.J. Barnes (Department of Chemistry and Applied Chemistry, University of Snlford, Salford M5 M-JT, U.K.)'

Introduction

It has long been recognised that matrix isolation is of great value in studying hydrogen bonding interactions - one of the earliest matrix studies was of methanol by van Thiel; Becker and Pimental [1). Trapping the hydrogen-bonded multimer in an inert environment at low temperatures minimises unwanted interactions and gives•sharp bands capable of yielding much more detailed information than could be obtained from gas, liquid, solution or solid phases. The technique is of equal value in studying molecular complexes,•and a number of unusual weak complexes (e.g. HC1 - N2) have been observed in low temperature matrices. Raman spectroscopy, as well as infrared, is of considerable value in studying intermolecular interactions. Comparison of the infrared and Raman spectra of the Nl^ stretching region of methyFamine in matrices [2] shows that in the Raman spectrum, unlike the infrared, the multimer bands are not intensified relative to the monomer and thus the monomer bands predominate even at M/S 100. This is most useful, since it is often impossible to use high M/S ratios in matrix isolation Raman spectroscopy because of the weakness of the scattering.

Cclf association of IICN

The spectra of monomeric HCN in low temperature matrices have been extensively studied [3-5], and the dimer was examined by King and Nixon [3] who concluded that, as in the gas phase, it has a linear structure. As part of a detailed re-examination of the infrared spectra of hydrogen cyanide in a range of matrices [6], wc have identified bands due to trimer and tetramer, as well as those due to monomer and dimer, from their concentration dependence in argon matrices. The HC stretching and HCN bending regions are easy to study, but the CN stretching region is difficult because the monomer and dimer bands are weak whereas the absorp­ tion due to high multimer is not only intense but occurs in the middle of the region, thus obscuring any trimer or tetramer bands that may be present. A normal co-ordinate analysis' of the monomer, dimer and trimer o-l frequencies gave the following force, constants (mdyn A ):

species KCH ' KCN “h CN

monome r 5.8 0 17.86 0.258 * dimer (1) 5.79 18.21 0.266 (2) ■ 5.29 17.79 0.308 trimer (1) 5.77 18.25 0.268 (2) 5.23 18.00 0.312 (3) 5.28 17.45 0.322

* the molecules are numbered:

H-CEN*•••H-CEN • • "H-CEN (1) (2) (3)

The CH stretching and HCN bending force constants behave much as might be expected, while the CN stretching force constant decreases for the non­ bonded group (the highest numbered molecule) but increases slightly for the bonded groups. The force constants reflect the increasing strength of the hydrogen bond as the length of the chain increases.

Self-association of ammonia

The infrared spectra of ammonia in matrices have also been studied by a number of groups [7-10], The structure of the dimer may be either' open chain or cyclic: 86

If the structure is open chain there should be bands lying close to the monomer band due to the non-bonded NH^ group. There is in fact some evidence for this. The monomer bending absorption in a nitrogen matrix acquires an asymmetry at higher concentrations which could be attributed to a dimer band, while the Raman spectrum of the symmetric stretching region shows a weak side band on the very intense monomer band. More convincing evidence that the dimer has an open chain structure is pro- 14 15 vided by spectra of ^ 3 / ^ 3 mixtures which give doublets for each dimer band, rather than triplets as would be expected for a cyclic structure. Bands due to trimer and tetramer species were also identified and the force constants derived from a normal co-ordinate analysis of the monomer, dimer and trimer frequencies show a similar trend to those ol)tallied lur hydrogen cyanide. ’

Ammonia-hydrogen chloride complex

Ault and Pimentel [11] recently published infrared spectra of the strongly hydrogen-bonded NH«HC1 complex trapped in a nitrogen matrix. -1 The spectrum showed an intense very broad absorption around 700 cm with weaker features at higher frequencies. We have examined the same complex in both argon and nitrogen matrices, and find considerable differences between the spectra in the two matrices. The very broad absorption near 700 cm ^ -in the nitrogen matrix sharpens up in argon to a band at 736 cm \ and lesser changes occur in the higher frequency region. The changes around 700 cm ^ are interpreted in terms of Fermi resonance between the NH^ rock and the overtone of the bend, the tesuuauce being stronger in a nitrogen matrix because matrix shifts bring Llie frequencies closer together. Infrared spectra of methylamine- hydrogen chloride complexes show similar differences between argon and nitrogen matrices.

Alkfeufe halugfen complexes

Fredin's group have studied a number of halogen complexes in matrices by infrared and ultraviolet spectroscopy [12]. Raman spectro­ scopy should give more information, since many of the "interesting11 modes of species such as ethylene-halogen or benzene-halogen complexes are either infrared inactive or very weak. We have observed the perturbed I-I stretching frequency in the resonance Raman effect for several alkene-iodine complexes, but were unable to locate the alkene vibrational 87 modes because of the high background due to the iodine. No such diffic­ ulty was encountered with chlorine complexes, and the Raman spectrum of the ethylene-chlorine complex [13] gave data complementary to the infra­ red results [14].

Acknowledgement

This paper is based on results obtained by (in alphabetical order) D. Cowieson, S. Holroyd, Dr. Z. Mielke, Dr. C.J. Purnell, M. Stuckey,- Dr. S. Suzuki and Dr. B. Walsh.

References

1 M. van Thiel, E.D. Becker and G.C. Pimentel, J. Chem. Phys., 27 (1957) 95. 2 C.J. Purnell, A.J. Barnes, S. Suzuki, D.F. Ball and W.J. Orville- Thomas, Chem. Phys., 12 (1976) 77. 3 C.M'. King and E.R. Nixon, J. Chem. Phys., 48 (1968) 1685. 4 -J. Pacansky and G.V. Calder, J. Phys. Chem., 76 (1972) 454. 5 J. Pacansky and G.V. Calder, J. Mol. Struct., 14 (1972) 363. 6 B. Walsh, A.J. Barnes, S. Suzuki and W.J. Orville-Thomas, to be published. 7 G.C. Pimentel, M.O. Bulanin and M. van Thiel, J. Chem. Phys., 36 (1962) 500. 8 K. Rosengren and G.C. Pimentel, J. Chem. Phys., 43 (1965) 507. 9 G. Ribbegard, Chem. Phys., 8 (1975) 185. 10 A.J. Barnes, C.J. Purnell and S. Suzuki, to be published. 11 B.S. Ault and G.C. Pimentel, J. Phys. Chem., 77 (1973) 1649. 12 e.g., L. Fredin and B. Nelander, Mol. Phys., 27 (1974) 885. 13 A.J. Barnes, D. Cowieson and S. Suzuki, Proc. 5th Int. Conf. Raman Spectroscopy, Verlag, Freiburg, 1976. 14 L. Fredin and B. Nelander, J. Mol. Struct., 16 (1973) 205. 88

SELF-ASSOCIATION OF OXIMES IN ARGON MATRICES STUDIED BY IR-SPECTROSCUPY

Andreas BEHRENS. Werner A.P.LUCK, Bernhard MANN Fachbereich Physikalische Chemie der Universitdt Auf den Lahnbergen, D-3550 Marburg/Lahn

The association of different oximes can be described very well by a coupled equilibrium of cyclic dimers and trimers, as indi­ rectly shown in previous papers by the study of the concentra­ tion dependence of the OH stretching vibration in the overtone 1 1 2 1 region 1 , . The energy per H-bond of a trimer is higher than that in a dimer, because of the angle dependent strength of the hydrogen bond^*. The favoured trimer structure of cy­ clic OH:N H-bonds with the optimal bond angle of zero is found in crystalline actone oxime by X-ray diffraction4-*. Studies of the OH stretching vibration in the. fundamental region of

CH, H Cli CH3 " C ' ' C acetaldoxim Jl, and actone oxime Jl % % A ' H in CCl^ solutions confirm the overtone measurements^*. The spec­ tra have a broad concentration dependent H-bond band caused by the overlapping of different dimer and trimer bands. A bigger frequency shift of the trimer band parallel to the H-bond ener­ gy is found. Matrix spectroscopic studies present separate bands with a different concentration dependence related to a dimer and trimer equilibrium as shown in fig. 1. The matrix technique give? the possibility to study the effect of hydro­ gen bonding on all IR fundamentals in the region between 4000 cm-1 and 250 cm-1 without disturbance by solvent absorption bands. The IR spectra of acetone- and acetaldoxime in argon ma­ trices at 4 K were recorded at various M/A values with a Perkin Elmer 325 spectrometer. Because of the low vapour pressure the matrices of acetone oxime with lower M/A values were prepared by mixing the argon and the oxime in a heated steel vessel. This mixture was sprayed onto a CsJ window through a heated tube at deposition rates between 1 and 20 umol/min. All the IR 89

Acetonoxim

M..0.

. M _ 1.78

M_ HO Acetonoxim

4000 3000 2000

Figure 1: Acetone oxime in Argon Matrix at 4 K. • bands in the observed region were assigned to normal vibrations of different aggregates. The molecular vibrations exist in three forms: The monomer species in the high diluted spectra have bands with small half width. At high concentration there are bands, which are still present in the spectra of pure oxime. Table 1: (Frequencies in cm"') Acetone oxime Acetaldoxime Assignment A' 3633 36 33 monomer V0H 34oosh,328o 34oosh,3295 dimer 32o5,3140,29oo 32oo,3135,29o8 trimer A' 1339 125o monomer d0H 141o ? 1435 ? dimer 15oo 1Soo trimer 365 36 7 monomer A' ' d0H 68o 7So dimer 8oo 8oo trimer 9 0

They were assigned to absorptions of the cyclic OH:N bonded trimer. The third one, normally in the middle of the others, be­ haves like a band of a dimer species. For example: the anti­ symmetric CC stretching vibration (A1) of acetone oxime has a monomer peak (1250 cm''), a dimer peak (1260 cm’ ') and a tri­ mer band (1270 cm- '). The OH stretching and deformation bands (in plane and out of plane modes), given in table 1, show a si­ milar behaviour. The good qualitative agreement between the ma­ trix results and the association model of the liquids stimula­ ted us to interpret.the matrix spectra quantitatively. Taking into account the demixing effect during spraying onto the IR window^ and the intensity of the CHj stretching vibrations, we calculated the amount of oxime and its concentration in the ma­ trix. With these data we get the extinction coefficients of the absorption bands. To prove the association behaviour in the ma­ trix, methods known from solution spectroscopy are used. The extinction coefficient at infinite dilution is determined and the average association number m = 2.8 is calculated for both oximes, comparable to m = 2.97 for acetaldoxime in highly con­ centrated CCl^ solutions. Fig. 2 demonstrates the determination of the association constants of dimers and trimers K^.

$ g

[Cl - [Mol 2 3 id— LJL = — ♦ — [Mo] [Mo] 2 K 12 K 13

6-

Mp<10 M ot/cm 1)

Figure 2: Determination of and for acetone oxime in matrix 91

With these results we can show, that the angle dependence of' the OH frequency shift of oximes is caused by the angle depen­ dence of the H-bond energy. This experience is in agreement with the interpretation of dif­ ferent water association bands observed by the matrix technique as angle dependent H-bond bands..The contradictionary interpre­ tations of the water matrix spectra can be ordered by a reas­ signment given in fig. 3. The water matrix spectra limit the continuum model of liquid water and show the preference of certain H-bond structures.

1375 9 24 A 10 6 11 12

3700 3600 3500 3400 3300 cm"1 Position of H20-bonds in an Ar (probably doped) matrix V, = 11 Monomer: ^ cycl. Trimer: ^ Polymer: V. = 2 V, = 10 V3 = 12

Dimers:

I in.=

Hs cycl.: z - , H (splitting caused by coupling X = 8 of the two molecules) H

Figure 3: Reassignment of water matrix bands

Literature: 1) Luck, W.A.P., Ber .Bunsenges ., £5, 355 (1961) 2) Geiseler,G., S.LUck, J.Fruwert, Spectrochim.Acta 31A, 789 (1975) 3) Luck, W.A.P., Naturwissenschaften, S2_, 25 (1965) 4) Bierlein, T.K., E.C.Lingafelter, Acta Cryst., £,450 (1951) 5) Behrens, A., Diplomarbeit, Marburg 1975 6) Mann, B ., A.Behrens, Paper presented on this meeting 92

Cage size analysis with diatomic molecules trapped in rare gas matrices

V. CHANDRASEKHARAN and E. BOURSEY L.I.M.H.P. Centre Universitaire PARIS-NORD 93430 Villetaneuse -'France

Tnfrndnrtinn :

The interactions in molecular solids are generally investigated by the comportment of the shifts of the transitions from gas to solid. As for example the experimentally observed shifts of the vibrational or 1 2 3 vibronic transitions of CO (or NO) isolated in a Ne ’ ’ matrix are inconsistent with the respective Lennard-Jones diameter (or

General formulation

If there are vacancies and impurity molecules, the atoms of the perfect lattice will relax until the force on every atom and molecule is zero. As this general problem is intractable one generally makes the hypothesis that only atoms within the interaction radius Rq around the impurity relax and outside the interaction sphere RQ the atoms do not move. Let there be M atoms, two vacancies and one molecule within the sphere of interaction around the molecule. Since the atoms at the edge of the sphere R will interact with all atoms within a sphere of radius RQ around them we have to consider a sphere of radius 2Rq around the impurity I Containing N + M atoms + 2V + 11, where V is a vacancy.

The development of the pair potential around the unrelaxed configuration is • ■

V__, = V„ + £ !+ J £7^ where A. "ol ( - ) \ 3 n -rn„ ' a*; and . I 2!2--- ) °* (* X "da • 3 a „ /n.” a ” - the equilibrium condition of particle m is then :

A , u. . + A _ = o

In order to simplify this problem we use a trick. A vacancy can be expressed as the sum of an atom (Gv) plus a negative atom (S„) i.e. vacancy = atom - atom = Gv + Sv 94

Then G + rest of the atoms form a perfect lattice. Therefore the crystal can he represented by crystal + 2V + 11 = perfect lattice + 2Sy + II (3)

As in the perfect lattice there are no forces on the atoms, forces on each atom arise only from the interactions with 2S v +11. This brings out clearly the origin of the lattice relaxation. The effect of the atoms in rigid lattice part is thus completely eliminated. The restoring forces on an atom in the perfect lattice is independant of the origin due to translational symmetry. Then the linear set of equations (2) can be solved.

In the next step of the iteration when all the atoms have moved we introduce at each positions of the perfect lattice an atom (G^) and a negative atom (5^). Now the forces arise only from the set of "dipoles" formed by atoms (M) and negative atoms (S^) plus those due to 2SV + 11. In this stage off-diagonal elements of the interaction matrix may be ignored. The iteration is continued until the energy is minimized. If Epis the energy per particule of the perfect lattice, then the energy of M atoms in (M+2) sites before relaxation is

ss (4) where Egg is the interaction between two negative (S ) atoms.

If we introduce now the impurity I the energy becomes

O) where Ef is the interaction energy of the guest molecule with the lattice. 95

Symmetry considerations

To carry out the calculations we assumed that for a homonuclear molecule placed in a di-vacancy site, the D2^ site symmetry is preserved during relaxation. Then only the totally symmetric displacements are allowed. For example this reduces the number of independant coordinates for the 18 nearest neighbour to 8 and similarly for the second and third neighbour to 12 and 25.

References

1. H. DUBOST Chem. Phys. 12, 139 (1976) 2. E. BOURSEY and J.-Y. RONCIN J. Mol. Spectry 55, 31 (1975) 3. E. BOURSEY J. Mol. Spectry 61^, 11 (1976) 96

Induced Infrared Spectra of Homonuclear Diatomic Molecules in Zeolitic Matrices

Horst Forster and Manfred Schuldt

Institute of Physical Chemistry, University of Hamburg, Laufgraben 24, 2000 Hamburg 15, Federal Republic of Germany

Obviously, matrix-isolation studies are not only restricted . to noble-gas crystals. Trapping molecules within zeolitic cavities has several attractive features: - The theoretical descriptions of these systems lie some­ where between the extremes "Uncharged molecules in non­ polar environment" and "Charged molecular defect in po- 1ar environment". - In comparison with solid noble gases, zeolites offer lar­ ger cavities for external motions (i.e. 11 8 for zeolite A), but also much stronger anisotropic forces. - The transmittance of the host lattice is quite good be- tween 4000 and 1200 cm . Radiation loss is essentially due to light scattering from the compressed crystallites. In contrast to clathrates, however, the far-infrared pro- 1 2 perties are not good due to phonon and impurity-induced phonon modes so that only one study is published^. - Since the sorbed molecules occupy sites in the neighbour- hood of the cations in the cage , at best axial site sym­ metries exist, and examples of infrared-"forbidden" bands are found , which are of considerable intensity due to the large electric fields inside the cavities. - Molecular motion studies are not only of theoretical in­ terest, but offer a possibility to understand catalytic processes, Only few are conducted so far^’^. V/e have decided to investigate homonuclear diatomic mole­ cules to see whether the fundamentals will get sufficient intensity in the infrared. It is known that in other envi- 8*9 10 11 ronments , including the silica surface ’ , only very weak bands have been found. Also, complications due to 12 translational-rotational coupling are minimized Thin self supporting wafers of zeolites NaA and NaCaA of 0.1 mm thickness and 19 mm diameter were pretreated in UHV 97

13 at 675-K in an all-metal dual-range IR-FIR cell , which also enabled-the. determination of the temperature distribution ' across the pellet. The adsorbates were of analytical grade ■ and used without further purification. The spectra were ob­ tained with a Fourier transform spectrometer Digilab-FTS 14, averaged over 512 scans, and ratioed against .those of the outgassed zeolite.

0 3 . D2/NaCaA Ng/NaCaA Og/NaCaA 90 Tori- 0.02 Torr 0.2 Torr 135 K .165 K 135 K

Dg/NaA Ng/NaA 02/NaA 0.3* 10 Torr • 2 Torr 10 Torr . 105 K 175 K 110 K

0-

3050 2300. 1600 1500 FREQUENCY (cm"1 ). . Fig. 1. Field-induced infrared fundamentals of mole­ cular deuterium, nitrogen, and oxygen inside zeolites —1 —1 NaCaA and NaA. Resolution 1 cm . (0.5 cm only for Ng/NaCaA),; gas phase fundamentals marked by a trianr- . gle. .

In all cases we succeeded in inducing infrared absorptions of the physisorbed molecules. Typical examples of the ob­ tained bands are shown in Fig. 1. 98

In order to explain the different shapes it is assumed that the adsorbate molecules favour sorption sites on a axis at the entrance of the cage in case of NaA, and on a three­ fold axis inside the cage in case of NaCaA. The encaged species may be regarded as AP molecules rotating in a cy- 14 lindrical field with eigenvalues calculated by STERN and 15 PITZER et al. . The selection rules for induced absorption result from symmetry considerations of the polarizability following the KILLER-DECIUS method^. As only transitions b e tw e e n symmetric or antisymmetric levels can occur, the lowest transition will be a J=0-»J=0 transition in case of 0-D2 and o-^, J=1—»J=1 transitions, however, for the split- ted m=0 and m=+1 components in case of p-Dg, P-N2 and Og. In this picture the low intensities of the oxygen absorp­ tions in both zeolites are due to the lack of the J=U—»J=0 transition. The larger shifts, and a more pronounced asym­ metry of the bands indicate stronger interactions in zeolite NaA. It is obvious that I^NaCaA represents the system with the highest degree of rotational freedom, where, following 14 -1 -1 STERN , potential barriers of 5^ cm in the v=0 and 55 cm in the v=1 state can be estimated. Until now we only suc­ ceeded in getting a far-infrared absorption for the system D2/NaA at 1 2 5 cm-^ , which seems to be of translational ori­ gin. The different shifts of the fundamentals are at pre­ sent explained by the small mass in case of D9 , and by the 17 smaller quadrupole moment in the v=1 state in case of Ng . In order to calculate the electric field inside the cages of NaCaA from the integrated absorbance, we assumed a fixed Q A nitrogen molecule along the threefold axis, and got 7x107Vm 10 —1 in good accordance with 10 Vm normally assumed for zeo­ lites. It is still difficult to explain the. different band shapes as orientation-independent contributions from the isotropic part of the polarizability, and those from the anisotropic part sum up, and are in either case convoluted with vibrational relaxation and translational terms, the latter coming from the electric field dependence. Rotational diffusion seems to prevent the observation of higher rota­ tional excitations. Motional narrowing is also present, 99 since in NaCaA the increase-of the sorbed amount (Dg)'or low­ ering the temperature (Ng)’leads to an abrupt increase of the linewidth.

References

1 I.A. BRODSKI, S.P. ZHDANOV and A.E. STANEVICH, Fiz. Tverd.' Tela 1£, 2661 (1975) 2 M. SCHULDT, R. SEELEMANN, unpublished results 3 K. MOLLER, D. KUNATH and H.-J. SPANGENBERG, Spectrochim. Acta 27 A , 353 (1971) 4 J.W. WARD, Adv. Chem. Ser. 122, 118 0976) 5 H. FORSTER, M. SCHULDT and R. SEELEMANN, Z. physik. Chem. NF. 22, 329 (1975) 6 E. COHEN DE LARA, Mol. Phys. 2£, 355 (1972) P.J. FENELON and H.E. RUBALCAVA, J. Chem. Phys. £1, 961 (1969) 7 K. KLIER, J. Chem. Phys. £8, 737 (1973) 8 M.F. CRAWFORD, H.L, WELSH and J.L. LOCKE, Phys. Rev. 22. 1607 (194-9) 9 J. DE REMIGIS, H.L. WELSH, R. BRUNO and D.W. TAYLOR, Can. J. Phys. 4$, 3201 (1971) 10 N. SHEPPARD and D.J.C. YATES, Proc. Roy. Soc. A 238. 69 (1956) 11 R.P. EISCHENS and J. JACKNOW, Proc. Int. Congr. Catal., 3rd, Amsterdam, 1964 (North-Holland, Amsterdam 1965), P. 627 12 H. FRIEDMANN and S. KIMEL, J. Chem. Phys. 4£, 3925 (1965) 13 H. FORSTER, V. MEYN and M. SCHULDT, in preparation 14 T.E. STERN, Proc. Roy. Soc. A 13 0 . 551 (1931) 15 R.F. CURL, JR., H.P. HOPKINS, JR. and K.S. PITZER, J. Chem. Phys. 48 , 4064. (1968). 16 R.E. MILLER and J.C. DECIUS, J. Chem. Phys. 22. 4 8 7 /| (1973) 17 G. ZUMOFEN and K. DRESSLER, J. Chem. Phys. 64, 5198 (1976) 100

VIBRATIONAL SPECTRA OF MALEIMIDE AND BARBITURIC ACID IN LOW-TEMPERATURE MATRICES

L. Le Gall (Laboratoire de Thermodynamlque Chlmique, Universite de Bretagne Occidentale, 6 Avenue le Gorgeu, 29283 Brest Cedex, France) and A. J. Barnes (Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 AWT, U.K.) introduction MalRlm-fde (I) and its N-deuterated counterpart have been the subject of several recent studies in vapour, liquid and solution phases [1-3] , ■ There is. disagreement on the assignment of several of the fundamental modes, particularly the NH(D) bending modes. The infrared and Raman spectra of barbituric acid (II) derivatives, particularly the pharmaceu-

H H

H H \ / C = C 0/xNyx/'■ \ 0

H O I II tically important 5,5-disubstituted compounds (barbiturates), have been extensively studied. However the main purpose of these studies has been either the characterisation of barbiturates or correlation of the spectra with pharmacological activity, Detailed interpretation of the spectra has been largely confined to the N-H and C=0 stretching modes, and even in these regions of the spectrum there has been some disagreement [4-6], Matrix isolation vibrational spectroscopy is a valuable tool [7] in the study of hydrogen bonding substances since monomer and dimer bands may be distinguished by varying the solute concentration, while the small 101

bandwid ths, obtained- in a.low-temperature matrix enable near-degenerate

bands to be separated. Barbituric :.aci.d„ .unlike maleimide, is not yplatile

at a sufficiently.,.low-temperature to make.it practicable, to obtain a

spectrum in .the, .vapour phase, but the monomer .form can be readily .trapped • ■

in a low. temperature matrix,... Furthermore only the .triketo form is

obtained in the matrix, whereas'solution spectra may. contain enol .forms, ; ■

Experimental

The sample was evaporated from the solid contained in a tube held

at a controlled temperature in the range 0 to 35°C for maleimide, 135

to 210°C for barbituric acid or 52 to 105°C for 1-methyl barbituric acid,

The vapour was mixed with argon or nitrogen diluent gas just prior to

deposition on the cold window, Refrigeration was provided by a C.T.I,

Cryodyne and infrared spectra were recorded on either a Beckmann IR-9

or a Perkin-Elmer 180 spectrophotometer, Raman spectra on a Cary 82

spectrometer. The experimental arrangement for obtaining Raman spectra

has been described previously [8].

M a l e i m i d e

The 24 normal modes of the planar (C 2 V symmetry) maleimide molecule

divide into 9A^ + 3 A 2 + 8 B ^ + 4 B 2 modes, where the A 2 a n d B 2 s p e c i e s

are out-of-plane modes. The assignment of the majority of the funda­

mental modes from the matrix spectra agrees with that of Woldbaek et al,

[1], although there are several regions where the matrix spectra

provide a new or more certain assignment [9],

The position of the out-of-plane NH.bending mode (I^) has been

disputed. The solid phase frequency is ca. 730 cm' , and Woldbaek et al.

placed the solution frequency at ca, 6.70 cm ■*". Le Gall et al. ,[2] ,

suggested that the vapour phase band at 504 cm. ^ coul'd be the, monomer .

counterpart.! .This is confirmed by the matrix spectra,, which .show, clearly

the loss of-,intensity of the strong monomer band,at .505 cm-'*" in' an argon..-

matrix as the!dimer band grows in at ca. 740:cm . The monomer, band :

exhibits a large matrix shift from argon (505 cm to nitrogen (546 cm "*"),

presumably due to an interaction between the NH group and nitrogen-

giving rise to a steric effect on this out-of-plane bending mode, ■ A

similar effect is observed if the argon matrix is doped with .1% nitrogen, '

This matrix shift leads to interesting changes.in the 1060.cm-'*' region.

Apparently, a Fermi resonance interaction occurs between the A^ CH bending 102

mode and the overtone of the 8 2 NH bend, giving rise to a large shift of

the bands in this region from an argon matrix to a nitrogen matrix as a

result of the change in the NH banding frequency. The vapour phase

frequencies will be similar to those in the argon matrix, whereas the dimer frequencies are closer to those in the nitrogen matrix since the NH bending mode has shifted to much higher frequency.

Maleimide is thought to form a cyclic dimer (similar to succinimide [1 0 ]):

H H \-/ / \

/ X / c \

H

X / " x / V/ / \

Doping studies carried evt with water and with hydrogen chloride in argon matrices showed that the NH modes were little affected, whereas the C=0 modes showed shifts of a similar nature to those from the monomer to the dimer i.e. the dopant hydrogen bonds to a carbonyl group. Nitrogen, on the other hand, was shown to interact specifically with the NH group.

Barbituric acid

Raman spectra of barbituric, acid could not be obtained because of its high luminescent background, thus the 1 -methyl substituted derivative

(which does not suffer from this problem) was also studied in detail to assist with the assignment. The Raman spectra of the C=0 stretching region showed that, contrary to previous assignments, the highest frequency 103

band of the three observed in the infrared spectrum is due to the symmetric

stretching mode. Detailed comparison of the infrared and Raman matrix

spectra of barbituric acid and 1,3,5,5-tetradeuterobarbituric acid [11]

and 1-methylbarbituric acid and 1-methyl-3,5,5-trideuterobarbituric acid :

[1 2 ] enabled a vibrational assignment of each compound to be madei ,

The spectra of the dimers in argon matrices were found to be quite

similar to the spectra of the respective solids. Comparison of 1^

methylbarbituric acid (MBA) with barbituric acid (BA) showed that the NH

modes exhibited similar shifts from monomer to dimer, but that MBA did

not show a large shift of the 2-C=0 stretching mode as found for BA,

Thus MBA multimers must hydrogen bond through the 4- or 6 t C = 0 positions,

i.e. an open chain structure is most likely, whereas BA dimer probably has a cyclic structure (solid BA comprises such units further linked in hydrogen-bonded chains [13]).

R e f e r e n c e s

1. T. Woldbaek, P, Klaboe and C, J, Nielsen, J, Mol, Struct,, 27 (1975)

283.

2. L. Le Gall, J, Lauransan and P, Saumagne, Can, J, Spectr,, 20 (1975)

136.

3. M, G. Giorgini, B, Fortunato and P, Mirone, Atti Soc, Nat, Mat, di

Modena, 106 (1975) 89,

4. S. Goenechea, Z. Anal, Chem., 218 (1966) 418,

5. R. J. Mesley, Spectrochim, Acta A, 26 (1970) 1427.

6 . J. N. Willis Jr., R. B. Cook and R, Jankow, Anal. Chem,, 44 (1972)

1228.

7. H. E. Hallam (Editor), Vibrational Spectroscopy of Trapped Species,

Wiley, London, 1973.

8 . A. J. Barnes, J, C, Bignall and C. J. Purnell, J, Raman Spectr,, 4

(1975) 159.

9. A, J. Barnes, L, Le Gall, C, Madec and J, Lauransan, J. Mol, Struct,,

i n p r e s s ,

10. R, Mason, Acta Cryst,, 9 (1956) 405; 14 (1961) 720.

11. A. J. Barnes, L, Le Gall and J, Lauransan, to be published,

12. A. J. Barnes, M, Stuckey, W, J, Orville-Thomas, L, Le Gall and

J. Lauransan, to be published,

13. W, Bolton, Acta Cryst,, 16 (1963) 166, 104

DISTORSION OF DOPED NITROGEN AND RARE GAS MATRICES STUDIED BY INTERMOLECULAR

POTENTIAL DEVELOPMENT. DISCUSSION FROM HC1 AND HBr INFRARED SPECTRA.

C.GIRARDF.T. D.ROBERT Laboratoire de Physique Moleculaire, Universite de Besangon, route de Gray La Bouloie, 25030 Besangon.

D.MAILLARD, J .P .PERCHARD, A.SCHRIVER Laboratoire de Spectrochimie Moleculaire, University Pierre et Marie Curie 10 rue Cuvier Paris 5e

An analytical expansion of the intermolecular potential including dipolar and quadrupolar electrostatic interaction, R and R induction and -12 - 1 3 dispersion terms and R and R repulsive contributions has been used

to calculate the structures and the intermolecular distances of small aggre­ gates in Ne, A r , Kr and Xe matrices. The studied aggregates are •:

i) HX-nNg, with n = 0,1,2,3 and X = Cl, Br

ii) (HX)^ with n = 2,3 and (HXj )m (®^2^n n+m = 2,3 and

X = Cl, Br.

The results about the intermolecular distances inside the aggregates are the following :

For case i) the N g - ^ distance strongly increases with the lattice parameter, the N^-N^ pair being sensitive to the influence of the matrix because of the low values of the ^ Lennard Jones parameters.

For case ii), on the contrary, the HX-HX distances are nearly matrix independant because of the.strong interaction between these molecules.

The distorsion of the first matrix shell surrounding.the aggregates has also been studied and is found rather weak except for Neon because of its small Lennard Jones a d i a m e t e r .

A self consistent method using the.some analytical potential has been developped to determine the static orientational relaxation of the first shell of the a crystal around a substitutional HX. impurity .’This iterative methbdnshows that the orientation of a few molecules is strongly modified because of the HX neighbourhood with, as a consequence, an important distorsion of the crystal.

From the determination of equilibrium positions, HX vibrational frequencies have been calculated, using dipole moment derivative of the gas phase for aggregates containing only one HX molecule. Agreement with experimental data is generally good. 105

EXTERNAL FIELD INDUCED VIBRATIONAL SPLITTING

IN TRAPPED SPECIES ON ALKALI HALIDE CRYSTALS

Joachim Heidberq, Ram D. Singh and Helmut Stein

Institut fur Physikalische und Theoretische Chemie, Universitat Erlangen-NUrnberg, 852 Erlangen, Germany

In vibrational spectroscopy of polyatomic trapped species it is well known that the frequency shifts produced by the environment may be descri­ bed as the sum of external field and anharmonicity terms. In each of the numerous known systems, particularly defects in alkali halide crystals, it has been found that the anharmonicity terms dominate.^ In an extensive study it has been shown that the vibrational split­ ting of degenerate modes in trapped species may be largely due to the ex­ ternal field term. For carbon dioxide on an alkali halide surface a splitting of the internal bending mode should occur and be determined mainly by the external field term. By transmission measurements of the infrared absorption by carbon dioxide' - sodium chloride a splitting of the Pg band was detected and found to be in satisfactory agreement with the calculated external field splitting. Evaporated alkali halide films appear to be useful as matrices in trapped species studies because of their thermal sta b ility , their trans­ parency in a wide spectral range, and the easy accessibility of the trapped species by other molecules.

Two different types of matrix isolation systems.have been widely studied: 1. Ionic crystals (primarily alkali halides) doped in low con­ centration with' impurity ions e.g. CN , N^ , NCO . 2. Solid matrices of inert molecules (usually rare gases) with trapped molecular species. Though the experimental procedures are quite different, from a descrip­ tive point of view a third group of important systems appears to be analogous to these systems: Adsorption systems with weak interaction between the adsorbed molecules and the adsorbent. Just as in matrix iso­ lation systems, in general the correlation field coupling is expected to be absent at least in low coverage systems. In fact until now no corre­ lation field coupling has definitely been observed even in high coverage adsorption systems. Also rotational and translational degrees of freedom 106 are ordinarily "frozen out", though in adsorption systems zero or nearly zero frequency external vibrational modes may not be excluded. In the adsorption system NaCl (100) - C02 a normal mode of symmetry species B^, which may be represented as a coupled translational-rotational motion of the C02 molecules in the plane normal to the surface along the diagonal through the positive ions, was found to be a slightly hindered low fre­ quency motion (28.6 cm"1).

In seeking to answer the basic question, what happens to the motions when a molecular species becomes trapped on the crystal, the use of simple molecules trapped on an alkali halide appears to have distinct advantages: 1. The spectral absorption bands are sharp. 2. The lattice surface appears to be well-defined, even that of films, and it should be possible to give a sufficient mathematical description of it. 3. The interaction potential may be expressed adequately in a simple, much used form (Born & Lennard-Jones). 4. Also those few molecules such as CO., and N20 can be studied, for which the relation between the anharmonicity con­ stants in the empirical expressions for the vibrational energy and the cubic and quartic coefficients in the potential energy, as well as the other vibrational constants (Fermi resonance), for the "free" molecule are known.

A serious limitation of the use of alkali halide surfaces in isolation studies may be that the spectral signals are weak because of the small number of molecules available for radiation absorption and/or emission. This limitation may partially be overcome by the in situ preparation of high surface area alkali halide films, which have specific surface areas 2 of 100 m /g with still rather well-defined surfaces yielding sharp adsorp­ tion signals with half widths of ca. 15 cm"1.

The potential function V governing the motion of the molecule in the field of the environment, neglecting the dynamic coupling between molecule and environment, may be written as

V - Vf i U , (1)

(2 ) vf = IX akk i + X aklm Qk Q1 Qm k k,l,m

Here V^. is the potential function of the "free" molecule and U the poten­ tial energy of the interaction between molecule and environment, the are the normal coordinates of the "free" molecule. The force constant for the k-th normal mode of the trapped species is approximately given by

F,kk

(3)

where the second deri'vates are to be taken at the new equilibrium con­ figuration which the. molecule assumes in the external field, the firs t term being the harmonic force constant of the "free" molecule, the second the sum of the anharmonicity terms and the third the field term. In every of the numerous known cases the anharmonicity terms have dominated the frequency shifts. In contrast to this the splitting of the vibrational fundamental modes is in approximation (3) completely determined by the external field term because the cubic potential constants in (3) must be the same for the different components of a degenerate fundamental. Thus the study of the splitting of degenerate modes provides a direct technique for probing the lattice field.

Extensive calculations yielded the energetically most preferred ad­ sorption site with symmetry C2v and an energy of adsorption, U, in satis­ factory agreement with the measured value. For this and many other sites the vibrational splitting was calculated. Again best agreement with earlier experimental results from Folman, Kozirovski ^ and us and our recent measurements under improved conditions was obtained for adsorp­ tion site C£y . Representative results are:

CO., trapped free CO. on the NaCl (100) face '2 (v2a - V2b)exp site symmetry C2v D o o h 1.08

Improving the approximation (3), in this study also the new normal coordi­ nates Q^, of the adsorbed molecule were calculated, assuming an unperturbed kinetic energy matrix G for the molecule on the crystal surface.

Experimental. A stainless steel ultra high vacuum system was built with a liquid Helium cryostat for the in situ film evaporation, the adsorp­ tion and infrared measurements. With KBr/Viton windows pressures below -9 -10 2.5 x 10 Torr, with sapphire windows pressures in the 10 Torr range were regularly achieved. The temperature on the copper sample holder was 4 K, on the sample 10+2 K (minimum usually reached) without using Indium or other thermal contact material (because of baking). During the evapor- _g ation of the salt the total pressure was below 4 x 10 Torr, the partial water pressure below 1 x 10"** Torr.

1. W. F. Sherman and G. R. Wilkinson, Infrared and Raman studies on the vibrational spectra of impurities in ionic and covalent crystals, in Vibrational Spectroscopy of Trapped Species, Ed. H. E. Hal lam, John Wiley & Sons, London 1973. G. Turrel1, Infrared and Raman Spectra of Crystals, Academic Press, London 1972. A. Maki and J. C. Decius, J. Chem. Phys. 31, 772 (1959).

