GAS-PHASE ION CHEMISTRY OF Mg+', (C - C5H5)Mg"and (C - C5H&Mg+': THEORETICAL AND EXPE-NTAL STUDIES
REBECCA K. MILBURN
A thesis submitted to the Faculty of Graduate Smdies in the partial fulfillment of the requirement for the degree of
Doctor of Philosophy
Graduate Programme in Chernistry
York University
Toronto, Ontario, Canada
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The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts £tom it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. + + Gas-Phase Ion Chemistry of Mg , (C-C~H~)M~and
(c-C H M~+: and Experimental 55L) , Theo~etical Seudies
by Rebecca Katherine Milburn
a dissertation submitted to the Faculty of Graduate Studies of York University in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
0 1999 Permission hzs been granted to the LIBRARY OF YORK UNIVERSITY to lend or seIl copies of this dissertation, to the NATIONAL LIBRARY OF CANADA to microfilm this dissertation and to lend or seIl copies of the film, and to UNIVERSITY MICROFILMS to publish an abstract of this dissertation. The author reserves other publication rights, and neither the dissertation nor extensive extracts from it may be printed or otherwise reproduced without the author's written permission. The gas-phase CO-ordination of Mg*, (c-C5H5)MgC and (c-C~H~)~M~in their electronic ground States with a variety of inorganic molecules, cyanides, and sanuated and unsaturated hydrocarbons has been investigated using experimentai and theoretical techniques. Reaction rate coefficients and product distributions were measured with the Selected-Ion Flow ~ub&ollision Induced Dissociation
(SIFï/CID) technique operating at (294 c.3) K and a helium buffer gas pressure of
(0.35 I0.01) Torr. The results of rnulti-CID studies give insight into bond comectivities and suengths. Molecular orbital caiculations have been performed on a variety of magnesium-contauiuig neutrals and cations in order to determine smictural and themochemical information. For my dad, 1 love you. 1 have a lot of people to thank that have helped me achieve this goal. Most importantly are my mom and dad. Thanks mom and dad! 1would like to express my appreciation to Professor AC. Hopkuison for his patience and support without which
1might not have even started graduate school. 1 would also like to sincerely thank
Professor D.K. Bohme for aLl the vatuable guida&e and for allowing me to expand my education in an experimentai settbg. 1have also been fortunate to be helped by a lot of great people; Professor LM. Goodings, Dr. C.F. Rodriquez, Dr. V.I. Baranov,
Dr. Y. Ling, Dr. A-E. Ketviais, Dr. J. Sun, Aiwin Cunje, Steve Qum, Tamer Shoeib,
Greg Ko yanagi, Doina Caraiman, Diana Sargla, Rob, Claire and Avi. LIST OF PUBLICATIONS
Gas-Phase Co-ordination of Mgt, (c-CsH5)Mg' and (c-c5H&~gCwith Saturated Hydrocarbons- Milburn, R.K.; Hopkinson, AC; Bohme, D-K. J. Phys. Chern. A (submitted)-
A Smdy of complexes of Mg(NH3)." and Ag(NH3),,', where n = 1-8: Cornpetition Between Direct Coordination and Solvation Through Hydrogen Bonding. Shoeib, T-; Milbuni, R.K.; Koyanagi, G.I.; Lavrov, V.V.; Bohme, D.K.; Siu, K.W.M.; Hopkinson, A.C. (in press).
Themochemical Properties of Molecules and ~ahonsof MgN,H, (n=1,2 and m=l- 6): Enthaplies of Formation, Proton Minicies and Ionization Energies. Miiburn, R.K.; Hopkinson, AC.; Bohme, D.K. (in preparation).
Sm&-ring Carbenes Carrying a Positive Charge: the Effects of Substituting by CN in c-Ca3+. Milburn, R.K; Hopkinson, A.C.; Bohme, D.K. Int. Mass Specrrom. (in press).
Dimer Cations of Cyanoacetylene: Theoretical Isomers and Their Laboratory Production in the Absence and Presence of cao2f Implications for InterstellarICircumstek Chemistry. Milburn, R.K.; Sun, I.; Baranov, V.I.; Hopkinson, A.C.; Bohme, D.K. Phys. Chem. A 1999,103,7528.
Gas-Phase Co-ordination of Mg, (c-C5H5)Mg+ and (C-C~H~)~M~+with Srnail Inorganic Ligands. Milbum, R.K; Baranov, V.I.; Hopkinson, A.C.; Bohme, D.K. J. Phys. Chern, A 1999,103,6373.
Experimentai and Theoretical Studies of the Basicity and Proton-Affinity of Si& and the Structure of SiFX. Ling, Y.; Milburn, R.K; Hopkinson, AC.; Bohme, D.K. J. Am. Soc. Mass Spectrom. 1999, 10, 848.
Magnesium Chemistry in the Gas Phase: Calculated Thennodynamics Properties and Experimental Ion Chemistry in Hz-Oz-Nt Flames. Chen, Q.F.; Milbum, R.IC ; Hopkinson, AC.; Bohme, DK; Goodings, J.M. Int. J. Mass Spectrom. Ion Processes 1999,184,153.
Sequential Ligation of Mgf, Fe+, (c-C5Hs)Mg', and (c-c5H5)Fe+with Ammonia in the Gas Phase: Transition from Coordination to Solvation in the Sequential Ligation of Mgt. Milburn, RK; Baranov, V.I.; Hopkinson, AC.; Bohme, D.K. J. Phys. Chern. A 1998,102,9803.
vii Destabilised Carbocations: A Cornparison of the CtH&SC and c2H4NOf Potential Energy Surfaces. Rodrïquez, CF.; Vuckovic, D.Lj.; Milburn, R.K-; Hopkinson, A.C. J. Mol. Struct., Theochem, 1997,401,117.
Theoretical Enthalpies of Formation of N'&Cl,: Neutra1 Molecules, Cations and Anions. Milbum, RK;Rodrïquez, CF.; Hopkinson, A.C. Phys. Chern. B 1997, 101, 1837. TABLE OF CONTl3NTS
Page
Abstract iv
Acknowledgements vi
List of Publications vii
List of Tables xiv
List of IUustrations xvi
METHODS OF INVESTIGATION
2.1. Experimental Methods
2.1.1. IonSource
2.1.2. Quadmpole Mass Analzyer 19
2.1.3. Venturi Gas Aspirator
2.1 -4. Gas Inlet S ystem
2.1.4.1. Carrier U-aç Flow
2.1 .M. Neutral Reactant Gas 23
2.1.5. Multi-Collision Induced Dissociation 25
2.1.6.1. Reactions with no Si3-cant Reverse or 26 FherReaction 2.1 -6.2. Higher Order Reaction with no 5i-cant Reverse Reaction 2.1.6.3. Reaction with Si,anificant Reverse Reaction
2.1 -7- References
2.2- Theoretical ,Methods
2.2.1 - The Schr6dinger Equation
2.2.2. Hartree-Fock Theory
2.2.3. MQller-Plesset Theory 40
2.2.4. Coupled Clusters and Confiovation Interaction 41 Theory
3-25 Density Functionai Theory 42
2.2.5.2. Becke-3 Lee-Yang-Pan (B3LYP) 45
2.2.6. Basis Sets 46
2.2.7. Thermochemical Corrections 47
2.2.8. References 49
3. INORGANIC MOLECULES (H7, m, H20, &, CO, NO, 02¶COt, NO, NO3 3.1 - Introduction 50
3 2. Results and Discussion 55
Reactions with MgC 55
Reactions with (c-~sHs)Mg+ 62
Reactions with (c-CsHs)lMg" 80
Variation in the Rate of Ligation with the Number 83 of (c-CsH5)Ligands 3.2.5, Variation in the Rate of Ligation with the Nwnber 84 of Ligands, L
3.2.6. Structure and Bonding 87
3 -3. Conclusions 91
3 -4. References
4. SATURATED, UNSATURATED AND CYCLIC ENDROCARBONS
4.1. Introduction 96
4.2. Results and Discussion 98
4.2.1. Saturated Hydrocarbons
4.2.2. Unsaturated Hydrocarbons
4.2.3. Cyclic Hydrocarbons
4.3. Structure and Bonding 124
4.4. Variation in the Rate of Ligation with the Number of Ligands 126
4.5. Conclusions 128
4.6. References
S. CYANIDE AND ISOCYANIDE MOLECLXES
5.1. Introduction
5 2. Results and Discussion
5.2.1. Reactions with HCN
5.2.2. Reactions with CH3CN and CHaC
5.2.3. Reactions with H2C=CHCN
5.2.4. Reactions with HC* xi 5.3. Strucnue and Bonding of Mg+/HC3N 147
5.4, Possible Mechanism for the Formation of Cyclooctatetraene 162
5.5. Conclusions 166
5 -6. References 169
THERMOCHEMICAL PROPERTIES OF MgN& AND MgOnH,
6.1. Introduction 170
6.2. Results and Discussion 17 1
6.2.1. Structural Details 17 1
6.2.2. Themochemical Properties
6.2.2.1. Enthdpies of Formation
6.2.2.2. Proton Affinities
6.2-2-3. Ionization Energies
6.2.2-4. Binding Energes
6.3. Conclusions 190
6.4. References 19 1
DIMER CATIONS OF CYANOACTYLENE: A THEORETICAL STUDY
7.1. Introduction 192
7.2. Results and Discussion 193
7.2.1. Review of Previous Experimental Results 193
7.2-2. Theoretical Results 197
7.3. Conchsions 212
7.4. References 217 8. FUTURE WORK
8.1. Future Work
8.2. References
9. APPENDIX
9.1. List of Illustrations and Tables LIST OF TABLES
Table Page
3.1. Measured rate coefficients for the reactions of ground states of Mg*, (c-CsHs)Mg+ and (c-C5H5)?Mf with selected inorganic ligands.