2. Y. Kozirovski and M. Folman, Trans. Faraday Soc. 62, 1431 (1966). J. Heidberg, S. Zehme, C. F. Chen and H. Hartmann, Ber. Bunsenges. physik. Chem. 75, 1009 (1971). MATRIX SPECTRUM OF PYRUVIC ACID AND ISOTOPIC MODIFICATIONS'

H.Hollenstein Physical Chemistry Laboratorium Swiss Federal Institute of Technology Universitatsstr.22, CH-8092 Zurich, Switzerland

Matrix spectroscopy represents an excellent method to get approximate gas frequencies of more complex molecules. In particular, it provides easy access to small isotopic shifts which contributes to a significant improvement of the vibra tional analysis of such species. An investigation of the in frared spectra of matrix isolated pyruvic acid and 15 iso­ topic species was part of an extended study on chemical and spectroscopic properties of this molecules performed at our laboratory [1]. For molecules of this complexity, the deter mination of a physically meaningful force field requires a large amount of vibrational data. Therefore, 16 isotopic modifications, including 33C-, 330- and D-labelled species were investigated. Among these species, each atom was labelled at least once. The main part of the present investigation is concerned with the determination of a reliable valence force field. The pyruvic acid molecule has 16 a' and 8a"fundementals and therefore 172 independent quadratic force constants. Though the experimental data is very comprehensive (90 frequencies from H- and D-species, 112 frequencies from 13C- and species), it is by far not sufficient to allow a determination of the whole set of quadratic ; potential constants. Criteria were worked out for a ade­ quate reduction of this set to a treatable size. Diffe­ rent calculations were performed using frequencies in the case of all isotopic species bn one side, frequencies in the case of H- and D-isotopic species and 13 frequency shifts in the case of C- 110 and ^0-isotopic modifications on the other side. In addition several types of weightings of the data were tried in the lat­ ter case. The different calculations yielded significantly different force fields. In order to check the physical feasi­ bility of the various solutions, the known and fundamen­ tals of acetone [2] were calculated using the force constants associated with the C-C'C^ fragment. The force field based upon frequencies in the case of all isotopic species showed best transferability with high significance and is therefore considered to be the most reliable solution. This force field is given in Table 1. More details will be given by the poster presentation, which will involve among other aspects an impressive example of a Fermi Resonance showing drastic changes in structure by going through the series of ^ C - and ^O-isotopic species. More­ over the pyruvic acid spectra will be used to demonstrate the enormous discrimination power of matrix IR-spectroscopy for isotopic species and impurities.

[1] Ch.Dyllick-Brenginger, A.Bauder and Hs.H.Gunthard, Chem.Phys., submitted. [2] G.Dellepiane and J.Overend, Spectrochim.Acta 22, 593 (1966).

Dwflnllion uf internal coordinates 111

Table 1 Force constants in mdyn/8 . (angular coordinates are multiplied by. 1-X)

Kr = 4.957(025) H -H = 0.231(074) \ = 0.083(012) cp2 cp1 p 2cp2 Kd = 7.445(027) = 0.525(008) h = 0.351(031) \ ecp-L nc 1

Kn =12.590(217) >- =-0.096(013) h = 0.115(026) CM U 1 1 Kn =12.082(192) = 0.222(001) h00 = 0.011(043) 2 \ h = 0.037(053) Kc = 6.507(173) = 0.140(001) cpcp x ' 1 \ Kc = 4.608(207) = 0.032(000) h = 0.065(022) 2 "’ 3 1 2

K q = 4.731(091) = 0.025(004) kgs = 0.004(062) hap 3 H = 0.514(002) = 0.028(005) kDS = 0.997(162) a l b p p H -H =-0.027(003) =-0.076(003) fD0 =-0.687(170) a 2 a l b p Y Hp = 0.591(009) h = 0.035(003) fSa =-0.212(014) T 1Y 1 Hp -Hp =-0.008(012) =-0.024(008) fS0 = 0.231(037) V NT V2 He = 0.850(006) h =-0.049(005) fScp = 0.626(073) YY

H. = 2.110(060) = 0.197(019) fs = 0.507(032) 6 1 ^ 1 0 2 H. -H. =-0550(089) =-0.069(023) 2 8 1 2® 2 H = 1.817(062) =-0.081(016) cpl ^ 1 ^ 2 112

MATRIX ISOLATION SPECTROSCOPY OF STABLE ORGANIC MOLECULES IN THE FAR INFRARED REGION

hy

E. Knozinger and M. E. Jacox Lehrstuhl fur Physikalische Chemie der Gesamthochschule Siegen/ Institute for Materials Research, National Bureau of Standards, Washington

The significance of the MI technique applied to vibrational spectros­ copy of stable molecules is based mainly on two effects: - band sharpening due to cryogenic temperatures - the reduction of the influence of intermolecular interactions on the molecules under invcstignation. These effects are of particular importance in the low frequency part of the vibrational spectrum, where intramolecular as well as inter- molecular vibrations are present.

The relativ error of band positions Av/v becomes increasingly signifi­ cant below 400 cm"1 due to relatively great half band widths and small values of the wavenumber. The MI technique guarantees a more accurate determination of band positions. - Furthermore, the vibra­ tional assignment of organic molecules in the far infrared region is extremely complicated under normal conditions. The reasons are: - the lack of characteristic frequencies - me existence ot 'hot bands' - the existence of 'intermolecular bands', i.e. bands originating from intermolecular interactions. Under the proper conditions the MI technique reduces the intensity of both 'hot bands' and 'intermolecular bands' and thus gives the basis for a precise assignment.

Both accuracy and precision are necessary, if band positions are used to calculate such important physical characteristics of molecules as 113 potential barriers and thermodynamic functions. - As an example one may refer to methyl formate which has a CO single bond with partial double bond character. Using the frequency of the torsional vibration around this bond axis the respective potential barrier opposing to free internal rotation may - under certain conditions - be calculated. A sufficiently accurate and precise determination of this frequency necessarily implies the availability of Raman data (depolarization ratios) and - particularly - the application of matrix isolation at cryogenic temperatures (fig. 1).

-100 Otherwise this objective cannot be Fig. 1 pursued due to the great band width

100i and to spectral overlap. Under the described conditions the considered torsional vibration is located at 332. 5 cm *.

Another interesting application of MI spectroscopy in the far infrared is the direct study of intermolecular interactions such as H bonding or dipole dipole forces under well defined 350 300 cm" HCOOCHg-Dampf physical conditions. - As an example, ps.100 Torr, I = 50 cm T= 300°K ; A^= 2cm ' associates of acetonitrile will be dis­ 350 cm*' 300 HCOOCHq in A r-M a trix , cussed. Acetonitrile shows a strong 1 ;200.2 0 0 . f 1 - Vi °K, Av- 1 cm '1 and broad absorption band around 80 cm" . It varies only slightly on going from pure liquid to solutions of different concentrations in CCl^. Intramolecular vibrations and diffe­ rence bands may be excluded as its origine. Associated molecules or clusters must be assumed [l] in spite of the slight dependance upon concentration.

Under matrix conditions the broad band observed in pure liquid and in solution splits into four (fig. 2). The relative band intensities depend in different ways on the concentration of acetonitrile in Ar: The bands at 114

v in cm"' Vio. 2 200 so"— Too so 122 and 105 cm increase relative

100 to the bands at 98 and 72 cm 1 when CH3CN in the concentration is increased (fig. 2). Argon-Matrix 1:1000 This indicates that two types of asso­ T=M °K ciated molecules are present. Consi­ dering the low concentration (1 in 1000) and the fact that only 2 bands are observed for each of them, 100 CHjCN in we may conclude, that the associated A ryun-M uli ix molecules are dimeres with an anti­ 1 : 100 T = tt° K parallel (a) and a linear (b) configu­ ration:

9 © j ® - — — — © ..© ------© 200 150 6 b) a) The antiparallel dimer gives rise to a stronger interaction than the linear one; the two bands above 100 cm 1 have, therefore, to be as­ signed to the antiparallel configuration whereas the linear one accounts for the two bands below 100 cm'1. This picture is confirmed by the isotopic effects observed on fully deuterating acetonitrile (fig. 3): the bands assigned to the antiparallel dimer are distinctly shifted (approxi­ mately 10 cm” ). The IR active vibrations of this dimer have rotational character (the vibrations with translational character are only Raman active). This means, that a particular moment of inertia and not the mass of the single acetonitrile molecule accounts for the frequency of each of the vibrations in question. The contributions of the H and D atoms to the different moments of inertia of acetonitrile are - of course- greater than to the total mass of this molecule. On the contrary in the case of the linear dimer vibrations with translational character are IR active and subsequently should lead to a much smaller isotopic shift than that observed in the antiparallel dimer. This is actually the case. - On heating the Ar matrix containing the acetonitrile molecules to 38°K and then recooling it again down to 14°K an increase of concentration of the antiparallel dimers at the expense of the concentration of the 115

Fig. 3 linear dimers is observed (fig. 4).

100 - As expected the antiparallel form CHjCN in A r-M atrix appears to be the more stable con­ 1:100 figuration which is separated from T =14 °K the linear configuration by a po­ tential barrier. On the other hand .1 a the antiparallel configuration is - | at least at very low concentrations 100

V L t g V I I III the one which is formed with A r-M atrix 1 :100 smaller probability (fig. 2). This T=U °K may be understood on the basis of some sort of Langmuir Hinshel- wood mechanism (known from 20 2 00 150_____100 heterogenous catalysis):an aceto­ v in cm"* nitrile molecule on the surface of the matrix attracts another one from the gasphase.

v in cm-* All measurements were performed in Ar and Ng matrix with sim ilar C0-.CN in Argon-Matrix results. An interference of phonon 1:100 bands may, therefore, be excluded. T=U °K .ncch Erwarmung auf 38 °K 10 m ini Reference [l] A. Loewenschuss and N. Yellin, Spectrochim. Acta 31 A, 207 (1975)

CD-jCN i Argon Matrix 1 :100

150.____ 100 50 v in cm -' 11 6

DISCUSSION OF THE CHANGE WITH INTERMOLECULAR DISTANCE OF THE HX DIPOLE MOMENT DERIVATIVE IN HX AGGREGATES FROM IR INTENSITIES AND SPECTRAL SHIFTS, COMPARISON WITH MINDO/3 SEMI-EMPIRICAL CALCULATIONS

D.MAILLARD. J.P.FERCHARD; A.SCIIRIVER, B.SILVI Laboratoire de Spectrochimie Moleculaire, Universite Pierre et Marie Curie 4 Place Jussieu Paris 5e

C.GIRARDET Laboratoire de Physique Moleculaire, Universite de Besangon, Route de Gray, La Bouloie, 25030 Besangon

The I.R. spectra of dimers (HX)2(X = Cl, Br) isolated in rare gas matrices have been reassigned in the light of the results obtained for rare-gas nitrogen mixed matrices. The two bands Dj and D^, IR active,-ans this rules out the cyclic geometry-, lie in different spectral regions : D j , in the monomer region, is very weak and D_ in the polymer region is on the contrary intense. -1 . • Both are only slightly affected by decoupling effects (Av= 1 cm ) ; this fact demonstrates that the molecules are not equivalent (and have different force constants). Calculations using an analytical intermolecular potential development,including electrostatic, induction, dispersion and exchange (repulsive) interactions, allow the determination of : i) the geometry and intermolecular distances as a function of molecu­ lar parameters (dipole and quadrupole moments, polarizability and hyperpola- rizability tensors ; e and a Lennard-Jones parameters). ii) The frequencies of vibration as a function of molecular parameter derivatives. The geometry leads to nearly orthogonal orientations of the two molecules, which confirm their non equivalence. Reasonable agreement with the experimental results for the frequencies and intensity ratio of Dj and D 2 can only be', obtained if the dipole derivatives have very different values for both molecules ( ~ 2.5 for HC1 or 6. for HBr) . The ratio, adjusted to fit the experimental frequencies and plotted as a function of the intermolecular distance, decreases when distance increases going from Ar to Xe. Justification of these results can be obtained with MINDO/3 semi-empirical method of quantum chemistry, using the dimer structure obtained from the previous calculations. This MINDO method allows the determination of- the dipole moments and their first derivatives for the two molecules. The results are in qualitatively good agreement with the experimental ones. 118

A complex between water and formaldehyde

Bengt Nelander, Thermochemistry Laboratory, Chemical Center, University of Lund, S-220 07 Lund, Sweden

The interaction between water and formaldehyde has been studied with IR-spectroscopy in solid argon at 10K. When water and formaldehyde are codeposited in an argon matrix, a few absorption bands are observed, which are absent with water and formaldehyde alone. The most conspicous new band occurs close to the (bound) OH stretching fundamental of the hydrogen donating water molecule of the water dimer (1). The absorbance of this new band varies with the water and formaldehyde concentrations as expected for a 1:1 complex. It therefore seems reasonable to assign it to the (bound) OH stretching fundamental of a water molecule, which is hydrogen bonded to formaldehyde. No other new peaks are observed in the water fundamental absorption regions. However, the peaks assigned to the bending and (free) OH stretching fundamentals of the donor part of the water dimer and to non­ rotating water monomer (1) increase by roughly a factor of two (depending on formaldehyde concentration). The increase of the nonrotating water bands is expected whenever anything is added to a water containing argon matrix (1). The increase of the two dimer bands is consistent with the assignment above of a band to a hydrogen bonded water formaldehyde complex. Only two new bands are observed in the formaldehyde spectrum, the C=0 stretching fundamental, v ,, gets a satellite 5.7 cm"^ below the monomer band and 2.0 cm below the dimer band (2). The other new band is a satellite to the out of plane fundamental, v^, 6.0 cm ^ above the monomer band. Both the new bands vary with water and formaldehyde concentrations as expected for a 1:1 complex and are therefore assigned to a water formal­ dehyde complex. Apart from these bands in the water and formaldehyde

fundamental regions, a new band is.observed at 440 cmfor H2 O-H2 CO and at 330 for DpO-HjCO. The ratio between the H^O and Opl) freqijpnrips is quite close to the square root of the ratio between the moments of inertia of

11^0 and D 2 O, indicating that the corresponding vibration is approximately a 1ibration of the complexed water molecule. From the results given above, it seems likely that only one water formaldehyde complex exists in significant concentrations in arqon matrices. The water part of the complex is hydrogen bonded to formaldehyde. We 119 expect the water molecule to be hydrogen bonded to the formaldehyde oxygen. The observed spectrum is entirely compatible with such a complex. From the observed spectrum it is impossible to rule out completely an interaction via the carbonyl carbon, but it seems unlikely that such an interaction can exist without perturbing the CH stretching and inplane bending vibrations. The HDO formaldehyde complex was found to be exclusively D-bonded. A similar behaviour is observed in the water dimer, where it also has been found (1,3) that donor HDO prefers to form a D-bond. Since formaldehyde is heavier than water, it is tempting to assume that HDO will prefer to form a D-bond to an H-bond (at very low temperatures) whenever i t is hydrogen bonded to a heavy molecule. If that is true, it should be the result of a difference in the zero point energies of the rigid molecule motions of D- and H-bonded HDO (3). The three vibrations which result from the three translational motions of HDO and the libration around the axis orthogonal to the HDO plane give approximately equal contributions to the zero point energies in the two cases. Only the two librations around axis in the HDO plane can give unequal contributions. One of these librations is the rotation around the hydrogen bond which is expected to give a vanishing contribution to the zero point energies. The other libration around an in plane axis is approximately a deformation of the hydrogen bond, for which the contribution to the zero point energy from H-bonded HDO is expected to be higher than from D-bonded HDO. Therefore D-bonded HDO is expected to have a lower zero point energy than H-bounded HDO and consequently to be more stable at very low temperatures. If this explanation of the preference for D-bonds is correct, the number of H-bonded HDO molecules is expected to increase rapidly with temperature, since the torsion energy levels are more closely spaced in this case. It is interesting to compare the water carbon dioxide and the water formaldehyde complexes. In the carbon dioxide case water prefers to form a complex with a water oxygen to carbon dioxide carbon, "bond" (4,5), while in the formaldehyde case it forms a hydrogen bond. Snyder and Basch (6) give the gross atomic populations of carbon and oxygen from double zeta ab initio SCF calculations on formaldehyde and carbon dioxide as 5.985 and 8.306 (HgCO) and 5.413 and 8.293 (COg). Thus the oxygen atoms have roughly the same net charge in the two cases. The formaldehyde carbon is approximately neutral while the carbon atom of carbon dioxide has a large positive net charge. This change in the carbon population is apparently what makes water sh ift from hydrogen to "oxygen bonding". 120

References (1) Pullin, A.D.E. and Ayers, G.P. Spectrochim. Acta 32A 1629, 1641, 1689, 1695 (1976). (2) Khoshkhoo, H. and Nixon, E.R. Spectrochim. Acta 29A 603 (1973). (3) Fredin, L., Nelander, B. and RibbegSrd, G. J. Chem. Phys in press. (4) Fredin, L., Nelander, B. and Ribbeglrd, G. Chemica Scripta 1_ 11 (1975). (5) Jonsson, B., Karlstrom, G. and Wennerstrom, H. Chem. Phys. Letters 30 58 (1975). (6) Snyder, L.C. and Basch, H. Molecular Wave Functions and Properties. Wiley, New York, 1972. 121

NUCLEAR SPIN CONVERSION OF HzO AND D zO IN ARGON MATRICES G. P. Ayers* and A. D. E. Pullln Department of Chemistry, Monash University, Clayton, Victoria, Australia * Present address: CSIRO, Division of Cloud Physics, Box 134, Epping, N .S.W ,, 2121, Australia INTRODUCTION It is known that H20 and D20 molecules can rotate in argon matrices and that the pattern of vibration-rotation transitions observed is similar to that calculated from gas phase data for the appropriate temperature. Interpretation of such matrix spectra necessarily involves consideration of ( 1 2 3) nuclear spins and nuclear spin interconversion in matrices ' ' since for the ground vibrational state of H2 0 rotational states such as 1 (0 1 ) with (K_j + K) odd must go with symmetrical (ortho) nuclear spin states while states such as 0(00) and 1 (11) with (K ^ + K) even go with antisymmetrical (para) nuclear spin s ta te s . RESULTS: H O/Ar (4) In the course of an i.r. study of the water dimer in argon matrices we re-examined some of the earlier monomer vib-rotor lines with particular reference to their intensities and nuclear spin. Methods were devised for checking the reliability of our measured (relative) line intensities. Our observations concerning nuclear spin effects for H20 /Argon matrices can be summarised: (a) Nuclear spin conversion was found to be dependent on M/A ratio, being faster at lower M/A. At high dilution, e.g. ^2000, little or no conversion is observable in the spectrum over several hours or more. (b) Partial conversion from the initial room temperature gas phase ortho/ para ratio to the low temperature ratio takes place as the matrix is deposited. The extent of conversion is dependent on the temperature of deposition, the o/p ratio in the matrix being roughly half-way between the room temperature and the deposition temperature values.

Temperature cycling experiments clearly showed that in H 2 D /argon matrices of high dilution (e.g. M/A » 2,200) nuclear spin conversion was not occurring and that para and ortho H2

1 (01) respectively). The vib-rotor spectra of Fig. 1 illustrate this.

V ! ; f 2M -1(io)V i, j

r v L » 2(02) -Koi) 'j 1(od-CKoo)

1 8 T : 20 K 'i Kod -0 (o o ) .

3790 3750 3700 3800 3750 3700 CM" CM"

Figure: H^O/Ar M/A « 2,200, deposition at 8-10 K, vib-rotor i.r. spectrum. Foi interpretation, r,no text. Confirmatory spectra were found for However in the matrix M/A ~ 2,200, deposited at 8-1 0 K, and recorded at 1 0 K the actual ortho/para ratio was approximately half way between the room temperature ratio and that calculated for 10 K, The absence of spin conversion at M/A 2200 was confirmed by returning the matrix quickly from 20 K to 1 0 K (approxi­ mately one minute). This returned the original spectrum which remained unchanged when the matrix was held at 1 0 K for a further 110 minutes. In further experiments slight nuclear spin conversion was observed at M/A « 880 and, e.g. , a relatively fast nuclear spin conversion at M/A 45. This indication of an inverse dependence of nuclear spin conversion rate on M/A value also showed up as a rough correlation between the ratio of the intensities of lines originating from ortho H^O to those originating from para HjO and the M/A value, since spectra were usually recorded about half an hour after deposition. Initially this M/A dependence had Been tor u 6 9 pilf.zl.l riy fea lu te. Since clearly nuclear spin conversion must be occurring during deposition, matrices at M /AK 2200 were deposited at 15 and 22 K. For 1 5 and 22 K a considerably greater equilibrium ratio of ortho/para is calculated than for 1 0 K. Examination of these matrices at 1 0 K or below showed that the ortho/para ratio was greater than for the matrix deposited at 10 K but that again the ortho/para ratios were approximately half way between the room temperature and deposition temperature ratios. 123

H20 /A r : INTERPRETATION

The observed, roughly inverse correlation between the rate of nuclear spin conversion and M/A value (observation a, above) suggests direct nuclear magnetic H.O - H-O interaction as the cause, though the observed 2 rates seem high. However oxygen contamination is also a possible cause , despite careful gas handling. The observed correlation would require an amount of oxygen approximately independent of the amount of argon in the matrix. The observation that partial conversion takes place on deposition is however not explicable in terms of oxygen contamination and this we now consider. Our proposed explanation is tentative. An H20 molecule near an argon atom will perturb it and this perturbation can be described in terms of mixing in of excited argon states. Because of spln-orbit coupling, 2 5 1 3 Ar(3s 3p 4s) for example, will be appreciably admixed with Ar Pj , thus conferring paramagnetic character on the argon. If the H20 is unsymmetrically placed with respect to the argon atom the two protons will experience different strengths of magnetic field; this is an essential requirement1,1 for spin conversion by this mechanism. We must suppose that once the matrix is formed the argon atom cages surrounding the water molecules are too symmetric for this mechanism to be then effective. D 20/Ar The nuclear spin statistics of D20 make observation of nuclear spin trapping and/or conversion much more difficult than for H20 . Infra-red vib-rotor spectra obtained by us at 7 K are consistent with intensities calculated for free nuclear spin conversion rather than no conversion. This conclusion holds for dilute D20/Ar matrices (M/A 4,600; 2,300) through to concentrated DnO/Ar matrices. Temperature cycling and quenching experiments gave a pattern of intensity changes consistent with complete or almost complete conversion in the time of the temperature changes (a minute or two). We consider nuclear spin conversion in D.O/Ar to be effected by g spin-rotation interaction . This mechanism is likely to be much more effective for D20 than H2

REFERENCES

1 . R. L., Reddlngton and D. E. Milligan, J. Chem. Phys., 37_, 2162 (1 962). 2. R. L. Reddington and D. E. Milligan, J. Chem. Phys., 3£, 1276 (1 963). 3. H. P. Hopkins, Jr., R. F. Curl, Jr., and K. S. Pitzer, J. Chem. Phys. , 48, 2959 (1 968). 4. G. P. Ayers and A. D. E. Pullin, Spectrochim. Acta, 32A. 1629, 1641 , 1 689/ 1 695 (1 976). 5. E. Wigner, Z. phys. Chem. B, 23^ 28 (1933). 6. R. F. Curl, Jr. , J. V. V. Kasper and K. S. Pitzer, J. Chem. Phys. , 46, 3220 (1 967), 125

A matrix infrared study of the NH2 vibrations of amides

MARKKU RASANEN, JUHANI MURTO and ANTTI KIVINEN

Department of Physical Chemistry., University of Helsinki, . Meritullinkatu 1 , SF-00170 Helsinki 17, Finland

There are discrepancies in the literature in the assignments of the NHg group vibrations of amines and amides. The NHg torsion .in the IR spectra of amines is usually assigned to a band at about 3 0 0 cm ”*, whereas in the case of amides some workers favour the 30 0 cm- 1 .region, while others (e.g.,[1 ]) prefer the 600-800 cm"”' region.

We have studied the IR spectra of the amides: formamide (FA), acetamide (AA), monofluoroacetamide (MFAA) and trifluoroacet- amide (TFAA) and their deuterated analogues in both argon and nitrogen matrices. All these molecules have a very intense band in the far-infrared region of the spectrum and in almost all cases this band is.the most intense in the whole spectrum.' Its medium shift from argon to nitrogen is about 80 cm-1 and doping the argon matrix with a small amount of nitrogen ( 1- 2 % ) leads to bands that resemble the "nitrogen complex" bands of alcohols [2]. All observed phenomena indicate a great similarity between the band in question and the OH torsion band of alcohols in matrices. The band will hence be assigned to the NH2 torsion (see Tabie 1 . ).

Several bands in the matrix isolation spectra of N-methyl acetamide in argon are split, which suggests that the nitro­ gen atom is pyramidal in argon and inverts through tunnel effect. There is no splitting in nitrogen [ 3,4j .

In the spectra of the amides under investigation more bands are split in the argon than in the nitrogen spectra. This is most clearly seen in the spectrum of dideuterated formamide. The splittings are too large to be due to matrix effects. In nitrogen this kind of splitting is almost entirely absent. If the amide nitrogen were planar there would hardly exist two spectroscopically non-equivivalent conformers. Thus we are 126

TABLE 1 . Observed torsional frequencies in matrices (in cm-^). Deposition temperature 15 K. The spectra were recorded at 11 K.

ArZN2 A i- — ®2/Ai' /N tND2 /1n2 N2 shift

FA 311w, 3 0 6 m, 402m, 396ws 93 303vs

FA-ND- 2 2 6 m, 2 2 0 w s 289vs 69 1.376 1.369 AA 276sh, 270vs |357ws, 348m 87 aa-nd2 206m_, (?) 279w, 2G5ws , 73 1.338 1.346 25^m aa-d3 272m, 267ws, 358vvs, 350m 91 265m aa-d5 280w, 267s, 1-339 257w MFAA 35?w, 3 5 4 w s 4?2s, 425m, 78 4i9s, 4l5m, 4 n m , 395m "-nd2 263vs 324m, 3l8w, 61 1.345 1-334 313m, 3l0w, 307w, 297w TFAA 368s, 365m, 450m, 443s, 83 3 6 0 w s 438m ”-nd2 288w, 282w, 340m, 335s, 65 1-333 1-322 274m, 2 7 0 w s 33im

led to conclude that the amide group is non-planar at least in argon.

Many different explanations have been given of reversible tem­ perature effects in matrices. With alcohols the reversible changes often relate to conformational changes [5j- Especially interesting in connection With the present work are the rever­ sible changes in the spectra of formaldehyde, which occur in nitrogen but not in argon [6 ]. A constrained rotation of the molecule has been proposed as the explanation.

In the case of amides, the NH^-torsional peaks are temperature dependent in nitrogen matrices. In argon there are no observ­ able changes in the temperature region from 10 to 25 K. 127

In nitrogen the NI^ torsion of FA and AA broadens upon warming the matrix and some of the fine structure disappears rever- sibly.

The NHg torsion of TFAA consists of three peaks both in argon and nitrogen matrices. On increasing the temperature of the nitrogen matrices there are first some irreversible changes; after these have occurred the structure of this band changes reversibly. It is notable that the structure and behaviour of the NH2 torsion of trifluoroethylamine (which occurs at about 270 cm-1 in argon [7]) are similar to those of TFAA. .

20K

I r_rT r 330 300

FIG. 1 . NHg torsion bands of a) MFAA and b) MFAA-NE^ in nitro­ gen at various temperatures. The matrix was deposited at about 15 K, after which the spectra were recorded in the order 11 K, 16 K, 20 K, 11 K. Abscissa scale in cm ^. Notice the similarity between a) and b). 128

The NHg torsion of MFAA consists mainly of one peak in argon but of at least seven components in nitrogen; the intensity variation with temperature is considerable (see Fig. 1 . ). Our preliminary results indicate that these changes are thermal in nature. The estimated AH is about 140 J mol--*. Several explanations for this phenomenon may be suggested:

(i) The equilibrium established between different conformers is temperature dependent. (ii) There are several "nitrogen complexes" of well-definied structure at equilibrium. (iii) The reversibility suggests that there might be some kind of rotational states, the populations of which change with temperature. The whole molecule would hardly be rotating in this matrix. (iv) The rotational constant of Ng is about 2 cm" 1 so the difference between the lowest rotational states of nitrogen is of the same order of magnitude as the obser­ ved splitting. There might be an interaction between the molecular dipole and the quadrupole of nitrogen.

REFERENCES: . L1] S.T. KING, J. Chem. Phys. 75, 405 (1971); Spectrochim. Acta 28A, 165 (1972). [2] J. MURTO, A. KIVINEN and I. MUTIKAINEN, Chem. Physics Letters, 3 6 , 369 (1975). [3J F. FILLAUX and C. deLOZE, J. Chim. Phys., 73, 1004 (1976). [4] F. FILLAUX and C. deLOZE, J. Chim. Phys., 73, 10.10 (1976). [5j J. MURTO, A. KIVINEN, M. RASANEN and M. PERTTILA, Spectro­ chim. Acta ,33A. (1977). in press. [6 ] H. KH0SHKH00 and E. NIXON, Spectrochim. Acta 29A, 603(1973) L7j J . MURTO et. al. To be published. 129

Spectroscopy of Transient Species in Matrices: Cs+Cl2 ~, XeF, CFaCl+ and Ca2 .

Lester Andrews Chemistry Department University of Virginia Charlottesville, Virginia 22901.

Matrix-isolation spectroscopy is.particularly useful for the study of transient species produced by metal atom reactions, by laser photolysis of trapped species, and by resonance photoionization of small molecules during conden­ sation in solid argon. Samples formed by cocondensing alkali metal atoms and chlorine molecules at high dilution in.argon exhibited very strong Raman signals near 250 cm 1 which showed a small alkali metal dependence. These .signals are assigned to the (Cl-Cl) mode in the M+Cl2 ~ species, where the. alkali metal dependence arises from interaction between the (Cl-Cl) .and + - 1 2 M --Cla modes. Owing to.the very strong 350 nm absorption of CI2 > these molecules produced resonance Raman spectra. The Cs+Cl2 " species is especially distinctive in this regard, using 457.9 nm excitation. Notice the regularly decreasing intensity in the progression of overtones out to 8 v and the increasing resolution of the 3SCl2 ~ and 3S Cl37Cl isotopes. Also characteristic of the resonance Raman effect is the. in­ crease in fundamental and overtone intensities as the laser exciting lines.approach the absorption maximum from the long wavelength side. . . . Laser photolysis :of chlorine in solid xenon produced a strong Raman signal at 254 cm 1. A detailed.wavelength study of the photosynthesis of the new Raman signal showed no pro­ duct with 514.5 nm light, slow production with 501.7 nm, rapid with 488.0 nm and very rapid with 476.5 nm laser lines. The 130 dissociation limit of Cla at 478.6 nm identifies the mecha­ nism as the reaction of a chlorine atom pair with a xenon atom in the matrix cage. 0 In order to confirm this mechanism, xenon-fluorine and krypton-fluorine matrix samples were exposed to blue laser ex­ amination. XeFa was produced immediately, as identified by a very strong Raman band at 512 cm 1; KrFa was synthesized more slowly, but a strong band was observed at 452 cm 1 . The mixed compound XeClF was prepared by blue laser photolysis of ClF in solid xenon and in subsequent infrared studies of Ar/Xe/CIF samples subjected to photolysis. The examination of Ar/Xe/Fa matrix samples with Che krypton ion laser ultraviolet lines at 350.7 and 356.4 nm produced XeFg Raman signal at 512 cm 1 and extremely strong emissions at 18,600 and 24,300 cm 1 Since XeFa absorbs strongly in the vacuum ultraviolet region, the strong emis­ sions were believed to arise from a new xenon-fluorine species, most likely XeF, produced by reaction of Xe and F atoms, the latter from the laser photodissociation of Fa. Samples of Ar/Xe/Fa = 400/2/1 were photolysed with a high pressure mer­ cury arc for 15 min and strong,.structured absorptions appeared 3 4 at 258 and 324 nm. ’ The vibrational spacings of approxi­ mately 300 cm” 1 are appropriate for the excited states of XeF which are ionic, Xe+F ~ , with. Cs+F as a model^ having a 313 om 1 argon matrix fundamental. The strong emission at 24,300 cm 1 exhibited partially resolved structure appropriate for a species with an excited state fundamental near 300 cm 1 and a 2 0 0 cm 1 ground state fundamental, which are in good agree­ ment with the gas phase values,b The matrix observations on XeF show that the ground state species has a potential mini­ mum near the excited state which indicates that XeF is more strongly bound than the van der Waals molecule XeNe, in dis­ agreement with theoretical calculations 131

Cations are important products of the interaction of high energy radiation with molecules. We have been working on techniques for producing and trapping cations in matrices over the past five years, first by proton radiolysis® and I 9 I more recently with argon resonance photoionization. The latter technique uses an argon discharge resonance lamp with a 1 mm orifice. Argon gas is flowed through the discharge tube and deposited with the matrix along with vacuum ultra­ violet radiation from the discharge. In experiments with a LiF window placed 1 cm in front of the discharge tube, the product yield was reduced to 15 + 3% of the observed in­ tensity without a LiF window.'*"® Since the transmission of LiF is 15-20% at 104.8 and 106.7 nm, the discharge tube func­ tions primarily as a windowless resonance lamp. In keeping with the recent interest in Freon compounds, and because C-F bonds are strong infrared absorbers, the Freon system was examined in detail,*"® and the CF3Cl study will be described here. Following proton radiolysis or argon resonance photoionization, Ar/CFaCl = 300/1 samples exhibited new bands in the C-F stretching region at 1512, 1415 and 1298 cm 1 . These bands behaved differently on mercury arc photolysis; the 1298 cm 1 band was destroyed by pyrex-water filtered light whereas the former bands were reduced by only one-third. The 1298 cm 1 band is assigned to the antisymmetric C-F stretching mode of CF3 C1+ produced by photoionization of the CF3C1 molecule whose analogous vibrational mode is 1205 cm 1 . The photoionization of CF3Cl requires 12.39 eV in the gas phase'*"*"; the 11.83 eV argon resonance line is capable of photoionizing CF3Cl with a 1 eV red shift in the 12 process by the argon matrix or photoionization may pro­ ceed directly with higher energy 13.8 - 15.5 eV radiation 13 + from the argon discharge. Photolysis of CF3 C1 with 290- 10 0 0 nm light probably involves chlorine atom elimination. 132

The 1512 and 1415 cm 1 absorptions are assigned to the daughter ion CF2C1+ produced by ionization of the CFa Cl radical, formed by photodissociation of CFaCl, which has 14 symmetric and antisymmetric C-F modes at 1208 and 1146 cm 1 . The decrease of CFaCl"*" upon 220-300 nm photolysis is believed to arise from neutralization by electrons photo­ detached from chloride ions trapped in the matrix. Calcium dimer, Ca2 , is a very interesting molecule. The ground state has a very strong van der Waals interaction with ho formal chemical bond; however, the excited state has 16 an electron-pair bond. Codeposition of Ca atoms with Ar, Kr or Xe at 1QK allows formation of Ca2 from diffusion and reaction of Ca atoms during sample condensation. Opti­ cal spectra revealed a very strong structured red absorption and red dye laser excitation produced very strong structured emission^ from 14,000 to 15,000 cm 1, which will be dis­ cussed for krypton matrix samples. The absorption spectra of 40 Ca2 and 44Ca2 showed a common origin at 14,430 + 3 cm 1 with initial vibrational spacings of 118 and 113 cm 1 , re­ spectively, for the excited state fundamental. Pumping each isotopic species with the dye laser tuned to the 11 <— 0 absorption for each isotopic molecule produced very strong emissions peaked at 14,000 cm 1 with 0 — > v" progressions

down to the band origin 1 and resolved v 1 — ^ 0 , v' — > 1 and v 1 — ^ 2 emissions (v ' = 1, 2, 3, 4, 5, 6 ) on the high en­ ergy side of the band origin. The ground state isotopic fundamentals arc 80 and 76 cm 1 , respectively. Isotopic substitution confirmed the vibronic assignments. The Ob­ servation of unrelaxed vibrational emission (v' = 1, 2, 3, 4, 5, 6) from a diatomic with a low fundamental frequency trapped in solid krypton at 10 K is of considerable interest. We h a v e discussed the formation of charge-transfer molecules, Cs+Cl2 ~, by matrix reactions and the resonance 133

Raman spectrum of the dichlori.de molecular anion trapped in solid argon. The matrix is ideally suited for the synthesis and stabilization of weakly bound molecules, like XeF and Ca2 , which have intense optical transitions to more strongly bound excited states. Finally, the solid matrix is a good trap for small molecular cations so that their infrared spec­ tra and photolysis behavior can be recorded.

References

1. W. F . Howard, Jr. and L. Andrews, Inorg. Chem. , 767 (1975). 2. L. Andrews, J . Amer. Chem. Soc. 9j3, 2147 (1976 ). 3. W. F. Howard, Jr. and L . Andrews, J. Amer. Chem. Soc. 96, 7864 (1974) . 4. B. S. Ault and L . Andrews, J. Chem. Phys. 64^ 3075 (1976); ■ 65, 4192 (1976). 5. C. A. Brau and J. J. Ewing, J. Chem. Phys. &3, 4640 (1975). 6 . J. Tellinghuisen, personal communication (1976). 7. D . H . Liskow, H. F . Schaeffer, III, P. S. Bagus and B . Liu, J . Amer. Chem. Soc. 95^, 4056 (1973). 8 . L. Andrews, J. M. Grzybowski and R. 0. Allen, J . Phys. Chem. 79, 904 (1975). 9. C. A. Wight, B . S. Ault and L . Andrews, J. Chem. Phys. 65, 1244 (1976). 10. F. T. Prochaska and L . Andrews, to be published. 11. J. M. Ajello, W. T. Huntress, Jr. and P. Rayermann, J. Chem. Phys. 64, 4746 (1976). 12. A. Gedanken, B . Roz and J. Jortner, J. Chem. Phys. 5 8 , 1178 (1973) . 13. A. R. Striganov and N. S. Sventitskii, Tables of Spec­ tral Lines, (Plenum, New York, 1968).