3.2. Room temperame equilibrium constants for ligation reactions with selected inorganic ligands.
3.3. Computed total elecnonic energies at B 3LYW6-3 l+G(d), vibrational zero-point energies and thermal energies of Mg(N&)n*, where n = 1-6.
3 -4. Cdculated binding enthalpies at 298 K at B3LYP/6-3 1+G(d) of Mg(NH3)n*, where n = 1-6.
3.5. Standard enthalpies of formation for (directly metal coordinated) adducts Mg(NH3)n*, where n = 1-6 as calculated at B3LYP/6-3 l+G(d).
4.1. Measured rate coefficients for the reactions of ground states of ~g*, (c-CsH5)~g+and (C-C~H~)~M 4.2. Room temperature equilibrium constants and standard free-energy changes for the ligation reactions involving selected hydrocarbon ligands. 4.3. The coordination numbers of Mg", (c-csH5)MgCand (c-CsHs)?Mg* for a variety of hydrocarbon ligands. 5.1. Measured rate coefficients for the reactions of ground states of Mgf, (c-C5H5)Mg+and (c-CsH5)?MJ with selected organic nitriles and rnethyl isocyanide. 5.2. Total energies, zero-point vibrational enerpies and thermal corrections for several isomers of Mg(HC3N),". 6.L Total energies, zero-point vibrational energies and thermal corrections for several cations and neutrals of MgN,H, and MgOnH,. xiv Standard enthalpies of formation at 298 K for several cations and neutrals molecules of the general formulae MgN,H, and MgOnH,- Proton affinities at 298 K for neutral molecules of the generd formulae MgN,Hm and MgOnHm. Ionization energies at 298 K for neutral molecules of the pneral fonnulae MgNnHmand MgO,H,. Calculated binding energies at 298 K for several cations and neutral molecules of the general fodaeM&Hm and MgOn&. Effective bimolecular rate coefficients for the reaction of ~60~~with HC*. Effective bimolecular rate coefficients for the reaction of HC~Wwith HCa. Total energies, unscaled zero-point vibrational energies thermal energies and relative energies for several isomers of (HC3N),+ as calculated at B3LYW6-3 li-G(d). Total energies and relative energies at B3LYW6-3 1l++G(Zdf,p) for several isomers of (HC$J),*. The calculated bond energies at 298 K from the parent ion, smcture 1, at B3LYP/6-3 I+G(d) and B3LYP/6-3 1I+iG(2df,p). The caiculated bond energies at 298 K from the parent ion, structure II, at B3LYP/6-3 l+G(d) and B3LYW6-3 1 Ii+G(2df,p). The caiculated bond energies at 298 K fiom the parent ion, saucture III, at B3LYP/6-3 l+G(d) and B3LYW6-3 11 W(2df,p). The total energies of the dissociation products at B3LYW6-3 1+G(d) and B3LYP/6-3 11++G(2df.~l. LIST OP ILLUSTRATIONS Figure Page SIFTICID apparatus. 16 Ion source for the SFUCID apparatus. 18 Mapesocene probe for the ion source. 18 The quadrupole geometry, coordinate system and potential bias. 19 Stability diagram for a mass quadrupole analyser. 21 Reaction profile for the reaction of (c-C5Hs)Mgf with N2. 27 Reaction profüe for the reaction of (C-C~H~)M~+with ninic oxide. 29 Reaction profde for the reaction of (c-c~H&M~*reaction with 32 1-hexene. Equilibrium plot for the reac tion of (c-~~H~)lMg*with 1-hexene. 32 Reaction profile and mulri-CID for the reaction of M$' with W. 59 Reaction profile and multi-CID for the reaction of (c-CsHs)Mg+ 63 with N?. Reaction profiles of (c-C5H5)Mg(IJC with L = CO, NO, COI, NtO. 64 Equilibrium plots for the reactions of (C-C~H~)M~(L)+with 66 L = CO, NO, COz, N20. Multi-CID'S for the reaction products of (c-C5H5)Mg(L)*wiih 68 L = CO, NO, CO2, N20. Reaction profile and rnulri-CID for the reaciion of (c-CsHs)Mg' 69 with NO2. Reaction profile and rnulti-CID for the reaction of (c-CsHs)Mgf 71 with H20. xvi 3.8. Reaction profile and multi-CID for the reaction of (c-C5H5)Mg+ with MI3. 3.9. Optimized structures for several isomers of Mg(NH$,+', where n = 1-6. 3.10. Reaction profiles of (C-C~H~)~M~+with H20 and MI3. 3.1 1. The rates of ligation of (c-C&)Mg' with a variety of inorganic ligands* 3.12. Some plausible structures for (c-C~H~)M~(L)~+,where L is an inorganic ligand. 4.1. Reaction profiles for the reactions of MgC with C3&, C&Ilo, C5H12 and -14- 4.2, The primary rate coefficients for ligation of several saturated molecules vs. the degrees of fieedom in the ligating molecule for Mg" and (c-C5H5)Mgf. 4.3. Mulri-CID's for the reaction products of Mg+*with C&, Calo, 106 CSHI?and CsHi4. 4.4. Reaction profiles for the reactions of (c-CrH5)Mg"with Cl&, CZ&, 107 C3& and C&IO- 4.6. The primary rate coefficients for ligation of several unsaturated 111 molecules vs. the degrees of frzedom in the ligating molecule for MgC and (c-C5Hs)MgC. 4.7. Reaction profiles for the reaction of Mg+' with three isomers of hexene. 113 4.8. Multi-CID'S for the reaction products of Mg"' with the three isomers 114 of hexene. 4.9. Reaction profdes for the reaction of (c-CsHS)MgCwith three isorners 115 of hexene. xvii Multi-CD's for the reaction products of (C-C~H~)M~+with three isomers of hexene. Reaction profiles of (C-C~H~)~M~*with three isomers of hexene. Multi-CID'S for the reaction products of (c-C5H5)&lgC with three isomers of hexene. Reaction profde and multi-CID for the reaction of (C-C~H~)~M~* with benzene. Reaction profile and equilibrium plot for the reaction of Mc with c-CaI2. Qtimized structures for adducis of various hydrocarbon molecules with MgC. Reaction profile and mulri-CID for the reaction of Mg" with HC3N. A sernilogarithmic plot of the rate coefficients vs. the nurnber of ligands for the reaction of MgC with HC3N. Reaction profde and multi-CID for the reaction of (c-csHs)MgC with HC3N A semilogarithmic plot of the rate coefficients vs. the number of ligands for the reaction of (c-C5H5)Mg+with HC*. Reaction profüe for the reaction of (C-C~H~)~M~"with KC3N. . Optimized geometrical parameters for structures at severai critical points 150 on the Mg(HC3N)n surface from B3LYP/6-3 I+G(d) caieulations. A possible mechanism for the formation of the 1,2,5,6-temcyano- 163 1,3,5,7-cyciooctatetraenemagnesium ion in the gas-phase . A few possible themochemical pathways for the reactiora of Mgt* 167 with HC3N. Optimized geomenical parameters from B 3LYP/6-3 1+GCd) 174 (top numbers) and MP2(full)f6-3 1l++G(d,p) (bottom numbers) calculations for several molecules and cations of MgOnHm- Oprimized geometricai parameters from B3LYP/6-3 l+G(d) 175 (top numbers) and MP2(full)/6-3 1l++G(d,p) (bottom numbers) calculations for several molecules and cations of MgN,H,. Schematic illustrating the determination of the enthalpy of formation 180 of MgO(g). (Right side) Reaction profile for the reaction of ~so'+with HC*. 196 (Left side) Reaction profile for the reaction of HC3rwith HCsN. (Righr side) Mulri-CID of dimer of (HC3N)2" produced fiom the 198 reaction of c6$+with HCD. (Lefi side) Multi-CID of dimer of (HC3N)T produced fiom the reaction of HC~~with HC3N. Optimized stmctural parameters of several dimers of cyanoacetylene 200 at B 3LYP/6-3 1+G(d) (top numbers) and B 3LYW6-3 11 i+G(Zdf,pp) (bottom numbers). Relative energies for the structures at minima on the potential energy 201 surface for (HC3N)2? A possible pathway to the formation of dimer cation III of cyanoacetylene. A possible mechanism for the formation of the trans-cyclic and hear dimers of cyanoacetylene. CHAPTER 1 INTRODUCTION Magnesium is a member of the aikaline earth metal family and was first recognized as an element in 1755. It is the eighth most abundant element in the earth's cmst. Magnesium is not found in nature uncombined, but exists in large deposits in the form of magnesite, dolomite, carnaIlite, brucite and other minerais. The pure metal is prirnarily produced by electrolysis of fiised magnesium chionde derived from seawater. Pure magnesium is light and silvery white and when finely divided readily ignites upon heating in the air and bms with a dazzling white flame. The chemistry of magnesiurn is diverse and wide-ran%n,o. It plays a centrai role in the biochemicai health of ail living orgmisms, but it is also involved in the chemistry of the earth's atmosphere and those of other celestid bodies. Synthetic organic chemists use magnesium in Grignard reagents, RMgX, which are classical compounds that allow chemists to convert two organic molecules into one Iarger one through nucleophilic addition of the incipient R- ion. Commercially, magnesium is also co~mnonlyused in pyrotechnies, flashphotography and in ailoys for airplanes and missile construction. With all these applications the chernical understanding of magnesiw in the solution phase is well established, but our knowledge of its chemistq in the gas-phase is fairly limited. This study will expand the understanding of the chemistry of Mg* in the gas-phase, using both experïmental and computational studies. 1.2. Metai Ions The gaç-phase binding affhities of metal cations to a variety of ligands have become the focus of many recent experimental and theoretical stuclies.' SpecEcaUy, the gas-phase bondhg patterns and geometrïes of MgU containing molecules are currently the topic of much investigation?' The interaction of Mg* with many ligands is very weak and considered to be primarily electrostatic in nature. Calculations indicate that the electrostatic bond is approximately 2 A in length with a bond stren,@ in the range of 5 to 50 kcd mol". In addition, caldations on higher order adducts show that as the number of ligands is increased the binding energy decreases. The MgU binding &inities for a variery of alcohols, aldehydes, ketones, aromatics and inorganic Ligands have been investigated?