14. D. E. Milligan, M. E. Jacox, J. H. McAuley and C. E. Smith, J. Mol. Spectrosc • 4J5, 377 (1973). 1 34

15. R . S. Berry and C. W. Reimann, J. Chem. Phys. 38, 1546 (1963 ) . 16. W. J . Balfour and R. F . Whitlock, Can. J . Phys. 53^ 472 (1975). 17. J. C. Miller and L. Andrews, to be published. 135

HIGH RESOLUTION IR MATRIX SPECTROSCOPY WITH TUNABLE DIODE LASERS

M Dubs Physical Chemistry Laboratory Swiss Federal Institute of Technology Universitatsstr.22, CH-8092 Zurich, Switzerland

The problem of vibrational linewidth in matrices has been avoided by most matrix spectroscopist for several reasons. First, most theories of vibrational relaxation lead to line- widths that are not measurable with conventional spectro­ meters, other theories that include specific matrix effects lie outside the direct interest of spectroscopists as well as outside the interest of solid state physicists, whose methods might be applied successfully to the problem. Second, as every matrix spectroscopist knows, measuring the matrix linewidths is not unproblematic, since many disturbing parameters can in­ fluence the results: impurities, concentration effects,poorly defined matrix deposition conditions, unresolved splittings, unsufficient resolution or bad signal to noise ratio of the spectrometer. With the tunable laser spectrometer and its -3-1 excellent spectral linewidth of <10 cm together with its high spectral power density and its high signal to noise ratio, the main problem of determination of true lineshape is eliminated. Also another difficulty in matrix spectroscopy - heating of the matrix by the radiation of the spectral source - is greatly reduced, because the total emitted laser power is only of the order of 1 mW. At present, our efforts are concen­ trated on determining the effects of deposition parameters and optimization of deposition conditions in order to get repro­ ducible results. 136

The diode laser spectrometer we use is shown schematically in Fig.1. The radiation source of the spectrometer is a PbSxSe^_x diode laser with a coherent output in one or several modes spaced approximately 1 cm each mode with a linewidth of -3 -1 -1 <10 cm . A single mode can be tuned 0.5 to 3 cm by changing temperature and current of the diode, which is operated at a temperature of 10 to 40 K typically, in a.closed..cycle He cooler. A second cryostat with a Helitran continuous flow LHe cooler is used for cooling the matrix window to any tempera­ ture between 4,2 and 300 K, stabilized to within 0.1 K. For deposition of the matrix, the window is rotated and a premixed matrix gas of the desired M/A ratio is sprayed onto the window through a Knudsen cell. The spectrometer is calibrated with known absorption lines of a reference gas, the tuning rate is measured interferometrically with a Fabry-Perot etalon.Not shown in Fig.l is a recently installed double beam setup, that permits direct measurement of the transmittance of the sample. At the present time, absorption spectra of the v^(f2 )band of CH^, v^(b^) of SC>2 and V y ( a ' ) of CH^CHO isolated in Ar have been measured. For CH^ only fragmentary results could be ob­ tained because the single mode tuning range of the diode did not cover the whole absorption band. The observed linewidth of the principal absorption at 1306.3 cm was 0.26 ± 0.01 cm-'*" FWHM for a matrix deposited at LHe temperature, independent of the M/A ratio, which varied between 200 and 2000. Some results for v^(b^) of S02:Ar are published in [l]. Further experi­ ments with SO2 are planned but after several successful experi­ ments the output mode structure of the diode laser changed, so that t h e absorption range of SO2 could not be observed any more. The experiments with Acetaldehyde:Argon have confirmed several of the results obtained with S0 2 :Ar and can be summarized as follows: i. Matrix deposition conditions: M/A ~ 500; deposition rate = 1.1 pm/min; Thickness: 84 pun; deposition temperature: 16 K. 1 37 ii. The Vy(a') band of CH^CHO:Ar which was assigned and ob­ served unter low resolution at 1349 cm 1 by H.Hollenstein [2] shows under high resolution a splitting into three partly overlapping bands at 1348.690 cm ^, 1348.919 cm \ 1349.248 cm ■*■. As a wavenumber calibration the R7F1' gas absorption line of CH, at 1348.034 cm 1[3] was used. _ i iii. For the strongest band at 1348.690 cm a reversible tem­ perature dependence of the band center frequency was ob­ served. The measurements in the temperature range 5 to 30 K are described within experimental error by the fol­ lowing expression v = (1348.689 + 1.67 * 10"6 T3 ) cm"1- There is a correlation between the frequency shifts and the density of solid Argon which might be of importance. The other bands also show a temperature dependence which however is masked by the overlapping of the bands and cannot be determined precisely without a band shape analysis. iv. For the same reason only the temperature dependence of the linewidth of the strongest absorption band has been investigated so far. For the linewidth of of CHgCHO at least, two mechanisms are responsible, one leading to a irreversible narrowing of the linewidth with tempera­ ture cycling between 5 and 30 K, the other showing a reversible temperature dependence. For the observed band the linewidth at FWHM could be described with the following expression Avi(T) = [A2 + B2(t)]* , A describing the irreversible contribution (A ~0.145 cm ) and B the reversible part, giving a significant broade­ ning above approximately 15 K (B(30 K) = 0.10 cm 1).

[1] M.Dubs, Hs.H.Gunthard, Chem.Phys.Lett. (in press) [2] H.Hollenstein, Hs.H.Gunthard, Spectrochim.Acta 27A (1971) 2027 and private communication [3] J.Botineau, J.Mol.Spectr.41, (1972)182. 1 38

Ch

Mo De L3

Fig.1 Tunable diode laser spectrometer

D: diode laser, (Laser Analytics) L1,L3: Ge lenses, L 2 : ZnS lens Ch: chopper, M: monochromator, (Spex 1500 SP), G: gas cell, E: Ge etalon, M: matrix (CaF2 window, rotated 90° for deposition), K: Knudsen cell, V: needle valve, P: diffusion pump, De: HgCdTe detector (Hughes SBRC), R: X-Y-recorder and electronics. 1 39

Matrix Isolation, Infrared and Raman Spectra of Binary and Mixed Zinc Dihalides

A. Givan and A. Locwcnschuss Department of Inorganic 5 Analytical Chemistry The Hebrew University of Jerusalem Jerusalem, Israel

ihe systematic spectroscopic examination of a group of related molecules pro­ vides the means for the study of their intramolecular properties, how they are affected by substitution and of the dependence of macroscopic magnitudes upon mole­ cular parameters. We have reported (1,2) such investigations of matrix isolated binary and mixed cadmium and mercury dihalides. Irregularities were found in the trends of force constants upon substitution of a given halogen by a heavier one.

These irregularities occur for the stretching, interaction and bending force con­ stants of the halides of both metals and are focused on the heaviest molecules within each group. They were interpreted as indications of possible deviations from lin­ earity in the structures of the MI^ and MBrY molecules, as such assumption both restores the regularity in force constant trends and improves the fit between cal­ culated and experimental thermodynamic properties.

The present paper is concerned with the dihalides of zinc, the third and light­ est metal of group 113. The full infrared and Raman spectra of all ZnX2 and

ZnXY (X,Y = Cl,Br,I) are reported and their force constants evaluated. The Raman spectra of the ZnX2 molecules have not been reported previously and this is also the first experimental evidence of the existence of the ZnxY species. Isotoplc affects were resolved in all spectra and these facilitate a more detailed and re­ liable experimental analysis than was possible for the cadmium and mercury halides, as in the present case isotopic patterns due to the metal could be observed in addition to the isotopic band splittings due to the halogens. Irregularities in 1 40 force constants arc again revealed and are actually more extreme in the present case.

Thermodynamic functions of all molecules are computed and compared to calori- mctrically obtained values where available.

1. A. Strull, A. Givan and A. Loewenschuss, J . Mol. Spectrosc. 62_, 2S3 (1976).

2. A. Givan and A..Loewenschuss, J . Chem. Phys. 65, 1851 (1976) and references

therein.’ 141

High Pressure Raman Matrix Isolationspectroscopy.at low Tempe­ rature H.J. Jodi (Fachbereich Physik, Universitat Kaiserslautern, Pf af f enbergstralJe, 6750 Kaiserslautern, W.Germany)

Introduction: The effect of pressure on the Raman spectra of polyatomic mole­ cules, for example NC^, isolated in a matrix, especially Ar, is studied at 77K and 4.2K up to SKbar. The combination of different techniques - high pressure at vari­ ous low temperatures with Raman studies on matrix isolated spe­ cies - opens a new wide field of a series of experiments with the following advantages and aims. First, this method allows the separation of effects due to change of molar volume and of temperature. From the experimental point of view this means the variation of temperature at constant pressure, and the change of pressure at the lowest temperature. The results found here with hydrostatic pressure can be compared with data of Evans and Fifchen [l] who used zero pressure and unaxial stress in the case of NOj in KJ, and with data of Tevault and Andrews [2] who made their experiments with NC>2 in Ar at zero pressure. In recent years some work was published on high pressure in­ frared spectroscopy at low temperature [3] . Thus Raman data are necessary because coincidences between Raman and'infrared frequencies are not allowed. The technique of matrix iso­ lation spectroscopy provides the possibility to get informa­ tion about the impurity alone, about the coupling between defect and lattice and about the matrix indirectly. The effects may be classified in static and dynamic ones. Secondly,some aims of these kinds of combined measurements are the study of unharmonic phenomena which manifest themselves first in a volume dependence of the phonon frequencies and second in mul- tiphonon interactions. Pure alkali halides and rare gas so­ lids do not show first order Raman scattering, but the in­ clusion of the defect destroys inversion symmetry for the neigh­ boring particles and gives rise to a first order Raman spectrum. In addition phase transitions (fee—hep) will be expected which should be observable by Raman active quasi TO-modes.Both kinds of 142 induced Raman activities will be detected under pressure and temperature variation. A further question is whether some re­ lationship exists between temperature shift, pressure shift and matrix shift [4] . An analysis of the peak shifts and of the changes in halfwidth as a function of pressure and temperature provides the possibility to get lattice dynamical values, for example compressibility and Grtineisen parameters and to com­ pare them with the results of the pure matrix. Different kinds of modes, pure molecular ones (rotational,vibrational,vibronic), combined impurity-lattices ones (local or resonant) and pure lattice ones can be examined independently by the specially chosen impurity and matrices. This is of great interest because it predicts a change of the projected densities of states as a function of fractional changes in force constants [l, 5] . In these experiments the Raman scattering intensity is however, proportional to the phonon density of states; if the lattice is doped by an impurity the force constant will be varied and in addition the use of hydrostatic pressure increases the force constants. NC>2 as an impurity was favoured because this mole­ cule has symmetric ( V , V^) and asymmetric ( V^) normal modes; the last one should be hindered when isolated in an Ar matrix. For testing this technique, for comparison and for still unre­ solved problems NC>2 in KJ was used [l , 6] . Raman spectra and phonon densities of the pure substances KJ [?] and Ar [8,9] are al­ so k n o w n . Experiments and results: From the technical point of view the hydrostatic pressure is produced by the gas mixture itself (Ar:1% N0o) and controlled b> the measured shift of the R1 fluorescence line in ruby which is implanted in the matrix [lo] . The crystal is produced by carefully cooling down the gas mixture according to the known p-T molting curve uf Argon [ll] . Standard equipment was used to control temperature in the helium cryostat with an optical tail and to monitor Raman spectra. The Raman spectra are monitored at 300 K with p=o; 5.45 Kbar and 9.63 Kbar and at 77 K with p=o and 8.65 Kbar and at 5 K with p=o and 6.57 Kbar for 1% N0~ = KJ, and at 77 K with p=o and 3.7 Kbar and at 5 K with p=o and at 4.2 Kbar for 1% N 0 ? in Ar. They may be classified in an high energy part - 700 cm~lto 143

1300 c m - '1' far off the laser line - and in an low energy part - 20 cm * to 300 cm *. The first-part contains the pure molecu­ lar modes; they are known in literature already and their Raman intensities were strong enough to optimize the apparati ve con­ ditions. The second part was established by combined impurity - lattice modes and by pure lattice modes, so the structure is pretty complicated. The different peaks are tentatively assign­ ed. The optical quality of the crystals especially in the case of NO-:Ar, permits detectable Raman intensities with a reso- z -1 lution of 2-5 cm and a small enough Rayleigh scattering about 20 cm * away from the exciting laser line. Table - Modes measured in Raman Scattering (cm ):

high energy Substance Technique low energy p art References v , v 2 v 3 N02 G as IR 1329,6 750,1 1633.6 Abe 11973) NO 2 Theory 1356 771 ? Gillispie (1975) pure Ar Raman 15 K 20-120 Fteury (1973! C91 N02:Ar Raman 16 K 1325 752 - Tevault (1976) [2]

This work N02 : Ar p = 0 Kbar 26-35 - 62 767,5 (port of the sp ectra) Raman 5K p = 4,2Kbar 26-36 65-55 66 750,5

1305 N 02 : KJ Ram an 6 K 58 76 133 806 - Evans (1970) tl) 1316 N02:KJ Roman 5 K 36 59 77 130 1308 806 1252 Rebane 11975) C61 1317

1302,1 N 02 :KJ p = 0 Kbar 20-50 56,1 76,5 130,0 This work 1311,3 Ram an 5K p = 6,75 Kbar • 20-50- 56,5 80,8 151,5 1306,9 (part of the spectra) 1316,6

Discussion: The data will be discussed in general under the aspect of the mentioned aims in the introduction (unharmonic effects, induced Raman active modes, temperature-pressure-matrix shift ...) and published elsewhere [l2] . In this publication only one aspect in the low energy Raman spectrum will be analysed. For KJ: the various peaks shift differently with pressure at 5 K between 5 cm-1 and 20 cm 1 to higher energy, one peak shows almost no pressure shift. This might be explainable by the fact that hydrostatic pressure changes the fractional force constants; some projected densities of states with cer­ tain symmetries e.g. I ~ ^ , are not affected by a change of 144

central force constants and others are strongly perturbed e.g. I™, -> . For Ar: two shoulders at 30 cm '''and at 50 cm"' -1 and a peak at 62 cm is to be seen in the Rayleigh wing at 5 K and slightly shifted to higher energies (about 3 cm"') when pressure is applied. The peak can be attributed to a lo­ cal mode of NOj in Ar and roughly estimated by changing the ■ mass and keeping the force constants equal. The first hump may be a quasi TO-mode at 32 cm [5 p.36l] which becomes Raman active when the fee structure of Ar changes to hep with small amounts of impurities.

References

[l] A. R. Evans, D.B. Fitchen, Phys.Rev.B 2,4, p. 1074 (1970) [2l D.E. Tevault, L. Andrews, Spectr.Acta 60 A,p.969 (1974) [3] M. Jean-Louis, M.M. Thiery, H.Vu, B . Vodar, J. Chem. Phys . 55, 9 p . 4657 (1971) [4] J.Y. Roncin, M. Miladi, H. Damany, B . Vodar, In Vacuum ul­ traviolet Radiation Physics, p.67, Vieweg-Hamburg 1974 [5] M .L . Klein, J.A. Venables, In Rare Gas Solids, Acad.Press (1976) [6] T.J. Haldre, L.A. Rebane, Phys.Stat.Sol. b 70, p.359 (1975) [7] G. Dolling, R . A. Cowley, Phys .Rev. 147,2 p. 577 (1966) £i] N.R. Werthamer, R.L. Gray, T.T. Koehler, Phys.Rev. B 2, 10 p. 4199 (1970) fj] P. A. Fleury, J.M. Worlock, H.L. carter, Phys .Rev.Lett. 30, 13, p. 591 (1973) [in] G. J. Piermarini, S. Block et a l ., Journ. Appl .Phys . 46, 6 p. 2774 (1975) fll] M.S. Anderson, C.A. Swenson, J .Phys.Chem.Solids 36 p. 145 (1975) [12] H.J. Jodi, W.B. Holzapfel (to be published) 145

Mossbauer Studies of Rare-Gas Matrix-Isolated Halide Molecules Containing 97Fe, **9Sn and 191Eu + ~

F. J . Li t t e r s t , A. Schichl, E . Baggi o-Sa i tovi tch* • Physik-Department, Technische Uni versi tat MUnchen , D-8046 Garching, 8RD

H. Mi ckli tz Lehrstuhl fur Didaktik der Physik, Uni versi tat MUnchen, D-8046 Garching, BRD

J.M. Friedt Laboratoi re de Ch i mi e Nucleaire, Centre de Recherches Nucle- ai r e s , 67037 Strasbourg, Cedex, France

The i nterpretati on of the hyperfine interaction (hfi) data as obtained from Mossbauer spectroscopy of c ry sta llin e compounds is often hampered by the complex i ntermolecular interactions which occur in the c r y sta llin e sta te . Mossbauer studies of rare-gas matrix-isolated (RGMI) molecular systems offer in principle the possibility of avoiding these difficulties. The re su lts of such experiments are also expected to be p a r tic u la r ­ ly suited for theoretical treatments of isolated molecular cl usters .

We report about Mossbauer experiments on the following RGMI molecules: 57FeCl7 ,' 57FeBr? , 57Fe?C l,, 119SnFP, 119SnCl,, ' 119SnBr2 , 119SnI2; 119SnF4 , 119SnCl4 , •119SnBr4 , 119SnI4 and

19^EuC12 . The samples (molecular concentrations 0.1-1%) have been prepared by simultaneous condensation of an argon (or xenon) gas beam and a molecular beam of the species to be studied on an aluminum substrate at 5 K.

+Work supported by "Bundesministeriurn fur Forschung und Technologic11 "On leave from Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, Brazil 146

SnX4 :

' 119 The hfi parameters of Sn in argon-isolated SnX4 ( X = C1 , Br , I ) molecules are identical to those of the corresponding crystal­ line compounds. This fact reveals that the i ntermolecular interactions are negligible in these c r y sta llin e compounds as far as concerning the electronic structure of Sn*+.

SnX2 : r i g The Sn hfi spectra of RGMI SnX2 (X=F,C1,Br,I) show a super­ position of two quadrupole doublets which are assigned to monomeric SnX2 and dimeric Sn2X4. The hfi parameters d iffe r clearly from those measured in the c ry sta llin e compounds (see table 1). Using a simple bonding model we can explain the ob­ served isomer shifts and quadrupole interactions of SnX2 with a bonding angle 0 = 95±2° for all SnX2 molecules and a slight Increase or o trow Siil2 to SnF2 .

FeX2 :

In table 2 we have given the hfi parameters for argon-isolated 57 57 FeCl2 and FeBr2 together with the data for the paramagnetic crystalline compounds. The three quadrupole doublets A , B and C of the argon-i sol a ted compounds are due to isolated monomers (A), dimers formed by nearest neighbour monomers (B) and bridge- bonded dimers (C). The re la tiv e ly small isomer sh ifts and quadrupole sp littin g s of the linear monomeric molecules are d is ­ cussed in a simple MO model giving Fe electron configurations of 3d6.20±0.024s0 . 50+0.024p0 . 50+0.02

Fe2C16: For RGMI ^ ^ F e 2 C1^ a single quadrupole doublet is found . Its splitting of 2 mm/s is unexpectedly high for a configuration Fe"* + (3d^) and can be explained by a ligand e le c tric a l field gradient which is strongly enhanced due to the polarization of" the chloride ions.

EuC12 : 151 The asymmetric resonance line found for argon-isol a ted EuCl2 is due to a quadrupole interaction = (-235t5) MHz which is assumed to arise from nearly outweighed contributions from a 147 ligand and a valence electrical field gradient. Both the isomer shift and the quadrupole interaction are consistent with a pure s-p hybridization and an electron configuration. 4 f76s0 '^ 3" ° ' 056p0 '®3"0-05 for Eu in EuCl2 I

These experiments clearly show that Mossbauer spectroscopy on RGMI molecules is in addition to optical , IR and Raman spectro­ scopy a supplementary method to provide information about bonding conditions in molecular systems.

Table 1 '

Isomer sh ift (IS) and quadrupole sp littin g (AEq ) data of RGMI- Sn(11)-halide molecules (monomer and dimer molecules). The corresponding data for the crystalline compounds are also ' given. IS values refer to 3 3^mSn:BaSnOg at 300 K.

IS(mm/s) AE q (mm/s)

119— — / A monomer 2. 9 ± 0. 05 2. 65± 0. 1 SnF2/Ar dimer 3.3 ± 0.05 2. 5 ± 0.1

''^SnFg monoclinic 3. 57 ± 0. 05. 1 . 77 ± 0. 05 . ... form crystal 1 ine

1 19— —, / . monomer 3. 1 7 ± 0. 05 2. 81 ± 0. 1 SnCI./Ar . .. 2' dimer 3. 67 ± 0. 06 2. 26 ± 0. 1 1 1 9 SnClg- crystalline 4. 07 ± 0. 05 0.

1 19— _. / A monomer 3. 22 ± 0. 05 2. 5 ± 0. 1 2^ dimer 3. 50 ± 0.05 1. 25 ± 0. 1

1 1 9 S n B r2~cr ystal 1 ine 3. 93 ± 0. 05 0.

119— , /A monomer 3. 25 ± 0. 05 2. 69 ± 0. 1 SnVAr . dimer 3. 40 ± 0. 05 1.5 ± 0. 1

119/ Snl2/Xe monomer. 3. 27 ± 0. 09 2. 7 ± 0.2 dimer 3. 48 ± 0. 05 1.5 ± 0 . 2

1 1 9 Snl2-crystal 1 ine 3. 85 ± 0. 05 0. 148

Table 2

Isomer shifts (IS) and quadrupole splittings (AEq ) for argon- isolated (doublets A,B and C at -5 K) and paramagnetic 57 57 crystalline FeCl ^ and FeB^. IS values refer to metallic iron at 300 K.

IS(mm/s) AEg(mm/s)

A 0.88t0.02 0.63±0.02 B 1.04t0.02 1.79±0.02 ' C 1.10t0.05 2.80±0.10 crystal 1.27t0.02 1.30±0.02

A U.81-0.02 0.86-0.OZ 57FeBr B l.lOiO.05 1.7'0t0.10 2 ' C 0.99 + 0.05 3.10+0.10 crystal 1.24+0.021. 10±0.02 149

MATRIX ISOLATION RAMAN SPECTRA OF FeClj AND Fe2Cl6

A. Loewenschuss and A. Givan Department of Inorganic 6 Analytical Chemistry The Hebrew University of Jerusalem, Jerusalem, Israel

INTRODUCTION There has been considerable interest recently in the vibrational studies of metal trihalides. Of special interest is the form of the mono­ meric molecules, as the possibility of either a pyramidal shape of C^y sym­ metry or a planar geometry with D ^ symmetry has implications of both spectroscopic and thermodynamic nature as well as on theoretical con­ siderations. Additional interest in these substances originates from the fact that the dimeric species often predominate in the vapor phase. Both aspects apply to FeClj. Up to 600°K the vapor exists mainly as Fe^l^. At higher temperatures the monomer predominates. From electron diffraction studies (1) planar D ^ and bridged D ^ struc­ tures were suggested for the monomer and dimer, respectively, with the respective Fe-Cl bond lengths of 2.14*0.OlA and 2.17*O.OlX. Frey et al. (2) conducted infrared matrix isolation studies of FeCl2> FeCl^ and their dimers and interpreted the FeCl^ spectra in terms of the planar geometry. However, they could only obtain complex spectra of mixtures of Fe^l^, FeClj, FeCl2 and Fe^l^ which were analyzed into component spectra and then interpreted. In the following investigation well defined Raman spectra of either monomeric FeCl^ or dimeric Fe ^ l ^ were obtained. The results indicate unequivocally that the shape of the monomer is pyramidal and lead to as­ signments different from those of Frey et al. Thermodynamic functions are calculated and compared to a calorimetrically obtained value.

EXPERIMENTAL Samples were produced by simultaneous condensation of a krypton gas jet and an iron trichloride vapor beam onto a gold plated copper cold tip in­ clined about 30° to the incident laser beam (5145& and 488 oX excitation). Cold tip temperatures were maintained by an AC-2L-110 Air Products Cryotip. Fe2Clg spectra were obtained from vapors emanating from a single oven pla­ tinum crucible. Cracking of the dimer was accomplished by the double oven technique and a temperature gradient of 500-600°C was maintained between the two compartments. The spectrum of monomeric FeClj was "pure" to the extent that no bands attributable to the dimer could be observed. The use 150

of a third monochromator (TTM) in tandem with the main double monochromator (Spex 1401) proved indispensable for the recording of the low frequency region of the spectra.

RESULTS The Raman spectrum of monomeric FeCl^ in solid Kr at 20°K consists of four distinct bands, shown in Fig. 1 and summarized in Table 1. Agreement with the infrared study (2) is only partial. Isotopic structure consonant with three equivalent chlorine isotopes is revealed under high resolution in the 363 cm’1 band (Fig. 2 and Table I). The matrix isolation Raman spectrum of Fe„Clfi is shown in Fig. 3 and band frequencies are listed in «1 -1 Toble 2. Isotupic patterns are resolved in the 315 cm and 426 cm in­ tensity ratios expected from two equivalent chlorine atoms.

DISCUSSION The observation of four Raman bands in the monomeric FeCl^ spectrum must lead to the conclusion that this molecule is of pyramidal C^v rtruo Lure. ine spectrum may be considered to consist of two high-frequency lines (460.2 cm 1 and 360.3 cm *) and two low-frequency bands (113.8 and 68.7 cm ). The assignment of the former as the antisymmetric v (e) (460.2 -1 -1 cm ) and the symmetric v (a.) (360.3 cm ) is rather straightforward. It -1 should be emphasized that a weak spectral feature at 367 cm has been ob­ served in the infrared but remained unassigned by the authors (2). This is taken as further support for a pyramidal structure. The two low frequencies should be assigned to the bond angle deformations. We assign the sharper 68.7 cm 1 band to the totally symmetric bend (a^) and the 113.8 cm 1 fre­ quency to v^(c). This assignment is based on both the line shapes and in­ tensities as well as on the force constant calculations described below. The conflict with the infrared assignments should be noted (Table 1). The vibrational problem of FeCl^ (L1^ ) consists of two two-dimensional blocks. Five valence force constants (f , f , f , f and f ) and the r' a ’ rr rtv bund-bond angle a as a sixth parameter are necessary for a full solution. A first, ectimato of a can be obtained Ocmi a Simple Valence potential neg­ lecting all interaction constants (3). A value of a=114.6° results from our assignment while that of Frey et al. renders the unreasonable angle of 86.3°. A second approximation involves the separation of high and low fre­ quencies. It yields the value of f^-2.36 md/X and ct=lli,y° from the high frequencies and the correct relative positions of the low frequency band can then be reproduced. The assignment of Frey et al. cannot reproduce the re­ lative band positions. Finally, a computer fit using four parameters - 151

Table 1. Vibrational frequencies of FeCMcm ') Frequencies Kr m a trix R a m a n Frequencies Infrared (this work) assignments argon matrix assignments

460.2 t ' a ( e ) 464.8 ^(e*) 366.0 362.8 359.6 V i ( S i ) 367.06 356.8 113.8 vaie) 116.0 ^2(^2) 102.0 t'tio') 68.7

ncy accuracy is better than ± 0.5 cm " • Ref. 2 • b Not classified as monomer band in Ref. 2

Table 2. Raman vibrational frequencies of Fe2i (cm-1) Kr matrix (this work) gasb Ga2Ci3 (gas) -„) 455 w 467 1,-,,) 613 I,-,,) 466.0 Iw) ll'sl" Intensities, polarization and suggested assignments are given in parentheses. vs = very strong; s - strong; m = medium; w = weak; vw - very weak, vvw = very very weak, p = polarized. i'i = MX 2 stretch (a0 ); v 7 = ring stretch {a9 ); r 3 = ring bend (a0); i’4 = MX 2 b en d (a0 ); r7 = MX 2 w ag ( 5 1p); i-3 = MX 2 stretch (£?,„); i'u = MX 2 stretch (S2e); /'12 = MX 2 tw ist (b2fl); i'16 = MX 2 stretch (S3u). a = assignments from Ref. 2 h = from reference 5 c=* from reference 5

Table 3. Observed and calculated isotopic structure in the bond of FcCI3

Molecule Observed; ir Calculaed; „,(F eC I335) 366.0 366.0 3.2 3.22 „,(F eC I235CI37) 362.8 362.78 3.2 3.09 ,-,IFeC I35CI237) 359.6 3 59.69 2.8 2.95 „,

,-3(FeCI3)‘ 460.2 460.2

° Isotopic structure not resolved. 152

f , f , f , a and the full secular determinant was performed. The r a rr r best results are f =2.356 md/X, f =0.074 md/ft, f =0.0705 md/S and a= r rr a 115.07°. The parameters reproduce all frequencies within experimental er­ ror except for which deviates by 3.5%. A separate computer fit was based on the isotopic frequencies of and the position of Vj, a se­ paration of the high frequencies and the three parameters f , f and a. All five frequency values can be reproduced within experimental error (0.15 cm *) with the parameter values of fr=2.39 md/X, frr=0.0776 md/X and 0=115.66°. (Table 3) These results may be contrasted with the iso­ tope effect expected from a planar configuration which implies a se­ paration of 10.03 cm 1 between the extreme isotopic components versus the experimental value of 9.2 cm * and a value of 9.26 cm * calculated by us. Further support for the pyramidal structure is thus established. Thermodynamic, function calculated from the experimental frequencies, a bond length of 2.14^ (1) and a bond angle of 115° are presented in Table 4. Small changes in a (*2°) do not affect these entities significantly. An experimental entropy value of is given by Polyachenok et al. (4). A 03h symmetry implies a symmetry number value of a=6 (versus a=3 for and reduces the calculated entropy by 1.4 e.u. However, the effective value of o in a CJv molecule may well be higher than 3 due to a low in­ version barrier in the v2 mode. The assignments of Frey et al. (2) have the effect of even further increasing the calculated S°gg value by about 1.0 e.u. We therefore consider the comparison of the calculated and ex­ perimental entropy value to be consonant with our conclusions. The essentially centresymmetric structure of the dimer is revealed in the fact that no bands of significant intensity have the same frequency values in the infrared (21 and Raman. (There is a possible coincidence be­ tween a 315 cm * infrared shoulder and the 314 strong Raman band.) The assignment of bands will not be discussed in detail here but is sum­ marized in Table 2. It is based on the observation that the spectrum can again be separated into a high and a low frequency region representing the stretching and deformation modes respectively, on the observed isotopic effects, on approximate force constant calculation and on comparisons with similar molecules (5).

REFERENCES 1. E.Z. Zasorin and N.G. Rambidi; Izv. Akad. Nauk. SSSR 2, 705 (1964); J . Struct. Chem. 4_, 836 (1963). 2. R.A. Frey, R.D. Wender and Hs.H. Gunthard, J. Mol. Spectrosc. 3£, 260 (1970). 153

3. G. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules, p. 163, Van Nostrand, Princeton, N.J. (1945). 4. O.G. Polyachenok and O.N. Komshilova, Obshch. Prikl. Khim. 1969, 109 (1969). 5.a. I.R. Beattie and G.A. Ozin, The Spex Speaker XIV, 4 (1964). b. I.R. Beattie, H.E. Blayden, S.M. Hall, S.N. Jenny and J.S. Ogden, J. Chem. Soc. A666 (1966). c. I.R. Beattie, H.E. Blayden and J.S. Ogden, J. Chem. Phys. 64, 909 (1976).

V, F e C I ;

FeCh 366.0

3 I0 .I

450 400 350 300 250 200 150 100 50

Figure 1. Raman spectrum of monomeric FeCI3 trapped in solid krypton at 20 K. Double crucible, evaporation temperature 1 4 0 -1 5 0 °C, su p erh eatin g to 700-750 6C.

308.2

3 0 5 C m - ' 500 450 4 0 0 350 300 250 200 150 100 50 FIG. 2. Isotope effects in (A) (FeCl3) and (B) v2 ( F e ^ ) Figure 3. Raman spectrum of Fe2CI6 in solid krypton at 20 K. Single crucible, evaporation temperature 125-140°C.

Table 4. Thermodynamic Properties of FeCI, (ideal gas)

xm-Hio) icm-wo)] S( 7) < V C v T{K) k c e l/m o l -1 k c e l/m o l* cal/mole deg cel/mol" deg 298 4.30 19.80 18.02 80.81 1.12 300 4.33 19.94 18.04 80.92 1.12 400 6.17 28.31 10.75 86.22 1.12 500 8.07 37.15 19.13 90.45 1.12 600 9.99 46.38 19.34 93.95 1.11 700 11.94 55.93. 19.48 96.95 1.11 800 13.89 65.75 19.57 99.55 1.11 900 15.65 75.83 19.63 101.86 1.11 1000 17.81 86.12 19.68 103.93 1.11 1100 19.78 96.61 19.71 105.81 1.11 1200 21.76 107.28 19.73 107.53 1.11 1300 23.73 118.11 19.76 109.11 1.11 1400 25.71 129.09 19.77 110.57 1.11 1500 27.68 140.22 19.78 111.94 1.11 154

The Infrared and Raman Matrix Isolation Spectra of Some MFj and MXF Molecules (M = Zn.,Cd,Hg; X = Cl.Br.I)

A. Loewenschuss and A. Givan

Department of Inorganic 6 Analytical Chemistry The Hebrew University of Jerusalem Jerusalem, Israel

. While the infrared spectra of the difluorides have been studied previously their Raman spectra are not as yet reported. As for the mixed fluorides MXF, their existence has not been demonstrated to date. In the present paper we report the

Raman and infrared matrix isolation spectra of several of these molecules. In pre­ vious studies, mixed dihalides MXY were produced (2) through the evaporation of a mixture of the two binary parent dihalides MX£ and This procedure is impractical in the present case due to the very large difference in vapor pressures between the much less volatile difluorides and any other dihalide. We circumvented this dif­ ficulty by the use of "double oven" crucibles in which the parent difluoride and the second dihalide were maintained at two different temperatures.

From the recorded spectra bond stretching, bond-bond interaction and bending force constants can be evaluated for the binary difluorides while two bond stretching and the bending force constant can be derived for the mixed fluorides. In several bands isotope effects could be resolved and these are used to support the force con­ stant evaluations. The derived force constants are compared to values obtained for the other mixed and binary dihalides of group IIB and this comparison is discussed in view of the irregularities found in force constant trends of the other dihalides

(3) and the large difference in electronegativity between fluorine and the other halogens. 155..

Thermodynamic functions arc computed for these molecules for which all vi­ brational'frequencies have been observed. •

1. A. Loewenschuss, A. Ron and 0. Schnepp, J. Chem. Phys. 49^ 272 (1968); 50, 2502

(1969) .

2. A. Givan and A. Loewenschuss, J. Chem. Phys. 64, 1967 (1976).

5. A. Strull, A. Givan and A. Loewenschuss, J. Mol. Spectrosc. 62, 283 (1976);

A. Givan and A. Loewenschuss, J. Chem. Phys. 65, 1851 (1976). 1 56-

Some Recent Applications of I.R. Isotope Frequency and Intensity Patterns to Matrix Isolated Molecules

by J. S. Ogden, University of Southampton, England

Isotope frequency and intensity patterns have been used extensively in the last two or three years as an aid in the characterisation of 12.. 3 It 5 carbonyls * , dinitrogenyls and dioxygenyls , and it is generally accepted that the excellent agreement between observed and calculated spectra depends significantly upon the operational effectiveness of the Cotton-Kraihanzel approach. The purpose of this paper is to outline how similar frequency and intensity calculations may in principle be applied to a wide range of molecules for which the "high frequency separation" approx­ imation is only rarely considered. The recognition of non-degenerate vibrations from isotope patterns presents few difficulties, since the patterns frequently follow simple statistics, and even where this is not so, they may readily be interpreted using isotope intensity sum rules.^ Degenerate vibrations, however, present complications, since the isotope intensity patterns will bear little or no resemblance to a binomial distribution, and this has led to difficulties of interpretation in more than one chemical system. As a preliminary illustration, we may consider D and C XY3 species for which r , = /,v /, \ dn dv stretcn Aj + E If the stretching modes are uncoupled from the bending modes, 9 ... it has been shown that the frequency and intensity patterns in the stretching region for a mixture of scrambled isotopomers X Y ^ ^ Y ^ may be calculated using only two force constant parameters (F^ and Fnn) and the angle YXY, together with atomic masses and isotope abundances. For a given system, calculations can therefore be carried out over a wide range of sets of parameters F^, F and YXY, and it. turns out that there are essentially three different types of pattern which one might expect. Figure la summarises model calculations for a P ^ species with atomic masses X = 50, Y = 35, Y 1 = 37 and an abundance ratio Y: Y ' of 1.0. The E 1 mode of XY3 has been fixed at U50 cm 1 , and in the vicinity of this band one predicts either a four— line pattern with the outer bands more intense than the central pair, or two types of six line pattern in which the two extra bands lie outside the frequency range E' XY3 -*• E' XY3' . Figure lb summarises the correspond­ ing patterns expected for a trichloride in natural abundance (Y:Y' = 3) whilst Fig. lc shows the calculated patterns for a pyramidal trichloride with a bond angle YXY of 105°. The extent to which these calculations 157

mirror recent experimental results is shown in the accompanying spectra of matrix isolated FeClg, CrClg and OSO3F 2, For the trichlorides, the observed spectra indicate that the A j ' mode is well removed from the E' mode (almost certainly to lower frequency in both cases) but the spectrum observed for 180 enriched OSO3F2 provides an excellent example10 of perturbation leading to a six-line pattern. FIGURE 1.

V T H 1 (i) (i) (i) x 5

II | " f"" ' V 111

(ii) (ii) (ii)

1 1 ' 1 1 " I1 ■I1

(iii) (iii) (iii) '7 7 '

'I1 T 1 lr (iv) (iv) (iv)

Extension of this approach to more complicated molecules is relatively straightforward, and may be illustrated by reference to two important structures which contain more than one degenerate stretching vibration. First, if one considers the vibrations of a six-membered D ring X 3Y3, it is evident that rstretch = A 1' + A 2 ' + 2E' and since only one kind of bond dipole derivative is involved, it should be possible to perform a similar set of calculations without employing too many independent variables. This particular structure has been proposed for a number of molecular species, 7 11 12 notably S13O 3, 60303, and P3N3, and the approach is best illustrated again by reference to a specific system. The two E' stretching modes in Si31603 lie^ at 971.9 and 629.0 cm 1 (Ar matrix), and using these values as constraints, it is possible to derive sets of parameters F^, F ^ and OSiO which can subsequently be used to 158

compute isotope frequency and intensity patterns for 1 8 0 enriched samples. Here, F^ is again the principal stretching constant, and FRR is a single interaction constant involving adjacent bonds. Figure 2a summarises the results of these calculations for 8 1 3 0 3 in the region of the intense 9 7 1 . 9 cm 1 band, assuming 50%'180 enrichment. Using this simple model, it turns out tnat one would again expect three distinct types of i.r. spectrum - a simple essentially four line pattern similar to that in Fig. 1 with the possibility of six line patterns arising when there is perturbation from a nearby mode inactive in (in practice, the A2' ). The experimentally observed^ isotope pattern for 180 enriched S i 3 0 3 is shown in Figure 2b and the positions of the oontral components in the observed quartet allow one

FIGUBE 2 950 (6 )

HO

-T— r

(i») ' U)

to chuuse an optimum set of parameters F^, F ^ and OSiO. Figures 2c and 2d

show corresponding fits for Ge 3 0 3 and P 3 N 3 . It is also interesting to note that this approach yields an almost exact frequency and intensity fit for the complex isotope pattern frequently observed between 7 0 0 and 8 0 0 cm - 1

in 8 LiF/7LiF matrix isolation studies. This pattern is particularly evident in Snelson's work,® and is entirely consistent with a doubly degenerate mode of IU.3 F 3 , if one takes into account the possibility of a perturbation similar to that shown in Figure 2a (calcn. iii). 159

One simplifying feature which emerges from these calculations is that unless there is significant perturbation from a nearby fundamental, these isotope intensity patterns may be readily interpreted by sharing the intensity of an E 1 mode equally between the corresponding non-degenerate modes in the partially substituted species. This simplification has been successfully 13 used in a number of systems, and one of the purposes of these calculations is to assess quantitatively when this approximation is likely to break down and produce an "anomalous" spectrum. Clearly each system studied will provide a different estimate of what is "nearby" but these patterns indicate that provided the band spacings in the quartets are roughly equal, there will be minimal intensity perturbation. The advantage of this index in exploring new systems is evident, since frequency patterns may be calculated much more easily than relative intensities.

A final example of this approach comes from recent studies on the vibrational spectra of matrix isolated Asi^Og. The cage structure for this molecule has r^s_g = A t + E + Fj.+ 2F2 and Figure 3 compares the observed and calculated i.r. spectra of 1 8 0 enriched A s ^ O § in the region of the two F 2 fundamentals. Calculations show that there is little differential frequency perturbation from other fundamentals, and the intensity in each F 2 vibration is accordingly distributed equally among the resultant modes in the partially substituted species. The agreement between observed and calculated patterns is very satisfactory. FIGUBE 3 160

1. J.H. Darling and J.S. Ogden, J.C.S. Dalton, 2496 (1972). 2. R.N. Perutz and J. J. Turner, Inorg. Chem. , J_4 262 (1975). 3. E.P. Ktindig, D. McIntosh, M. Moskovits and G.A. Ozin, J.A.C.S., 95 7324 (1973). 4. H. Huber, W. KlotzbUcher, G.A. Ozin and A. Vander Voet, Can. J. Chem., 50 3746 (1972). 5. J.H. Darling, M.B. Garton-Sprenger and J.S. Ogden, Faraday Sympi Chem. Soc., 8 75 (1974). 6. See, e.g. Wilson, Decius and Cross, "Molecular Vibrations, McGraw-Hill, New York (1955). 7. J.S. Anderson and J.S. Ogden, J. Chem. Phys. , 5J_ 4189 (1969). 8. A. Snelson, J. Chem. Phys., 46^ 3652 (1967). 9. I.R. Beattie, H.E. Blayden, S.M. Hall, S.N. Jenny and J.S. Ogden, J.C.S. Dalton, 666 (1976). 10. T.R. Beattie, H.E. Blayden, R.A. Crocombe, P.J. Jones and J.S. Ogden, J. Raman Spectr. , 4^313 (1976). 11. J.S. Ogden and M.J. Ricks, J. Chem. Phys., 52^ 352 (1970). 12. R.M. Atkins and P.L. Timms, submitted for publication. 13. R.G.S. Pong, R.A. Stachnik, A.E. Shirk and J.S. Shirk, J. Chem. Phys., 63 1525 (1975). 161

The Spectrum and Structure of Aluminum Trihalides Richard G.S. Pong, Amy E. Shirk and James S. Shirk

Department of Chemistry Illinois Institute of Technology Chicago, Illinois 60616 U.S.A.

There has been some discussion of the structure of monomeric AlClg recently. While the IR and Raman spectra are consistent with Dg^ se­ lection rules, there was an electron diffraction study of AlClg which gave an average C1A1C1 angle of 118° ± 1.5°J Such a non-planar aver­ age structure can be consistent with a planar equilibrium structure if

V2 (A2 "), the out-of-plane deformation mode, is sufficiently low for a large shrinkage effect, due to averaging over several vibrational lev­

els, to occur. It has been calculated that a V 2=95 or 110 cm'^ is the range necessary.2.3 However we have measured V2=183 cm"^ in argon^iS and others have reported similar frequencies. It seems unreasonable to assume that \>z could be as low as 110 cm"^. Thus the possibility

that A1Cl3 was a f la t pyramid remained. The ternary aluminum chlorobromides can provide a very sensitive

test of the planarity of AICI 3 . If A1Cl3 and AlBrg were both planar, one would expect to be able to calculate vibrational frequencies of the ternary chlorobromides by a direct transfer of force constants.

However if AICI 3 were slightly distorted from planar, or even quasi- planar, one would expect that a transfer of force constants to be less successful. In particular one would expect ^>2(^2") > the out-of-plane

vibration in AIX 3 (v6 (Bg) in AlXgY) to be very sensitive to a change in structure. Here we report a study of the vibrations of the aluminum chloro­ bromides to elucidate the structure of AlClg. The monomeric aluminum trihalides were deposited from a double oven furnace with a large excess of Ar onto an 18°K Csl or single crystal­ line Si window in a typical matrix isolation apparatus. AlBrClg and AlBrgCl were generated by mixing varying amounts of AlClg and AlBrg, then cosubliming the mixture in vacuo prior to the deposition. 162

Far infrared spectra of monomeric AlClg and AlBrg isolated in argon matrices each exhibited three absorptions (Table I). The AlClg and AlBrg frequencies are consistent with recent reports, except for vg of AlBr., for which our frequency is different from a Raman study.®

Table I. Observed and Calculated Vibrational Frequencies Of AlClg and AlBrg, in cm~^.

A1C1 AlBr, Mode Obsd 3 Calc Obsd J Calc

v-| (A-|1) 393.5a 393.5 228.0b 228.0 Vp(Ao") 183.0 183.U 155.0 155.0 Vg(E') 618.6 618.5 508.4 508.4 617.0 616.8 61b.0 615.2 613.2 613.4 Ui > «d* 151.1 151.0 96.3 96.0

aRaman of AlClg in Ar matrix, reference 7.

bRaman of AlClg, gas phase, reference 6 .

The observed frequencies of AlClg and AlBrg were fitted to a four- constant valence force field which included an f interaction con- stant. Dgh symmetry was assumed.

Table II. Force Constants of AlClg and AlBrg.

f r (mdyne/A) f^fxlO ^erg/rad2) f g(xl 0 ^ e rg /ra d 2) f r r (mdyne/A)

AlClg 2.754 0.4218 0.0666 0.2170 AlBrg 2.185 0.4272 0.0626 0.1305

The spectra of the mixed halides exhibited absorptions due to AlClg, AlBrg, AlBrClg and AlBr,,Cl. The spectrum of each halide could be iden­ tified by studies of relative intensities and isotopic shifts. The assignment of bands to a particular vibrational mode was straightfor­ ward. We then calculated the vibrational frequencies of the mixed halides using force constants from AlBrg and AlClg. The stretch and out-of-plane deformation force constants fr and f were transferred directly to the mixed halides. The in-plane bending force constant f was transferred directly for XMX bends; for XMY bends the arithmetic 163

average of the fQ‘s of MXg and MYg was used. The f , Interaction force constants were transferred in the same way as the bends. The results of these calculations are in Table III.

Table III Observed and Calculated Vibrational Frequencie of AlBrClg and AlBrgCl in cnT^.

AlBrCl AlBr,Cl Mode Obsd 2 Calc Obsd Calc

v,(A,) 560.9 563.0 588.3 591. 559.4 561.2 583.6 587. 557.1 559.5

Vg(A 1 ) — 326.5 -- 275. 137.0 136.4 104.2 105. v3(V

v4 (B1) 615.3 616.9 511.2 511. 613.0 614.4 610.0 611.7

124.0 128.1 124.0, 1 2 2 . v5(B1) V B2> 173.9 173.8 164.7 164. Note: The correlation of vibrational modes from Dg^ for AlXg to C£v for AIX^Y i s :

v 1 (A1 1) v2 (A1 ) ■+ v1 (A-|) + v4(B1 ) V3( E '5

v2 (A2") v6 (B2) v4(e ' ) ■* VgtA-]) + )

The fit between the calculated and observed frequencies was quite good. The fit for the out-of-plane mode (v^ in the C2v molecules) is ■ very good. There is certainly no evidence for any anharmonicity. One can conclude that the mixed halides AlBrClg and AlBrgCl as well as AlClg and AlBrg have the same structure and that structure is surely planar.

References

1. E.Z. Zasorin and N.G. Rambidi, Zh. Strukt. Khim. 8 , 391 (1967).

2. E.Z. Zasorin and N.G. Rambidi, Zh. Strukt. Khim. 8 , 591 (1967). 3. S.J. Cyvin and J. Brunvoll, J. Mol. Struct. 3, 453 (1969). 4. M.L. Lesiecki and J.S. Shirk, J. Chem. Phys. 56_, 4171 (1972). 5. J.S. Shirk and A.E. Shirk, J. Chem. Phys. 64, 910 (1976).

6 . I.R. Beattie and J.R. Horder, J. Chem. Soc. 1969, 2655. 7. I.R. Beattie, H.E. Blayden, and J.S. Ogden, J. Chem. Phys. 64, 909 (1976). 164

HIGH-TEMPERATURE AND INTERSTELLAR MOLECULES

William Weltner, Jr. University of Florida Gainesville, Florida 32611

The matrix-isolation technique is perhaps at its greatest advantage in the spectroscopic investigations of molecules pro­ duced in high-temperature environments. Optical spectra of molecules at high temperatures are often complex, thereby difficult, to analyze, and do not necessarily involve the lowest electronic state. Electron-spin-resonance (ESR) and microwave spectra of these molecules in the gas phase are very difficult, if not presently impossible, to observe. Each of these objec­ tions can be overcome by the observation of molecules trapped in rare-gas matrices -at 4°K. This talk will review a few selected areas of high-temperature chemistry and also of interstellar spectroscopy where matrix-isolation has been, and promises to be, particularly helpful. The transition-metal diatomic oxide molecules exemplify, par excellence, a fruitful application of matrix techniques. This is particularly true of the heavier metal oxides where ab initio calculations are difficult. TiO is the classic example where infrared, visible, and ultraviolet spectroscopy have been applied at 4°K to establish its ground state (^A) and to complement the extensive, gas-phase work. The theoret­ ical studies of Carlson, Moser, and Nesbet on TiO probably represent the pioneering effort on high-temperature species and contributed greatly to the understanding of the molecular orbitals involved. From this has evolved a simple MO picture which is applied crudely across and down the Periodic Tabic in di.'vuunt for, and to perhaps allow predictions of, the elec­ tronic properties of other oxidoe. Recently, laser excitation in matrices (Brom and Broida) has aided in establishing the energy of the two low-lying singlet states (-*-A and •*•!) of TiO which have been of interest cinoc the original gas=*phase work ot Phillips (Brewer and 165

Green). Such states have apparently only been observed for TiO and ZrO in this series, the latter by a direct triplet <- singlet absorption in solid neon. The establishment of the relative energy of manifolds of different multiplicity is a general problem in most molecules which may be solvable by the application of matrix techniques. Our knowledge of the diatomic hydrides, carbides, and nitrides of the transition metals is much less than that of the oxides, and little matrix work has been done. The ground states of the hydrides are often more difficult to calculate (Scott and Richards) than one might have expected because of many states lying within electron correlation estimates. ScH, for example, is expected to have six states within about 5000 cm--*- of each other. Only one transition-metal carbide, RhC (2£), has been investigated in matrices via optical and also ESR spectroscopy, and only one nitride, MbN, has been observed (Green, et £l) via its infrared spectrum. A few triatomic oxides Ti02, Z r02, Ta02, M0O 2, and W O 2 have been identified, largely from matrix spectra. Evidence obtained from vibrational and electronic spectra indicates that they are all bent. Although we have contributed to the matrix work on tran- sition-metal halides by studies of ScF and very recently TiF2, TiFg, extensive investigations have been made by others (Margrave, Hauge; Gruen). There is also increasing interest in the oxides, etc., of the lanthanides and actinides, where understanding is generally at a much lower level than the transition-metal molecules. An area of interest for many years in our laboratory has been the carbon molecules C 2, C-j, C4 , C 2 and C 3 have been trapped at 4°K by several groups, and the spectra have confirmed that the ground state of C 2 is -*-£g+ (-^nu lying 610 cm--*- higher, Ballik and Ramsay) , and have provided vibrational frequencies for C 3 not seen in the gas-phase. Although the 1 1 o well-known xn «- XXE system of C 3 near 4050 A exhibiting the large Renner effect (Gausset, et al) is easily observed at 4° K and has been thoroughly analyzed, searches into the ultra­ violet out to 2000 8 have been disappointing in not revealing 166 another excited state (expected to be -*-£u~) . However, C g , with its low bending frequency of 60 cm--*- in the ground state, appears to be well understood. The larger molecules are difficult to produce in high concentrations since they consti­ tute less than 1% of the vapor over graphite at 2500°K. They can bo prepared by diffusion Of the smaller species C, C2, and Cg in solid argon, and are easily observed in the infrared, although the assignment of particular bands to particular species is very uncertain. Recently we have prepared by the photolysis of diaceLylene and detected it in the ESR as a triplet molecule, in accord with theory (Pitzer and Clementi). The same ESR spectrum is observed in matrices prepared from o graphite vapor and a system with (0,0) at 5200 A is attributed to C 4. Hyperfine splittings obtained from 13C substitution indicate that the linear molecule is a tt radical and the low zero-field-splitting parameter indicates that the two un-= paired spins are largely on the two outer carbon atoms. We. are presently attempting to prepare the Cg molecule. Cg— was first prepared in matrices by Milligan and Jacox by photolysis of acetylene, the method used in the gas- phase (Herzberg and Lagerqvist). It is intriguing in re­ quiring a countering cation which has not been observed but has been presumed to be C 2Hg+ or CgH"1". The mechanism of its formation has been at least partially explained (Brus and Bondybey). Attempts to observe C2“ (3E ground state) in the ESR have not been successful, although M+C2- ion-pairs, where M+ is a alkali-metal cation, have been detected. SiC is presumably (Lutz and Ryan) but it has not been observed,• however GiCC (1E) is an important stellar molecule first identified by Kleman. Matrix optical spectra 1 pfl to revioion of its vibrational assignment, and recently a gas-phase study (v»rma and Magaraj) has both supported and revised the solid-state assignment. The ground-state handing frequency is 147 cm-^-, and the Renner parameter in the ex-cited ^-H state is small. SigC, present in comparable amounts to SiC2 according to mass spectrometry, has not been observed, but the spectrum of a presumably unsymmetrical Si2Cg molecule appears strongly in neon and argon matrices. 167

ESR has presently revealed nothing about these mixed silicon and carbon molecules. Si2 (^Ig-) is easily seen optically in matrices when Si atoms are trapped. Sij and larger Si chains have not been characterized spectroscopically. Elemental boron also vaporizes to give some polyatomic molecules in low abundance. Only B atoms and B 2 (probably ^Zg) have been observed in matrices. BCC (2f) is the only boron-carbon molecule trapped and observed spectroscopically; it yields a distinctive ESR spectrum. The first interstellar molecule "discovered" via matrix techniques was hydrogen isocyanide (HNC) (Milligan and Jacox) since its observation in the solid led to its detection in the interstellar medium by the radio telescope. The C 2H radical is, however, the only species whose identification in space (Tucker, Kutner, and Thaddeus) presently rests heavily on laboratory matrix observations. Hyperfine splittings and g shifts measured in the ESR spectrum of trapped C 2H provide the evidence. The measured value of gj_ - ge = Ag^ yields a value of y, the spin-rotation constant, by use of the Curl equation Y = -2BAg^ , where B is the rotational constant. It is apparently a very abundant interstellar polyatomic molecule. Surprisingly, the optical (or microwave) spectrum of C 2H has not as yet been observed in the gas phase. A recent theoretical study (Shih, et al) compares its properties with those of isoelectronic CN and predicts its electronic spectrum. The microwave spectrum of CN has only recently been ob­ served (Dixon and Woods), and prior to that time the best determinations of its ground-state properties were obtained from observations of CN in the interstellar medium and from ESR data of the matrix-isolated molecule. The majority of the ever-increasing list of interstellar molecules are stable molecules, as opposed to free radicals. This is not surprising since optical and particularly micro­ wave spectra (Lovas and Lide) of stable species are the easiest to observe in the laboratory and therefore to identify in space. It seems very likely that ESR spectra of matrix- 168 isolated radicals will play an increasingly important role in the future in the identification of interstellar species.

Some references (mostly matrix) Reviews: C.J. Cheetham and R.F. Barrow, Adv. High Temp. Chem. 3^, 7 (1967). K.D. Carlson and C.R. Claydon, Adv. High Temp. Chem. 1, 43 (1967) . W. Weltner, Jr. Science 155, 155 (1967); Adv. High Temp. . Chem. 2, 05 (1969). D.M. Gruen, Prog. Inorg. C h e m . 14, 119-172 (1971). J.W. Hastie, R.H. Hauge, and J.L. Margrave, Ann. Rev-..Phys^ C h e m . 21, 475 (1970) . P.R. Scott and W.G. Richards, Specialist Periodical Reports (The Chemical Society, London), Molecular Spectroscopy, 4, 70 (1976). TiO, ZrO W. Weltner, Jr. and D. McLeod, Jr., J. Phys. Chem. 69^, 3488 (1965). L. Brewer and D.W. Green, High Temp. Sci. 1^, 26 (1969). J.M. Brom, Jr. and H.P. Broida, J. Chem. Phys. 63, 3718 (1975). L.J. Lauchlan, J.M. Brom, Jr. and H.P. Broida, J. Chem. Phys. 65, 2672 (1976). Triatomic Oxides W. Weltner, Jr. and D. McLeod, Jr., J. Chem. Phys. £2, 882 (1965). Transition-metal carbide and nitride J.M. Brom, Jr., W.R.M. Graham, and W. Weltner, Jr., J. Chem. Phy§, 57 , 4116 (1972) . D.W. Green, W. Korfmacher, and D.M. Gruen, J. Chem. Phys. 58, 404 (I'JVJ) . Carbon and silicon carh.i.da. E .A. Ballik and D.A. Ramsay, Astrophys. J. 137, 61 (1963). L. Gausset, G. Herzberq, A. Lagerqvist, and B. Rosen, Astrophys. J. 142, 45 (1965). W. Weltner, Jr. and D. McLeod, Jr., J. Chem. Phys. J_5, 3096 (1966) . 169

K.R. Thompson, R.L. DeKock, and W. Weltner, Jr., J. Amer. Chem. Soc. 93, 4688 (1971). W.R.M. Graham, K.I. Dismuke, and W. Weltner, Jr., Astrophys. J. 20£, 301 (1976). G. Herzberg and A. Lagerqvist, Can. J. Phys. £ 6, 2363 (1968). D.E. Milligan and M.E. Jacox, J. Chem. Phys. 52, 1952 (1969). W.R.M. Graham, K.I. Dismuke and W. Weltner, Jr., J. Chem. Phys. 61, 4793 (1974). L.E. Brus and V.E. Bondybey, J. Chem. Phys. 6_3, 3123 (1975). B.L. Lutz and J.A. Ryan, Astrophys. J. 194, 753 (1974). W. Weltner, Jr. and D. McLeod, Jr., J. Chem. Phys. ££, 235 (1964) . R.D. Verma and S. Nagaraj, Can. J. Phys. 5£, 1938 (1974). Boron and boron carbide W.R.M. Graham and W. Weltner, Jr., J. Chem. Phys. 6_5, 1516 (1976). W.C. Easley and W. Weltner, Jr., J. Chem. Phys. 5£, 1489 (1970) . Interstellar molecules D.E. Milligan and M.E. Jacox, J. Chem. Phys. 3_9, 712 (1963); 47, 278 (1967). K.D. Tucker, M.L. Kutner, and P. Thaddeus, Astrophys. J. 193, L115 (1974). E.L. Cochran, F.J. Adrian, and V.A. Bowers, J. Chem. Phys. £0, 213 (1964). W.R.M. Graham, K.I. Dismuke, and W. Weltner, Jr., J. Chem. Phys. 60, 3817 (1974). S. Shih, S.G. Peyerimhoff, and R.J. Buenker, J. Mol. Spectr. 64, 167 (1977). R.F. Curl, Mol. Phys. £, 585 (1965). L.B. Knight, Jr. and W. Weltner, Jr., J. Chem. Phys. 53, 4111 (1970). W.C. Easley and W. Weltner, Jr., J. Chem. Phys. £2, 197 (1970) . F.J. Adrian and V.A. Bowers, Chem. Phys. Lett. £1, 517 (1976). F.J. Lovas and D.R. Lide, Jr., Adv. High Temp. Chem. £, 177 (1971). 170

Can the production of diatomic high temperature molecules be optimized by MIS?

F.W. Froben, Institut fur Molekillphysik Freie Universitat Berlin Boltzmannstr. 20, 1000 Berlin 33, Germany

High temperature molecules like those of stellar interest have been studied during recent years by 3 M methods (mass spectroscopy, matrix-isolation spectroscopy, and microwave 1-3 spectroscopy ). During the last years chemiluminescence reactions , laser excitation , and high temperature photo­ electron spectroscopy have been added to the study of these molecules,.but the 3 M's are still the most important ».nd can he used f or all the species .

If we talk about high temperature molecules we are discuss­ ing the spectra of stars, where C2 , C3, CN, CH, SiC2 , NH, OH,

ZrO, LaO, YO, TiO, VO, ScO, CrO, A10, BO, SiO,

SiS, NS, SO and some hydrates

MgH, SiH, A1H, CaH have been identified, and about the molecules above the melts of refractory metals and in flames and plasmas.

The derivation of thermodynamic data for these molecules - the above mentioned are thought as examples - has many difficulties, which increase with the size of the molecule. Mass spectroscopic measurements can give the composition ot laboratory vapors but the precise estimation of for example the heat of formation requires the knowledge of spectroscopic da-ta which are not available for many of the high temperature molecules involved. 171

For many diatomic molecules microwave measurements have provided these data, but until now, this has been mostly i restricted to E-ground state molecules within the groups I-VII, II-VI, III-VII, and IV-VI of the periodic system with 8 and 10 valence electrons'*.

A rough estimate for the sensitivity of these spectrometers will be given.

In addition to these methods, matrix isolation spectroscopy has some great advantages- and disadvantages. The most important merit is the fact that at low temperature only very few low lying states are populated. As a result, the interpretation of the spectrum is simplified and a firm assignment of the ground state can be made. Other advantages are: the small amounts of material required, the small band­ width of the spectra so that several species with similar energy levels can be distinguished. Among the difficulties of the matrix approach are the following: the rotational energy levels are suppressed so that interatomic distances cannot be calculated. Vibrational assignments can only be made if isotropic frequency shift data are available. The matrix splitting of energy levels complicates the interpret­ ation. The most serious disadvantage is connected with a great advantage. The matrix technique allows the collection of a sample over an extended period of time and so it is very sensitive. But if only EPR measurements can be made, a beautiful spectrum may be due only to a minor species from the high temperature reaction cell.

At the' end it is very important to say that the question in the title cannot be answered in general. Certain experimental conditions for the production of diatomic molecules, especially unstable molecules, can be optimized by matrix isolation spec­ troscopy but that, of course, does not imply that a different production mechanism cannot produce the molecule of interest with a much higher rate. This will be demonstrated. 1 7-2

Due to the extensive literature the cited articles may serve only as examples.

1.a. J.L. Margrave The characterization of high temperature vapors J.Wiley u. Sons 1967

b . J.W. Hastie High Temperature Vapors Academic Press 1975

2.a. W. Weltner Adv. High Temp. Cnem. 2, 85 (1959)

b . A. Snelson Matrix-isolation spectroscopy of high temperature species Thermochemistry and Thermodynamics Chap.3 p 8l, H.A. Skimmer editor Butherworths 1976

3.a. F.J. Lovas and D.R.Lide Adv.High Temp, ) Chem -3, 177 (1971)

b. R. Honerj&ger, Z. Naturforsch. 32a, 1 (1977) R. Tischer

c. J . Hoeft, F.J. Lovas, Z. Angew.Phys. 31, 265 (1971) E. Tiemann, T. Torring

4. P.J. Dagdigian, H.W. Cruse and R.N. Zare J .Chem.Phys. 6£, 1824 (1975)

5. J.G. Pruett, R.N. Zare J. Chem.Phys. 62, 2050 (1975)

6. J. Berkpvfit? J . Chem. Phys. 6j_, 407 J (1974) 173

MATRIX ISOLATION STUDIES WITH FOURIER TRANSFORM IR

David W. Green and Gerald T. Reedy Chemical Engineering Division ■j Argonne National Laboratory Argonne, Illinois 60439

The combination of Fourier transform infrared (FT-IR) spectroscopy with the matrix-isolation techniques has advantages compared with the use of more conventional grating spectroscopy. Furthermore, the recent com­ mercial availability of Fourier transform spectrometers has made FT-IR a practical alternative. Some advantages of the FT-IR spectrometer over the grating spectrometer are the result of the computerized data system that is a necessary part of the FT-IR spectrometer; other advantages are a consequence of the difference in optical arrangements and these repre­ sent the inherent advantages of the FT-IR method. In most applications with the matrix-isolation technique, the use of FT-IR spectroscopy results in either an improved signal-to-noise ratio or a shorter time for data collection compared with grating infrared spectroscopy. Fourier transform infrared spectroscopy has been used in our labora­ tory to study several molecular species in low-temperature matrices. Some species have been produced by high-temperature vaporization from Knudsen cells and others by sputtering using the device shown in Fig. 1. With this sputtering device, Ar and Kr matrices have been prepared which con­ tain U atoms, UO, UO2, UO3, PuO, Pu02, UN, or UN2, depending upon the composition of the gas used to sputter as well as the identity of the metallic cathode.

Fig. 1

Sputtering device used to produce high temperature atoms and molecules.

Results for the uranium oxides have been published1*2 which show that UO2 is linear and that UO3 has a "T-shaped" geometry (C2V)• Preliminary results of a study of uranium nitrides^ show that UN is trapped in multi­ ple sites in both Ar and Kr matrices. The UN2 molecule is linear. 174

Figure 2 shows the infrared spectra of.UN and UN2 together with an un­ identified species labeled "X-UN."

”i 1 1 1 | 1 1 » 1 . r

Fig. 2

Infrared spectra obtained from an Ar matrix at 14 K. (A) The products of cocondensation of U atoms with NO2; (B) the products of sputtering urani­ V %W_,v um metal with a 1:1:800 14N2: N2:Ar X-ITN mixture; (C) the same matrix as (B) UN after annealing to 30 K and recooling 4n-u-hn fy to 14 K.

1050 1000 950

The reactions of UO and UO2 with NO and with NO2 have been studied4 by cocondensing two reactants in an Ar matrix. The infrared spectra of both UO2 and N0->, and their 18U-substituted isotopomers, have been ob­ served. The UO* ion is linear, whereas the NO2 ion is bent with a bond angle of 109°. The species UO2 was produced by either (1) UO2 + NO2, (2) U02 + NO, or (3) UO + NO2. The spectral data indicate that UCb> exists as part of an ion pair with either NO2 or NO . Figure 3 shows two of the infrared spectra recorded in these studies.

Fig. 3

Infrared sner.tra obtained ar 15 K by codeposition of the vapors over hypostoichiometric *80-enriched UO2 with N 180z (top spectrum) and with N 160 (bottom spectrum)+ in Ar matri­ ces. A peaks are U 1602 V 3 , B peaks are U 160*80+ V 3 , and C peaks are U 1 S 0 2 v 3 .

"00 01a t»40 uu euu More recently, plutonium oxides have been studied in Ar and Kr ma­ trices. 5 The sputtering device, Fig. 1, was used with a Pu metal insert in the hollow cathode to produce the vapor species. Figure 4 shows the effects of variation of the oxygen concentration upon the infrared spec­ trum. The PuO molecule is trapped in multiple sites in Ar matrices but not in Kr matrices. The PuO2 molecule was found to be linear.

t ei ozone _ muil. Itolopom ers I'3 pu

higher Pu aides T s

juWl Fig. 4

Infrared absorption spectra of plutonium oxides in Ar matrices £ & ' v Three different nominal concen­ trations of O2 in Ar were used Uu in the sputtering device.

Pul60

REFERENCES

1. S. D. Gabelnick, G. T. Reedy, and M. G. Chasanov, J. Chem. Phys. 58 4468 (19 73).

2. S. D. Gabelnick, G. T. Reedy, and M. G. Chasanov, J. Chem. Phys. 59 6397 (1973).

3. D, W, Green and G. T- Reedy, J. Chem. Phys.' 65, 2921 (1976).

4. D. W. Green, S. D. Gabelnick, and G. T . Reedy, J . Chem. Phys. 64, 1697 (19 76).

5. D. W. Green and G. T. Reedy, unpublished results (1977). 176

A STUDY OF THE GROUND STATES OF PtO AND IrO

K. JANSSON and R. SCULLMAN, Department of Physics, University of Stockholm, Stockholm, Sweden.

The molecules PtO and IrO both have a very complicated gas phase emission spectrum. Several bands constituting these spectra have been rotationally analyzed and some have been arranged into systems or sub-systems (1,2,3), 61 spite of this information, there is still doubt about PtO and IrO concerning their ground states.

An investigation of the corresponding gas phase absorption spectrum often solves the question about the ground state. This has however not been done for PtO and IrO because there are experimental difficulties in producing these oxides in large enough quantities for such an experiment due to the high melting points of plati­ num and iridium.

There is however another way of determining the ground state of a molecule and that is to study the absorption spectrum of the molecule when it is matrix-isolated in a solid rare gas. This should be a convenient method for the molecules PtO and IrO since a hollow cathode can then be used as the source for producing the molecules.

KXPKRTMENTA I.

The experiment was performed in the following way. Argon gas containing species produced in a hollow cathode discharge was frozen onto a cool KBr plate which could be cooled down to approximately 10K by a refrigerating unit, a CTi Cryo- dyne Cryocooler Model 21 (4), The hollow cathode consisted of a water-cooled copper cathode with a hole of about 2 mm diameter. The anode was a platinum or copper wire surrounded by a quartz tube which fitted into the cylindrical cathode. In the experiments with platinum the cathode was lined with platinum foil 0.1 m m thick. However, iridium metal is very difficult to bend into such a small diameter and it was found to be impossible to line the cathode with iridium foil and obtain a good thermal contact. Iridium powder was used instead. It was pressed by a hy­ draulic press onto a perforated copper foil which in turn lined the cathode. 1.77

The rare gas used in the hollow cathode discharge was used at the same time as matrix gas. The rare gas entered the system through the quartz tube. In order to produce the oxides, 0 . 5 - 2 % oxygen was added to the rare gas.

The absorption spectrum was photographed between 2500-8900 A in.an Eagle spectrograph with a i m grating. A tungsten band-lamp was used as a back­ ground lamp for the continuum except for wavelengths shorter than 4000 A where a 35 W high-pressure Xe-lamp was used instead.

T H E S P E C T R U M OF PtO

The absorption spectrum of PtO isolated in an argon matrix showed bands that could be arranged into two groups probably constituting three or more band sys­ tems. The bands interpreted as originating from transitions between the lowest vi­ brational levels, the 0-0 bands, are situated at 7594 A for one system and at 5260, 5229, 5225, and 5200 A for the remaining systems. However, in the wavelength region where an intense band system of gaseous PtO is situated around 5900 A, no diotinct bands are observed.

The corresponding bands were also observed using krypton instead of argon. The bands are then shifted to the red compared with those for argon. Moreover bands were also seen for the isotopic molecule Pt18Q when using 1 8Og enriched gas. Bands with higher vibrational quantum numbers were then somewhat displaced from those of Pt160.

The absence of bands originating from a molecule consisting of a mixture of 1 80 and 18C shows that the absorber is a diatomic molecule and not a triatomic or larger molecule. Furthermore, since the bands only appear in the experimonto using a platinum-coated cathode and with oxygen present, they have been inter­ preted as originating from the molecule PtO.

If one of the systems of bands of matrix-isolated PtO is identified with the gas- phase A 1£ - X system, this would mean a matrix shift of more than 2000 cm from the gas-phase transition which seems rather improbable. It is also very im­ probable that two broad diffuse' absorption lines with maxima observed at 5715 A and 5564 A should be identified with the A 1E - X 1£ system. In addition these 178 broad lines have not been verified as oritinating from PtO. The conclusion is 1 1 o thus that the A £ - X £ system with an intense 0-0 • band at 5902 A from gase-. ous PtO does not as earlier assumed belong to a transition to the ground state.

The investigation of the emission spectrum of gaseous PtO has however been continuously progressing (5). Recently, three new systems have been analyzed which seem to be three separate sub-systems originating from a lower sub-state with Q=1. The latter probably constitutes, together with the earlier designated 1 + 3 - X £ state, the two sub-states 1 and O of a £ state which has properties according to Hund's case (c). This result was based on the observed values of the vibrational and rotational constants of the lower states. Moreover a prelimi­ nary analysis of an intercombination band at 6923 A shows that the 0+ (earlier X *£) substate is still the lowest state, 936 cm 1 below the 1 sub-state. Further­ more, besides the earlier reported A - X and D - X systems, two more sys­ tems have been found which have the X state as their lower state. However, none of these earlier known, or recently found, systems seem to correspond to the bands of matrix-isolated PtO.

In addition two PtO bands have been observed, one at 762G A and one at 7776 A, with an iso topic structure due to platinum isotopes typical for a transition between the lowest vibrational levels, v1 = 0 -> v" = 0. These bands have not yet been ana­ lyzed but a preliminary investigation showed that they do not combine with the 0 + (earlier X 1£) sub-state or the 1 sub-state. Since they are not far from the position of the intense system at 7594 A of matrix-isolated PtO, these two new bands may be of great interest in the determination of the ground state of PtO.

THE SPECTRUM OF IrO

The experiments with IrO did not give any positive results. The failure to ob­ serve as absorption spectrum may however be explained in the following ways. One explanation might be the presence of sputtered copper. The great disposi­ tion of copper to react with oxygen could be the reason why no IrO is observed in the matrix although the appearance of an intense broad line with its maximum at 3645 A, probably an Ir atomic line, indicates the presence of Ir atoms in the argon matrix. From earlier gas phase experiments (2) it is also known that 179

IrO is formed under the existing conditions. That CuO is formed is indicated by the observation of earlier known bands of matrix-isolated CuO (6,7,8,9).

Another explanation is that the known gas phase IrO bands do not belong to tran­ sitions to the ground state. A further analysis of the gas phase IrO spectrum (10) shows that there may be systems at longer wavelengths outside the investi­ gated region, 4000 - 6800 A, with another hitherto unanalyzed lower state. This vr lower state may thus be the ground state of IrO.

REFERENCES

1. C. Nilsson, R. Scullman and N. Mehendal6, J. Mol. Spectrosc. 35, 177 (1970)

2. K. Jansson and R. Scullman, J. Mol. Spectrosc. 23, 208 (1972)

3. R. Scullman, U. Sassenberg and C. Nilsson, Can. J. Phys. 53, 1991 (1975)

4. K. Jansson and R. Scullman, J. Mol. Spectrosc. j5^, 299 (1976)

5. U. Sassenberg and R. Scullman (private communication)

6. J.S. Shirk and A.M. Bass, J. Chem. Phys, 52, 1894 (1970)

7. K.R. Thompson, W.G. Easley and L.B. Knight, J. Phys. Chem. 77, 49 (1973)

8. D.H.W. Carstens, J.F. Kozlowski and D.M. Gruen, High Temp. Sci. 4, 301 (1972)

9. R. F.' Barrow and M.J. Griffiths (private communication)

10. K. Jansson (Thesis 1977, University of Stockholm) 180

Section: Reactive Matrices Title: Metal Atom Chemistry and Surface Chemistry; Dinickel Monoethylene. NiJCLH..); A Localized Bonding Model for Ethylene Chemisorbed on Bulk Nickel. Author: Geoffrey A. Ozin Address: Lash Miller Chemistry Laboratory and Erindale College. University of Toronto. Toronto. Ontario. Canada and Sherman Fairchild Distinguished Scholar (1977), Department of Chemistry. California Institute of Technology. Pasadena. California 91125, USA.