-' The calculated bond strengths are consistent with an electrostatic association between the metal ion and the ligand. Calculations demonstrate that Mg* has the greatest afunity for nitrogen- and oxygen- containing ligands with binding energes between 30 to 45 kcal mol?' The interaction with the saturated hydrocarbons methane and ethane has been shown to be weaker with a calculated binding energy of less than 10 kcal mol''. Introduction of a ic-system into a hydrocarbon ligand results in almost a doubling of the ~r-ligandbond stren-gh due to the electron rich n-system interacting with an empty p-orbital of the magesium ion (Structure 1.1). Structure 1-1 Ab initi~studies indicate that N2 ligates the magnesium ion through one lone pair, forming a very weak hear ion-molecde complex with a dissociation energy of less than 3 kcal mol-'.* To date the focus of calcularions has been on the direct ligation of Mg*, usuw involving one or two ligands, but a more extensive study with variations in the ligand substrates and in the number of ligands bonding to the metal should provide additional insight into the chernical nature of Mg*. AU the theoretical results of Mg* have revealed an unexpected geometRcal feature when the metal center is coordinated by two ligands, L. With the unpaired electron formaUy located on the magnesium, the geometxy of MgLZC reveais a smaller than expected bond angle of -100° as shown below in Structure 1.2. Structure 1.2 According to Baushlicher the second ligand does not bind on the opposite side of the magnesium fkom the first ligand because this is a region of high electron density; the magnesium 3s orbital is polarized away fiom the first ligand to enhance the bonding between Mg9 and the first ligand? An alternative explanation for the small bond angle of < LM& WUbe presented in this study. Even within the same family of alkaline earth metals, this geometrïcal feature is not repeated. A parallel theoretical study of Mg" and SrU with COz shows that Sr(CO2)7 is linear, but Mg(CO&* retains the characteristic LMgL bond angle of -100O.~ Solvation versus direct coordination of the metal center has been studied with water clustering of akali metal cations: in an effort to determine relative bond stren,as, and geometries. Results show that x(H20).+ smail clustea (n = 1-3) favour structures in which each of the water molecules coordinates directly to the metal. When the clusters become larger (n = 4-6) the geometries favour one or two water ligands occupying a solvation shell and coordinating through hydrogen bonding. Bauschlicher has also investigated the higher order ligation of Mg* and Aif to ~ater.~This preliminary theoretical study showed that M~(H~O)~*prefers to be coordinated directly three tirnes and have one water in an outer solvation shell; the solvating water hydrogen bonds through two directly coordinated water ligands. This conformation is only slightly lower in energy, (3.6 kcal mol-1 at HF/6-3 l+G*) than a tetrahedral arrangement in which all water ligands are directly coordinated to the metal cation. To obtain a better understanding of the transition between direct ligation and solvation a more extensive study is required. The maximum coordination number of Mg* is another area of rnapesiurn chemistry that has not been fUUy probed. Bauschlicher has looked at four directly coordinating water ligands to ~g*,which fomially exceeds the octet of eight in the coordination shell of magnesium. If Mgw were behaving like a traditional non-transition metd, then the maximum number of electrons in the valence shell of magnesium would be eight. Mg* has fonnally one electron around the metal and each of the waters ligates through one of the lone pairs on the oxygen. Bauschlicher has shown that when four water molecules directly coordinate around the Mgw, the maximum number of electrons in the valence sheii of magnesiii2 is nine. Current literature does not show any results to suggest the maximum number of valence electrons of mapesium. Does the number of valence electrons of mapesium extend up to eighteen iike that of a transition metal? If the octet is fkther extended are the d-orbitds of magnesium involved in the bonding? Futher computational work is required to give a full description of the chemical behaviour of Mg*. Studies on biologïcal systems with magnesium and with ather metal cations show that solvation strongly influences the selectivity of the macrocyclic Ligands and determines the chemicai properties of the ligand to which it is bound." Knowing a metal ion's chemical effect on a ligand in a biological environment would be useful information *whendeveloping future dmgs to combat vhses that attack DNA. 11.12 1.3. Metailocenes The study of metd rc-complexes has grown rapidly into an important and major research area in chemistry over the past few years. These compounds cm be classified into three main groups: olef'in-, cyclopentadienyl- and arene-metal R-complexes (examples shown in Structure 1.3). olefm zxomplex cyclopentadienyl n-complex arene n-complex Structure 1.3 Cyclopentadienyl-metal R-complexes, more commonly referred to as metdocenes or sandwich complexes, are biscyclopentadieny1 denvatives of transition met als. The bonding in these types of n-complexes involves overlap of ns, (n-l)d and np orbitals of the metai with the molecular orbitals of the appropriate symmetry in each cyclopentadienyl ring.'' The resulting complexes often, but not always, contain two ~gsthat cm be paralle1 or at an angle, depending upoo which metal is present in the middle of the system. The metal also affects the geometry of the sandwich, determinhg the bond angle of the rings and the metal-ring distance. As well as full sandwich complexes, there also exist monocyclopentadienyl compounds referred to as half sandwiches. The first known fidl sandwich, nicknamed ferrocene, was reported in 1951 by Kealy and Paulson at Duquesne University in ~itrsbur~h.'~X-ray crystallography showed that the molecule contains an iron atom synimetrically located between two parallel pentagonal cyclopentadienyl rings.L3 In the gas-phase, infrared studies of Fe(c- C5Ej[5)2, Ni@-C5H5)z, M~(c-C~H~)~and Mg(c-CsH& have also shown that these compounds retain a sandwich type geometry.15 Metallocenes exhibit a high degree of aromaticity and undergo many typical solution-phase substitution reactions that include: Friedel-Craft acylation, metdati~n,sulfonation and domethylation. Metallocenes also have many industrial applications such as polymerization catalysts for ethylene, as moderators in high temperature combustion such as occurs in solid rocket fuels and octane boosters, and as anti-knock agents in liquid fuels, The chemistry of cyclopentadienyl cornpiexes has received increasing attention over the past few years, but the chemistry of cyclopentadienyl-magnesium has been somewhat neglected, although the cyclopentadienyl Grignard reagent, C&J@l3r, has been known for over 80 years.16 Cyclopentadienyl-metal n-complexes (both the full and half sandwich) that contain magnesiun as the sandwiched metal are the primary focus of this study. Magnesocene was first prepared in the early 1950's by the addition of cyclopentadiene to ethyhagnesium brornide followed by removal of the solvent, ether. l7 An alternative method to produce magnesocene is by the reaction of metallic magnesiurn with cyclopentadiene, shown in the reaction 1.1, and results in a greater than 80% yield.'8 2 c-Cs& + Mg + & + M~(c-C&)~ (1.1) To date there is sorne dispute as to whether the bonding involved in M~(C-C~H~)~is ionic or covalent, but it has been detennined to contain two paraIlel eclipsing five membered rings with D5h symmetry and a Mg-ring distance of 2.008 A." A schematic of mapesocene is shown in Structure 1.4. It is formaliy a l2x system with each of the cyclopentadienyl Rngs back-donating 6 x electrons into the M$'. Structure 1.4 Beryllocene is another example of an organometallic complex that contains two parallel rings.'0 crystflbas well as gas phase20s studies have shown that beryllocene prefers a ccslip-sandwich" arrangement, Le., Be has one ql-and one p5-coordinated ring. In contrast to magnesocene (12~system) that formally exceeds the octet, beryllocene (8x system) does not demmstrate the sarne ability and instead adapts into a "slip-sandwich" arrangement to accommodate the two rings- 1.4. Atmospheric Chemistry The existence of magnesium atoms and ions in the earth's lower ionosphere has long been known, but very little is understood of the chemistry of magnesium within this medium and the role that it plays in this environment. The magnesium ions are produced 8 during meteoroid ablation, are long-lived and are gathered together by wind shears in the presence of the earth's magnetic field." Previous snidies have reported the concentration of MgC in the localized layers between 105 and 120 km above the surface of the earth to be 1.4 x 1o3 ions cm3. It is known that some metd ions are important in controlling atmospheric properties. If this magnesium layer could interact with the ozone layer of the earth (located between 35 to 45 h above the surface of the earth), a very rapid reaction wouid occur, reaction 1.2 an equally rapid reaction would follow to then give The combination of these reactions would result in keeping the Mg" concentration constant in the atmosphere, but the net effect is destruction of ozone. As weli as being present in the eanh's amosphere, Mghas also been detected in many other areas such as in the circumsteliar envelope surroundhg the asymptotic branch star IRC +10216'" and in dense interstellar clouds? In solution, magnesium is commonly found in organometallic haiides or Grignard reagents (R-Mg-X).V. Grignard fist synthesized these reagents in 1904 by reacting Mg with an organic halide and using an ether as a solvent to stabilise the reagent as it forms, as shown in reaction 1.4.'