Abstract: In theoretical and experimental studies of chemisorption and catalysis, a great deal of discussion has focused on the validity of employ­ ing localized models for elucidating the surface bonding between adsorbate and adsorbent.1 For this model, the chemisorption bond is described in term s of a surface molecule, by coupling the adsorbate wavefunctions to only a limited number of neighboring surface atoms. From a computa­ tional standpoint, the concept of a surface molecule and localized orbitals at surfaces is an attractive one and one that receives a large measure of 2 support from a vast body of homogeneous organometallic analogues. In this context, the past decade has witnessed a renaissance in surface spectroscopy and reliable microscopic information is becoming available on the interaction between transition metal surfaces and a variety of 3 hydrocarbons and small molecules such as H2, CO, N2, 02, etc. Con­ comitant with these developments in surface chemistry and physics, significant advances have been made with approximate and highly accurate molecular orbital methods for handling transition metal systems, partic­ ularly those which bear a logical relationship to various aspects of horno- 3 4 geneous and heterogeneous catalysis. ’ Theoretical studies of adsorp­ tion have generally proceeded by placing the adsorbate molecules on a pseudo-metal surface, represented by two or mtire atoms. Usually the objectives are to test the localized bonding approach to chemisorption, to probe the electronic and geometrical structures of chemisorbed mole­ cules and to extract information for use in deciphering experimental photoemission spectra of surface complexes. Such computational proced­ ures appear to be justified in part by the fact that calculated electronic energy levels of clusters consisting of only a few metal atoms (as well as these clusters interacting with small molecules) already reveal striking 181

resemblances to the corresponding band structures of the infinite crystal analogues. 2’ ^ However, until very recently, a serious shortcoming of the localized model has been the difficulty of experimentally generating realistic molecular systems for interconnecting the surface molecular state and its localized bonding counterpart. 2c’6 In this particular study we have addressed ourselves to the experi­ mental challenge of producing an "ideal" localized bonding model of ethylene chemisorbed onto bulk nickel, a much discussed question in previous investigations of olefin hydrogenations, hydroformylations, and isomerizations over Group Vin catalysts.2c’3l3’ ^g’ 6b’e’7 As a model "surface complex" we have chosen to aim for a nickel-diatom interacting with an ethylene molecule, namely, Ni^CgH*), a not unreasonable synthetic goal in view of the recent metal atom cryochemical syntheses of ® and Ni(C 2H4)1>2}3. ®e If successful, the spectroscopic properties of Ni^CgHJ would be especially pertinent to the still controversial question of the geometry of chemisorbed ethylene.7 In brief, recent photoemission studies of ethylene on an Ni(ni) surface1** have yielded the first direct observation of ir-d bonding. The assumption here is that ethylene forms a n-complex coordinating symmetrically to a single surface atom as o originally proposed for organometallic complexes. However, from earlier infrared investigations and various surface reactions73’13’1® it had been inferred that ethylene chemisorbed onto silica supported nickel exists as a di-a-surface complex, in which each of the two carbon atoms forms a cr-bond to a different nickel atom. This picture has changed slightly as a result of some very recent infrared investigations7*3’** which provide evidence for a ir-bonded form of ethylene which probably 7f coexists with the di-a form on Group VHI supported metals. Interest­ ingly, a recent secondary ion mass spectrometer study of ethylene adsorbed on nickel11 showed the presence of two distinct types of adsorption, as indicated by the detection of the ions Nm^Hj"1" and Ni2(C2H4)+. These observations can be considered to indirectly support the idea of a di-cr and it -bonded form of chemisorbed ethylene on nickel. Along with these experimental probes of the chemisorbed state of ethylene there have emerged reliable SCF-Xa-SW calculations of the electronic structures of model surface ethylene complexes, one involving M(C-H4) 4b 4ff (where M = Ni, Pd and Pt) and the other concerning NLj and NiJC^HJ, B the latter having ethylene in both di-a and ir-complexed forms. In what follows we will describe in detail the cryochemical synthesis and spectro­ scopic characterization of Ni^CyiJ based on our previous experiences 182

W i t h N i,12 Ni,, 6e and Ni(C2H4)1;2)3. 6e

References: 1. (a) T. E. Madey, J. T. Yates, Jr., D. R. Sandstron, and R. J. H. Voorhoevre in Solid State Chemistry. B. Hannay, Ed., p. 1, Vol. 2, 1976; (b) L. H. Little, Infrared Spectra of Adsorbed Species (Aca- demic P ress, London, 1966); (c) M. L. Hair, Infrared Spectroscopy in Surface Chemistry (Marcel Dekker, New York, 1967); (d) J. E. Demuth and D. E. Eastman, Phys. Rev. Lett. 32, 1123 (1974); (e) H. Conrad, G. Ertl, H. KnSzinger, J. Kuppers, and E. E. Latta, Chem. Phys. Lett. 42, 115 (1976); (f) G. C. Bond, Catalysis by Metals (Academic Press, London and New York, 1962), and refer­ ences cited therein. 2. (a) R. Ugo, Catal. Rev. 11, 225 (1975); (b) E. L. Muetterties, Bull. Soc. Chim. Beiges 84, 959 (1975); ibid. 85, 7 (1976); (c) G. A. Ozin, Accts. Chem. Res. 10, 21 (1977) and references cited therein. 3. (a) A. Anderson and R. Hoffmann, J. Chem. Phys. (H, 4545 (1974); (b) A. Anderson, J. Am. Chem. Soc. 99, 696 (1977); J. Chem. Phys. 65, 1729 (1976); ibid. 64, 4046 (1976); Chem. Phys. Lett. 35, 498 (1975); (c) R. C. Baetzold, J. Chem. Phys. 55, 4363 (1971); J. Catal. 29, 129 (1973); Surf. Sci. 51, 1 (1975); (d) R. C. Baetzold and R. E. Mack, J. Chem. Phys. 62, 1513 (1975); (e) G. Blyholder, Surf. Sci. 42, 249 (1974) and references cited therein. 4. (a) R. P. Messmer, S. K. Knudson, K. H. Johnson, J. B. Diamond, and C. Y. Yang, Phys. Rev. B 13, 1396 (1976); (b) R. P. M essmer In The Physical Basis for Heterogeneous Catalysis. E. Drauglis and R. I. Jaffee, Eds. (Plenum P ress, New York-London, 1975); (c) K. H. Johnson and R. P. Messmer, Int. J. Quantum Chem. Symp. 10, 147 (1976); (d) N. RSsch and K. H. Johnson, J. Mol. Catal. 1, 395 (1975/76); (e) J. C. Slater and K. H. Johnson, Phys. Today 34 (1974); (f) C. F. Melius, J. W. Moskowitz, A. P. Mortola, M. B. Baillie, and M. A. Ratner, Surf. Sci. 59, 279 (1976); (g). N. RSsch and T. N. Rhodin, Faraday Soc. Chem. Soc. 58, 28 (1974); (h) I. P. Batra and O. Rtibaux, J. Vac. Sci. Technol. 12, 242 (1975) and references cited therein. 5. (a) The Physical Basis of Heterogeneous Catalysis. E. Drauglis and R. I. Jaffee, Eds. (Plenum P ress, London-New York, 1975); (b) Characterization of Surfaces and Adsorbed Species. R. B. Anderson 183

and P. T. Dawson, Eds. (Academic P ress, New York, 1976), Vol. 3; (c) Advances in Catalysis, D. D. Eley, H. Pines, and P. B. Weisz, Eds. (Academic Press, New York, 1976), Vol. 26 and references cited therein. 6. (a) G. A. Ozin and M. Moskovits in Cryochemistry. G. A. Ozin and M. Moskovits, Eds. (John Wiley and Sons, New York, 1976); (b) G. A. Ozin and W. J. Power, Inorg. Chem. 16, 212 (1976); (c) D. McIntosh and G. A. Ozin, Inorg. Chem. 16, 51 (1976); (d) H. Huber, D. McIntosh, and G. A. Ozin, Inorg. Chem. 16, 59 (1976); (e) H. Huber, W. J. Power, and G. A. Ozin, J. Am. Chem. Soc. 98, 6508 (1976) and references cited therein; (f) J. E. Hulse and M. Moskovits, Surf. Sci. 57, 125 (1976). 7. (a) B. A. Morrow and N. Sheppard, J. Phys. Chem. 70, 2406 (1966); Proc. Roy. Soc. Lond. S er., A 311, 391 (1969); (b) J. Erkelens and Th. Liefkens, J. Catal. 8, 36 (1967); (c) J. D. Prentice, A. Lesiunas, and N. Sheppard, Chem. Commun. 76 (1976); (d) Y. Soma, J. Chem. Soc. Chem. Commun. 1004 (1976); (e) G. Blyholder, D. Shihabi, W. V. Wyatt, andR . B artlett, J. Catal. 43, 122 (1976); (f) H. A. Pearce and N. Sheppard, Surf. Sci. 59, 205 (1976) and references cited therein. 8. J. E. Hulse and M. Moskovits, J. Chem. Phys. (1977), in press. 9. J. Chatt and L. A. Duncanson, J. Chem. Soc. 2939 (1953); M. J. S. Dewar, Bull. Soc. Chim. F r. 18, C71 (1951). 10. (a) G. I. Jenkins and E. K. Rideal, J. Chem. Soc. 2941 (1955); (b) J. H. Sinfelt, Advan. Catal. 23, 91 (1973). 11. M. Barber, J. C. Vickerman, and J. Wolstenholme, J. Catal. 42, 48 (1976). 12. W. Klotzbiicher and G. A. Ozin, Inorg. Chem. 15, 292 (1976). 1 84

Photoreactions in condensed NO 2 and N O 2 - acetone mixtures

H. D. Breuer, J. Kruger

Institut fur Physikalische Chemie II Universitat des Saarlandes 6600 Saarbrucken, West-Germany

The photochemistry of the oxides of nitrogen in the gas phase has been widely investigated during the last years. There is, however, very little information about photo­ reactions of these gases in the condensed state. The possi­ bilities for photochemical reactions may be markedly different from those in the gas phase not only due to the effect of low temperature but also by the lack of trans­ lational and probably rotational degrees of freedom.

In this study, we report on some photoreactions of pure nitrogen dioxide and of mixtures of N0 2 and acetone con­ densed on a NaCl substrate at L-N2 temperature. For compa­ rison the N 0 2 spectrum in a C0 2 matrix has also been re­ corded.

At 77 K the equilibrium between NOj and N2®4 :i-s almost completely on the side of the dimeric. From x~ray measure- monto the stable configuration of solid riuUy. IS known to Lll be planar "*. Two metastable configurations also have been observed, a nonplanar structure with an absorption band at 1715 cm 1 andam a configuration which is linked over an oxygen atom (1) 0 0 0 N < (I) 0

This structure has been observed so far only at liquid helium temperatures1* 185

Irradiation of N0 2 (N20^) condensed on NaCl by a Hg-resonance lamp leads to the formation of some new absorption bands which are listed in table 1 .

Table 1

Absorption bands appearing during irradiation (A = 254 nm)

v> [ c m 1J Assignment

1295 n o 2c i

1360 NO” (NaN03)

1680 . n o 2c i

2225 N0+ (N0C1)

The band at 2225 cm ^ can be attributed to the nitrosyl cation, N0+ . The very broad band between 1400 cm ^ and 1300 cm ^ which becomes sharper during warm-up and having' its center frequency at 13 60 cm ^ belongs to the NO^ ion. The formation of N0-,C1 is documented by two bands appea- -1 -1 ring at 1295 cm and 1680 cm

The absorption bands at 1360 cm ^ and 2225 cm ^ indicate that during the irradiation (I) has been produced which partly reacts with the substrate to form N0C1 and NaNO^:

0 2NN02 - hV>» 0N0N02 - NaN03 + N0C1 ' (II)

By comparison with NaNO, in a KBr pellet the extinction coefficient at 1360 cm has been determinded to ft|3gQ = 9 x 10 cm^ mole ^. Knowing the quantum flux of the 186 light source the rate of formation of NaNO, can be estima- 13 -1 J ted to be 3.5 x 10 sec , the quantum efficiency being about 10 ^ .

During warm-up the N 2°4 and N0C1 bands disappear at 180 K, the NOjCl bands at 210 K while NaNO^ is still observable at room temperature.

Irradiation of NO, in the gas phase by the Hg-resonance 2— 3 line leads to the formation of NO ( Tf) and 0 ( P). Photolysis in a condensation layer should, if at all, be much less efficient because of fast recombinations. However, atoms pro­ duced at the surface of the layer may be able to react with other molecules or radicals to form some new species. In order to test this mechanism we passed a constant stream of carbon monoxide (p = 10 ® torr) over the surface of the condensation layer. The formation of carbon dioxide was monitored by recording the asymmetric stretching mode at 2350 cm '. After 6 hours of irradiation the absorption reached a constant value of 3 % corresponding to approximately 50 CO 2 layers. Since the formation of C02 is independent of the thickness of the N 2°4 laYer it is assumed that 0-atoms are produced in the surface layer and that the reaction probability is limited by the diffusion of oxygen atoms through the carbon dioxide layer. From the initial slope of the absorption-vs-time plot the rate of CO, formation can be 12 — 1 ‘ estimated to be 5 x 10 sec

The pliu Lucliemistry of mixtures of NOj and acetone has been investigated in the gas phase at A = 313 nm and A = 366 nm^'4“. in these experiments a variety of products has been ob­ served, the most prominent being nitromethane, CH-jNO,. In contrast to these findings we only observed carbon dioxide as an additional reaction product. However, since the for­ mation of C0 2 only can occur via a radical splitting of acetone it is most probable that other products have been formed as well. For several reasons these products may remain undetected: 187

i: they are formed in concentrations below the detection limit of our spectrometer,

ii: they are volatile at L-N., temperature,

iii: their spectrum is masked by other absorptions of the NOj-acetone mixture.

Additionally the choice of NaCl as substrate limits the observable range at about 1000 cm . Direct photolysis of the products by the 254 nm radiation was excluded by the use of appropriate filters.

The authors gratefully acknowledge support of this work by the Deutsche Forschungsgemeinschaft.

References

1: J . Broadley, J . Robertson, Natur, 164 (1949) 915 2: W. Fateley, H . Bent, B. Crawford, J. Chem. Phys. 31. (1959) 204 3: R. Gray, Trans. Faraday Soc. 52 (1956) 145 4: E. Allen, K. Beglay, T r a n s . Faraday Soc. 72 (1968) 227 188

IKPRAR3D AMD HAMAH SF3CTRA 0? MATRIX-ISOLATED CYANOGEN 103123 AMD IODINE I30CYAZCTDS AND OTHER HELATSD SYSTS1.3 OBTAINED FROM THE SELP-SAiS MATRIX

by:

3. H. Carr, B. Ji. Chad?jjck, D. G-. Cobb old, J. il. Grsybuvaki,

D. A. Long, and D. A-. 15. Marcus-Hanks

(School of Chemistry, University of Bradford, Bradford, Viest Yorkshire, BD7 ID?, England)

We have obtained infrared spectra of cyanogen iodide, ICN, and

its u.v. photolytic product trapped in argon and nitrogen matrices.

Dilution (l'/3 = 20oj|5000)/injlBirm-up experiments indicate that the

parent molecule can exist as monomer, small aggregate, and large aggregate.

The behaviour of the bending vibration of the monomer and its first

overtone indicate that the effective site symmetry of the monomer is

C.j, Cg, Cg, or Cgv in argon and nitrogen.

U.v. irradiation of these matrices, especially Mg, with a medium pressure mercury arc results in the generation of monomeric iodine

isocyanide, INC, characterised by an intense NC stretching vibration

at 2058.5 on'' and a relatively weak IN stretching vibration at 492 cm-'.

To confirm and extend these findings we have obtained infrared and

Raman spectra from the self-same matrix' of Ng/ICN (400:1) before and

after u.v. irradiation. The two monomer stretching vibrations are polarised and the bending vibration depolarised, indicating an effective

Cj, or dpv site symmetry. All the stretching vibrations of the small and large aggregate are polarised. ICN is very sensitive towards aggregation.

Using the growth of the isocyanide stretching vibration of INC in the infrared spectrum tp monitor the progress of the u.v. photolysis, we have been able to detect the Iri stretching vibration in the Hainan 189

spectrum and to establish that the INC nolecule is destroyed by the

visible excitation (433.0 n s ) . IHC is thus relatively ".veil characterised.

These investigations and those on related systems

(BrCN, CgMg, or S i ’.-:e2 (CN)2 in Ar or Ngj C1CU in CgNg) illustrate some

of the value of Raman spectroscopy of matrix-isolated species and of the usefulness of obtaining infrared■and Raman spectra from the self-same matrix.

1 . J.M. Grzybowski, 3.R. Carr, B.M. Chadwick, D.G. Cobbold, and

D.A. Long, J. Raman Spectroscopy. 1976, 4, 421. 1 90

FORMATION AND LUMINESCENCE OF CH IN AN Ar MATRIX by M. Creuzburq, J. Duschl, and R. Heumuller Fachbereich Physik, Universitat Regensburg

Introduction

The system Ar: CH. has been studied in thin films using x-ray excitation 1) and luminescence spectroscopy '. Other investigators used VUV excitation 21 and absorption spectroscopy in thin films These experiments showed, to- 3 41 gether with ESR spectroscopy after irradiation ’ ', that CHg, CH 2 , CH, and H radicals are being formed. As has been shown in ref. 3), the dominant energy transfer to the methane molecule is from the host.

The experiments presented in this paper concern luminescence spectra of

Ar: CH4 during x-ray excitation. They exceed previous investigations1^ in three aspects: f ir s t, bulk crystals instead of thin films were used which is important both for the luminescence intensity and for the crystal qua­ lity ; second, temperature and concentration could be varied over extended ranges;-and third, the dependence of the luminescence intensity on the x- ray dose could be studied. CH^ was chosen as a dopand since this molecule f its both in size and in its boiling point very closely to Ar. It can there­ fore be assumed that, at least at low concentration, it is substitutionally and statistically distributed in the matrix.

Experimental

Poly-crystals were grown from the liquid or from the gas phase, using the method described in ref, 5 ), the supply qas being mixed in a gas handling system.

After the crystals had been grown in a Plexiglas tube of 15 mm length and

6 mm diameter, this crucible was removed leaving the crystal in the vacuum. The luminescence intensity was measured during x-irradiation of approxi­ mately IQ1* rontgen per hour with 1/2 m Czerny-Turner monochromator, Sodiuiil- salicylate converter and photomultiplier. The temperature of the crystal could be varied between 5 K and 35 K. Crystals with CH. concentrations be- -4-1 tween 1 0 and 1 0 have been investigated.

Results

A typical luminescence spectrum is presented in Fig. 1. The doping of CH^ 191

in Ar results in the appearance of three bands labelled A, B, and Bj which are attributed to the radiative decay of methylidine CH, see Fig. 2:

CH

U) (eVl

LU I—

'0-5 -6 -7 0-8

250 300 35 0

Fig. 1: Luminescence spectrum of CH. in an Ar matrix at 5 K. Resolution 2 nm. Fig. 2: Potential curves of the CH radical after ref. 7).

A-band: A2A -* X2tt, X = 433 nm

B-band: B 2Z" (v 1 = 0) - X2n (v" =0), X= 393 nm

Bj-band: B 2X” (v 1 = 1) -» X2n (v" = 0), X = 367 nm

Besides these bands, the Vegard-Kaplan bands of ^ between 220 nm and 370 nm and the Herzberg bands of between 350 nm and 550 nm are observed. These impurity bands will not be further discussed here.

We shall concentrate our interest neither in the shape of the bands A, B, and 8 ^ which show broadening and rotational fine structure, nor on the tem­ perature shift of their energetic positions, but consider in the following mainly the intensities of the three CH bands since these are determined by the number of CH radicals and by the energy transfer.

The intensities of the CH bands show a pronounced dose dependence which can be devided into two regions, see Fig. 3: the f irs t starts from zero and reaches the "initial value I " after a fraction of a minute; in the second region, the intensity is nearly linearly increasing with a slope a which is only dependent on concentration and temperature. The intensity is re­ stored after each intermittance of x-irradiation. 192

CD

Intermittance

x-irradiation

0 2 3

Fig. 3 : B-band intensity of 0.3 % CH. in Ar at T = 5 K, schematically.

No saturation was found during irradiation times of up to two hours. When the temperature of the crystal is raised just above 30 K, and then lowered to 5 K again, the "initial value I " is observed instead of zero for the "new" crystal. At higher annealing temperatures, I is reduced but restores to the previous value after a fraction of a minute.

I is temperature dependent; it is 5 times smaller at 30 K than at 5 K, independent of the concentration. There is a characteristic concentration dependence of IQ: it has, for all temperatures, a pronounced maximum at 0.18 % CH^ in Ar which exceeds the value of lowest and highest concentra­ tions by a factor of about 5. These dependences of I are common for the bands A and B.

The slope a in Fig, 3 is for all concentrations largest at S K and de­ creases to zero at about 25 K for the A band and 30 K for the B band. Above 25 K, the A band intensity decreases with irradiation, i.e. a < 0. At a concentration of 0.18 % CH^ in Ar, a has its maximum value which is about 4 times larger than at 0.02 %. At the highest concentration, 17 %, the intensity is practically independent of dose, i.e. a » 0. Since a and I show qualitatively similar dependences, the relative slope, a/I^, is neither very pronouncedly concentration nor temperature dependent: it -3 -2 -1 was between 4 x 10 and 10 min for all crystals. '

Discussion

We present a model for the interpretation of the above results which is based on the migration of atomic hydrogen. Of cause, in unradiated cry- 193 stals there are no CH radicals,resulting in zero intensity. CH is produced after CH^ excitation,producing new H atoms. When CH is stable it can be ex­ cited in a second step. „

The initial steep intensity increase can be explained by the trapping of H at a lattice site leaving the CH radical stable. Such lattice sites are known to exist below 12K, 23 K, and 39 According to our results, re­ duction of the number of CH radicals sets in only at temperatures above 30 K.

In the range of the slow intensity increase, see Fig. 3, a lesser number of H atoms find stable positions; they are recaptured or might even reduce the CH concentration by associating with neighboring CH. Also, the forma­ tion of H 2 molecules is possible which might explain that even after anneal­ ing at the highest possible temperature there is some initial intensity left.

While the temperature dependence of a can be understood in this model, we assume from the temperature dependence of IQ that there is also a radia- tionless transition competing with the luminescence, reducing the total in­ tensity at higher temperature. Further, since the excitation is accom­ plished by the host excitons, their reduction in lifetime should be reflec­ ted in a luminescence decrease.

The concentration dependence of I can be discussed along the same lines. There is no luminescence at zero concentration and none found for CH^ cry­ stals. Somewhere in between, at 0.18 % CH^, the maximum number of stable CH radicals are being formed, this concentration leaving enough space for

H2 formation and stable H traps, and inducing not too many clusters of CH^ that quench both formation and luminescence of the CH radicals.

References

1) Keyser: Thesis,Calif. Inst, of Technology (1965) 2) D.E. Milligan and M.E. Jacox: J. Chem. Phys. 4^, 5146 (1967) 3) W.V. Bouldin and W. Gordy: Phys. Rev. 135 A, 806 (1964)

4) D.W. Brown, R.E. Florin, and L.A. Wall: J. Chem. Phys. 6 6 , 2602 and 2672 (1962) 5) E. Schuberth and M. Creuzburg: phys. stat. sol. (b) 2I> 797 (1975)

6 ) E.L. Cochran, V.A. Bowers, S.N. Foner, and C.K. Jen: Phys. Rev. Lett. 2, 43 (1959)

7) G. Herzberg and J.W.C. Johns: J. Astrophys. JJ> 8 , 399 (1969) 1 9.4

Thermoluminescence following U V irradiation of molecules trapped in rigid matrix at 6 K .

J. FOURNIER, J. DESON, C. LALO, and C. VERMEIL ESPCI - Equipe 57 CNRS, 10, Rue Vauquelin, Paris 5e.

We have studied thermoluminescence following vacuum

U.V. excitation of small molecule*trapped in rigid matrix

at 5K.

The sample already prepared is irradiated through

a lithium fluoride window by a xenon resonance lamp. The

light emitted by the sample is detected by an Hammamatsu

photomultiplier through an H R S Jobin Yvon spectrometer.

A large holographic grating is used 110 x 110 mm. The

corresponding wavelength range is 200-600 nm. For thermo­

luminescence studies the irradiation is stopped, the tem­

perature is continnously measured by a carbon resistor

close to the sapphire window and the liquid helium circu­

lation is reduced to permit the temperature to decrease

at a rate of 10 K/min. The wavelengh can be measured

with an accuracy of1 nm . 195

The thermoluminescence spectrum between 350 .and

500 nm-is recorded several times during the increase of the temperature from 9 to 40 K. From these spectra it

is possible to measure the variation of the intensity of the different emission as a fonction of the tempera­

ture.

Results

Two kinds of light emissions must be distinguished:

those arising during the irradiation of the sample at 6 K and those duo to an increasing of the temperature.

Photolysis of PCS

The emission observed after irradiation by 147 nm of a mixture of 0.1% OCS in Ar matrix is that of excited atomic sulfur S ( ^ ) wich appears at 456 nm.

Consistent with a dissociation process

OCS + hv — > C0(1Z+ ) + S(1S)

After irradiation the matrix is vv'armed up and a thermo­ luminescence spectrum is observed fig 1 : it is the

$2(8 ^ X emission. The glow curve fig2 196 shows that this emission appears from 10 K . But the Sj emission never appears during irradiation of the sample e v e i ' i a t Q'1 ^ 1 0 K i t

The photolysis of C$2 will be described.

All this results suggest that thermoluminescences are due to recombinaison of atoms at very low temperatu­ re. 197 fIC ±

A<2^147nrr» 0.1%COS/Ar

Anm 500 450 400 350 198 f I/arbit rarvn Cs v unit's } Thermic luminescence X, = i47nm 0.1% COS /A r

1------1 T “ “ »— r — '— r 3 0 4 0 5 0 6 ° K 199

Section: Reactive Matrices Title: Rhodium Atom Chemistry Authors: Lee A. Hanlan and Geoffrey A. Ozin Address: Lash Miller Chemistry Laboratory and Erindate College, University of Toronto, Toronto, Ontario, Canada

The ability of metal atoms being deposited on a cold surface (10-20°K) to diffuse either on the matrix surface or within a narrow region near the surface (the reaction zone) before the kinetic energy is dissipated sufficiently to immobilize them, results in the formation of small well defined metal clusters. The statistical pathway to binuclears as well as the kinetics of the surface diffusion processes occurring in the reaction zone were analyzed and showed a simple expression for the ratio of the absorbances of binuclears (D) to mono­ nuclears (M) of the form

A n X2 -. = K[M]0 a M where [Ml. is the concentration of metal in the matrix reaction and K (1) is a constant.' ' This dependence of the concentration of various species on the rate of metal deposition has led to the formation and identification of many transition metal diatomic molecules. ^ Quantitative metal concentration and matrix uv-visible studies have shown that large amounts of molecular Rh, can be synthesized from the cocondensation of Rh vapour and Ar at 10°K. Using Rh/Ar 1/10 mixtures, the spectrum obtained shows good correlation with the gas phase atomic spectrum. However, on moving to progressively higher Rh/Ar ^ 1/10 to 1/10 ratios, absorptions associated with atomic Rh decrease in intensity while absorptions at 344, 325 nm and at least two around 312, 317 nm begin to grow in. Using the expression mentioned above we have shown that the absorbance at 344 nm represents an electronic transition of Rh,. The absence of these four absorptions when Xe m atrices are used and their reduced intensities in Kr matrices supports their assignment to Rh, since the more rigid Kr and Xe matrices are expected to favour isolation of pure atoms. ^ With the use of controlled Rh atom depositions in reactive matrices, it is possible to manipulate the formation of both mononuclear 200

and binuclear Rh complexes. For example, using Rh atom deposition rates which favour the formation of mononuclear complexes, the cocondensation of Rh with pure CO led to the formation of Rh(CO)4, exhibiting a doublet infrared absorption at 2020/2010 cm-1. The 12 16 13 16 12 16 .12 18 . complex was characterized using C 0/ C O and C 0/ C O/- 13 IB 13 16 C 0 / C O mixed isotope substitution experiments revealing it to be very close to a regular tetrahedral molecule. ^ Experiments with CO diluted in Ar resulted in the lower stoichiometry complexes Rh(CO) 12 10 13 10 (n = 1-3) which were identified using C 0 / C O/Ar isotopic gas mixtures. ^ When Rh/CO ratios were increased to favour binuclears, the resulting infrared spectrum showed four new absorptions, in addition to those for Rh(CO)4. This ir spectrum (2060, 2040, 1852, and 1830 cm"1) closely resembled that of matrix isolated Co2(CO)a' ' as well as that of a purported bridged form of Rh^COlg reported by Whyman^ and so we believe it to be a matrix isolated bridged Rh2(CO)e. On allowing the optical window to warm slowly we observed a gradual decrease in the absorptions of Rh(CO)4, while those for Rh^COjg gradually increased in intensity. At abcxit -48°C, we observed the disproportionation reaction: Rh2(CO)8 — Rh4(CO)12 + 4CO as monitored by infrared spectroscopy. This species remained until room tempera­ ture where it disproportionated to Rh6(CO)16 by further loss of CO. ^ The reaction of Rh atoms with dioxygen best exemplifies the formation of mononuclear, binuclear and trinuclear Rh complexes. Using low Rh deposition rates and O^ diluted in Xe , designed to favour mononuclear complex formation, two binary Rh dioxygen complexes were identified; Rh(02)2 absorbed at 1048 cm-1 and Rh(O^) at 900 cm-1, dioxygen stoichiometries being determined by 08/ Og/Xe and 10O2/°O 18O/10O2/Xe isotopic substitution. ^ Rh metal concentration studies in pure 02 revealed four species whose absorbances were Rh concentration dependent. One of these represented the mononuclear complex RhfC^ (1038 cm"1) the absorbance of which was used in the oiiproooion: Rhx(q)v X - l a [Rh]0 [Rh(02)2] The results indicated that of the remaining three species, 1266 cm-1 and 1078 cm-1 were binuclear while 1130 cm "1 was a trinuclear Rh dioxygen complex. Isotopic substitution identified the binuclears to be 201

Rh2(02)4 at 1078 cm-1 and Rh,(02) at 1266 cm"1. Limited isotope data on the trinuclear suggested it may be Rh^Q.^ or Rh3(02)6. The species Rh2(02)4 can be viewed as a dimer of the mononuclear RH(02)2 while the complex Rh^O,) is of particular interest due to its uniquely high 0-0 stretching frequency. When dilute 0 2/A r mixtures and high Rh deposition rates were used, new absorptions in the 890 to 930 c m '1 region were observed. Based on isotope data, metal concentration and matrix warm up behaviour these complexes were identified as the lower stoichiometry binuclear species Rh2(Oz) at 922 c m '1, Rh2p 2)2 at 900 c m '1 and Rh,(02)3 at 890 cm '1. Also associated with RhjpJj was a high frequency absorp­ tion at 1080 cm ' . The small frequency shifts of these binuclear molecules from mononuclear Rh(Q) (910 cm '1) and the 40 cm '1 blue shift on going from Rh(02)2 to its dimer Rh2(Oz)4 suggest very little perturbation due to additional dioxygen ligands or the presence of a second Rh atom. Subsequently these molecules have been viewed in terms of localized bonding models for associative chemisorption of dioxygen on dispersed Rh catalysts. ^ Preliminary experiments with Rh/C2H4 reactions reveals convincing evidence for three complexes at low metal concentrations namely Rh(C2H4), Rh(C2H4)2 and Rh(C2H4)3. The presence of binuclear Rh ethylene complexes Rh2(C2H4)n is indicated under high metal con­ centration conditions but work is required to conclusively identify these molecules, which incidently may also be useful for modelling ethylene chemisorption on supported Rh catalysts. ^ Collectively, these studies reveal the ease with which Rh atoms dimerize and the synthetic uses this property can be ixit to. The ability to tailor-make small, well defined metal cluster complexes enables suspected catalytic intermediates to be modelled on atomic, diatomic and higher order metal cluster sites and to relate the data to the chemisorbed state. References 1. E. P. Kiindig, M. Moskovits and G. A. Ozin, Nature, 254. 503 (1974). 2. see for example: ref (1). R. Busby, W. Klotzbticher and G. A. Ozin, J. Amer. Chem. Soc., 98, 4013 (1976); T. C. DeVore, A. Ewing, H. F. Franzen and U. Calder, Chem. Phys. L ett., 35, 78 (1975); J. Hulse and M. Moskovits, Surf. Sci. , 97, 125 (1976). 202

3. L. A. Hanlan and G. A. Ozin, Inorg. Chem., in press. 4. L. A. Hanlan and G. A. Ozin, unpublished results. 5. E. P. KUndig and G. A. Ozin, unpublished results. 6. R. Whyman, Chem. Commun., 1194 (1970); J. Chem. Soc., Dalton Trans. , 1375 (1972). 7. L. A. Hanlan and G. A. Ozin, J. Amer. Chem. Soc., 96j 6324 (1974). 203

Matrix Isolation I.R., and- EPR Studies of Reaction intermediates:

Lithium Metal Reactions with HgO, CHgOH, and NHg

R. H. Hauge, P. F. Meier and J. L. Margrave

Rice University - Houston, Texas 77025

EPR spectra clearly show partial electron transfer between lithium

and HgO, CHgOH, and NHg. For HgO, a diwater complex with Li is also observed.

In all cases, the complex can be completely photolyzed within minutes by full

irradiation from a xenon arc lamp. Li(HgO) reacts to form LiOH and H, Li(HgOjg forms Li(H^OjOH and H. Radical species are produced from Li(NH^) and Li(CH^OH) complexes but are not yet identified. It is clear, however th at photolysis does not occur in the same way as the Li(H 2 0) complex since H atoms are not produced in either reaction. Nor is the CHg radical produced in the methanol reaction. It would appear in the latter cases that H 2 is removed with resul­ tant radicals being the NHandCHgO anions. EPR and I.R. spectra for the above complexes and reaction products will be presented. 204

G.®1°_Q a_n_d E SR at £_o£_UiJ^j_rn_e^ta_l_l lc _m_olecul^s _Ag- M_.

Paul H. Kasai, D. McLeod, Jr. Union Carbide Corporation, Tarrytown, New York.

A series of internetallic diatonic molecules Ag-M (M = Mg, Ca, Sr, Zh, Cd, and Hg) were generated in argon matrices and were examined by ESR spectroscopy. The observed spectra are compatible with the axially symmetric spin Hamiltonian of the form: H = g|( gH S + 9, 6 (H S + Hy Sy) + A^' I^S + Z Z X X Z 2 Aj_' (I'Sx + Iy Sy) where Aj| and Aj_ represent the hyperfine coupling tensor to the Ag nucleus, and the last two terms involving A' and Ajl' are to be added when the a t m M possesses a magnetic nucleus. For all the cases ex­ amined the coupling tensors to Ag were found to be essentially isotropic, ranging from 0.39GHz for AgSr to 1.56 GHz for AgHg. For this series of AgM, the unpaired electron is expected to occupy the orbital given by an anti­ bonding combination of the valence s orbitals of Ag and M. Thus, the varia­ tions observed in the Ag coupling constants can be readily correlated to the ionization potentials of the atoms involved. The coupling tensors determined for the magnetic Cd and Hg nuclei revealed small but clearly resolvable anisot­ ropies indicating admixture of the valence pz orbital of M. Such admixture of the pz orbital would account for the negative shift of the g-tensor (g-ge) observed in each AgM. The smallest g-shift was noted for AgMg and the largest for AgHg. 205

Matrix Isolation I.R Studies of Hydrocarbons

in Fluorine Matrices

R. H. Hauge, J. Wang, J. L. Margrave

Rice University - Houston, Texas 77025

CH., C0 H0, C0 H. and C.K, were isolated in solid fluorine a t 10K. 4 2 2 t 4 bo Infrared spectra show sharp well defined bands where band shifts from the gas phase resemble those for a neon matrix. All of the above molecules could be isolated without reaction. CH^, C 2 H2 and CgHg could be kept in- . definitely at 20K. However, C 2 H^ underwent slow reaction. Reaction products and reaction rate will be discussed. Photochemical Oxidation of Iodosilane in Solid Argon

J. P. Ogilvie. University of Kuwait. V.R.. Salares and M.J. Newlands, Memorial University of Newfoundland, Canada.

The photochemical decomposition of iodosilane, alone on in the presence of oxygen, in argon matrices at 4-8 K has been studied by methods similar to those used previously for iodomethane (Can. J. Chem., S3, 289 - 275, 1975). Vibrational absorption spec­ troscopy has been used to detect the formation of iodosilyl radicals and oxygenated iodosilane derivatives. The use o f deuterated iodo­ silane and oxygen- 18 has permitted identification of structures of likely products. Attribution of absorption lines to the various species has been possible through the use of high- and low-pressure mercury lamps which produce different sets of line intensities. The effect, of the relative concentration of oxygen also aids in product identification. The formation of azone and the production of photo­ chemical explosions with silane/o'zone mixtures, a t 4 K w ill be discussed. 207

Section: Reactive Matrices Title: Metal Atom Olefin Chemistry; Interaction of Group VIII Metal Atoms with Ethylene Authors: Geoffrey Ozin and William Power Address: Lash Miller Chemistry Laboratory and Erindale College University of Toronto, Toronto, Ontario, Canada

The existence of binary complexes of the form Ni(ol)n was first demonstrated by Wilke et _al.' ' from the reaction of Ni(acac)2 and C2H4 at 77 K, to form Ni(C2H4)3; this complex was subsequently synthesized directly by metal vapour techniques in the temperature range 77- (2 3 1 10 K, * ’ and further characterized by various spectroscopic techniques. Standard matrix dilution and warm-up techniques, under conditions of metal dilutiondesigned to produce mononuclear complexes, allowed the isolation and characterization of the reactive intermediates Ni(C2H4) and Ni(C2H4)2. This area of research was extended to other group VIII metals in view of the great importance of metal-olefin interactions in a large number of heterogeneous catalytic reactions of industrial signifi­ cance. In addition, this type of research allows one to address one’s self to the problem of producing localized bonding models for ethylene che mis orbed onto the bulk metal. With cocondensations of palladium and ethylene at 15 K, three complexes analogous to the Ni(C2H4)n species were observed and characterized by 13C2H4/ 12C2H4 isotopic substitution, ^ namely Fd(C2H4), Pd(C2H4)2, and Pd(C2H4)3, the latter complex having previously been identified.' ' At this point, it is pertinent to discuss both the infra­ red and ultraviolet-visible data for these M(C2H4)n complexes, as summarized in Tables I and II. With respect to the infrared spectra for directly corresponding species, there is very little difference as the metal is varied. The insensitivity of the vibrational spectra is in direct contrast to the sensitivity of the optical spectra. For all these ethylene complexes, only one optical transition is observed, in all cases in the uv region. For a given metal, the absorptions monotonically blue shift with increasing stoichiometry of ethylene, a fact borne out by extended Huckel calculations if the transitions are assigned as metal - ligand charge transfer (MLCT). In addition, on going from Ni to Pd in each pair of complexes, there is a noticeably large blue shift (as opposed to 208 a negligible energy change in the y-*-.), easily rationalized when /c% compared to recent SCF-Xer-SW calculations, ' ' in that as we pass from Ni(C2H4) to Pd(C2H4), the metal d-band becomes more stable, while the upperv *-state remains approximately unchanged; the energy of the MLCT should therefore blue shift, as is observed. In view of the great sim ilarity of the infrared spectra of (C2H4)Pd and (C2H4ads)Pd, (6) we have discussed this complex in terms of a localized bonding model for C2H4 chemisorbed on alumina- snppnrted palladium. ^ The small blue shift or passing from Pd(C2H4) to the adsorbed species implies a slight strengthening of the C-H and C=C bonds on going from a single Pd atom to a surface Pd atom site. The success at modelling the n-complexed form of ethylene chemisorbed by a single metal atom led us to speculate that the "ideal" localized bonding model might be well represented by a dinickel species,'( 8) especially in view of the theoretical studies which have proceeded by placing the adsorbate molecule on a pseudo-metal surface, represented by two or more atoms. The metal atom technique lends itself very well to experimentally generating realistic molecular sys­ tems to compare with these studies. In brief, we have isolated the binuclear complexes NL2(C2H4) and NL,(C2H4)2, whose spectroscopic data are summarized in the Tables, indicating once again ii-complexed ethylene in these species. The striking resemblance of our vibrational data for Ni(C2H4) and Ni2(C2H4), and the sim ilarities in UV-MLCT transition energies, suggests that the overall effect of a neighbouring Ni atom on the spectroscopic properties of ff-(C2H4)Ni is minimal. The relatively small frequency perturbation of the and on passing into the Nix(C2H4) regime, with n>2, and the noticeable resemblance of these infrared spectra to those of chemisorbed C2H4, lend credance to the proposal that cluster species like Ni2(C2H4) are valuable models for evaluating the properties of surface complexes. This aspect of the work will he discussed in greater detail in the invited paper to be presented at this conference by Professoi Geoffrey Gain, References 1. K. Fischer, K. Jones and G. Wilke, Angew. C hem ., 85, 620 (1973); Angew Chem. Int. Ed. Engl., 12, 565 (1973). 2. R. M. Atkins, R. McKenzie, P. L. Timms and T. W. Turney, J. Chem. Soc., Chem. Comm., 764 (1975). 209

3. H. Huber, G. A. Ozin, and W. J. Power, J. Amer. Chem. Soc., J)8, 6508 (1976). 4. H. Hubert G. A. Ozin, and W. J. Power, Inorg. C hem ., (in press). 5. R. P. Messmer, "The Theoretical Basis for Heterogeneous Catalysis," E. Drauglis and R. K. Jaffee, Eds., Plenum, New York, 1975. 6. J. D. Prentice, A. Lesuinas, and N. Sheppard, J. Chem. Soc., Chem. Comm., 76 (1976). 7. G. A. Ozin and W. J. Power, Inorg. C hem ., 16, 212 (1977). 8. G. A. Ozin and W. J. Power, Inorg. Chem. (submitted for publication). 210

Table I. Infrared Dataa for Some Matrixrlsolated Ethylene Complexes

Ni(C2H4) Ni(C2H4)2 Ni(C2H4)3 Assignment 1499 ' 1465 1514 UC=C 1160 1223 1246 6ch 2 Pd(C,H„) Pd(C2H4)2 Fd(C2H4)3 1502 1463 1524 1223 1242 1255 6ch „ Ni,(C2H4) Nii(C2H4)2 1488 1504 "c=c 1208/1180 1232 5ch2 W(C2H4)ads 1510 ^ c not Observed ®CH, aIInits in cm-1.

Table II. Ultraviolet3 Spectral Data for Some Matrix-Isolated Ethylene Complexes

Ni(C2H4) Ni(C2H4)2 Ni(QH4)3 Assignment 280 250 236 MLCT N i^ H ,) 243 MLCT M(C,H4) Pd(C,Hj, Pd(C2H4)3 240 221 204 MLCT

a TTnits in nm. 21 1

Fluorescence Studies of CuO in Ar Matrix

HOSSEIN ROJHANTALAB Department of Chemistry, College of Science Jundi Shahpur University, A h w a z , Iran.

LOU ALLAMANDOLA, AND JOSEPH NIBLER Department of Chemistry Oregon State University Corvallis, Oregon, U.S.A.

The band spectrum of CuO has been the subject of several matrix isolation (1, 2), and gas phase studies (3-6). Many bands have been observed throughout the visible region for this molecule, howeve^ by far two systems of blue and red bands have received the most attention. Although all these authors agree on a /7.ground state, the symmetry of the upper electronic states, their position with respect to the ground state, and the spacing of the l/J; states are still unanswered. The two alterna­ tive assignment"; are summarized in Figure 1.

21 122

275

275

Figure 1. Energy level diagram for the blue and red systems in CuO. a) The system as reported by Shirk and Bass, and Ant ic-JoVannvi r.. The matrik values of the former are shown in parantheses.' b) Appelblad and Lagerquist system. Ail -1 values are in cm

In the red band system two weak features are observed between 16600 and 15900 cm ' with a vibrational spacing of 600±10 cm '. For this system Shirk and Bass (1) have reported a doublet with 200 cm splitting and they have assigned it to. the 177^— substates of the — Z/Tc transition. In order to distinguish between these choices, a ye I low-red 212

tunable dye laser was used to probe the state. An enhanced emission in any of the red bands observed in the previous experiments would unequi­ vocally support Appelblad and Lagerquist's assignment. When the matrix was scanned from 17200 to 16000 cm ' even at 10 cm 1 slit width, no emi­ ssion was observed for the same dye laser range. This result thus places

the final state of the red system at 3900 cm 1 above the ground electronic state supporting Figure-la option. In the blue region the emission spectrum is more complicated. Seve­ ral experiments were performed at different Cu0:02: Ar ratios for varying

photolysis periods and the matrix in each case was scanned with different exciting laser line. The fluorescence spectrum showed strong dependency on all these factors. A typical fluorescence spectrum of CuO/Ar with Ar:0^ sz 20000 at five different exciting laser line is shown in Fiqure 2. With 2(880 A exciting laser line, three reproducible progressions were observed in the blue band system as shown in Figure 2a. In each case a very intense band is followed by two weaker ones each shifted 200+10 cm to the red from the previous band. The spacing in each prog­

ression is 660± 10 cm ' in good agreement with the previously reported values for the ground vibrational spacings both in the gas (3, 5) and the matrix studies (1). For a lTT^ — ln i system, the A765 A exciting line is energetic enough to excite the molecules to the *77, fv/'-O level, however the emission observed is ‘77,. (o) --- X 1 IT. l(u"7 indicating ‘ l 1 rapid vibrational relaxation in the upper 77; state. Figure 2c shows the emission obtained using A727 6A line which resembles spectrum 2a with the exception of a strong band on the blue side of the most intense progression band. This is attributed to a di­ fferent site in the matrix. The fluorescence spectrum observed using

2(658 “a exciting line shows two progressions for both ^ ana i TT\,

17000 19000 21000

CuO/Ar 14 *K

20487

20981

21150

21463

218311

-L J 17000 FREQUENCY(cm1) 21000 Variation of the fluorescence spectrum of CuO/Ar with exciting laser line. 214 ted a vibrational spacing of 600 cm ' with v1 = 0 level just below the v1 = 0 level of the B state. We place the ground level of this system at 20700 cm ' with a vibrational spacing of 625 ± 10 cm '. Such overlap may contribute to rapid v 1 relaxation due to intersystem crossing. The 200 cm ' spacing observed in the blue region spectrum has also been observed by Shirk and Bass (1) in the blue and the red region. Since repeated annealing of the matrix up to 33 °K did not alter the spectrum, and it is unlikely that CuO is trapped in 4 or 5 different stable matrix sites, we believe these splittings are due to 1 — * / 7 ^ substates of the exciting electronic state. The results of this emission study is thus most cons i stent with: •1 _-l C ‘■/7V 20700 cm'1 625+10 cm" ' ir J/, A B l/T, 20490 620+10 "/t /-ZOOcm"1 A 3900 600+10 ----- X t/7. 0 660+10 " " 7 In every experiment matrices were formed by depositing a premixed ratios of Ar:02 ranging from 100 to 20000 from a jet at 45* to the Cu jet held near 550 C. Strong fluorescence was observed at 20, to 100 mw laser exciting powers and 0.5 to 2.0 cm slit width after 5 to 25 minutes in- situ vacuum UV photolysis at 1216 *A.

References: (1) U.S. Shirk and A.M. Bass, J. Chem. Phys. 52, 1894 (1970). (2) K.R. Thompsen, W.C. Easley and L.B. Knight, J.Phys. Chem. 79,, 52 (1973). (3) 0. Appelblad and A. Lagerqu i st, Physical Scripta, ,10,, 307 (1974). (4) A. AulIc-JuvanuvIc, 0.5. Peslc, and A.ti. Uaydon, Proc. Roy. S o c . London, A307, 399 (1968). (5) A. Antic-Jovanovic, and D.S. Pesic, J.Physics, jte, 2473 (1973)- (6) A. Lagerqui st, and U. Uhler, Z. Naturforsch, 22b, 551 (1967). 215

International Conference on Matrix Isolation Spectroscopy, West Berlin, Germany, 21-24 June 1977

CHEMILUMINESCENT MATRIX REACTIONS Richard R. Smardzewski Chemistry Division Naval Research Laboratory Washington, D.C. 20375 U.S.A.

The matrix-isolation technique, whereby highly exothermic reactions are examined in low temperature, inert media (viz., solid argon at 8 °k) has been applied to an examination of the elementary processes of combustion and chemiluminescence. Ultraviolet and laser photolysis of dilute argon matrix samples containing O 3 and H 2S molecules at 8 °K produced new infrared absorptions at 3425.0, 1177.0, 763.0 and 444.8 cm-3- which were assigned to the previously unobserved HSOH inter­ mediate species, hydrogen thioperoxide (1). The HSOH molecule is believed to be formed by the following reaction in the argon matrix cage.

|o(3P) + H 2S - [H2S=0 ] - HSO + H - HSOH > ( J cage

Extensive ^®0 and deuterium isotopic studies had shown this species to contain one oxygen atom and two nonequivalent hydro­ gen atoms. When the photolyzed Os-H2S matrix samples were warmed to 18-24°K, an intense blue-violet chemiluminescence was observed which lasted, in some instances, as long as 30 minutes. The unstable nature of the intensity of the emission was such, how­ ever, that it was not possible to employ a single element photo­ multiplier tube to record the spectrum after monochromatic dis­ persion. Two types of experiments were conducted to analyze this visible emission. The first involved photographically recording the chemiluminescence (after dispersion) using high speed (ASA 10,000) Polaroid film. Exposure times were generally of the order of 5-20 minutes with the entrance slit of the mono- chrometer (0.6 meter, f/5.2) maintained at 50-100 microns. In this manner, the resultant chemiluminescence was shown to be 216 primarily S02 phosphorescence. The ^B^-^A^' emission spectrum, observed in the general wavelength region of 390-470 nm in both the S ^ 0 2* and S^®02* cases, consisted of the intense vibration­ al progression a(000) - x(v^v20). The electronically excited S02 is believed to be formed by the o(3P) + HSO(SO) diffusion reaction in the argon matrix where rapid -• T^ intersystem crossing leads to phosphorescence.

Matrix phosphorescence band maxima (cm-3-)* of S^-^02 and Sl°02 produced as a result of o(3P) + HSO(SO) recombination in solid argon at 20°K

Assignment s 16q 2 Sl8o2 A(16-18) 000 25,443 25,526 83 010 24,938 25,011 73 100 24,320 24,417 97 110 23,768 23,90b 138 120 23,226 23,363 137 130 22,674 22,829 155 220 22,135 22,284 149 230 21,517 21,772 255

3 20 - 21,245 * Photographically approximated.

A second type of investigation was conducted in which the photographic film was replaced by an optical multichannel analy­ zer which employed 500 simultaneous detection channels (experi­ mental details will be presented). Using this technique in related experiments involving the ultraviolet photolysis of H2S in an argon matrix, ground state atomic sulfur, s(3p), was Ob­ served to undergo radiative recombination at temperatures of the order of 20°K. The intense banded emission recorded in the 360-500 nm region was characterized as B32~ - X3I!” fluorescence of S2.with a strong (0,v") progression. Subsequent doping of the Ar/H2S matrix sample with small quantities of 02 (or 1®02) produced intense S02 phosphorescence which overwhelmed any S2 fluoresence. This is believed to occur by the alternative chemiluminescent reaction: s(3p) + 0 2 - S 0 2* (2). As the temperature of the matrix was increased, the S02 phosphorescence band maxima were observed to shift to higher wavelengths. 217

This same behavior was observed in studies involving the radiative recombination of ground state chlorine atoms, Cl(2p3/2), in argon matrices. Intense emission from excited Cl2 was observed in the 650-850 nm spectral region which con­ sisted of a long vibrational progression. The frequencies of the band maxima were observed to. red-shift by ca. 100 cm--*- for a 50°K increase in matrix temperature. Details of this and other chemiluminescent systems will be discussed.

(1) R. R. Smardzewski and M. C. Lin, J. Chem. Phys. 66, 3197 (1977). (2) S. R. Long and G. C. Pimentel, J. Chem. Phys. 66, 2219 (1977). 218

Photoemission Studies on Rare Gas Matrices E.E. Koch (a), R. Niirnberger (b), and N. Schwentner (c)

(a) Deutsches Elektronen-Synchrotron DESY, Hamburg (b) II. Institut fur Experimentalphysik der Universitat Hamburg (c) Institut fur Experimentalphysik der Universitat Kiel

In this paper we illustrate the power of photoelectron energy distribution (PED) measurements for (i) determining the energy levels and bands in pure and doped solid rare gases and for (ii) studying energy transfer processes in rare gas matrices. The capabilities of photoemission studies with a tun­ able monochromatized synchrotron radiation light source for matrix isola­ tion spectroscopy are as follows: (a) distinction between initial and final state effects, (b) possibility to prepare well defined excitonic or conti­ nuum states, (c) detection and capability to study fast transfer processes in matrices on a time scale shorter by an order of magnitude than that generally resolved in luminescence experiments, (d) discrimination between true primary and scattered secondary electrons, (e) monitoring volume pro­ perties by choosing electrons with low kinetic energies which penetrate several hundred X thick rare gas films, and (f) relatively simple inter­ pretation. Thus we fined photoemission studies to be a useful complement to optical and luminescence experiments. Details of our experimental set up have been given elesewhere1>2 .

I. Valence states in pure Xe and Ar and in Xe/Ar alloys: The structure of the valence bands of solid Ar and Xe formed by the atomic 3p and 5p states has been the subject of extensive theoretical work.3 Some common features are predicted: (i) a spin orbit splitting of the np 3/2 and np 1/2 states in the center F of the Brillouin zone of 0.2 eV for Ar and 1.4 eV fui. Xe, (11) a direction dependence banding to lower energies in going from F to the border of the Brillouin zone and (iii) a splitting of the np 3/2 bands, which are degenerated at F , into two subbands. A quan­ titative comparison with recent FED's shows considerable discrepancies con­ cerning the calculated widths and splittings of the bands especially for the heavier rare gas solids.11

We have studied the gradual formation of the valence bands of Xe and Ar by a seccessive variation of the mean nearest neighbour distance with the con­ centration of Xe in Ar, in order to separate the influence of spin orbit COUNTING RATES I mils I 0 Xe . 00V 1 r e/A X V. O . V 1 9 . V 0 9 . V 50 10V. . V 0 7 i 5 32 and are 3/2 1/2 states is5p The observed. bandwidth increasing 5p mic aaino temxm f h est f ttsna tebre f theof theborder near states of thedensity of themaxima of paration dis­ tobe energy in separated well are andAr Xe of states valence The for alloys Xe/Ar for curves distribution energy Photoelectron 1£ig. od fr h rvlnebns A xlnto o tewdhad split­ and thewidth for explanation An bands. valence the Ar for holds interpretation corresponding The distance. neighbour nearest Xe creasing in­ with reduction exponential an using Xe pure from determined been have integrals overlap The ppo.and ppx integrals overlap depended centration con­ and splitting orbit spin atomic the with approach tightbinding a elce ya raeigo theconcentra­ of with in broadening structures thePED's a by reflected ato­ two the Xe of concentrations low For limit.band split the in cussed o ten 23 n p 1/2holes np and 2/3 thenp for by satisfactorily explained can be formation band This zone. Brillouin se­ the to attributed are Xe pure for the in PED's splittings The tion. ting in the PED's of pure Ar and Xe by different relaxation energies relaxation different by Xe and Ar pure of PED's the in ting interaction and banding and interaction dashed lines. The energy scale for all FED1s has been fixed at fixed been energy The lines. allforhas scale FED1s dashed msina ,. =0. E,at emission . the by indicated is background estimated The rate. counting maximum same the to normalized are spectra All parameter. a as given concentration the Xe with energies photon different three the Xe 5p (1/2) maximum. The arrows indicate the onset of photo­ of the onset (1/2) maximum. indicate 5p arrows The Xe the 0 3 2 1 10 eSp X e n Ar in Xe IDN EEG RLTV T Xe5p'/2 | ) V |e 2 / ' p 5 e X TO RELATIVE ENERGY BINDING kin .5 Rsls r hw nFg 1. in Fig.shown are Results 1 3 2 1 0 1 - 2 - 3 6 has to be rejected. tobe has r3p A XeSp e n Ar in Xe -1-3-2-10123 2 1 0 1 - 2 - 3 - 5 1 - Ar 3p Ar e n Ar in Xe 219 220

II. Time hierarchy for energy transfer and relaxation In rare gas matrices efficient energy transfer from the matrix to guest atoms has been observed in luminescence and photoelectron yield spectra. The radiative decay time of the emission centers is of the order of nsec up to usee7. In general these times are long compared to nonradiative re­ laxation processes and emission from relaxed states is observed. On the other hand photoemission processes are fast (I0~13 sec) and photoelectron spectra contain information about the energetics immediately after the excitation processes. For example Xe atoms in an Ar matrix can be ionized by an energy transfer from excited states of Ar. The energy of the ejected electrons yields, together with the known binding energies (Part I), the amount of transfered energy. Thus the PED's show whether relaxation in the matrix takes place before energy transfer to the guest atom or not. We have applied this type of spectroscopy to the study of energy transfer from exciton states of an Ar or Ne matrix to Xe guest atoms1 (see Fig. 2).

13 79 tV

g

»> 07«V

KINETIC ENCRO? |t>v|

Fig. 2 Photoelectron energy distribution curves versus kinetic energy of I % Xe in Ar and I % Xe in Ne for several excitation energies (A : spin orbit splitting of Xe guest with n and n 1 exciton stiSes of the host matrix and their spin orbit partners are denoted.

At liv - 11.3 eV for Xe/Ar and 16 ev tor Xe/Ne the Ar respectively Ne ma­ trices are transparent and the PED’s show those parts of the Xe valence states, from which electrons are directly excited above the vacuum level. At hv = 12.07 eV and 17.5 eV the n=I host excitons are predominantly ex­ cited. These exciton states are located below the vacuum level and cannot decay directly by photoelectron emission. The observed photoemission is produced by energy transfer to the Xe guest states lying in the bandgap 221

of the matrix. Thus the PED's for these photon energies show again a struc­ ture caused by Xe valence states. They are shifted to higher kinetic energy corresponding to the higher excitation energies. If no energy is lussed by relaxation processes prior to energy transfer the PED's should follow the diagonal lines with increasing excitation energy. Evidently no relaxation takes place for the Ar n=l exciton and its spin orbit partner n'=l whereas the slight shift to lower kinetic energy for the n=I exciton in the Ne ma­ trix indicates a relaxation process.

For the n=2 and n'=2 states in Ar the PED's follow the diagonal line (maxi­ mum B and C) but an additional maximum A appears. Thus the spectra show energy transfer from the unrelaxed n=2 excitons as well as from partly re­ laxed excitons (A) which do not correspond in energy to the n=I exciton. In the Ne matrix the PED's for n=2 and n=l are identical as far as the ener­ gy of the maxima is concerned, thus demonstrating a complete relaxation of the higher excitons to n=1 before energy transfer. For primary excitation into continuum states no appreciable energy transfer is observed (see Fig.1). In Table 1 a time hierarchy for the competition of relaxation and energy transfer is compiled from our measurements together with time constants f ruin theoretical estimated.

Table I Time hierarchy for decay processes in Xe-doped Ar and Ne. radiative decay; relaxation to trapped excitons; t^( 1 ',2->-l), relaxation of the n=1',2 to n=1 state; energy transfer to Xe guest atoms; relaxation to the phonon-dressed free-exci- ton state.

experimental time constants s v s t em ______time hierarchy___ including theor. estimates______

1-at. % Xe in Ar n=l td>tr>tt td^10_9>t^%10_12>tt

n= 1 1 tr(1

n=2 td>tr (2-+1)>tt>tr TD% 10-9>TR(2+l')%10-n >TT>TR*l(r12

1-at. % Xe in Ne n=l Tj)>T-f tr>Tq%10-9>tr%!0~12 (2->-l )%5x 10- 1 3 n=2 td >ti>JTR (2">I) Tt>TD% 1 0 - 9>TT%10 124 V lTph LTph 222

References

1. N . Schwentner and E.E. Koch, Phys.Rev. B14 , 4687 (I 976) 2. N . Schwentner, Phys.Rev. B14, 5490 (1976) 3. U . Rossler, in: Rare Gas Solids, ed. M.L. Klein and J .A. Venables, Academic Press, London (1976), Vol. I, p. 505 4. N. Schwentner, F.-J. Himpsel, V. Saile, M. Skibowski, W. Steinmann, and E.E. Koch, Phys-Rev.Lett. 34^ 528 (1975) 5. R. Niirnberger, F.-J. Himpsel, E.E. Koch, and N . Schwentner, phys.stat.sol. (b) 1977 in press 6. M. Parinello, E . Tossatti, N.H. March and M.P. Tosi, preprint August iy/b, Lu be published 7. U. Hahn,’N. Schwentner, and G. Zimmerer, Optics Communications (1977), in press 223.

VIBRATIONAL RELAXATION IN MATRICES

Henri DUBOST Laboratolre de Photophysique Moleculalre Universlte Parls-Sud, ORSAY , France.

HISTORICAL SURVEY

.Since the theoretical prediction In 1 9 6 5 of a long vibrational lifetime for Ng In solid argon there was a consi­ derable advance in the field of vibrational relaxation of ma­ trix-isolated molecules. Several experiments using UV or vi­ sible emission spectroscopy showed that In an excited state vibrational relaxation can be slow on the electronic lifetime scale. In 1 9 6 8 , the Ng molecule in the A ^ £ u state trapped In rare gas solids was found to have a vibrational lifetime of

1 s. In the meantime laser technology was developing very fast and In 1 9 6 9 the laser excited fluorescence technique was used to estimate the vibrational relaxation of CuO In the

B2Z state. In 1972, IR laser excitation of matrix-isolated CO was achieved and the vibrational fluorescence was found to have a long lifetime (ms). The IR double resonance technique was soon applied to measure the lifetime of the v>2 (v = 1) state of NH^ In N2 . In 197^ an elegant optical double reso­ nance experiment showed a long vibrational lifetime in the. ground electronic state of Cj? in Ar and N2. Then in 1975 se­ veral time and wavelenght resolved studies were performed.

Fluorescence from atomic nitrogen coupled to N2 molecules and laser excited vibrational fluorescence of CO gave evi­ dence for lntermolecular V-V transfer processes. In a laser excited visible fluorescence study of OH(A2£+ ) and NHtA^ji) the vibrational lifetime was found to be . short (^is.ec) des­ pite the large frequency. In addition deuteratlon Increased 224 the lifetime, Indicating that V^.Rotation transfer was taking * place. Recently ND->CO direct transfer has been studied and IR fluorescence from lsotoplcally enriched CO has shown the Im­ portant role played by migration of the vibrational excitation.

Vibrational lifetime of Cjj (a**£) and of 1)2 CF2 have been mea­ sured. Relaxation of ground electronic state HC1 (v = 1,2) and

1)^ (v = 1) CH-jF have been recently Investigated using IR fluo­ rescence and double resonance respectively. All these studies were performed In low temperature (4 - 40 K) rare gas or N2 solids,

From 1973 to 1976 several elaborated theories of some of these processes have been worked out. Conversion of vibra­ tional excitation into lattice phonons or multlphonon process has been studied In detail. Temperature and energy dependence of the relaxation rate was predicted. However these theories were unable to estimate Its absolute magnitude. In a similar way V-V transfer has been treated. The major theoretical lack is that rotational motion which Is often involved in the vibra­ tional deactivation processes is not taken Into account. It Is now obvious that a variety of processes can remove energy from an excited vibrational level. We will try in the following to understand how they occur and in which conditions,

RELAXATION PROCESSES

The molecules are anharmonic oscillators whose fre­ quencies are only slightly shifted from their gas phase value due to the Interaction with the matrix host. The perturbation of the rotational motion Is stronger because of the smaller rotational energy. However the potential barrier hindering rotation is rather low i 30 - 50 cm-* In the rare gases,

200 - 300 cm-* in N2 . In addition rotation-phonons interac­

tion brings out an additional perturbation. But in high rota­ tional energy states lying well above the barrier the rota­

tional motion can be considered as nearly free. Even light molecules trapped in solid argon have their lowest J levels near or above the barrier and thus almost unperturbed. The motion of heavier molecules is restricted to llbration at

least for small energies, Fhonons or lattice vibrations cons­

titute a thermal bath in which vibrational energy can ultima­ tely decay. In the case of a monoatomic solid their frequency

is in the range 0-80 cm-1. Isolated molecules can also in­ teract through long range multipolar f o r c e s , The relative strenght of the lntermolecular coupling and of the molecule

lattice interaction will determine the behaviour of a vibra- tlonally excited molecule.

- Radiative relaxation 1

The radiative lifetimes of vibrational levels are well known in the gas phase. They are deduced from absorption measurements of IR bands. For dipole allowed 1—> 0 transitions they are typically•between 10 and 100 msec. In an host matrix where electronic Interactions are weak such as rare gase.s or

Ng, the radiative lifetime of a trapped molecule is reduced by a small factor («< 2) depending on the refractive index of the medium. The radiative process has been shown to be the only efficient to remove the vibrational energy from CO and NO trapped in rare gas matrices. 226

- V -> Phonon process $

The coupling between the medium oscillators and the

molecular oscillator Is responsible for the radlatlonless

process In which the vibrational energy Is converted Into a

large number of phonons. Several theories dealing with this

process have been derived but are not able to calculate the

absolute magnitude of the relaxation rate. However they pre­

dict the temperature dependence and the existence of an

energy gap law. The temperature effect Is bigger for larger

energy gaps and Increases with Increasing coupling.

It Is predicted to be small In the temperature range T <0 , 3 T Debye. Also the short range repulsive por­

tion of the Intermolecular Interaction Is assumed to be res­

ponsible for vibrational relaxation. Experimental data on

the VPhonon process are rather scarce. However the re­

laxation rates of CF2 , CuO, CD^F, N2 and C£ seem to confirm

the energy gap law. They range from 1 s-* for V =1400 cm"1

to 10® sec-* for V =500 cm-*. The magnitude of the rate Is

correlated with the spectral shift with the bluest shift

having the shortest lifetime thus supporting the assumption

that relaxation Is Induced by the repulsive part of the po­

tential, The rate of Cj> whose vibrational frequency Is

strongly blue shifted from Its gaseous counterpart IS seve­

ral orders of magnitude faster than the rate of neutral mo­

lecules. In this case the temperature dependence also Indi­

cates a strong coupling. V-> Rotation process «

The vibrational excitation can be transferred to 227

the rotational localized modes Instead of directly to the tra­

veling wave modes of the unperturbed lattice. The rotational

energy Is then readily converted Into phonons by low order ef­

ficient B - * P h processes. In many cases rotation Is the most.

Important accepting mode, as shown by the absence of general

energy gap law, the Inverse deuteratlon effect and the tem­

perature Independent rates. The V— yR process will be more ef­

ficient than the V— *Ph one when the number of rotational quan­

ta necessary to match the vibrational energy Is smaller than

the number of lattice phonons. This Is actually the case for

several light molecules with large rotational constants (OH,

NH, HC1, CH^F). There Is a good correlation between the rela­

xation rate and the order of the V— > R process. Unfortunately

no elaborate theory Is available.

- V-*V Transfer 1

Resonance transfer of vibrational energy does not

change the population of an excited level but produces an ex­

citation migration through the crystal. But the range of non resonant energy transfer Is considerably extended. In this

last process lattice phonons compensate for the energy discre­ pancy &E and then simultaneous lntermolecular and molecule lattice coupling Is required. At the low temperature of the thermal bath there are no phonons available for absorption and then only processes Involving phonon emission can take place.

The probability for one phonon processes Is large enough that they occur even when the acceptor concentration Is extremely small. For Instance transfer with AE = 50 cm-*- occur between

12CO (v=l) and ^ C O (vs=l) in solid argon at a 10-1* relative 228 concentration of natural CO (1 ppm of *^00 I). A similar pro­ cess can populate high levels of: the vibrational ladder as ob­ served In matrix Isolated CO and NO. Energy transfer can still take place when several phonons are required to match ^E. The rate Is much smaller but large enough to efficiently quench a long lived state. As an example diffusion .controlled transfer has been found to occur between CO and NO (IE = 220 cm-*), CO and CH^ (&E = 620 cm-1,). Direct transfer between ND* and CO has been observed but only when the acceptor concentration Is large.

CONCLUSION

According their mode of vibrational relaxation mole­ cules can be classified Into three groups :

- Those having a large vibrational frequency (1400-2500 cm-*) and a small rotational constant (B < 2 cm-*) i V— ^ P h and V_*R processes are quite Inefficient. However their long lifetime ( y 1 msec) can be shortened by V— transfer. - Molecules with large V(1000-3000 cm-*) and large B (2-20 cm-1) are relaxed by V->R transfer on a 1-100 |isec time scale.

- The vibrational energy of molecules with small V(< 900 cm*"1) and small B (< 1 cm-*) Is directly converted Into lattice pho- nons. Their lifetime Is smaller than 1 jisec. However excepted for molecules having frequencies smaller than 500 cm-* the relaxation Is not faster than In gases at STP and then pheno­ mena so far studied In the gas phase sych as stimulated emis­ sion and IR photochemistry will be readily observed In matrices. 229

INFRA-RED LASER-INDUCED PHOTOCHEMISTRY IN MATRICES

M. Poliakoff. B. Davies, A. McNeish and J.J. Turner Department of Inorganic Chemistry, The University Newcastle upon Tyne , NE1 7RU.

Ab s t r a c t

IR laser irradiation of '*'^C'*"®0 enriched Fe(CO)j,, isolated in Ar, Xe and CHI) matrices at 20K provides the first examples of selective IR laser-induced reactions in the solid state. The reactions display not only isotopic and stereo-chemical selectivity but also selectivity between molecules isolated in different substitutional sites in the matrix.

Introduction In this paper we describe the use of IR lasers to promote reactions and rearrangements in matrices. The technique provides a solution to one of the major problems of matrix isolation; the study of processes with activation energies much lower than the photon energy of conventional photolysis sources but too high to be overcome by annealing the matrix. In addition, IR lasers introduce a very high de­ gree of selectivity into matrix photochemistry. This select­ ivity allows one to follow not only the chemistry but also the sterochemistry of a reaction, and we describe the first intra­ molecular ligand exchange process to be characterized by a technique other than Dynamic Nuclear Magnetic Resonance. Furthermore, the laser provides a new and powerful method for studying "matrix splittings", as it is possible to react selectively molecules in particular matrix sites. Our preliminary study has involved the laser-induced re­ actions of Fe ( CO ) ^ . This compound is produced'*' by UV photolysis of Fe (CO ) ^. UV 20K Fe ( CO ) j V,— IR (Ar) Fe(CO ) ^ + CO

In Ar matrices, the reaction is reversible. Fe(CO) ^ can be regenerated by irradiating Fe(CO). with near-IR light (13000- -1 1 2 9000 cm ) from the Nernst Glower of an IR Spectrometer. ’ After prolonged UV photolysis of the matrix, however, the formation of Fe(CO)^ is only partially reversible because the photoejected CO molecule diffuses away from the Fe(CO)^ frag­ m e n t .'*'3 Thus, in Ar, Fe(CO)^ can be obtained in two forms, "reversible" or "irreversible", which behave differently under IR laser irradiation. 230

Figure 1 shows the structure of Fe(CO ) ^ . The molecule has C2v symmetry and a shape similar to .that of SF^. For the present experiments Fe(CO)^ was generated from Fe(CO)^ en­ riched with 13C16 0 or 1 ^C^O . In these circumstances the matrix contains a mixture of nine possible isomers of Fe (■*" ^C^^O ) ;)_x ( ^C “0 ) x , each of which gives rise to character­ istic absorptions in the C-0 stretching region of the IR , lb, 2 spectrum. ’

Experimental Mote An Edinburgh Instruments c.w. CO laser"* was used for IR irradiation. This laser can be tuned to pro­ duce output, with a linewidth of less than 10- ^ cm'-1 and a power of 0.5 - 3W , at one of a series of frequencies separated by ~ U cm--'-. All experiments were carried out at 20K on CsBr substrates, with an automatic temperature controller which prevented any significant" rise in the temperature of the mat­ rix during laser irradiation.^ Reactions were monitored by IR spectroscopy , using a polished Ge filter 1 to remove visible and near-IR radiation from the spectrometer beam.

Isotopic and Stereochemical Selectivity Laser irradiation promotes the following reactions of matrix isolated Fe(CO)^.

F e (CO ) ^ + CO hV l a s e r » Ar Fe ( CO ) ,-

Fe(C0)u ♦ Xe -hV *a^ r- Xe-> FetCO^Xe hv laser, CH, Fe ( CO ) ^ + CH ^ ------> F e ( C 0 ) 1)CH u

These are the first selective IR laser-induced reactions to be observed in the solid state. The reaction with CH v is an 4 example of true IR photochemistry since it does not occur on

eq

CO e q

CO ax

Figure 1_ The structure of Fe (CO) 4 as determined from C 0 enrichment and IR spectroscopy .111 Bond angles are estimated from v(C-O) band intensities with an experimental error of +5 . Ax. and Eq. denote axial and equatorial groups respectively. 231

annealing the matrix, 2b presumably because the activation energy is too high. All of the reactions can also be promoted 1 2 .... by near-IR radiation. * However, the laser irradiation is highly selective, while near-IR is not. Only the IR band ir­ radiated by the laser (and other bands due to the same mol­ ecule) decrease in intensity while the bands due' to all of the other Fet^C 0 ) U- x ^ l"'Z® sPec*es remain virtually unchanged. The selectivity is such that it is even possible to differ­ entiate between isomers of Fe("*'^C^^O)[t_x ( ^CzO ) x which differ only in the stereochemical arrangement of the same number of 1, isotopic groups around the central Fe atom. A single molecule of Fe(CO)^ appears to require the absorption of only one photon to undergo any of these react- 2 4 -1 ions. ’ This allows an upper limit of 1900 cm or 23kJ/mole to be put on the activation energy for these processes. Al­ though these molecules are excited in C-0 stretching vib­ rations , the reactions involve C-M-C bending modes. It is clear from the high selectivity of the reactions that intra­ molecular energy transfer between these modes is far more efficient than intermolecular energy transfer between diff­ erent matrix-isolated Fe(CO)^ molecules.

Laser-Induced Isomerization There is considerable interest in the permutational processes which occur during intra­ molecular ligand exchange. In the case of a C^v four co­ ordinate molecule, such as Fe(CO)^ or SF^ , there are two possible permutational processes'1; either exchange of one axial with one equatorial ligand, or simultaneous exchange of both axial ligands with both equatorial, see Figure 1. The thermal rearrangement of SF^ in solution has been shown to involve the second process-*. IR laser irradiation of "irreversible" Fe(CO) ^ in an Ar matrix promotes intramolecular ligand exchange. The observed rearrangements, together with the laser frequencies which in­ duce them, are shown in Figure 2. The interconversions of the di-substituted molecules , (_t, 5. and 6 in Figure 2) clearly show that only one axial and one equatorial ligand are ex­ changed at once. Thus Fe(CO ) ^ undergoes a different rearrangement mode from SF^, despite its similar structure. Figure g Laser-induced, isomeri zations of Fe ( C 0 ) ^ - (1 ic J-^0) x species, as deduced from changes in the IR spectra, and the laser frequencies, cm_l, which promote the rearrangements. X represents the 13cl8o group. The mol­ ecules are numbered, 2; - j?, as in Ref. la.

If the vacant co-ordination site (i.e. between the two X groups in jt) is imagined to be a ligand these isomerizations represent the first known example of a non-Berry pseudo- rotation.^ This laser-induced rearrangement of "irreversible" Fe(CO)^ has also enabled us to detect an isomeri zation of Fe(CO)^ induced by near-lK light.® UV photolysis of Fe(CO)r, with statistical ^ C 0 enrichment, necessarily produces Fe(CO)^ also with statistical labelling. After the laser- induood rearrangement of paiticalai isvme i s uf Fe (" ^ C ^ O ) _x ( ^C"*"% )^ , the labelling is no longer statistical However, irradiation of the matrix with near-IR light (13000- 9000 cm ) rapidly restores the statistical distribution of isotopes. This shows that near-IR radiation must promote intramolecular rearrangement of Fe(CO)^ . Since near-IR also causes Fe(CO ) ^ to react with CO, CH^, Xe etc. this rearrange­ 233 ment may well form an integral part of the mechanism of these' r e a c t i o n s .

Site Selectivity An aspect of matrix isolation, which is particularly poorly understood, is the reason why single vib­ rational modes sometimes give rise to several closely spaced IR bands. The b Q mode of "reversible" Fe(CO)^ , for example, is split into three bands, Figure 3a. These bands are thought to be due to molecules of Fe(CO ) ^ occupying different pos­ itions relative to the photo-ejected CO molecule, but the precise nature of the interaction between the Fe(CO)^ and CO is not clear. Laser irradiation at 1881 cm almost completely removes the central band of the Fe(CO)^ triplet, while leaving the other two bands largely unaffected, Figure 3b. This immed­ iately confirms that the three bands are not due to a single molecule. More importantly, it should be possible to develop Or

u z < o <

1881 1 8 8 5 1 875 0

o z <

o < 0.1

1881 1 885 1875 Figure 3 Illustration of the site selectivity of laser ir- radiation. ( a) Split b 2 mode of "reversible" Fe ( ) i, in Ar, (b) after 1 min laser irradiation at l 88l cm 2W, (c ) bp mode of "irreversible" Fe ( in Ar , (d) after 30 min laser irradiation at 1881 c m ' l , 2W. 234 a model for the matrix sites by correlating which components of each split C-0 stretching mode belong to the same molecule. The IE bands of "irreversible" Fe(CO)^ , on the other hand, are unsplit but substantially broader than those of "reversible" Fe(CO ) ^ , Figure 3c. Laser irradiation of the bg mode of "irreversible" Fe(CO)^ results in a "hole" appearing in the broad band, which does not decrease in intensity uni­ formly across its width, Figure 3d. This shows that the broadness of these bands is caused by molecules in different sites, having narrow .absorptions too close together to be resolved by our spectrometer, and that even "irreversible" Fe(CO) ^ does not occupy a single site in the matrix.

Acknowledgements We thank the S.E.C. for supporting this res­ earch and for a grant to B.D., and members of the Physics Department of Heriot-Watt University, for their encouragement and technical assistance.

References

1. M. Poliakoff and J.J. Turner, (a) J. Chem. Soc. Dalton. Trans., 1351 (1973) ; (b) ibid. 2276 (1971*).

2. D . Davies, A. McNeish, M. Poliakoff and J.J. Turner, J. Amer. Chem. Soc.. submitted for publication.

3. M.J. Colles, R.B. Dennis , J.W. Smith, J.S. Webb and R.L. Allwood, Optics and Laser Technology. J , 73 (197 5 ) •

1*. A. McNeish, M. Poliakoff, K.P. Smith and J.J. Turner, Chem■ Commun.. 859 (1976).

5. W.G. Klemperer, J.K. Krieger, M.D. McCreary, E.L. Muet- terties , D.D. Traficante, and G .M . Whitesides, J. Amer. unem. soc. . y_/_, fu2j U y ft). 235

VIBRATIONAL RELAXATION OF CH F IN A KRYPTON MATRIX'AT LCW TEMPERATURES. 3 INFLUENCE OF THE ROTATION . (1) .

L.Abouaf-Marguin, B.Gauthier-Roy and F.Legay.

Laboratoire de Photophysique Moleculaire (Bt 213)

Universite de Paris XI - 91405 - ORSAY -

A spectroscopic study of CH F in a krypton matrix shows' that the gas 3 -l phase frequency ( V = 1048.6 cm ) is shifted towards the red in the matrix -1 3 -1 ( 1035.8 cm ).Other features appearing between the dimer ( 1027.6 cm ) , and -1 the pure solid ( 988 cm ), are concentration dependent and probably due to aggregates. The molecule undergoes an almost free rotation in the lattice, -1 the barrier to rotation being evaluated around 5 to 6 cm .

In the present work, we have used the good coincidence between the P(32) line of the V - 2 V transition of CO lasers,and the v =0->v =1 transition 3 2 2 3 3 of the lowest vibration of CH^F in the krypton matrix, to perform a double resonance experiment with two CO^ lasers (fig.1).The registered signal is the time dependence of the probe transmitted energy, which monitors the population of the v =i level of the CH F molecules after their excitation , on the same 3 3 transition, by the pump laser.Owing to the fact that the excitation by the probe is weak enough to be neglected, and also that the total excitation by the pump is weak, the variation of the probe intensity is proportional to the population of the v=3 level.