~ The magnesium center becomes connected to the CH3CH20CH2CH3 R-X + Mg r R-Mg-X (X = Cl, Br, 1) Grignard reagent 9 same carbon that was previously comected to the halogen, X, and therefore the allcyl group remains intact during the preparation of a Gnpard reapent. The carbon- magnesium bond is covalent but highly polar (R:Mg), with the carbon pulling electrons fiom the electropositive magnesium while the magnesium-halogen bond fMg4:IC) is essentially ionic, as shown in the schematic below. Grignard reagents can be made from a variety of akyl, vinyl and aryl halides and are one of the most useful and versatile reagents to the organic chemist. They are highly reactive towards a variety of compounds including carbon dioxide, oxygen and nearly every organic compound containhg carbon-oxygen or carbon-nitrogen multiple bonds. 1.6. Biocheniistry Magnesium is an essential rnacronutrient for aii flora and fauna. Within the molecular makeup of plants, mapesium is Iocated in a porphyrin-type compound. Porphyrins are conjugated tetrapyrroles with the magnesium atom formally coordinated by four nitrogens in a rigid planar environment. An example of a porphyrin is chlorophyll, shown in Structure 1.5. The chlorophyl photosynthetically reduces CO-, using the water present in the plants to provide a source of electrons to the plant that may conrinue to be supplied for a Ume in the dark? The derivatives of chlorophylIs and related compounds play an important role in the photosynthesis of plants. Structure 1.5 Direct and circumstantial evidence indicates that cation-x; interactions are important in a variety of proteins that bind cationic ligands or substrates. 11.12 They are the strongest non-covalent type f~rcesand have an important biochemical catalytic role in protein structures, functions and protein-ligands interactions. Studies with phenylalanine, tyrosine and tryptophan, whose major components are benzene, phenol and indole, indicate that the binding of Mg+* and other metal ions affect the chernical lifetirne of these amino acids." Further computational research on the human immunodeficiency 11 virus (HIV)has shown the role of aromatic components of the host to be quintessentid in aliowing the binding of HIV. The interactions of the divalent metal through binding with the aromatic segments of the host aUow the vird RNA of HIV to reproduce itseIf in the host cell's chromosome and to continue propagation.26 1.7. Other Use. Magnesium is used in a wide range of commercial and military applications such as batteries, flares, fI ash bulbs, welding kits, tracers, light generators and in autornotive airbag inflators. Prim to this the ~huiesefust used magnesium around 1865 in fxeworks2' and today magnesium is still used as the fuel to propel the projectile, as the i-piter, and to produce the vivid white colour when it explodes. 1.8, References (a) Kebarle, P. Ann. Rev. Phys. Chem 1977, 28, 445. (b) Castleman, A.W .; Hoiland, P.M.; Lindsay, D.M.; Peterson, KI. J. Am. Chem. Soc. 1978,100, 6039. (c)Petrie, S. Mon. Not. R. Astron. Soc. 1999,302,482. (d) Gardner, P.J.; Preston, S.R.; Siertsema, R.; Steele, D. J. Cump. Chem 1993, 14, 1523. (e) Yeh, CS;Pilgrim, I.S.; WiiIey, KIF-;Robbins, D.L.; Duncan, MA- Int- Rev. Phys. Chem 1994,13,231 . Bauschiicher, C.W.; Sodupe, M.; Pamidge, H. J. Chern. Phys. 1992,96,4453. Sodupe, M.; Bauschlicher, C-W. Chem. Phys. Ler. 1992,195,494. Bauschlicher, C.W.; Partridge, H. J. Phys. Chem. 1991,95,9694. Bauschlicher, C.W.; Pariridge,J. H. Phys. Chem. 1991,95,3946. Bauschlicher, C.W.; Partridge, H. Chem Phys. Let 1991, I8I; 129. Sodupe, M.; Bauschlicher, C.W.; Partridge, E Chem. Phys. Let 1992,192, 185. (a) Maitre, P.; Bauschlicher, C.W. Chem Phys. Let. 1994,225, 467. @) Tachikawa, H.; Yoshida, H. Mole. Struct. 1996,363,263. Glendening, E.D.; Feller, D. J. Phys. Chem. 1995,99,3060. 10. Glendening, E.D.; Feller, D. Phys. Chem 1996,100,4790. 11. Dougherty, D.A. Science 1996,271,163- 12. Ma, J.C.; Dougherty, D.A. Chem. Rev. 1997,97, 1303. 13. Parker, S .P. McGraw-Hill Encyclopedia of Sciences and Techriology, 7" ed., Toronto, McGraw-Hill Inc.: Toronto, 1992. 14. Kealy, T.J.; Paulson, P.L. Nature 1951,168, 1039. 15. Cotton, F.A.; Reynolds, L.T. J. Inorg. Chem. 1958,80,269. 16. Grignard, V.; Courtani, C. CR.Heed. Seances Acad. Sci. 1914,158, 1763. 17. (a) Wilkinson, G.; Cotton, F.A. Chem. Ind. (London) 1954, 307. @) Fischer, E.O.; Hafner, W.2- Naru$orsch 1954,95,503. 18. Barber, W .A. Inorg. Synth. 1960,6, 1 1. 19. (a) Faegri, KyJr.; Almlof, J.; Luthi, H.P. J. Organometallic Chern. 1983, 249, 303. @) Halland, A.; Lusztyk, J.; Brunvoll, J.; Srarowieyski, K.B. Organometallic Chern. 1975,85,279. 20. (a) Mar& P.; Schwarz, K. J. Am. Chem. Soc. 1994, 116, 11177. @) Nugent, K.W.; Beattie, J.E; Hambley, T.W.; Snow, M.R. Aust. J. Chem. 1984, 37, 1601. cc) Alrnemingen, A.; Haaland, A.; Lusztyk, J. J. Organomet. Chem. 1979,170,271. 2 1. Lyons, J.R. Science 1995,267,648. 22. Cernicharo, J.; Guelin, M. A &A1987,183, LlO. 23. Wakker, B.; Howk, C.; Schwarz, U.; Van Woerden, H.; Beers, T.; Wilhelm, R.; Kalberla, P.; Danly, L. Asnophys. J. 1996,473,834. 24. Lai, Y.H. Synthesis 1981,585. 25. Conon, F.A.; Wiikinson, G. Advanced Inorganic Chemisiry, 5" ed., Toronto, John Wiley & Sons, Inc., 1988. 26. Nicklaus, M.C.; Nearnati, N.; Hong, H.; Mazumder, A.; Sunder, S.; Chen, J.; Milne, G.W.A.; Pommier, Y. J. Med. Chem. 1997,40,920. 27. Mark, H.F.; Othmer, D.F.; Overberger, CG.; Seaborg, G.T. Encyclopedia of Chernical Techology, 3d ed., New York, John Wiley & Sons, Inc., 1997. CHAPTER 2 METHODS OF INVESTIGATION 2.1. Experimental Methods Experirnents were perfonned using a Selected Ion Flow Tube (Sm) apparatus,' see Figure 2.1. The ions were produced in a low-pressure ion source by electron-impact dissociative ionization of magnesocene vapour ac electron energies between 5 and 50 eV. The ions produced in the source were mas-selected by a quadrupole mass fdter, introduced into the flow tube via a Venturi aspirator,la and dowed to thermalize by coilision (about 4 x 10') with helium buffer gas before entering the reaction region further downstream. At the end of the reaction region a second quadrupole mass fdter sampled ail ions. Matheson Gas Products, Sigma Aldrich or Air Products supplieci the neutral reactants, which had a minimum p~~3tyof 99.99%. Rate coeffkients and product distributions were measured in the manner that be described in detail in this chapter.' AU rneasurements were perfonned at room temperature, 294 + 3 K, and ar a helium operating pressure of 0.35 t 0.01 Torr. Bond comectivities in the product ions were probed with multi-collision induced dissociation (CD) experiments by raising the sampling nose- cone potential from O to -80 V without introducing mass discrimination.' In the multi- CID operation, the potentials applied to the front- and exit-lenses are varied to ensure the ion signai intensities remain constant. The potential adjustments are mass-independent parameters that are determined manuaily and incorporated into the control program of the power supply and range between O to -500V. The dissociation threshold for a particular ion is determined from the intercept of the baseline with the fastest-growing portion of 15 Neutra1 Reactant Gas Inlet, L Roots Pump + Region Region to Pump to Pump Ion Source Flow Tube Analyzer/Detector Figure 2.1 : SIFTJCID Apparatus the appearance cuve for the product ion. The quantitative interpretation of the measured thresholds in terms of absolute binding energies is problematic since the theory ofmulti- collision induced dissociation is not completely understood; consequently the CD measurements are used primarily to ascertain bond comectivities and to provide insight into relative binding energies. 2.1.1. Ion Source The function of the ion source is to generate ions £iom a sample using one of two methods, electron ionization or chernical ionization (Fiapre 2.2). Electron ionization is Mapesocene is a crystalline soiid that is htroduced into the ion source by a stainiess steel probe as shown in Figure 2.3. The direct heat of the ionizer is sufficient to produce a vapour of magnesocene which can then enter the ionizer chamber. The ions MT, (c- CsH5)Mg+ and (C-C~H~)~M~+'were produced in a low-pressure ion source by electron- impact dissociative ionization of mapesocene vapour at electron energies between 5 and 50 eV. These ions are then transported to the entrance plate of the quadrupole bgthe use of electrostatic lenses. The electrostatic lenses are used to accelerate and focus the ions of choice from the ion source into the upstream quadrupole mass filter. The elecmostatic lenses are merely metal plates with voltages appiied to them and when working with this particular set of ions the typical set of potentials is as follows. Typical Potential Applied Repeller 80 V Extractor -15 V Lens 1 -55 V Lens 2 -27 V 17 Figure 2.2: Ion Source. The probe inlets and filaments are 90 degrees apart and in a crossbeam arrangement. probe i nlet Figure 23: Magnesocaie probe for ion source. 2.1.2. Quadrupole lMass Analzyer (QMA) The theory of operation of the quadrupole mass filter and its application as the analyzer portion of a mass spectrometer has been described in detail in Paul and steinwedeL3 There are two quadrupole rnass fdters in the SIFï/CID instrument. They each consist of four pardel rods of circular cross-section (Extranuclear 162-8, diameter 0.95 cm, leno& 20 cm, maximum mass 1400) auanged symrnetrically about the z-axis and housed in a solid outer case.45 A voltage made up of a dc component, Udc,and an ac radio-frequency component, Vpswe c, is applied between the opposite rods, shown in Figure 2.4. The potentials applied ro the two pairs of rods are equal in ma-oninide, but the dc potentiats are opposite in sisand the ac porentials are shified in phase by 180°. The result is a set of negative rods lying in the yz plane and a set of positive dc rods lying in the xz plane. Figure 2.4: The quadrupole geometry, coordinate system and potential bias. Ions are injected into the filter with a small accelerating voltage, usually -0.W for the upstream quadrupole and -0.6V for the downstream quadrupole, and are made to oscillate in the x and y direction by the electric field. This allows ions of a given m/z to be transmitted through the mass Glter axially (the z direction) while placing the ions of different m/z on unstable orbits, leadhg to these ions being thrown out radially. An ion with mas, m, and an electric charge, &, enters the filter with a velocity, v,, in the z direction and will undergo an oscillation that is described by the Mathieu equations.4" Whether the trajectory is stable (bound amplitude) or unstable (unbound amplitude) is determined solely by the values of a and q, given by the relations; The field radius, r,, is defmed as half the distance between the two diagondy opposite rods. Only for certain values of a and q are the oscillations of a panicular ion stable. Figure 2.5, which is also known as a Mathieu stability diagram, shows the values of a and q for which these conditions apply. From the relations for a and q it is seen that for ftxed values of a, r,, Udcand Vg ail ions of identical mass will have the same operating point (a,q). The ratio of a/q is given as 2UdcN$which is independent of the mass, so that ail the ions of different masses lie on the same straight line in the stability diagam. The slope of this line depends only on the ratio of Udcto V* For masses that fall within the region of stable oscillation, ions continue on the oscillatory path within the bounds of the rods to reach the detector, whereas for the other ions the oscillation becomes unstable and the ions are Iost on the rod assembly, and hence mas separation is achieved. The mass specuum is scanned by varying Udcand Vfi while maintaininp the ratio UdJVq constant (a/q remains constant as well). This mode of operation generates a recorded mass that is constant x unstable 9 Figure 2.5: Stability diagram for a mass quadniipole anaiyser. proportional to V,.f. To increase the resolution of a QMA, so that the operating he passes closer to the apex of the stability region, the dope of a/q is increased. To alter the resolution of a QMA, requires adjusting the dc voltage. At high resolution, a passing band can be reduced to a single mass unit, so that a signal peak with unique mass can be obtained at the QMA exit Ar lower values of a/q the range of the stabilization is extended and the resolution is decreased. In theory the mass resolution of a QMA can be very high, but in practice the attainable resolution depends upon the initial ion velocity in the x and y directions and upon the position at which the ion enters the filter. The quadrupole is essentially a low resolution mass fdter because of these limitations. 2.1.3. Venturi Gas Aspirator The ion selection region at low-pressure (10" Torr) is separated from the main now tube at high-pressure (0.35 Torr) by an interface called a Venturi gas aspirator.la The ion beam is injected against a pressure gradient established by the pumping action of the Venturi gas aspirator. The aspirator reduces the back streaming into the upstream quadrupole mass filter region by establishing hypersonic flow through the buffer nozzle. 2.1.4. Gas Inlet System In order to make quantitative measurements, it was necessary to know both the reaction time (which can be determined from the cder gas flow, pressure in the tube and the reaction length) and the reactant gas concentrations which could be calculated fiom the reactant gas flows. 2.1.4.1. Carrier Gas Flow All of the experiments were perfomed using helium as carrier gas. The helium cylinders were co~ectedto the flow tube through a singe-stage output readator valve and a zeolite trap. The zeolite trap was cooied with liquid nitrogen to assist in the removal of any impurities in the carrier gas, particularly water. To re-establish the carrier gas to room temperature prior to re-entering the flow tube, the carrier gas was passed through several coils of tubing. The flow of the carrier gas was regulated in the system by a needle-nose valve. The carrier gas flow into the flow tube was determined using a Mass Flow Meter, Mode1 GFM17 inserted afier the coils of tubing and before the needle- nose valve. The pressure in the flow tube was maintained at 0.35 TOITand measured ushg a Baratron Pressure Gauge (MKS Baratron, 3ûûBHS-IO, senes 170M-6A). 2.1.4.2. The Neutra1 Reactant Gas There are two reservoir systems made of stainless steel in the SIFïKID apparatus used to store and mix the neutral reactant gases. One reservoir system is connected to the ion source region and the other is connected to the fiow tube, downstream of the Venturi idet at one of two ports. Typical operating pressures in the reservoirs range from 200 to 1000 Torr. The neutrd reactant gas is allowed to expand into the resentoir to a pressure that never exceeds 75% of its vapour pressure at 20°C, in order to avoid condensation. The gas was diluted with the carrier gas, helium, to raise the pressure in the reservoir. The neutral reactant flow of gas is measured by determining the pressure ciifferenrial across a capiliary using a pressure rransducer (Pace Engineering, rnodei WD). The reagent is introduced into the flow tube at a constant fiow that is maintained by a leak valve. The capillaries have a range cf flow fiom 10'' to 10" molecules sel. Each capillary is calibrated by relating the transducer output to the flow of the reservoir, determined fiom the rate of fall of pressure in the reservoir, given by Poiseulle's formula, equation (2.3). is defined as the flow of gas out of the reservoir rhe radius of the capillary the length of the capillary the pressure difference at the tube ends of the capiiIary the viscosity of the gas Equation (2.3) is valid if the pressure &op across the capillary is srnail cornpared to PI. In terms of the reading fiom the pressure transducer, 2, which is directly proportional to AP i. equation 2.3 becornes where c = (2.5) 812 By substituting v1into Boyle's Law the results are given in equation (2.6) and if the viscosity of the gas mixture, and the volume of the init: system are known, then C can be determîned fiom the slope of the graph of Z vs. PeIIPi. The relationship between the molecular and the volume fiow is defmed in equation (2.7). Loschmidt's number, L, = 2.687 19 x 10'' molecules cm$ is defined as the number of molecules per unit volume of an ideal gas at 273 K and normal atmospheric pressure (760 Torr). The viscosity of a gas in the reservoir at a certain temperature is given by equation (2.8). Equation (3.9) relates the volume of the reservoir system, the capillary volumes and the temperature, viscosity and pressure of the gas in the reservoir system to the molecular flow out of the reservoir and is derived from combining equations (2.5), (2.7) and (2.8). where Q = 760 Capillaries were recalibrated every few months and Q values have been found to be stable to within -96. 2.1.5. Multi-Collision Induced Dissociation (CID) The downstream samphg nose cone is an interface that separates the relatively high-pressure in the flow tube from the lower pressure region of the downstream analysing quadrupole chamber.' The cone shape and the 1 mm nozzle on the tip of the nose cone were designed for sampiing the reaction mixture with minimal fiow distortion. Using normal SIFT operation, a small NC voltage (-7 V) was used to inject ions into the analysing mass quadrupole. Under CID conditions a bias from O to -80 V was applied to the NC while concomitantly varying the potential of the front and rear quadrupole focussing Ienses so as not to introduce mass discrimination. Ion-sipal ratios were kept constant to within L3% over the entire NC range. The voltage adjustments which were required to maintain constant ion-signal ratios were determined manually using ~r+, parameterized and incorporated into the control program of the power supply. The CID technique is very usefhl for the deteenation of bond connectivities in the reactant and product ions thereby distinguishing between isomeric ions, and for the 25 elucidation of dissociation and reacrion mechanisms. The ions are deliberately dissociated by collision with a buffer gas and the extent of dissociation of the sampled ions is controlled b y the magnitude of the NC voltage- 2.1.6. Curve Fitting AU curves were fitted to obtain anaiytic solutions to differential equations in order to deternine rate coefficients for the reactions under investigation. All curves were fitted using the Sigma Plot program and this method of curve fitting ailowed flow to be used as the kinetic variable rather than the. The resulting fits have errors associated with them that accumulate as the nwnber of curves being fit increases. This is due to each successive curve being fit based upon the previous cuve fit. There are three cases under consideration in this study: the fust case snidied is a one-step irreversible pseudo-frst order reaction with no branching reactions, the second case considers irreversible hipher order sequential reactions with no branching reactioos and the third case is a senes of equations with no branchhg reactions where equilibrium is established. 2.1.6.1. Reactions with no Signif~cantReverse or Further Reaction The rates detennined in the first analysis are pseudo fust order with no reverse reactions considered and only one product ion chamel, Figure 2.6. Consider the following ion molecule reaction: N, Flow /(1018 molecule S-') Figure 2.6: Reaction profile for the reaction of (C-C~Y)M$ with N,.