Depending upon the temperature and the concentration, different signals are observed :

- 75004 M/R 425000 - 25K ^ T ^ 50K - exponential decay time ,T =11. 5 ± 0.5 (us.

It corresponds probably to the relaxation time of isolated molecules inter­ acting with the lattice.

- T<2 5 K - non exponential signal. Superposition of two decays :T 3.5 (js andT= 11.5 ys. The percentage of molecules following the shorter decay increases as temperature decreases. This can be explained by an inhomogeneous linewidth, the fast decay corresponding to the energy 236.

C W C 0 2 laser

Beckman SQ 1.R.9 Spectr.

Monochromator

Rotating ^mirror

Qswitched CO2 laser Ampli PAR 1H Digital recor der|- averager • AI NE Scop Ampli ATNE

Figure 1 - Block diagram of the experiment .

ti: system of two mirrors turning the polarisation of the probe.

D: diaphragm. P: polarizer. L:NaCl lens. D^.D^iGe-Au or Ge-Cu

detector..

transfer and equilibration to molecules in other sites, not seen by the probe,

the frequency of which is the same as the pump one.

- 1 0 0 0 j t M /R < 7500 - The decay time f measured at 30 K decreases as the con­

centration is increased (fig.2). The shorter signal disappears gradually at

low temperatures as T: becomes Itself shorter,

- M /R < 10UU - The signal is exponential over the whole range of temperatures,

and. t he t i m e T decreases as concentration is increased (fig.2). It is to be

noticed that, a t T ^ 20K., we observe the same signal when the frequencies of

the probe and the pump are different.

The strong concentration dependence observed for M / R < 7500, suggests that

the dimers,and probably hygher polymers, take part in deactivation .

The relaxation mechanism will be discuss then, so that it can also explain

others observations made in our laboratory:

-ammonia has a longer relaxation time in nitrogen matrices (15 ^is) , than in

rare gas matrices (2) . 237

T (jis) at 3 0 K

I-M

M/R 10000 20000

Figure 2 - Decay time at 30K as a fonction of the mole ratio.

-CD^F has a relaxation time ten times longer than CH^F in a krypton matrix(3).

Considering the direct relaxation to the phonon bath, via multiphonon

transition, most of the theories predict a strong temperature dependence,

when the molecular vibration-phonon interaction is described by a short range

repulsive force (4). But in case of a coupling of the B o m Oppenheimer type,

other theories (5) give a much smaller temperature effect, which can fit with

our results. Nevertheless, the deuteration effect cannot be explained . Now,

following Brus and Bondybey (6), and supposing that the rotation is the

accepting mode in the relaxation, we can calculate the minimum J of rotatio- m quanta to match the vibrational energy gap. Then the agreement between our

results and results of the litterature with a relaxation rate independent of

temperature in a large domain, and varying exponentially with (7), is surprisingly good.

R e f e r e n c e s

1-L.Abouaf-Marguin, B .Gauthier-Roy and F.Legay, Chem. Phys. to be published.

2-L.Abouaf-Marguin, H.Dubost and F.Legay, Chem. Phys. Lett. 22, 603 (1973)

L.Abouaf-Marguin, Thesis, University Paris VI (1973).

L.Abouaf-Marguin, B.Gauthier-Roy, to be published. Recent experiments,with 238

probably cleaner samples, give a vibrational relaxation time of 15 (j s in nitrogen, instead of 3^>s'.

3-B.Gauthier-Roy, L .Abouaf-Marguin, to be published.

4-A.Nitzan, S.Mukamel and J.Jortner, J.Chem.Phys. 63, 200 (1975).

S.H.Lin, H.P.Lin and D.Knittel, J.Chem.Phys. 64, 441 (1976).

5-J..Tortner, Mol.Phys. 32, 379 (1976),

D.J.Diestler, J.Chem.Phys. 60, 2692 (1976) .

6 .II.Lin, J.Chem.Phys. 63, 1033 (1976) .

6-L.F.Brus and V.E.Bondybey, J.Chem.Phys. 63, 786 (1975) .

V.E.BOnaybSy And L.ti.tirus, J . C h e m .P h y s . 63, 794 (1975) .

7-K.F.Freed and H.Metiu, Chem.Phys.Let. ,to be published .

It.F.Fiend, II.MuLlu and D.L.Yuugur, to be published . 239

VIBRATIONAL ENERGY TRANSFER STUDIES OF MATRIX ISOLATED C~ Louis Allamandola, Hossein Rojhantalab and Joseph Nibler Department of Chemistry, Oregon State University Corvallis, Oregon 97331

A great deal of experimental and theoretical effort has been expended on elucidating energy transfer processes for molecules isolated in low tem­ perature matrices. Of recent interest have been the energy loss mechanisms available to vibrationally excited diatomic molecules isolated in a frozen monatomic array since theoretical calculations are most feasible in this simple case. The theoretical models for the vibrational decay generally in­ volve dissipation into many lattice phonons by means of various coupling mechanisms. Most models predict a strong dependence on phonon order and tem­ perature with concentration playing little or no role. Quantitative calcu­ lations have not been possible since the different molecule-lattice coupling parameters have not been available. Between 1972 and the present Dubost et al have measured vibrational re­ laxation of CO via direct infrared fluorescence and while considerable migra­ tion from one isotopic species to another was observed they measured life­ times which are mainly radiative^. From 1975 to the present Bondybey and Brus have investigated energy transfer occuring in the upper electronic state for 2 a number of molecules isolated m matrices . They have shown that intersystem crossing between interleaved low lying electronic states leads to rapid vibra­ tional relaxation and that pseudo-rotation is important in the rapid relaxation of OH and NH. Therefore, because of the added vibrational decay channels for electronic­ ally excited states, studies of vibrational relaxation in the ground electronic state are more desirable and, in view of the apparent dominance of the radiative decay path for matrix isolated molecules, the ideal model system would be a homonuclear diatomic molecule. Unfortunately, it is not presently possible to selectively excite and probe specific vibrational levels of the most obvious test cases (H^, D^, N^, 0^). However, the homonuclear ion does absorb and fluoresce in the visible rfegion and, in 197^, we reported some preliminary direct lifetime measurements for v" = 1 of matrix isolated C using a pulsed 3 • • dual laser scheme . Results from the extension of this work are presented here and reference 4. is readily formed in a matrix via 1216 8 photolysis of The positive ion counterpart has not yet been established. The absorption remains essentially unchanged on extended insitu photolysis although the ab­ sorption continues to grow by as much as a factor of three. We also observe no 2 4 0 significant change in Cg emission intensity for matrix samples of similar thickness with (only) CgHg concentrations varying by a factor of 200. We conclude therefore that the CL concentration reaches an upper limit and we 16 - calculate a number density in argon of 2. *ix 10 Cg/cc suggesting that a volume of h.2x10^ 8^ surrounds each Cg. If it is a spherical volume, a ra­ dius of ~2l0 A is implied. To measure vibrational lifetimes a 50(^ts pulse of li727 8 pump radiation (V^) excites the v" levels of C” via the absorp­ tion fluorescence cycle shown in figure 1. After a variable delay the resi­ dual v" - 1 population is excited to v' = 0 by a 1 ^s probe pulse at 57^*3 8 (Vpr ) and during the probe pulse on-time the number of B(0')-»x(0") pho­ tons emitted (t)g) are counted. This number is proportional, to the v" = 1 population remaining after the delay period. Thus plotting this number versus delay time yields the decay curve. The salient features of the v" = 1 lifetimes (Tv) can be summarized as follows: 1. The decay docs not follow first order kinetics 2. Tv depends strongly on C^Hn concentration 3. There is no measurable temperature effect in the range 15 - 32° K, in order to fit the decay data (figure 2) a sum of at least 2 simple ex­ ponential terms is necessary yielding a slow (T^) and fast (7^,) component to each curve. Varying the starting Ar/CgHg ratio from 100 to 20,000 in- creases'T'g and T^. from 0.3*11 and 9.6ms to 1.3 and 1U7 ms. These and other results are presented in Table I. These results imply that the relaxation process involves the preferential transfer of the vibrational quantum to CgHg even over long distances rather than into the phonon modes of the lattice. Further support of this picture is obtained from the fact that T"v measured when CpD? is used is 2-5 times faster then with CgHg (utllike in CgHg, the symmetric stretch of C0D0 is nearly resonant withl/cc). If a fi ion is at the origen of its particular matrix region the v" = 1 relaxation rate would be expected it) depend on the distance to the nearest CgHg accep­ tor. We have calculated the radial probability densities for the different concentrations Used (figure 3) and further divided these into two regions thuo providing a. semi-quantitative indication 01 the change m decay con­ stants with changing Cg ... CgHg separation. Since Cg has no dipole moment one would expect the coupling to C H, to be quadrupole in nature and thus -1 0 . 7 show an r dependence. However it is apparent that the interaction drops off much less rapidly (approximately r ^ ) than would be predicted by a simple electrostatic interaction model and it seems likely that the coupling involves a more direct short range interaction through the matrix atoms. In this picture, a particular Cg ...(Ar)fi..-CgHg configuration might be 241

viewed as a large "molecule" in which the Cg group is weakly coupled to the CgHg unit through the C? ...Ar, Ar...Ar, and Ar...CgHg bonds. When Xe and Kr matrices are used even longer vibrational lifetimes are measured (Table I) 12 - 13 - On the bars of Cg, Cg mixture studies in which one isotope is pumped, the other probed we conclude that very little if any resonant exchange from one Cg (v" = 1) to its nearest Cg (v" = 0) neighbour occurs (about 200 X away). Thus it appears that the rate determining step does not involve diffu­ sion among many lattice phonons or. other Cg species but rather the process by which the vibrational quantum is transferred from Cg over distances as great as 50 X to acceptor acetylene molecules.

1. H. Dubost and R. Charneau, Chem. Phys. J_2, 1*07 (1976) 2. V. Bondybey and L. Brus, J. Chan. Phys., 65., 311*6 (1976) 3. L. Allamandola and J. Nibler,' Chem. Phys. Lett., 28, 335 (197-**) 1*. L. Allamandola, H. Rojhantalab, J. Nibler, and T. Chappell, "Vibrational Relaxation Studies of Matrix Isolated Cg" submitted to J. Chem. Phys., Jan. 1977.

Table I: Cg v" = 1 Vibrational Lifetimes T ” 18 ° K

sample conc. 'T^.(msec) 'T'(msec)

CgHg/Ar 1 /100 0.31* + 0.02 1.32 + o.oi* C2Hg/Ar 1/500 2 .11* + 0.19 15.7 + 0.8 CgHg/Ar 1/2000 5-1 + 0 .1* 37-1 + 1 .1* CgHg/Ar 1/20,000 9.6 + 3.1 11*7.0 +20.0

CgDg/Ar 1/500 0.53 + 0.03- 3.5 + 0.2 CgDg/Ar 1/20.000 7-8 + 0.8 93.0 + 3.0

CgHg/Ng 1/1*00 0.81 + 0.03 7.9 + 0.3 CgHg/Kr 1/2000 16.9 + 3.6 288.0 +66.0 CgHg/Xe 1/2000 12.8 + 1.8 165 +11.0 iue1 ipi idEeg ee igo Figure 2: V"sldecay curves for C^as a function of C Figure 1; Simplified Energy Level Diogrom 42 2

A727A fluorescence iue:Naet probability density distribution for the concentrationsFigure3: Nearest used. A 2 100:1 NEAREST NEIGHBOR PROBABILITY NEAREST DENSITY . 00:1 A 6 Lnl 8 1 R/Ao - 10 500:1 20 12 20000:1 AO 2000:1 16 60 i V"= Ci 1 decoy 18 25 20 Argon: C 0 msec 80 2 5.3 & H . 000:1 000:1 2 concentration. i 11* 243

Vibrational energy transfer and relaxation in pure and CO-doped solid N2

G. Zumofen Physical Chemistry Laboratory, ETH-Zentrum, CH-8092 ZUrich

Experimental evidence for vibrational energy transfer from N„ 1 2 i (32 as shown in Fig. 1. The two-phonon-assisted process is strongly temperature depen­ dent. Calculations have been performed developing the vibron- perturbed phonon potential to 2nd order. The resulting two- phonon hopping rates are smaller than the one-phonon assisted rates for 4.2K but dominate for i u k and higher temperatures n,i :ihown i n Fi £. 1 . The off-resonant process 2

N 2 (v) + N2 (0) + { (q)} - N 2 (v-1) + N2 (l) is endothermic and hence strongly temperature dependent. The total absorbed phonon energy Gg (q) must match the energy gap A which is due to the molecular anharmonicity, here 245

Fig.1: lifetimes of highly-exci­ ted vibrational levels: --•-- phononless v-*0, O+v resonant trans­ fer; one-phonon-assisted v+Oj0+v transfer with trap; two-phonon-assisted v+0, 0-*v (Orbach type) transfer, ---—; multiple-phonon-assisted off-re­ sonant v+v-1,0+1 energy transfer. Horizontal bars cover the tempera­ ture dependent region in which vibrational levels have been ob­ served with lifetimes longer than -0.1 s e c .

A=(v-l)2coexe . The rate of process 2 has been calculated from the golden-rule type relation t "1 = (2tr/fi)£iE2G (i )e'A/kT, where e is the resonance energy between two molecules, i labels the lattice sites, and G(ti) represents an energy depen­ dent Franck-Condon factor per unit energy for multiphonon excitation. The one-phonon part of G(A) is displayed in Fig. 2. The computed lifetimes, shown in Fig. 1, explain why the v=2,3 levels have not been observed, but the temperature dependence is not as large as the observed one. Process J, the relaxation by the creation of multiple phonons, has been treated by the same model as processes 1 and 2. It yields astronomical lifetimes and Is therefore out of qucotion for the explanation of the observations. In the case of CO-doped N2 the relaxation processes

N 2 (v ) + C0(0) ■> N 2 (v -1) + C0(1) ± {ng (q) } reduce the lifetimes observed in pure N 2 . With or 1^C1^0, these reductions are largest for the transitions 246

Fig. 2: The histogram stands for G(A), the one-phonon Franck-Condon factor per unit energy. Crosses , circles mark the observed relaxation rates , r e s p e c ­ t ive l y , as a function of energy gap A , on an arbi­ trary vertical scale. x o 2 0 N2(6+5) or N2(8*7) resp.2 , wnlle the transitions (7"*"6 ) and N2(9-*-8), resp., would be nearest to resonance, ng (q)=0. The experimental rates due to ^ “CO energy transfer4 are dis­ played as a function of the energy gap in Fig. 2 and are compared with G(A). The detailed agreement may be partly fortuitous but it nevertheless demonstrates that the N^-CO relaxation rate is small for small energy gaps, where the density of phonon states is small, and that it reaches its maximum for A=30 cm

1K. Dressier, 0. Oehler, and D.A. Smith, Phys. Rev. Lett. 34, 1364 (1975) W.W. Duley, 0. Oehler, and D.A. Smith, Chem. Phys. Lett. 21, 115 (1975). 30. Oehle.r and K . Dressier, in Molecular Spectroscopy of Dense Phases, Elsevier (Amsterdam, 1976). p. 715 • J|. 0. Oehler and K. Dressier, unpublished. •’g. Zumofcn and K. Dressier, J. Chem. Phys. b_4, 5198 (1976). 6A.A. Ovchinnikov, Sov. Phys. JETP, 30, ■147 (1970). 247

Calculation of matrix shifts and splittings in the vibratio­ nal IR and Raman spectra of solid and matrix isolated CO.

G. Zumofen Physical Chemistry Laboratory, ETH-Zentrum, CH-8092 Zurich

It has been shown1>2 that matrix shifts and resonance splittings of vibrational bands can be calculated from intermolecular forces which are derived from gas phase and crystal structure properties. General relations1 and numeri­ cal factors2 have been derived for the isotropic and aniso­ tropic parts of the dispersive interaction, for the electro­ static dipole-dipole and quadrupole-quadrupole interaction, for the dipole induced dipole-dipole and the quadrupole indu­ ced dipole-quadrupole interactions. The repulsive contribu­ tion can be calculated assuming a Lennard-Jones 6-12 atom- atom potential. The corresponding atomic Lennard-Jones para­ meters can be computed from molecular parameters by equating atomic and molecular potential energies in the nuclear equi­ librium position of the crystal. The matrix shifts of diffe­ rent isotopes can be calculated from the isotope dependence of intramolecular vibrational amplitudes. The principal results for pure CO and CO isotopes isolated in CO and matrices are listed in Table I and displayed in Fig. 1. The Table I : Matrix shifts in units of cm . N? matrix 12-16,, 7 ! 7 ! T~ ^ C O matrix calc. incl. obs. calc.______obs .______calc, resonance 12Cl60 -3-5 -2.08 -4.9 -2.53 -3.89 12C170 -3-7 -1.97 -3-9 -2.44 13Cl60 -3-5 -2.12 -3-9 -2.56 12Cl80 -3.2 -1.86______-3-8 -2.28______tabulated experimental results are collected from the data of different authors published during the last fifteen years. The resonance interaction matrix elements have been calcula­ ted on the same basis as the shift. Energies corresponding to the irreducible representations A and T are presented as functions of different types of short-range interactions, but 248

Pig. 1. Matrix shifts. 6.86 j. 7.03 7.17 720 Calculation: — I is 1----- (---- n — CO isotope isolated in , CO isotope -1 isolated in 12c l6o. Experiments: Crosses for CO isotopes isolated in , the other symbols fur CO isotopes in "*"2C"*'^0.

+ 8 -4

x D u b o s t j CO in N2 + IVIaki □ Pimentel a Vu.Vodar CO __ o Maki c m

Table II: Resonance energies in units of cm

dispersive electrostatic induced repulsive o E(A) 0.77 -0.23 o = 0. 42 E(T) 0.10 0.05 <0.01 U . 14 exclusive of the electrostatic dipole-dipole interaction, in Table II. The resonance dipole-dipole interaction needs special attention. .Short-range dipole interactions contribute further uu trie a -t splittings Of Table 11, long-range dipole interactions' split the T mode into transverse (t) and longi­ tudinal (l ) components. Both of these two types of splittings are significantly influenced by the background dielectricum as demonstrated by the following equations: 249

E(A) = -'8.668 ^ l ^ y ) 2 (P10)2 a"3 (1)

E(Tt )= {2.89 7 ( ^ ) 2 ■ ( ^ y ) } . (p10)2 a' 3 ' (2)

E(T£ )= {2.89 7 '[^y ]2 + J ^ ) } ' (P10)2'a"3 (3)

E(T£ )-E(Tt ) = T T 1 ^ ] (P10)2' a' 3 ' . (4)

P 10, a, e stand for the molecular transition .dipole moment-, lattice constant, and dielectric constant., respectively. The calculated Z-t splitting is 4.62 cm 1 in good agreement with the observed3 splitting of 4.54 cm ^.-E(T^) represents an additional shift of the transverse mode by resonance, inter­ actions. This, calculated extra shift..agrees with the observa-. tions in pure solid "*"2C"*"^0 as. shown in Pig. 1. • •. The t/z intensity ratio depends on the angle of incidence 8: I(T-):I(T ) = e2 (cos2e+l)/sin20 .... (5) This relation demonstrates that the intensity of the longitu­ dinal mode is attenuated by. the background dielec;tricum. For .

a -C O R a m a n

2135 2140 c m 2135 2140 Pig. 2:• A synthetic IR .spec- Fig'.. J>: A calculated histogram . trum of the stretching mode of the vibron band is compared is compared with the obser- with' the Raman experiment of vation of Dubost. 3 Anderson et al .4 2 5 0

8=45° the theoretical intensity ratio is 10.1 in reasonable agreement with the observation of Dubost.3 A synthetic trans­ mission spectrum can be calculated by introducing a frequency-dependent complex dielectric tensor which takes the influence of crystal thickness, oscillator damping and the angle of incidence into account (Fig. 2). It is known from heat capacity measurements that CO molecules are distributed with little discrimination between the CO and OC orientations. The IR spectrum is only weakly influenced by this disorder because the molecular transition dipole moment is responsible for the resonance interaction between the molecules as well as for the intensity. An inversion of one molecule changes simultaneously the phase relationship of its resonance interaction and of the contribution of its transition dipole to the intensity. Hence eigenvalues and IR selection rules are not affected by the disorder. In the Raman spectrum, on the other hand, the molecular pola- rizability tensor does not change upon inversion and there­ fore the polarizability is independent of the phase change due to the disorder. Therefore the states of the entire Bril- louin zone obtain intensity and the Raman band becomes a map of the density of states. A theoretical histogram is com­ pared with the Raman spectrum measured by Anderson et al.4 in Fig. 3* The calculated spectrum reproduces the observed asymmetry but the total linewidth is about 2 cm too small. The experimental spectral resolution of 0.8 cm 1 has not been taken into account in the calculation and may be partly responsible for this difference. Short-range interactions depend only weakly on the disorder; details are given in Ref. 2.

. Zumofen and K. Dressier, J. Chem. Phys. 6j), 5198 (1976). 2 G. Zumofen, to be published. ^H-. Dubost, Thesis, Paris (1975). 4 A. Anderson. T.S. Sun and M.C.A. Donkersloot, Can.J. Phys. £8, 2265 (1970). 251

The Application of Ma^ietio Circular Diohroieo __ in Matrix Isolation Studies.

T.J. Barton. H. Orinter and A.J. Thomson. School of Chemical Sciences, University of East Anglia, Norwich.

The theory of magnetic circular diohroism (mod) is now well documented (1-3). In many oases there are considerable advantages in measuring mod spectra at low temperatures. However, the use of polarised light limits single crystal studies to oubio or uniaxial crystals. Low temperature studies can be made on frozen solutions and polymer films, but in practice these methods are of limited use. Thus matrlx-l'solation provides a means of extending the range of molecules available for low temperature study, as well as being the only means of examining unstable or high temperature species. MOD has several advantages over conventional absorption spectroscopy. Assignment data not available by other means can be obtained from mod studies, and with paramagnetic compounds mod signals may be detected when absorption bands oannot. The appartus for matrix- ieolation mod has been described elsewhere (4, 5)•

The following examples illustrate some of the applications of mod to matrix-isolated compounds. Benzene (6-8) is a good example of the simplification of spectra in a matrix, enabling individual vibronio bands to be identified. (See Figures 1 & 2). The matrix absorption spectra (Fig. 2) show a single progression in the alg vibrational mode. Four alg progressions built onto the four e2g vibrations that allow the lA^g-* transition would be expected. In mod three of the four progressions are observed, one series showing a negative AA and the other two a positive AA. Calculation together with results from other sources (9) enable the assignment of the negative progression 6, 11 A 16 to V ^ and the positive progressions 5» 7 & 12 and 8, 13 & 18 to Vg and Vg respectively. The fourth e2g mode is predicted to lie under band 16 and thus oannot be observed. 252

A® A'.

- 0.

002

- 0 0 1

-002

23 0 253 260 WAVELENGTH / nm Fig. 1. The mod and absorption spectra of benzene vapour.

240 250 255 260

WAVFI FNfiTH j n m pig, 2, The mod and absorption spectra of benzene in an argon matrix. In the oaee of osmium tetroxide (10) the matrix spectrum is again much improved over the gas phase (Mg. 3). This enables the reassignment of the' spectra and reveals a very close relationship to permanganate, which was not previously apparent.

5-

.O

- O'

WAVE NU MBER x 10"3 / cm '.1

fig. 3. The mod spectra of OsO^. Aigas phase. B:argon matrix.

A good example of a temperature dependant mod spectrum is provided by vanadium hexaoarbonyl. The spectrum consists of several C-terms and exhibits strong temperature dependence between 5 - 35 K. The plot of &A versus T-1 is non-linear due to some irreversible photolysis and possibly saturation effects. 254

With carbonyla It Is possible to carry out reactions In the matrix. Iron pentacarbonyl In an argon matrix has a temperature Independent mod speotrom consisting of several A- and B-terms. When the matrix Is exposed to mercury vapour lamp radiation for about one hour the spectrum changes completely and becomes temperature dependant. Under the above conditions Infrared spectroscopy (11) Indicates that the principle species present will be iron tetracarbonyl, and it is theoretically predicted that this compound would be paramagnetic, thereby giving a temperature dependant mod spectrum.

References: (1) P.N. Sehatz and A.J. MoCaffery, Q. Rev. chem.Sop,, 2^, 552 (1969) (2) A.D. Buckingham and P.J. Stephens, Ann.Rev.physio,Chem,, 11, 399 (1966) (3) P.J. Stephens, Ann.Rev.physio,Chem*, 25> 201 (1974) (4) I.N. Douglas, R. Orinter and A.J. Thomson, Moleo.Physios., 28, 1377 (1974) (5) T.J. Barton, I.N. Douglas, R. Orinter and A.J. Thomson, Beriohte der Bunsen-Qeaellschaft., 80, 202 (1976) (6) I.N. Douglas, R. Orinter and A.J. Thomson, Moleo.Physios., 26, 1257 (1973) (7) I.N. Douglas, R. Orinter and A.J. Thomson, Moleo.Physios., 22, 673 (1975) (8) T.J. Barton, I.N. Douglas, R. Orinter and A.J. Thomson, Mol no. Physi os,, 30. 1677 (1975) (9) P.M. Oarforth and C.K. Ingold, J.chem.Soo., 416 (1948) (10) T.J. Barton, R. Orinter'and A.J. Thomson, Chem.Phys.Lett., AO, 399 (1976) (11) M. Poliekof et al., J.Amer.chem.Soo., in press. 255

40 41 Ar : K A Model System for Matrixisolation

H. Coufal, U. Nagel, E. Ltischer ' Physik-Department E13, TO MUnchen, 8 0 4 6 Garching

Matrix isolation plays a steadily increasing role in atomic and molecular physics, particularly in chemical physics.

Normally the samples are produced by the vapour deposition technique; this technique has several serious disadvantages:

1) many atoms and molecules cluster easily in the gas phase; 2) doping is not homogeneous throughout the entire sample; 3) doping occurs under the influence of additional impurities because of insufficient vacuum conditions.

40 41 These problems can be studied using Ar :K as a model system for matrix isolation.

Argon polycrystals can be produced with highest chemical purity. By neutron irradiation a small percentage of argon 40 41 atoms is activated: Ar (n,^)Ar . With a decay time of 1.8h, these argon atoms decay into potassium atoms by the reaction Ar^ — % and are statistically distributed in the matrix. This method shows several advantages:

1) after neutron irradiation, the chemical purity of the doped sample can easily be proven by jf-activation analysis. Samples produced up to now showed a total impurity con­ centration of 4 ppm. 40 2) Argon gas is nearly isotope pure (99.6 % of Ar ) which is Why the dopand consists of one single isotope. 3) Since the cross section of argon for the capture of ther­ mal neutrons is small, the sample is homogeneously doped 41 41 with K . Up to now, 0.1 ppm of K is a typical concen­ tration for this doping method.

The great disadvantage of this method is that it is prac­ tically restricted to argon crystals. With the model system 40 41 Ar :K , however, it is possible to study the trapping of impurity atoms in a host lattice and their interactions. 256

Till now optical /1,2,3/, ESR- /4/, and double resonance- 41 experiments /5/ on K -atoms implanted by neutron activation in Ar^° crystals were performed.

A typical transmission spectrum of an argon crystal doped 4 1 with .2 ppm of K is shown in Fig. 1. There are three ab­ sorption bands between 500 nm and 800 nm. By ESR and double resonance experiments and by intensity comparison of optical and ESR-spectra these three absorption bands could be assigned to potassium atoms on three different trapping sites in the argon matrix. It could be proved that absorption band 1 and 2 in UV correspond to the 5p 4s transitions of the same argon atoms causing band 1 and 2 in the VIS and IR-region by their 4p ^ 4s transitions.

A comparison of all optical measurements performed up to now on Ar:K (Fig.2) show a rough agreement independent of sample preparation. Meyer /6/ and Weyhmann et a l . /?/ studied vapour deposited rare gas films, whereas neutron activated crystals were used by Neubauer et al. /1/ and in our experiments. Band 1 in IR, the so-called "red" band, was observed by all authors, band 2 blueshifted relative to the 4p * 4s transit­ ions of free potassium atoms - therefore, often called the "blue" band - was seen in all, but not in Neubauer1s , experi­ ments. As far as we know band 3 in VIS and bands 1 and 2 in UV were seen for the first time.

G-factor shifts and relative shifts of the corresponding zero- field hyperfine-splitting constants, derived from the spin-Ha- miltonian,tor the ESR-experiments on K-atoms in Ar crystals are shown in Fig. 3. There is an excellent agreement of measurements on vapour deposited films /8.9/ and nnr samples /'!/.

From the ESR-data it was possible to conclude the number and average distance of the argon atoms neighbouring a potassium impurity /10/. Band 1 corresponds to potassium atoms surround­ ed by 8 argon atoms, whereas band 2 is caused by K impurities with 10 nearest neighbour argon atoms. Band 3 should be a potassium atom on a substitutional lattice site with a 257 coordination number of 12.

All the observed effects as well as the annealing of the samples (Fig. 4) can be understood within this simple model. Moreover it demonstrates that the temperature during vapour deposition and the annealing temperature plays an important • role for all spectroscopic properties of impurity atoms in rare gas matrices.

/1/ H. Neubauer, G. Press, H. Jodi, J. Hingsammer, E. Luscher Z .Angew.Phys. 2J3, 309 (1 970)

/2/ H. Coufal, U. Nagel, M. Burger, E. Luscher Phys.Lett. 47A, 327 (1974)

/3/ H. Coufal, U. N a g e l , M. Burger, E. Luscher Z.Phys. B25, 227 (1976)

/4/ H . Coufal, M. Burger, U. Nagel, E. Luscher, K. Boning, G. V o g l , Phys.Lett. 4 8 A , 143 (1974)

/5/ H. Coufal, E. Liischer Phys.Lett. 4 8A , 445 (1974)

/6/ B . Meyer, J.Chem.Phys. 4_3, 2986 (1965)

/7/ W. Weyhmann, F.M. Pipkin, Phys.Rev. 166, 207 (1961)

/8/ K. Jen, V .A. Bowers, E.L. Cochran, S.N. Foner Phys.Rev. 1^6,.1749 (1962)

/9/ J .P. Goldsborough, T.R. Koehler Phys.Rev. A 1 3 3 , 135 (1964)

/10/ H. Coufal, E. Liischer, Proc .LT14, 2_, 44 (1975) transmission Tarbitro'y units} 258 0 40 0 60 0 800 700 600 500 400 300 F i g . 1 Optical transmission at an argon polycrystal doped polycrystal argon an at transmission Optical .1 g i F i. Otcl bopindt totsim tm tapd in trapped atoms ootassium at data absorption Optical 2 Fig...... We t. a t e n n a m h ey .W W .ivieyer B 6 684 668 M with potassium with argon matrices argon ? •!!:

■\4r.hh-,. .■"'•I'.. U> 76 * 771 WQVfi Icfiytli Enin] Enin] Icfiytli WQVfi . cc < z tn tn o Z U) z Couf et . ' t. a t e t fa u o .C J H Neubauer et al. a t e r e u a b u e .N J K \ 1 __ ■ * * ■ o " * iw * . mo" • . NANOM ETER - ETER NANOM f :

1 w His* w r i0 K

259

£ C.K.Jwt et at. ® J.P Goldsborough el ol. ° 41 K J.P.Go4daborcv$.h et al. C O 4'K H.J.Coefa! et ot.

-30 -20 -10

K)2 - A A /A Fig. 3 ESR-data of potassium atoms in argon crystals

100 , f ------

-j £ f

0 10 20 30

Tq -k Fig. 4 Annealing dependence of the three observed Ar:K- configurations 2 6 0

MOD AND MCPE STUDIES OF TRANSIENT AND STABLE MATRIX ISOLATED SPECIES

E.R. Krausz, R.L. Mowery, and P.N. Schatz

Department of Chemistry, University of Virginia, Charlottes­ ville, Virginia, 22901 USA

Although much spectroscopic work has been done on matrix isolated species, very few magneto-optical studies have been reported1'2. Matrix hosts in fact can provide an excellent vehicle for such studies since under favorable conditions samples with no significant depolarization can be prepared. Until now, matrix isolation magnetic circular dichroism (MCD) studies have involved creation of a sample "in situ" in the bore of a superconducting solenoid which is subsequently ro­ tated into the optical path of the spectrometer1'2. This technique has several limitations. Most importantly, the ex­ periment is "one-shot". Specifically, if an unsatisfactory matrix is deposited, the entire magnet system must be warmed, the vacuum broken, and the deposition window removed and cleaned. Turn around time is more than a day. Furthermore, the low thermal conductivity of matrices together with the lack of a radiation shield necessitated by the spray-on pro­ cedure, creates serious sample temperature uncertainties and substantially limits the lowest temperatures which may be reached. This can be a matter of crucial concern in studying C terms (T-1 dependence) both in MCD and in magnetic circu­ larly polarized emission (MCPE), especially fthere, in the latter measurement, there is the added thermal load of the exciting light energy. A vastly superior technique is to generate the matrix sample conventionally in an optical cryostat and, after de­ termining that its optical properties are satisfactory, "in­ ject" or transfer it into a standard exchange gas, top-loading split-coil superconducting solenoid magnet cryostat. The mag­ net cryostat is not committed to matrix isolation spectroscopy until actual injection and may be used for independent stud­ ies. The apparatus is shown schematically in figure 1. 261

Here, A is modified Ox­ MATRIX INJECTION SYSTEM ford Instruments CF204 con-,, tinuous flow cryostat fit­ ted with Circle Seal valves top and bottom, and.a Ley- bold vacuum flange (B) to interface it with an Oxford. Instruments SM4 6T five (CF204) window cryostat (C) fitted axially with 50 mm (i.e., huge) synthetic fused quartz windows. Initially the CF '204 is an indepen­ dent' system with the bottom valve closed. A long rod (D) is introduced through the top vacuum lock, and the sapphire sample depo­ sition window" is mounted C(SM4) on the bottom of- the inner tube of the rod. The rod is then thermally contact­ ed to the CF2 04, and the window is maintained at a temperature appropriate > for deposition. After the sample has been prepared and confirmed as suitable for magneto-optical study (note that the rod may be withdrawn through the vacuum lock and the win­ dow replaced' in a matter of minutes if the sample is unsatisfactory), the CF204 is interfaced with the SM4 as in figure 1. After precooling the sam­ ple space of the SM4 with 262 liquid helium, the internal liquid helium supply valve (needle valve E) is closed and the space evacuated to lO'^mm Hg. The circle seal valve is then opened, the sample is drawn behind the outer tube of the long rod, which then serves as a ther­ mal shield for the matrix, and the rod is lowered to the bot­ tom of the SM4 sample space. The needle valve is then opened thus immediately immersing the sample in liquid helium. The matrix is treated as a crystal at the end of the rod; free ro­ tation and height adjustment are possible. Critical features

KrF/Ne MCD 5.25T 3.4K ABS 5 K SATURATION

m O X < <

-10

H (Tesla)

24 28 32 36 40 44 48 52 56 ENERGY IcK 263 of the apparatus are the demountable thermal link between the rod and the CF204, the leak tight operation of the low temp­ erature needle valve, and the strain free nature of all the magnet cryostat windows. The system is described in detail elsewhere^. We have no evidence that the sample is in any way degraded or warms much above 15K during injection. Fur­ thermore, a sample immersed in liquid helium seems to suffer no degradation over a period of days, except under high power laser excitation. Neon matrices are too volatile to be in­ jected with the present apparatus and are studied using the in-bore deposition technique. Further development of the rod may allow neon matrix injection.

C€2 /Ar EMISSION

1 : 400 H =5T

v> H; z 4.2K 3 1.5K H = O > - o e < 06 4.2K I— H iP

> -

UJ 13.8K Z HiP

■••Mi 13.56 13.57 13.58 13.59 ENERGY kK 264

K r F

High pressure mercury arc photolysis of a matrix of argon . or neon with 1% Kr/F2 gives rise to broad, weakly structured absorptions in the ultraviolet- region- attributed to the radi­ cal species KrF^. In order to further characterize the spec­ tra, we have studied the MCD of the systems as a function of temperature and field strength, and to some extent as a . . function of photolysis conditions. Figure 2 shows the, MCD and its dependence on field strength at 3.4K together with an absorption spectrum taken from reference 4.

In neon, C terms of opposite sign are observed for the lower energy (X->B) and higher energy (X -► D ) transitions, the pattern, being the same .as in XeCl but opposite to that in XeF2. In argon, the situation is more complex since the MCD pattern depends strongly on photolysis conditions. The ground,state magnetic properties can be deduced from C/D ratios (D=dipole strength) but we are hot able to measure D reliably either in neon (because of the steeply rising background) or in argon (because of interfering absorptions of other species). How­ ever, essentially the same information can be obtained by studying the non-linearity ("saturation"), of MCD C terms as a function H/T for (3H/kT>,l. This technique has the added ad­ vantages of being independent of depolarization effects and interference from weakly dichroic absorptions. Saturation curves for XeF correspond to g values close to". 2 ( % pure ground state), but KrF curves are much mope strongly saturated. We have been able to reproduce the ob­ served saturation curves for KrF in neon quite well by assum­ ing that the gruund state is an unperturbed fluorine atom in,. the ^Pg/2 state. One finds for this model that the C term ,iea (X ■+ B or X-vu) Is directly proportional to ■M' y - / £3 [ e V- / } + f. e - ] \ / [ e i- e ^ e 1 ^ e ^ '-l where V=2BH/kT.• This expression contains no adjustable para­ meters. Slightly poorer agreement is obtained for the X •+ B transition in argon. 265

The sharp emission spectrum reported for CI 2 in argon"’ has some intriguing features when®magnetic field is applied. We have studied in detail the most strongly detected emission at 13600 cm-1 using 150 mW of Kr+ UV laser excitation. Charac­ teristic Zeeman profiles are seen (figure 3) with detected emitted light propagating parallel (P||) and transverse (Pj_) to an applied magnetic field; the spread of Zeeman energies is %48H. Strong temperature dependence of these profiles is seen in the range 1.5K to 20K consistent with Boltzmann popu­ lations of the Zeeman components of the excited state. How­ ever, no MCPE is seen to a limit of (IL-IR )/I ,<10 * where 1^ and IR are left and right circularly polarized emission in­ tensities. This lack of MCPE in a system where a Zeeman splitting can be resolved is inconsistent with an electric or magnetic dipole radiation mechanism. However, electric quadrupole radiation can have zero MCPE under such circum­ stances. We also see a strong magnetically induced linear polarization of emission (MLPE) in the transverse field ex­ periment, and this also is consistent with an electric quadru­ pole mechanism. We would like to acknowledge helpful discussions with Prof. L. Andrews with whom some of our magneto-optical work on noble gas monohalides is being done. This work was sup­ ported by the National Science Foundation.