- The rate of decay of the parent ion, A+, is given by equation (2.11) Since the concentration of the reactant neutral, B, is much larger than that of the reactant ion, AC, the rate equation 2.1 1 can be treated as fust order linear differential equation, equation (2.12). -dm- = ki'[~7 where kl'= kl[BI (2.12) dt Integration of equation (2.12) results in the mathematical formula used to fit the decay of the parent ion, equation (2.14). In a flow system, time is kept constant while the concentration of the reactant neutral is varied, The method we use allows us to interconvert time with flow and in a plot of ln[Aq vs. PI the slope is -kit. By keeping the reaction length fuced and varying the concentration of the neutral reactant we are able to study the change in the reactant ion concentration and kl' for this reaction. [AT = [~']~e-~l'' [AT = [A+]&~~~ 2.1.6.2. Higher Order Reactions with no Signif~cantReverse Reaction The curve fitting methods for rates of higher order successive reactions with no significant reverse reaction are a Little more complicated. An example of a higher order reaction is shown in Fi,we 2.7. After the initial decay, shown in Equation 2.10, the product ion C+ reacts further to give D*. NO ~low/(1018 molecule se') Figure 2.7: Reaction profie for the reaction of (c-c,H,)M~*with nitric oxide. To fit the cuve for the fxst adduct ion, [CT, is a Little more involved. The simultaneous differential equations that are required to be solved to detede ki are given in equations (2.12), (2.16) and (2.17). Taking equation (2.13) and substituting this into equation (2.16) results in equation (2.18). Rearranging and inte,gratioon of equation (2.19) results in equation (2.20); which is the formula used to fit the cwefor [CT and to determine the rate cofficient, ki. Reactions with even higher order additions were also considered in thi study and the rate coefficients that were determined were solved in an analogous manner to that shown above. 2.1.6.3. Reactions with Significant Reverse Reaction In the second reaction type investigated Figure 2.8, the loss of one primary ion gives a single product ion that does not react any further, shown in equation (2.21). At the, t = O, [Cq = O and when the reaction reaches equilibrium, at a temperature T, then &q = k&. (2.2 1) At equilibnum a plot of [O/[~flvs. @] should be hear (Figure 2.9). At sm concentrations of B, non-linearity is usually observed due to the approach of the system to equilibrium. To determine & nom Figure 2.9 use the relationship given in equation (2.22) that is derived from the equilibrium expression in te- of partial pressures. k+&+ the ion signai ratio of CC to A+ Ptoml the helium pressure in the flow tube (0.35 Torr) f~e the flow of helium fneumi B the flow of the neutral B The ion signal ratio of C+/ACand B is obtained directiy from the plot shown in Fi,we 2.9. The fH,is determined by converhg the density of hehm into a flow using equation (2.23). N the density of He molecules in the flow tube (1.14 x 10~molecules/m3) v the velocity of hehm in the flow tube (50 ds) A the area of the flow tube (radius of the tube is 4.45 cm) C& ~low/(1018 rnolecule s-'1 Figure 2.8: Reaction Profile for the reaction of (c-c,H,)~M$ reacting with 1-hexene. 2.1.7. References Mackay, G.I.; Vlachos, G.D.; Bohme, D.K.;Schifî, HI. Int. J' Mass Spectrom. ion Phys. 1980,36,259.(b) Raksit, A.B.; Bohme, D.K. Int. J. Mass Spectrom. Ion Phys. 1983,55,69. Baranov, V.I.; Bohme, D.K- Int. Ji Mass Spectrom. Ion Proc. 1996,154,71. Paul, W.; Steinwedel, H. 2. Naturfhchg. 1953,8a, 4-43. Muntean, F. Int. 3- Mass Spectrorn. Ion. Proc. 1995,151, 197. Trajber, C.; Simon, M.;Boh&ka, S.; Fut6,I. Vacuum 1993,44,653. Dawson, P.H. Quadrupole Mass Specrr~metryand its Applications. Elsevier: Amsterdam, 1976, Granville, W.A.; Smith, P.F.; Longley, W.R. Elemenrs of Calculus. Blaisdell Publishing Company: Toronto, 1964,379. 2.2- Theoretical Methods Molecular orbital calculations were performed using the GAUSSIAN 94 and GAUSSIAN 98 progams.l A v&ety of ab inirio and density functional methods were incorporated into this theoretical study and combined with a wide range of basis sets trr opllmize structures and charactede the critical points by harmonic frequency calculations. The frequency calculations also yielded zero-point energies, which were left unscaled, and thermal corrections. Calculations were dso performed with Becke-3 Lee-Yang-Parr (B~LYP)", Meller-Plesset (MP)~,Coupled Clusters (CC)' and Quadratic Configuration Interaction (QCQ~theory to determine thennochernical propertïes such as ionization energies, proton affinities and enthalpies of formation. 2.2.1. The Schrodinger Equation The solution of the theindependent SchrGdinger equation7 yields the wavefunction, Y, of a molecule or atom: --h2 V'Y+W=EY 82m and is often wntten in shorthand fonn as H,,Y,,, = Et0,Yt,,,where Hmtis the complete Hamiltonian operator that includes both electronic and nuclear contributions, = Hei + HnucThe Hamiltonian operator, Hel, cm also be written in terms of kinetic (-112~1' - 1/2~2)and potential (-Z/ri -Dr2 +l/riz) energy operators and for helium, a two-electron atom, is shown below. For molecules the introduction of the Born-Oppenheimer approximation8 simplifies the solution to the Schrodinger equation by separating the nuclear and electronic motions, by fwng the motions of the nuclei relative to the electrons. An electronic Hamiltonian, Hd,can then be constructed that neglects the kinetic energy terrn for the nuclei. The electroaic Hamiltonian, ai,may be used in a modified SchrGdinger equation, HeiYel = EelYei, the solutions of which are purely an electronic wavefunction, describing the motion of the electrons in the field of the fixed nuclei. Molecular orbital theory is only concemed with electronic wavefunctions and calculation of electronic energies; incorporation of the nuclear-nuclear repulsion energy, permits the total energy of the system to be calculated. 2.2.2.Hartree-Fock Theory (BF) The basic idea of Sartree-Fock (HF) theory is that each eIectron moves in an average field due to the nuclei and the remaining electrons. The Hamee self-consistent field equationsg are based on the fact that if the wavefunction for a molecule is a single product of orbitals, @, then the energy is the sum of the one-electron energies and Coulombic interactions between the charge clouds of dl pairs of the electrons. Hartree equations axe not norrnally used since they are based on the idea of a wavefunction for a molecule being a single product of one-electron orbitals and not an anùsymmetrized product. The Hartree-Fock equations do consider wavefunctions to be 35 antisymmetrized products and are sometimes referred to as the Roothaan-HaiI equations10 and they play a central role in ab inirio molecular orbital calculations- The Hartree-Fock equations are obtained by finding the condition for the enerav to be a minimum and at the same time demanding that the molecular orbitds obtained be orthonormal. The molecular orbitals are expressed as linear combinations of atomic orbitals and the total electronic energy of a molecule or ion is the surn of HF orbital energies and two-dectron tenns, J and K. [Hel + WJj(l) -K/(l)]Qi(l) = &&jf 1) An analytical solution to the Hamiltonian operator is unavaifable and instead the Fock operator, F , is used. The F operator is a one-electron operator and it describes the average field created by alI the remaining electrons. The above equation reduces to F(l)Qi(l)= x~&~(l) The moIecular orbital energies are eigenvalues of the F operator. By including the nuclear repulsion energy of the system, V,,, the HF limit is the lowest energy value that can be achieved for a single-determinant wavefunction. Etot = Eei + Vm The HF energy is not as low as the true energy of the system because no electron correlation is included. In the HF method there is nothhg that States that two electrons in the same orbital cannot occupy the same space. This is physicaily unreasonable and we have to build into the method some way of correlating the motion of the efectrons, This is achieved by using either perturbation theory (Ml?,,) or Configuration Interaction (CI)" methods. 36 The strate3 followed by self-consistent field (SCF) ab initio wavefunction progams is outlined on the following page. The fist step is to make a guess of a set of molecular orbital coefficients, usually fiom a semi-empirical calculation like CNDO'' or IN DO'^, for a given specified geometry. Then a set of HF equations is constructed fkom previously stored atomic inte,ds. The HF equations are solved for the total energy of the systern and resulr in an improved set of coefficients. This is the fmr cycle of an SCF procedure and if the wavefunction obtained satisfies the criteria for convergence, Le. when the energy reaches a minimum at a fxed geometry, then the calculation has converged and is frnished; if the cnteria have not yet been achieved then the program continues with MerSCF cycles. The optimization of a rnolecular geometry proceeds the same way, except that after the SCF cycles have been cornpleted, the ener,v gadient14 is evaluated. The energy agadient is the fussr denvative of the energy with respect to displacements in the nuclear coordinates and detemiines the direction on the potential energy surface in which to step in order to reach a stationary point. Afier the gradients have been calculated, the optimization re-enten the SCF procedure with a geometry predicted from the gradient calculation and a coefficient matrix obtained from the previous SCF cycle. input guess geometry 1 cdcuiationof I Iguess coefficients calculation of improved 1 diagonalize Fock 1 matrix 1 1 NO 1 ES 'I calculate ener-oy Most of the wavefunctions in this thesis have been for ions that have ground state doublet electronic open shell confi-mations, i-e. systeas with m Eneven nunber of a-spin electrons and P-spin electrons. The open shell wavefunctions have been caiculated by a spin-unrestrictedL5model, W, that separates the a-spin electrons and P-spin electrons thereby resulting in two unique detemiùiants, an a-deteminant and a P-dezenninant. By cornparison, a ~~in-restricted~~open sheil wavefunction, ROHF, forces each electron pair into a single spatial orbital represented by a determinant and the unpaired electron is represented by another determinam Unrestricted wavefunctions have a betîer energy for the system compared to the restricted wavefunction, but spin contamination from higher restricted unres tricted system system a P 4-4 uy3 excited States might cause problems.16 The unrestricted approach to molecular orbital calculations is more suitable than the other approaches since a more realistic description of the unpaired spin density is obtained." 