REFERENCES

1. A.J. Thomson in Electronic States of Inorganic Compounds: New Experimental Techniques, P. Day editor, D. Reidel Pub. Co, (1975), pages 241-753.

2. M.A. Goetschalckx, R.L. Mowery, E.R. Krausz, W.C. Yeakel, P.N. Schatz, B.S. Ault and L. Andrews, Chem. Phys. Lett. (1977) - in press.

3. E.R. Krausz and P. McDonald, - submitted to J. Phys. E.

4. B.S. Ault and L. Andrews, J. Chem. Phys. 65_, 4192 (1976).

5. B.S. Ault, W.F. Howard, Jr., and L. Andrews, J. Mol. Spectrosc. 55^, 217 (1975); V.E. Bondybey and C. Fletcher, J. Chem. Phys. 64, 3615 (1976). 266

DEMIXING EFFECTS DURING MATRIX PREPARATION.

Bernhard MANN. Andreas BEHRENS Fachbereich Physikalische Chemie der Universitat, Auf den Lahnbergen, D-3550 Marburg/Lahn

In matrix spectroscopy the M/A ratio of the prepared gaseous mixture is usually given as a measure of concentration. For a quantitative treatment of absorption bands - for example for a calculation of equilibrium constants of molecular associations - both the exact concentration and the amount of the investigated species are needed in the matrix. Because of difficulties in measuring the concentration in the matrix and the partial pres­ sure of the components in the storage vessel during matrix prepa­ ration, the demixing and the change of the M/A ratio is calcula­ ted.

For a study of the influence of some parameters on the streaming velocity we adjust the gas flow by a suitable capillary instead by a needle valve.

For the decrease of pressure during the flow of a gas (visco­ sity , molecular weight M, temperature T) out of a volume v0 through a capillary (radius r , length 1) into a high vacuum Knudsen^ found the equation : a p + b • Ot(p) • p CD

(constant of laminar flow)

frt R T1 (l u h s tan L uf molecular 2 M 1 (Knudsen) flow)

(R = gas constant; ptfp) empirical)

For oC(p) = ot = const, eq. 1 may be integrated and we get:

P(t) ( 2 )

(P0 starting pressure) 267

With oc = 0.8 eq. 2 describes well the gas flow of He and Ar through a capillary (r = 0.075 mm, 1 = 7.5 cm) for p > 100 torr as fig. 1 shows. For smaller pressures oc becomes pres­ sure dependent until finally we have to put oc = 1 for very small pressures.

capillary: Fig. 1: Pressure d = 0.15 m m vs. time diagram I = 7 .5 cm for Ar and He for a flow through a theor. capillary with exp. r = 0.075 mm, 1 = 7.5 cm into a high vacuum.

IUU 5 10 15 20 25 30 35 — > t/h

To get a flow rate of 1 - 10 jjmol/min over a pressure range from 10 - 500 torr we have to use capillaries with radii of 0.03 - 0.1 mm if we take a constant length of 10 cm. With these capillary dimensions the amount of Knudsen flow is no more than 201 for a flow rate f > 1 pmol/min and a starting pressure pQ > 100 torr. Therefore the M/A ratio does not change by more than 101 in this region during spraying on for a molecular weight ratio of matrix gas to absorber of 0.5 - 2.

For lower pressures or very small flow rates the influence of the molecular flow increases. At pQ < 1 torr or f ^ 0.1 pmol/min we get a nearly pure Knudsen flow for the capillary dimensions mentioned above. Then demixing is given by

M/A (cryostat) = y M A/MM • M/A (gas) (3)

(M/A (gas): M/A ratio of gas mixture prepared in the storage 268 vessel; MA , MM molecular weights of absorber and matrix gas, respectively). Due to demixing another effect may occur: For a small gas mixing vessel one must spray on a considerable amount of the total gas available in order to get a good matrix. Therefore the process of demixing changes the concentration in the storage vessel and thus the concentration in the cryostat is influenced again.

Over the whole usual pressure range we can calculate the demi­ xing for all the effects described above by the following equa­ tions:

^(mom) = X • ^(gas) • (1 - N)X " 1 (4)

J(int) - SjCgas) 1 N ' D " (1 " N)X] (5>

M/A(mom) = momentary M/A ratio in the cryostat M/A(int) - integral M/A ratio in the cryostat N = NA/NA (gas) = turnover of absorber particles (NA = number of absorber particles sprayed on, NA (gas) = number of absor­ ber particles put into the storage cylinder at the beginning).

X = (Vm a /Mm)^, 8 empirical, 0 ^ 8 1 , 8 = 0 for a pure laminar flow (no demixing because of an average viscosity), 8 = 1 for a pure molecular (Knudsen) flow. 8 depends on the flow rate and molecular weights of the species used. For 8 = 1 (pure Knudsen flow) eqs. 4 and 5 are represented in figs. 2 and 3 for MA/M = 4, 2, 1, 0.5, and 0.2.

As figs. 2 and 3 show, the degree of demixing is given by ‘^MA/MM for a small turnover of particles. Because of a higher flowing velocity of the lighter molecules the concentration of heavier molecules increases in the storage vessel so that finally only the heaviest molecules are left. Therefore the M/A ratio in the cryostat is changed strongly. NX/NA(gas) NX/NA(gas)

Fig. 2: s. eq. 4 Fig. 3: s. eq. 5

For this reason it may be recommended to spray on a very small amount of the prepared gas mixture to keep the M/A ratio in the cryostat nearly constant. If only a very low vapour pressure of the species .to be investigated is available or if a very small deposition rate is wanted (e.g. at studies of metals in rare gas matrices), the use of a big gas storage volume is necessary. Another possibility to get a constant M/A ratio is to take two different gas streams which are regulated by separate special needle valves as described by Fredin^. This, however, is very difficult to achieve.

Lit. : 1) M.Knudsen, Ann. d. Phys. (4), 2J[, 75 (1909); W. Klose, Ann. d. Phys. (5), 1_1_, 73 (1931); R. Jaeckel, "Kleinste Drucke, ihre Messung und Erzeugung", Springer-Verlag, Berlin 1950

2) L.Fredin, Chemica Scripta j>, 193 (1974) . 2 7 0

The isolation of metal atoms in solid noble gas m atrices.

W . Schulze, H.U. Becker, H. Abe Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6,

D -1 0 0 0 Berlin 33, Germany

The isolation of silver atoms has been studied in dependence on the condensation parameters, i.e. the metal/gas ratio c, the condensation temperature T^ and the gas deposition rate ™gas* The yield of isolated atoms is measured by evaluating the integrated absorbancy J A (y*)d y (A = Ab- sorbancy Y* = Frequency) corresponding to the S—*P resonance transitions of silver atoms;

The yield of isolated atoms in dependence on the concentration c is shown in fig. la for different Xe gas rates and at a condensation temperature of 8 K, Generally it is observed that with increasing c the yield of isolated atoms de­ creases due to an increased aggregation of the metal atoms. Furthermore the gas condensation rate needs a minimal value of about 101® cm “*s * above which an optimum isolation of the metal atoms is found. At such a rate a quantitative isolation of the metal atoms can be achieved for metal/gas ratios below 10 **. The same efficiency of isolation can also be achieved for a lower gas rate but only at much lower metal/gas ratios. The upper curve (fig. la, b) presents a theoretical result / l / giving directly the probability that in a lattice containing two types of atoms A and B randomly distributed over the lattice sites, a type A atom will be totally surrounded by type B atoms. With increasing c an enhanced deviation between the theoretical and experimental values does occur - a reasonable result since in this calculation any inter­ action between the two types of atoms has been neglected.

The dependence on c and m for Kr and A is quite the sam e as for X e. E,**® 16 -1 -2 Especially these gases also need a minimal m^ag of about 10 s cm above which an optimum isolation of the metal atoms is found. Using such a rate and a condensation temperature of 8 K, the dependence on the concentration is identical for the three gases (cf fig.lb); i.e. the gas type doesn't influence the isolation behaviour. g. Yed f sltd ivr tm i dpnec o the on dependence in atoms silver isolated of Yield .l ig F yield of isolated atoms yield of isolated atoms 1.0 10 ol gss eA o gs ae i 10 i rates gas for Xe-A gases noble pe cre n oh iue i a hoeia result. theoretical a is figures both the in for and curve (a) upper rates gas Xe different for concentration =8K T T-= 8K 15 f r a c t i o n a l c o n c e n t r a t i o n c 61 -2 16-1 16 s ^cm ^cm s 2 () The .(b).

271 272

1.0 .8 .6 "Gas Xe

.2

1.0 T " 1

10 20 30 AO VK

F ig. 2 Yield of isolated silver atoms in Xe, Kr, and A layers for

gas rates X 10*® s *cm “ and a. concentration of at least -3 ■ 10 in dependence on the temperature of formation (a) in comparison with properties of pure gas layers, i.e. the density (b).

Fig. 2a shows the yield of isolated atoms in Xe, Kr, and A layer for m 1 0 - 1 - 2 -3 ^aa > 10 s cm and c < 10 in dependence on the temperature of forma­ tion. The shape of the three curves is identical. Only at condensation tempe­ ratures below about 0.13 of the respective matrix triple point temperature (Xe: 21 K, Kr: 15 K, and Ar: 11 K) a quantitative isolation can be achieved which is impossible at higher temperatures even at the lowest values of c and the highest values of rh . The yield of isolated atoms decreases with gas f increasing T, and becomes zero at T, « , 0.25 T (Xe: 38 K, Kr: 30 K, k k trp A: 22 K). 273

Ag — -

Ag Ag Ag, n m

JH u 2 < m K O. CO .« <

5000 4000 ^ 3000 2000

Fig.3 Absorption spectrum of silver atoms and silver clusters in a

Kr matrix (T. = 29.8 K, c = 2 -1 0 '2, m . = 1 .2 -1 017 cm"2), k Ag

Properties of pure gas layers in dependence on the formation temperature- are shown in figure 2b. The density has been measured by an optical method during the layer growth (fig. 2b). First of all these results clearly demonstrate that the matrix layers have a porous structure and that there exists a close parallelism with the isolation behaviour and the properties of the gas layers.

It is interesting to know what happens under experimental conditions where the yield of isolated atoms deviates from unity. Fig. 3 shows an absorption spectrum- of a Kr "matrix formed at 29.8 K, ih = 8-1016 s 1cm 2, c = 2-10 2 , m 17-2 - Ag = 1.2-10 cm . Besides the absorption lines of silver atoms, absorption bands of 5 independent molecular silver aggregates can be distinguished clearly. Some of these bands could be unambiguously assigned to Ag^-Ag^ using other measurements. An other interesting feature is the existence of a broad back­ ground absorption between -4000-3000 R with its peak maximum at approximately 274

3600 R. This absorption band is due to larger silver aggregates showing the collective phenomenon of plasma oscillations. It is obvious that two processes are operating, one leads to the isolation of atoms and to the formation of mole­ cular aggregates, whereas the other leads immediately to the formation of larger aggregates.

To explain these results qualitatively we propose a simple model using results concerning the porous layer structure and the nucleation behaviour of the noble gases.

I'Or practical purposes it is important that an appropriate choice of the con­ densation parameters (c < 10 3, ih V IQ16 s 1cm 2 and T, <. 0.13 T. ) ' gas •*“ k s trp' allows the formation of the matrix in minutes or even in seconds. The maximum rate of reactive species which can be isolated under the conditions given above isj about . . ln13 10 s -1 cm -2 .

A/ R.E. Behringer, J. Chem. Phys., 29 537 (1958) INTERNATIONAL CONFERENCE

ON

MATRIX ISOLATION SPECTROSCOPY

DISKUSSIONST AGUNG

DER DEUTSCHEN BUNSEN-GESELLSCHAFT

FOR PHYSIKALISCHE CHEMIE

SUPPLEMENT EXTENDED ABSTRACTS

WEST-BERLIN, GERMANY

JUNE 21-24, 1977 THIS PAGE

WAS INTENTIONALLY

LEFT BLANK 3

Chemical Reactions in Cryogenic Solids

b y

George C. Pimentel Chemistry Department University of California Berkeley, California 94720

The original motivation for developing the matrix

technique for use with infrared spectroscopic detection was to

prevent the chemical reactions that interfere with IR study of

free radicals and other transient species.^" In the course of

this development, it became clear that there were interesting

things to learn about chemical reactions when they do occur

under these unusual environmental conditions. The potentiali- 2 ties were only dimly foreseen in Pimentel's 1958 paper on

kinetic studies in matrix samples. Today, there is signifi­

cant activity on three frontiers connected with the progress

of chemical reactions of highly reactive species as they

take place in cryogenic solids. These frontiers, which will be considered in turn, are -

• Diffusion-inhibited reactions in cryogenic solids

• Infrared-induced reactions in cryogenic solids

• Chemiluminescent reactions in cryogenic solids

Diffusion-Inhibited Reactions

An early and still useful premise in matrix isolation

studies of free radicals was that reaction (recombination, if no other) would take place with very low or zero activation energy if diffusion took place.^ Hence, the success of the matrix isolation technique was predicated and proved to be dependent upon the possibility of avoiding diffusion-induced 9 contact between the reactive species under study. Pimentel- speculated on the possibility of studying kinetics in matrix samples for reactions for which there was a small, but sufficient activation energy to negate this premise. With a

1-3 kcal activation energy, perhaps the rate of the reaction would still be governed by the gas phase activation free energy ratlxei than by diffusion.

Only recently there have at last been a pair of matrix kinetic studies to examine. One of these, by Guillory and

Smith^, investigated the slow formation of N^O^ when nitric oxide was suspended in solid oxygen. They showed that the reaction actually occurred between NO dimers, N2°2' an<^ °2 from the surrounding cage. Temperature-dependent measurements led them to an activation energy of 103 calories. Of-course, the reaction of NO in the gas phase has a termolecular rate law, but the presently accepted mechanism involves NO^ as an intermediate rather than N 2 ° 2 • These new results must be relevant to the course of the reaction in the gas phase, though how is not yet clear.

4 Separately, T,uras and Pimentel examined the kinetics of the oxidation of NO to NO., by O^ in solid argon. Since the gao phase activation energy is either 2.4 or 4.2 kcal/mole

(depending on whether gound state or electronically excited

NO 2 is formed^) no reaction at 10-20°K is to be expected.

Nevertheless, slow growth of product NO 2 was observed in

NO:0^:N2 = 1:1:100 to 200 matrix samples in the temperature range 12 to 20°K. Isotopic studies showed definitively that oxygen atom transfer from ozone to nitric oxide was responsi­ ble. They did not, however, suggest a tunnelling mechanism. 5

The possibility of infrared or other photolytic excitation

was carefully excluded. The temperature dependence revealed

an apparent activation energy of 106 calories. This value is

fortuitously, laut nevertheless, startlingly close to that

obtained by Guillory and Smith for the (NO)2 + reaction.

Our present interpretation is that the reaction of NO + 0^ is

controlled, in the nitrogen matrix, by an orientational

activation energy barrier within the cage, opening an atom-

transfer reaction channel that is not observable under normal

reaction conditions because it is overwhelmed by more rapid

high-temperature channels. It is conceivable that this

result indicates a highly angular-dependent activation energy

for the reaction.

Infrared-Induced Reactions

Access to powerful and, soon, tunable infrared laser

sources makes it possible to excite molecules (including

matrix-isolated molecules) into selected vibrational states.

This dramatically reawakens the study of infrared-induced

chemical reactions in cryogenic samples. The conditions would

simulate selective, high temperature vibrational excitation in absence of corresponding translational excitation. It is to be expected that local functional groups in a complex molecule might be excited to reactivity while other, fragile parts of the molecule are held quiet. Such speculations are still on the horizon, but the unique possibilities for control of reactions warrants examination of a few of the pioneering w o r k s . The slow cis-»trans isomerization of nitrous "acid, HOMO in solid nitrogen at 20°K was first observed by Baldeschwieler and Pimentel® and then elucidated by Hall and Pimentel.7 The slow conversion of the cis- form to the more stable trans­ form was found to take place under infrared irradiation

(v=3650-3200 cm "*") and then the reaction can be reversed to give a random mixture through electronic excitation in the visible-UV region. Evidently, when the cis-HONO absorbs a quantum of excitation in the 0-H stretching mode, there is sufficient energy, sufficient coupling to the torsional mode, and sufficient lifetime before matrix deactivation to permit the isomerization to take place.

Another early detection of radiation-induced isomeriza­ tion chemistry was connected with the suspension of NC>2 in solid N2 by Baldeschwieler and Pimentel.® Clarification was Q finally provided by Varetti and Pimentel, who showed that NO and N02 together in a matrix cage could be converted through near-infrared irradiation, 7000-9000 A, from one structure, characterized by one set of infrared frequencies, to another structure characterized by an entirely different spectrum.

Once again, the matrix reaction prove to be reversible under electronic excitation, in the ultraviolet range 3700-4800 A.

A third study, again from the Berkeley laboratories, remains to be elucidated. Bondybey and Pimentel"*"® passed H2 and Ar or Kr through a glow discharge immediately in front of a cold window at 15°K. The resulting matrix sample absorbed at a single frequency which, by isotopic studies, was shown to be due to a species containing a single hydrogen atom. Pro­ longed infrared irradiation at the absorbing frequency caused the irreversible loss of the absorbing species, deuterium more slowly than hydrogen. There remains uncertainty about the absorber. Bondybey and Pimentel'*"® considered the possibility that H+ had been trapped, perhaps in the form of an ion, HAr+ o r H A r 2 + / but concluded that the H atom was more likely to be 11 12 neutral, as did Ogilvie. Mil-lig n and Jacox present arguments that an ion is involved, proposing HAr^+ . There seems to be no doubt that the absorbing species contains only one H atom and that its disappearance is promoted through its own infrared absorption.

The fourth example to be mentioned points more directly ' to the future in this type of study. In the Newcastle labora­ tories of J . J. Turner, a tunable, spin-flip infrared laser has been used to excite chemical reactions in matrix samples..

Only brief reference is needed here because this work is reported here separately by Dr. Poliakoff from this laboratory.

However, one of their recent publications'*"^ clearly displays the potentialities. Selective conversion of ^CO-labelled iron carbonyl complexes into new tautomeric species is effected with the tunable laser while non-resonant species are unaffected.

Plainly this is only the precursor to other types and examples from the Newcastle laboratory and from other laboratories as - the equipment becomes more generally available.

Chemiluminescent Reactions

Chemiluminescent reactions are of great interest because they display the role of electronic degrees of freedom'in the disposition of energy released by an exothermic reaction. This type of information is of particular current interest because of the active search for chemically pumped electronic lasers. It is to be expected that the matrix technique will prove to be specially informative in the study of chemiluminescent reactions. The cryogenic temperatures coupled with the relatively ineffective deactivation efficiency ,of solid inert gas environments should simplify and accentuate chemilumi- , nescent spectra obtained in this fashion. Forbidden transi­ tions normally not observed because of competing deactivation.. processes may become important. Furthermore, the reactants are surely known to be in the ground electronic state,ja specification sometimes uncertain for reactiye species usually found only in high temperature sources.

One,of the earliest matrix chemiluminescence studies was

14 that of.Goldfarb and Pimentel . These workers .photolyzed . diazomethane in matrix samples and the resultant solid, on , warming, produced a diffuse chemiluminescence in the red. spectral region. It was attributed to a highly forbidden transition of ethylene, formed from CH^ molecules as they were allowed to diffuse. The interpretation was supported by observed growth of infrared absorption due to but uu confirmatory evidence has since been_reported. . .

This type of study is now under much more active investigation in a number of laboratories. J. Fournier1"’ has coupled vacuum UV-excited.fluorescence studies with subsequent spectral study of the thermoluminescence observed on warming the matrix sample. Using the equipment of Fournier and Deson,

Lalo and coworkcrs16 examined the phosphorescence and thermo­ luminescence obtained through vacuum UV photolysis of S02 in 1 7 “ - ' ’ argon. Using similar techniques, Brom and Lepack observed an "afterglow" on photolyzing carbonyl sulfide in argon which 9

they attributed to OCS formed in a recombination reaction' between CO and ground state sulfur atoms. This same system was studied by Fournier and Deson1® and in the Berkeley

Laboratories by Long3^ and by Lee3®. Only the Berkeley inter­ pretations will be discussed briefly - they illustrate the potentialities of the method.

Photolysis of OCS in'solid argon at 10°K produces a suspension of ground state sulfur atoms S (^P^) . On warming the sample a few degrees, sulfur atom diffusion causes the formation of S2 in electronically excited states. The most 3 - 3 - 19 intense emission is due to the B I .->■ X t transition , g corresponding closely to the gas phase progression.' In the 20 ■ near infrared, two other are observed that have never been detected before, either in absorption or emission, by any technique. When a small amount of oxygen is added, the thermo­ luminescence becomes much brighter above 13°K and the phos­ phorescence spectrum of SG>2 is observed: (a 3B1-*-X .

Isotopic substitution verifies the identification and shows that ground state sulfur atoms will react with oxygen to produce SC>2 in this cryogenic environment.

Bibliography

1. E. Whittle, D . A. Dows, and G. C. Pimentel, J . Chem.

Phys. 22, 1943 (1954). '

2. G. C. Pimentel, J. Am. Chem. Soc. 80, 62 (1958) .

3. W. Guillory and G . Smith; Private communication.

4. D . Lucas, Ph.D. Dissertation, University of California,

Berkeley, California (1976). 10

5. M.A.A. Clyne, B. A. Thrush, and R. P. Wayne, Trans.

Faraday Soc. 60, 359 (1964). ,

6 . J. D. Baldeschwieler and G. C. Pimentel, J. Chem. Phys.

33, 1008 (1960).

7. R. T. Hall and G. C. Pimentel, J. Chem. Phys. 38, 1889

(1963). .

8 . J. D. Baldeschwieler, Ph.D. Dissertation, University of

California, Berkeley, California (1959).

9. E. L. Varetti and G. C. Pimentel, J. Chem. Phys. 55,.

3813 (1971).

10. V. Bondybey and G. C. Pimentel, J. Chem. Phys. 56, 3832

(197.2).

11. J. F. Ogilvie, J. Chem. Phys. 59, 3871 (1973).

1 2 . D. E. Milligan and M. E. Jacox,•private communication.

13. A. McNeish, M. Poliakoff, K. P. Smith, and J. J. Turner,

J. Chem. Soc. Chem. Comm. (1976) , -859.

14. T. Goldfarb and G. C. Pimentel, J. Chem. Phys. 33, 105

(1960).

15. J. M.-Fournier and J. DeSOil, j . P h ys. E. (in preoo) .

16. C. Lalo, L. Hellner, J. Deson, and C. Vermeil, J. de

Chemie Physique (1976), 237.

17. J. M. Brom and E. J. Lepak, Chem. Phys. Letters 41, 185

(1376) ,

18. J. Fournier, private communication. ;

19. S. R. Long and G. C. Pimentel, J. Chem. Phys. 66, 2220

(1977).

2 0 . Y. P. Lee and G. C. Pimentel (in-preparation). VIBRATIONAL SPECTRA, ISOTOPIC EFFECTS, FORCE CONSTANTS AND GEOMETRY

OF MATRIX ISOLATED BINARY AND MIXED ZINC DIHALIDES

A. Loewenschuss and A. Givan

Department of Inorganic and Analytical Chemistry

The Hebrew University, Jerusalem, Israel

The infrared matrix isolation spectra of the binary zinc dihalides have been reported previously (1). In the present study, Raman spectra

of these molecules as well as infrared and Raman spectra of all ZnXY

molecules (X,Y = Cl,Br,I) were recorded and analyzed. These spectra

are also the first experimental evidence for the existence of the latter m o l e c u l e s .

Samples were produced by the codeposition of a molecular beam of.;

e i t h e r Z n X 2 vapors or of a mixture of (ZnXj+ZnYj) vapors and a krypton

jet o n t o a liquid hydrogen cooled Infrared window or Raman sample holder.

Band frequencies and their assignments for all molecules studied are

summarized in Table 1. Isotopic effects due to either zinc or halogen ■

isotopes (or both) were resolved for all molecules.

The high quality of the observed spectra facilitate the unequivocal

assignment of all three fundamental vibrations of the monomeric molecules.

TVo stretching force constants (f and f for ZnX, , f and f for ® r r r 2 r s - ZnXY) as well as the bending force constant can be evaluated from the

average band positions. However, the availability of isotopic fine

structure as well as some calorimetrically measured entropy values (2 )

motivated us to extract additional spectroscopic parameters from the

experimental data, specifically the interactions force constant f

of the mixed dihalides and the bond-bond angle of all molecules. The

latter quantity is of interest as it has structural, bonding and thermo­

dynamic implications. It has been generally assumed that the Group IIB

dihalides are linear, but recently it has been pointed out (3) that

irregularities in force of the mercury and cadmium dihalides and

thermodynamic considerations suggest the possibility of deviations

from linearity.

The major difficulty in the determination of interaction force constants and angles is that these quantities are highly sensitive to small changes in frequency values. Therefore, rather than aiming for the 12

parameters which best reproduce the observed band positions a propram was designed which first selects all the parameter values of fr> fs> frs,a that reproduce the observed frequencies within experimental error (±0.75 cm"*) and then determines the values which reproduce best all the frequency differences between the isotopic components of a given band. Such compu­ tation reflects the experimental fact that the isotopic splittings can he evaluated with much better accuracy than band positions. The results of these computations are summarized in Table 2. Table 3 provides an example of the obtained fit between observed and calculated frequency and frequency difference values. Calculated entropies are listed in Table

1 and compared tn available experimental values.

ZnCIBr

4 8 0 4 7 0 4 6 0 c m ' 1 /

I

223 220

£jgj_2. Isotope effects in infrared Fig. 1. Isotope effects in spectrum of v(Zn-Cl) of ZnClBr Raman spectrum'of V] (ZnClg) a) recorded spectrum' b) computer and V](ZnBr2). simulation. 13

Table 1. Vibrational Frequencies and Assignments of ZnX^ Fundamentals

(Kr Matrix)

M o l e c u l e Ram a n A s s i g n m e n t I n f r a r e d Assignment .

357.6 vi (Zn 3 5 C l 3 5 Cl) 50 8 . 5 * v 3 (6 4 Z n 3 5 C l 3 5 Cl) 352.5 u 1 (Zn 3 5 C l 3 7 Cl) 505.0* v 3 (6 4 Z n 3 5 C l 3 7 Cl) 347 . 5 • v , ( Z n 3 7 C l 3 7 Cl) 50 2 , 0 * v 3 (6 4 Z n 3 7 C l 3 7 Cl) Z n C l 2 504.6* v 3 (6 6 Z n 3 5 C l 3 5 Cl) 50 1 . 3 * v 3 (6 6 Z n 3 5 C l 3 7 Cl) 4 9 8 . 0 * v 3 (6 6 Z n 3 7 C l 3 7 Cl)

103 . 0 v2 ( Z n C l 2 ) 100 . 3

224.0 v i ( Z n 7 9 B r 7 9 Br) 4 0 4 . 0 v 3 (6 4 Z n B r 2 ) 7 2 2 2 . 6 v r ( Z n 7 9 B r 8 1 Br) 39 9 . 5 v 3 (6 6 Z n B r 2 ) n 2 2 2 1 . 2 V ! ( Z n 8 1 B r 8 1 Br.) 395.5 V3 (6 8 Z n B r 2 ) 7 1.0 v 2 ( Z n B r 2 )

163.10 V l ( Z n l 2) 346.0 v 3 (6 4 Z n I 2 ) 342 . 0 V3 f6 6 Z n ! 2) Znl 339.7 v 3 (6 7 Z n I 2)

337.9 V3 (6 8 Z n I 2 ) 6 1 . 0 v 2 ( Z n l 2 )

4 7 3 . 2 v 3 (6 4 Z n 3 5 C l B r > . 4 6 9 . 6 V3 (66Zn35CiBr) 4 6 7 . 6 V 3 (6 4 Z n 3 7 ClBr) Zn C l B r 4 6 4 . 0 v 3 (ZnClBr) 4 66 . 1 v 3 (6 8 Z n 3 5 ClBr)

4 6 3 . 8 v 3 (6 6 Z n 3 7 ClBr) 4 6 0 . 5 v 3 (6 8 Z n 3 7 ClBr) 27 3 . 8 vi(Zn C l B r ) 2 7 5 . 0 v i (ZnClBr) 8 8 . 7 . • v 2 (ZnClBr)

4 6 2 . 9 v 3 (6 4 Z n 3 5 ClI) # 460 v 3 ( Z n d I ) 4 6 0 . 5 v 3 (8 6 Z n 3 5 ClI) # 456 . 1 v 3 (8 4 7 n 3 5 ClI) 454. 3 v 3 (6 6 Z n 3 5 ClI) ZnClI 4 5 1 . 7 v 3 (6 4 Z n 3 7 ClI) 4 4 8 . 6 v 3 (6 6 Z n 3 7 ClI) 4 4 5 . 6 v 3 (6 8 Z n 3 5 ClI) 229.5 vi(ZnClI) 228 V! (ZnClI) 8 0 . 8 v 2 (ZnClI) 79.0 v 2 (ZnClI)

378 V 3 (ZnBrI) 3 78 . 8 v 3 (6 4 Z nBrI) . 190.4 v i ( Z n 7 9 BrI) . 373,9 v 3 (6 6 ZnBrI) , ZnBrI 189.6 v j ( Z n 8 3 BrI) 3 7 0 . 3 v 3 (6 8 ZnBrI) 6 4. 0 v 2 (ZnBrI) 65,0: .v 2 .(ZnBrI)

* F r o m r e f e r e n c e 1 # thermally unstable site 14

Table 2. Force Constants'(md/X) and Bond Bond Angles of ZnX^ and ZnXY Molecules

M X Y f a M-X *M-Y fM X , MY f a Zn Cl . Cl 2.59 0.05 0.0504 180 Zn Br Br 2.27 0.08 0.0344 180 Zn I T 1.83 0 ' 0.0310 162.5 Zn Cl Br 2.585 2.2 0.02 0.0469 159.5 ‘ Zn Cl I 2.54 1.85 0.048 0.0392 163.7 Zn Br I 1.96 2.16 0 0.0302 172.5

Table 3. Isotopic Effects in ZnClBr Zn Cl Br '’zn-Ci vZn-Br ubs. A calc. A obs. calc.. 64 35 79.904 473.2 472.75 3.6 3.66 66 35 79.904 469.6 469.09 2,1 2.06 64 37 79.904 467.5 467.03 1.4 1.39 68 35 79.904 466.1 465.64 2.3 2.35 66 37 79.904 463.8 463.29 3.3 3.53 68 37 79.904 460. 5 459.76 65.37 35.453 79.904 273.1 273.76

(2) Table 4. Calculated and Experimental ' Entropies (e.u..) of ZnX2 and ZnXY Molecules

Molecule T(°K) S(calc.) S(exp.) Molecule T("K) S(calc.)

ZnCl. 798 7 66. 17 65.1x1,7 Znl., 298.2 77.85 777 79.86 80.1x1.5 1000 94.44 1000 83.58 ZnClBr 298.2 72.99 Zh Bta 298.7 77.. 1 3 1000 89.34 768 85.89 86,1+2 ZnClI 298.2 74.44 1000 091 80 1000 90,86 ZnBrI 298.2 75 . 82 1000 92.35

References (1) A. Loewenschuss, A. Ron and 0.'Schnepp, J. Chem. Phys. 49, 272 (1968). (2) D. Cubicciotti and H. Eding, J. Chem. Phys. AO, 978 (1964); L. 0. Polyachenok, K . Nazarov and O.G. Polyachenok, Zh. Fiz. Khim. 50, 2120 (1976).

(3) A. Givan and A. Loewenschuss, J. Chem. Phys. 6 £, 1967 (1976), 6 5 , 1851 (1976); A. Strull, A. Givan and A. Loewenschuss, J. M o l . Spectrosc. 62^, 283 (1976). 15

GENERATION AND ESR STUDY OF INTERMETALLIC MOT,HOULES AgM (M = GROUP II METAL)

Paul H. Kasai and D. McLeod, Jr. (Union Carbide Corporation, Tarrytown Technical Center, Tarrytown, N.Y. U.S.A.)

The nature of metal-metal bonds found between pairs of metal atans in complex compounds^ as well as those existing between ligand-free metal 2 atoms has been the subject of many recent investigations. We report here the generations in argon matrices and observation by electron spin reso­ nance (ESR) spectroscopy of a series of intermetallic diatonic molecules AgM (M = Mg, Ca, Sr, Ba, Zn, Cd, Hg).

The cryostat-spectrcmeter assembly that would permit trapping of high temperature vapor phase species in a rare-gas matrix at -4°K, and observa­ tion of the resulting matrix by ESR has been described earlier^. In the present series of experiments the Ag atcms were vaporized from a resistively heated tantalum cell and were trapped in argon matrices together with the . atons of other metal independently vaporized frcm the second cell. The frequency of the spectrometer locked to the sample cavity was 9.41 GHz and all the spectra were obtained while the matrices were maintained at ~4°K.

The molecular synmetry of AgM dictates that their ESR spectra be com­ patible with the spin Hamiltonian of the form:

+ A„ I S .+ A. (I S + I S ) II zz -L- x x y y

+ a' i's + (i's + i ' s ) (1 ) II 2 2 -L X x y y where A u and A_,_ represent the hyperfine coupling tensor to the Ag nucleus, and the last two terms involving Aj'j and A^_are to be added when the atcm M possesses a magnetic nucleus.

The ESR spectrum of Ag atoms (4d'*"®5s1) isolated in an argon matrix is 4 known . It consists of two.sets of sharp isotropic doublets with the respec­ tive spacings of -650 and 750 G attributed to the couplings to 1(^A g 1 0 9 (natural abundance = 51%, I = 1/2, y =-0.1130^) and Ag (natural abundance = 49%, I = 1/2, y = -0 .1 2 9 9 ^ ). 16

The ESR spectra of argon matrices containing Ag and the atocms of the group II element M shewed, in addition to the sharp doublets due to the Ag atoms described above, two additional sets of doublets attributable to the intermetallic diatonic species AgM. The intensities of the AgM doublets relative to those of Ag atans were in the range of 2 - 5%. For AgMg, AgCa, AgSr, and AgBa the doublet spacings due to the Ag nuclei were found to be considerably smaller «400G) than those of the Ag atons, and the spectra appeared essentially isotropic. For AgZn, AgCd, and AgHg the doublet spacings were close to those of Ag and the spectral patterns were readily recognized as those expected frcm randomly oriented paramagnetic species having an axially synmetric spin Hamiltonian. In the case of AgCd satellite signals were noted and recognized as the perpendicular components of the spectra due to AgCd possessing ^ ^Cd (natural abundance = 13%, I = 1 /2 , li = -0.5922^) or 119 Cd (natural abundance = 12%, I = 1/2, |i = -0.6195ft.) nucleus. In the case of Ag Hg the perpendicular components 199 of AgHg possessing Hg (natural abundance = 17%, I = 1/2,y =. 0.4933^) or 201Hg (natural abundance = 13%, I = 3/2, y = -0.607^) nucleus were observed. The corresponding parallel components of these species were too weak to be observed.

In each case of AgM discussed above the large hyperfine coupling inter­ actions with the magnetic nuclei prevented the accurate evaluation of the spin Hamiltonian parameters using the usual seoond-order solutions. The g and the hyperfine coupling tensors of AgM were, therefore, determined from the observed signal positions resorting to the exact diagonalization of the Hamiltonian (1). The results are given in Table I. The Ay 's of the magnetic Cd and Hg nuclei were evaluated frcm the observed perpendicular components of the relevant species. This is possible because, when the magnetic field is perpendicular to the symmetry axis, the off-diagonal elements of the

Hamiltonian (1) are related to Ay + A_]_ ana Ay - a ' The large un­ certainties indicated for the Ay 's are due to this indirect approach.

The coupling tensor to the Ag nucleus of AgM was thus found to be com­ pletely isotropic in each case. The coupling tensors to the ^ ^ C d and 199Hg nuclei are also essentially isotropic. For Ag atans isolated in an argon 107 matrix the coupling constant to Ag has been measured to be 1.809 GHz. Using the Goudsmit's relation® and the known coupling constants of Ag 4 and Au atans , the isotropic coupling constants of unpaired electrons localized 111 199 respectively, in the valence s orbitals of Cd and Hg are estimated to be 12.5 and 40.0 GHz. 17

TABLE I

SPIN HAMILTONIAN PARAMETERS OF AgM

a a -/ c M I.P. of M A „ (=Ax )b a ' c g ll 9J- till AJ_ (eV) (GHz) (GHz) (GHz)

Mg 7.64 2.0001 0.837 Ca 6.11 1.9982 0.435 Sr ■ 5.69 1.9823 0.372 Ba 5.21 1.9538 0.218 Zn 9.39 2.0025 1.9905 1.324 Cd 8.99 2.0014 1.9711 1.327 2.18 .1.99 (+0.25) (+0.03) Hg 10.43 1.9958 1.9136 1.562 3.13 2.52 1*0.20) (40.03)

Ag atom 7.57 1.9998 1.809

(a) Accuracy: + 0.0002. For M = Mg, Ca, Sr, and Ba the observed spectra were analyzed as being isotropic.

(b) Coupling to 1®7Ag. ' Accuracy: j_ 0.003 GHz , . „ ^ 111,,. 199, (c) Coupling to Cd or Hg.

I 18

In the present series of AgM the unpaired electron should occupy the orbital given essentially by an antibonding combination of the valence s orbitals of Ag and M.

4> = a <|>Ag (5S) - b $M (nS)

The coefficients a and b may be estimated frcm the usual ICAQ-MO approxi­ mations using the ionization potentials of the atans involved. The ioniza­ tion potentials of M are also listed in Table I. The isotropic coupling constants to the Ag, Cd and Hg nuclei estimated by this schene are in reasonable agreement with the observed values. The small but clearly 111 199 resolved anisotrpies of the coupling tensors to the Cd and Hg nuclei indicate admixture of the valence pz orbital of the atcm M, however. Such admixture would also account for the deviations of the observed g tensors from the spin only case (g = 2.0023). The magnitude of the deviation in­ creases with the increasing atonic number Z of the atom M.

REFERENCES

1. F. A. Cotton, Accounts Chem. Res., 2, 240 (1969). 2. K. A. Gingerich, J. Cryst. Growth, 9, 31 (1971). 3. P. H. Kasai, E. B. Whipple, and W. Weltner, Jr., J. Chem. Phys., 2581 (1966). 4. P. H. Kasai and D. McLeod, Jr., J. Chan. Phys., 55, 1566 (1971). 5. See, for example, Ref. 4. 6. S. Goudsnit, Phys. Rev., 43, 636 (1933).

<