2.23. MQLler-Plesset Theory (MP) Currcntly the most efficient way of calcuiaùng correlation energy is to use MQller-Plesset perturtxion rheory, Mi?; this however, suf5ers fron one serioüî deficiency in that it is a non-variationai method. The MP method takes a wavefunction, Y, and operator, H,which combine to give a Schrodinger equation which we cannot solve, W = EY. Suppose we have a slighdy different operator ff for which we can solve the equation, H"v = E0P. Consider Ho to be an unpemirbed system and H a pernirbed system, then the ciifference is the perturbation, H. H=HO+rn For the pembed system H, E and Y ail depend on a perturbation factor, â. (HO + W)Y = EY Expanding Y and E in terms of a Taylor series results in the following ulL= Y?~)+XYI")+hVa+ ... + PYXk)+... k (k) EL = E'O' + E"'+ J?E(~'t .. . + h E + .. . For MP perturbation theory, consider h = 1, a fuLly pemirbed system and equal to the sum of the one-electron Fock operators, F HO=CF and it then follows that The starting point energy for MP theory is the sum of the HF one-electron energies, MPO. The first-order, MP1, brings in the appropriate electronic, coulombic, J, and exchange, K, 40 integrals resulting in the correct HF enerav. Second order perturbation, MP2, is the cheapest method computationally to include correlation ener=T into the calculation. Going to higher and higher orders of perturbation wiü not necessarily cause the calculateci tnergy to approach closer to the me ener,oy of the system. 2.2.4. Coupled Clusters (CC) and Configuration Interaction Theory (CI) The coupled clusters and c~nfi~gmttioninteraction rnethods are very similar to one another, and represent a high-level treatment of electron correlation beyond MP4 and usuaUy provide even greater acc~rac~.'~UnLike HF theory, which attempts to describe a many-electron wavefunction by a sinple detenninant, CI theory prornotes electrons to virtual orbitals produced from HF calculations, thereby using a large number of electronic confi,ou~ations. These virtual orbitds are not necessarily antibonding, they are just functions of the appropriate symmetry left over from the HF calculation. The result of promoting an electron to one of these vimial orbitals is equivalent to exciting an electron to a hîgher energy orbital. The electron promotion creates a new wavefunction, y3, which combines with the ground state wavefunction, Yi, giving A CI wavefunction is a linear combination of the ground-state electronic c~nfi~ouxation, Yi, and the wavefunctions obtained from excited ~onfi~gxations,Y2Y 3-..'£'n. The accuracy of the wavefunction depends of the size of n. This is often dictated by the computational expense. The exact solution to the the-independent Schrodinger equation is obtainable if all the possible stares of a system are included in the wavefunctions, Le. doing a fidl CI. One major draw back in this variationai method is that the wavefunction 41 suffers from an incorrect dependence on the number of particles, i-e. a size-consistency egect, which has been overcome in the quadratic confï,guration interaction wavefunction, QCI. 2.2.5. Density Functional Theory (Dm) Traditionaily developed and applied to the study of solids, DFT is being increasingly applied to the study of gas phase molecules. In this method, the correlation and exchange energies are treated as a functional of the three-dimensional electron density. While the exact functional is not known, many approximate functionals have been developed and have been used successfblly for a variety of problems. The attractive aspect of such a method is that computationally they are comparable to or cheaper than MF2 calculations, but their major deficiency is that they cannot be systematically improved. In the 1960's, Hohenberg and Kohn fmt suggested the fundamental theorems that underscore aIl DFï methods, basically staring that the elecnon density uniquely detemines the energy and the properties of the ground state.19 They also proved that the exact electron density functional is the one that minimizes the energy and thereby provided a variational method to find the -electronic density of system. Using the electronic density, p, the Schrodinger equation can be rewritten as Combining the above equation with the variation principle reveals that there is a one-to- one relationship between the density, p, and the extemal potentid, V, and thÏs irnplies that the energy of the system is a function of the electronic density of the ground state, E = E, [pl. The Kohn-Hohenberg equationslg suggest the existence of a functional that includes electron exchange and elecuon correlation energies, F[p(r)], but says nothing about how to construct the functional. If the F[p(r)] is not known then the exact energy of a system cannot be determined- The Kohn-Sham equations20 are designed to hdthe universal functional, F[p(r)J. The HK~is the Kohn-Sharn one-electron orbital operators and EKSthe orbital energies. The HKS term in this relationship attempts to descnbe the electron correlation of a system; if this term is left out then the following relationship is analogous to the HF equations. Kohn and Sham assumed that it is possible to constmct an artificial reference system of independent non-interacting electrons in a potential, VKS7which has exactly the same electron density as the real molecular system of interacting electrons. HKsYi= EKSYi -1/2v2'Pi + VmYi = EKSYi with a loc 1 potential, VKS,defined such that the non-interacting density equals the real density of a system. Then the total electronic energy of a real, fully interacting system can be expressed as EeI= T + Ipvdr + %IIp(-i; )p( q)dr,dt, + Ex= r12 where T is the kinetic energy of the non-interacting reference system, the second term is external potential of the system, the third term is the classical Coulomb energy and the last term is the exchange-correlation energy, Ex=. This equation defines the Kohn-Sham exchange-correlation energy. The Exc depends on the total electronic density distribution of the system and the Kohn-Sham local potentid is Vei is the electronic coulombic potential. The Vxc rem can also be expressed as follows and is comprised of one electron exchange and electron correlation. * Vxc=aExc7 a~ Herein Lies the problem; the Exc term is the value that must be approximated in the DFI' rnethod (with an exact Exc value then the total energy of a system can be detennined exactiy). The simplest fxst approximation for Exc in a uniform electron gas is the local density approximation, LDA." EXC~*= I &XCU)*p(r)dr The EXC L9A exchange-correlation energy distribution per electron depends only on the point at which it is evaluated. The LDA rnethod was developed to reproduce the exchange energy of an ideal electron gas of uniform density, but has shortcornings when describing molecular systems. The current DFT procedures follow the oudine fïrst proposed by Kohn and Sham. Refinements have been made to the Exc to account for any deficiencies found in previous methods. The exchange-correlation functional, Exc, is composed of two components that are both functions of the electron density, p- Exc = Ex + Ec These two components; Ex (exchange term) and Ec (correlation term), cm be funher refmed into two subtypes: local and non-local (or ,gadient) corrected functionals. A local fünctional depends only on the electron density of a system while the non-local corrected functionals depend also on the gradient. In an attempt to incorporate the strengths of HF theory and Dm, hybnd methods like B3LYP have been developed. B3LYP separates the Exc functional into a gradient-corrected exchange'' (B3) and gradient-corrected correlation'' functional (LYP) as weli as incorporating, HF exchange te-. 2.2.5.2. Becke-Three Lee-Yang-Parr (B3LYP) Recendy Becke has formulated functionals that include a mixture of Hartree-Fock and DFT exchange dong with a DFT conelation Ex, = cWF+ cDF%xcDFT Spe~~callythe Becke-three Lee-Yang-Parr (B3LYP)-91-23 parameter functional maybe defined as follows ExrnA = electron exchange energy fiom LDA functional E~~ = electron exchange energy from HF AExBS8= non-local correction to exchange energy fiom B88 functional" EcVWN3= local correction function fiom VWN functiona15 ECLYP= correlation energy from non-local LYP functional cl = 0.20, C? = 0.72, c3 = 0.8 1 The c's are constants defined by Becke, which he determined by fihg to thermochemical properties Like atomization energies, ionization potentials and proton af%nities. 2.2.6. Basis Sets A crucial choice in ail molecular electronic structure calculations is the choice of the atomic orbitai basis set. There are a few important criteria to kee:, in mind when selecting a basis set, normally ease of integral evaluation, computational feasibility of the size of the basis set and correct behaviour of the atomic orbitals. The best description of an analytical orbital is the SIater orbital,26 characterïzed by an exponent factor (-CC) where a is the orbital exponent. These are simpler than H-like atomic orbitals as they have no radial nodes, but many-centre two-electron integrals are time consuming. Another option is to use Gaussian-type orbitals," with an exponential factor, -a?; thh reduces many-centre two-electron integrals to a much simpler form that is easier to evaluate because the product of any two Gaussians can be expressed as a single Gaussian, thereby making the calculations faster. The main disadvantage of the Gaussian function is that it does not resemble very closely the form of a real atomic wavefunction, especially near the nucleus region and at large distances. These defecü may be partly 46 overcome by using a hear combination of a large number of primitive Gaussian functions with suitably chosen exponents then contracting them to make up a basis set. A split-valence basis set increases the number of valence orbitah per atom and aiiows the orbital to change size, but nct shape. For example, in the triply-split valence of 6-311'~ three sets of functions in the valence region provide a more accurate representation of the molecule in the bonding region. The inclusion of polarization functi~ns'~allows the orbitals to change shape by adding functions with angular momentum beyond what is required for the ground state to the description of each atom. This is important when describing polar molecules. Diffuse function~?~denoted by +, are included in basis sets to aUow the molecular orbital being descnbed to occupy a larger region of space. This is important in systems where the elecuons are relatively far hmthe nucleus as in anions, transition states or excited states. For example the bais set, 6-3 l+~(d)~'adds a d-polarization function and a diffuse function to all the heavy atoms. For geometry optimizations, 6-31+G(d) was used and previous studies3' have shown that this basis set reproduces experimental values. For properties that required higher accuracy, iike enthalpies of formation, a large basis set of 6-3 1~++G(~CE,~)~~ was employed. 2.2.7. Thermochemical Corrections The results of electronic energy calculations on a molecule are at O K and for systems with fuced nuclei. in reality there are always frnite nuclear movements resulting in zero-point energy. Also experiments are not performed at O K and it is necessary to correct for the thermal energy. Using a set of normal-mode vibrational frequencies, vi 47 and statisticd mechanics, zero-point energy (ZPE) and thermal corrections are calculated assuming ideal gas behaviour. To account for the effects of molecular vibrations at O K, the ZPE of a system is added to the total energy of a system. This expression is based on the hamonic approximation of the potential energy surface and has been found to systematically overestimate the ZPE, with the overestimation depending on both the basis set's size and the method of calcuiation. nod 1 modes Hvib(0)= EZpE=- hC ~i i Zn order to detennine the energy of a system at higher temperatures than O K a thermai energy correction must also be added to the ZPE-corrected total energy of a system. The thermal correction includes the contributions fiom the molecular translation, rotation and vibration at a specific temperature and pressure and is usually a srnail component. f&,,(T) =*RT- (RT for linear molecules) nonnid modes =Nhv Vi 2.2.8. References 1. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Gill, P.M.W.; Johnson, B.G.; Robb, M.A.; Cheeseman, J.R.; Keith, T.; Petersson, GA.; Montgomery, J.A.; Raghavachari, K; Al-Laham, M.A.; Zakrzewski, V.G.; Ortiz, J.V.; Foresman, J.B.; Cioslowski, J.; Stefanov, B.B.; Nanayakkara, A.; Challacombe, M.; Peng, C.Y.; Ayala, P.Y.; Chen, W.; Wong, M.W.; Andres, J.L.; Reploge, ES.; Gomperts, R.; Martin, R.L.; Fox, D.J.; Binkley, J.S.; DeFrees, D.J.; Baker, I.; Stewart, J.P.; Head-Gordon, M.; Gonzalez, C.; Pople, J.A. GAUSSIAN 94, Revision B.2, Gaussian, Inc., Pittsburgh, PA, 1995. 2. Becke, A.D. J. Chern. Phys. 1988,88, 1053. 3. Lee, C.;Yang, W.; Parr, R.G. Phys. Rev. B 1988,37,785. 4. (a) MQller, C.; Plesset, M.S. Phys. Rev. 1934, 46, 618-623. (b) Binkley, JS;; Pople, J.A. Int. J. Quant. Chem. 1975,9,229. 5. (a) Cizek, J. J. Chem Phys. 1966,45, 4256. (b) Cizek, J. Adv. Chem Phys. 1969, 14, 35. (c) Cizek J.; Paldus, J. Int J. Quant. Chem. 1971,5, 359. (d) Paldus, J.; Cizek, J.; Shavitt, 1. Phys. Rev. A 1972,5, 50. (e) Bartlett, R.J. J. Phys. Chem. 1989, 93, 1697. (f) Bartlett, R.J. Theor. Chim. Acta 1991, 80, 71. (g) Bishop, R.F. Theor. Chirn. Acta 1991, 80, 95. (h) Kutzelnigg, W. Theor. Chim Acta 1991, 80, 349. (i) Pople, J.A.; Krishnan, R.; Schlegel, H.B.; Binkley, S.S. In?. Quant. Chem. 1978, 14, 545. a) Bartlett, R.J.; Purvis, G. D. Int. J. Quant. Chern. 1978, 14, 561. (k) Purvis, GD.; Bartlen, R.J. J. Chem. Phys. 1982, 76, 19 10. (1) Noga, J.; Bartiett, R.J. J. Chem Phys. 1987,86,704 1. 6. Pople, J.A.; Head-Gordon, M.; Raghavachari, K J. Chem Phys. 1987, 87, 5968. (b) Pople, I.A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys. 1989, 90, 4635. (c) Raghavachari, K-;Head-Gordon, M.; Pople, I.A. Chem. Phys. 1990,93, 1486. 7. Schrodinger, E. Ann. Physik 1927,79,36 1. 8. Born, M.; Oppenheimer, R. Am. Physik 1927,84,457. 9. March, N.H. Self-consistenr Fields in Aroms. Pergamon: Oxford. 1977. 10. (a) Roothaan, C.C.J. Rev. Mod Phys. 1951, 23, 69. (b) Hall, G.G. Proc. Roy. Soc. London 1951, A205-541. 11. Daudey, J.P.; Heully, J.L.; Malrieu, J.P. J. Chem. Phys. 1993,99, 1240. 12. Segal, G.; Pople, I.A. J. Chem. Phys. 1966,3289. 13. Pople, J.A.; Beveridge, D.; Dobosh, P. J. Chem. Phys. 1967,47,2026. 14. Pulay, P. Mol. Phys. 1969, 17, 197. @) Schlegel, H.B.; Wolfe, S.; Bemardi, F. J. Chem. Phys. 1975,63,3632. (c) Schlegel, H.B. J. Comput. Chem 1982,3,2 14. 15. Pople, J.A.; Nesbet, R.K. Chem Phys. 1954,22,57 1. 16. Sasaki, F.; Ohno, K. J. Math. Phys. 1963,4, 1 140. 17. Pople, J.A.; Beveridge, DL. Approxirnate Molecular Orbital Theor-y. McGraw-Hill Book Company: Toronto, 1970. 18. Foresman, J.B.; Frisch, AE. Exploring Chemistry with Elec~onicSrrucrure Methods. Gaussian, Inc: Pittsburgh. 1993. 19. Hohenberg, P.; Kohn, W. Phys. Rev. B 1964,136,864. 20. Kohn, W.; Sham, L.J. Phys. Rev. A 1965,140, 1133. 21. Becke, A.D. J. Chem. Phys. 1988,88, 1053. 22. Lee, Cm;Yang, W.; Parr, R.G. Phys. Rev. B 1988,37,785. 23. Becke? A.D- J- Chern. Phys. 1993,98,5648. 24. Becke, A.D. Phys. Rev. A 1988,38,3098. 25. Lagowski, J.B.; Vosko, S-H. J. Phys. B 1988,21,203- 26. Slater, J-C-Phys. Rev. 1930,36,57. 27. Boys, C.F. Proc. Roy. Soc. London 1950, A200,542. 28. Wachter, A.J.H. J. Chem. Phys. 1970, 52, 1033. (b) Hay, P.J. J. Chem. Phys, 1977, 66,4377. (c) Krishnan, R.; Binkley, J.S.; Seeger, R.; Pople, J.A. Chem- Phys. 1980, 72, 650. (d) McLean, A.D.; Chandler, G.S. J. Chem- Phys. 1980, 72, 5639. (e) Raghavachari, K.; T'rucks, G.W. J. Chem Phys. 1989,91, 1062. 29. Frisch, M.J.; Pople, J.A. Binkley, J.S. Chern. Phys. 1984,80, 3265. 30. Clark, T.; Chandrasekhar, J.; Spianagel, G.W.; Schleyer, P.v.R. J. Comput. Chem. 1983,4,294. 3 1. (a) Ditchfield, R.; Hehre, W.J.; Pople, J.A. J. Chem, Phys. 1971, 54, 724. (b) Chandrasekhar, J.; Andrade, J.G.; Schleyer, P.v.R. J. Am. Chem. Soc- 1981, 103, 5609. (c) Chandrasekhar, J.; Spitznagel, G.W. Schleyer, P.v.R. J- Comput. Chem. 1983,4,294. (d) Hariharan, P.C.; Pople, J.A. Chem. Phys. kit. 1972, 16,2 1% 32. (a) Jursic, B.S. Molec. Struct. 1996,370, 65. (b) Jursic, B .S. J. Molec. Struct. 1996, 366, 103. 33. Frisch, M.J.; Pople, J.A.; Binkley, J-S-J, Chem. Phys. 1984,80,3265. 3-1-Introduction The chemisay of magnesium cations continues to be of interest in a wide range of disciplines including atmosphenc chemistry, organometallic chemistry and circumstellar/interstellar chemistry. Pioneering measurements of the reactivity of Mg" in the gas phase were reported as early as 1968 by Ferguson and ~ehesenfeldlwho were motivated by the roie of this ion in the chemistry of the earth's ionosphere. In 1963 rocket-borne mass spectrometers had identified Mg-? in the earth's upper amosphere in stratified layers at 105 and 120 km where magnesium is thought to be deposited by meteor ablation.' Ferguson and Fehsenfeld drew attention to the non-reactivity of Mg", expected on the basis of energetics, in birnolecular reactions with the abundant stable atmosphenc gases Oz,Nz. and CO2, but they also were able to demonstrate with rate measurements the substantial bimolecula. reactivity of this ion with the Iess stable, less abundant atmospheric constituents 03,Cl2, Br2, &O2, HNOj and CCL.' There is continuing interest in the atmospheric chemisûy of Mg" panicularly as it proceeds on meteor trains: the atmospheres of stars: and the atmospheres of Neptune and the other giant planets;6 more kineric data for reactions of this ion, for example with Hz and hydrocarbons, has been called for.6 Little progress has been made in this regard, although systematic studies of the kinetics of reacûons of Mg* with a large variety of inorganic and organic molecules using a Selected-Ion Flow Tube (SIFT)/glow discharge technique have been initiated in the laboratory of ~abcock~Preliminary results from this laboratory iodicate no reactions with the inorganic molecules Hz, N2, CO, NO, HZOiSOZ, NH3, NOz and N20 at room temperature in a helium carrier gas of 0.3 Torr, althouph addition reactions were reported for CO2 and H2S as well as hydrocarbons gnerally, except methane? A prime focus in organornagnesium chemistry has been the intrinsic nature of MgC4igand interactions and the influence of such interactions on the chernistry of bare and Iigated MgU. Theoretical investigations, as well as systematic gas-phase experiments have led to structures and total energies of isolated ligated Mg" ions as weLlM@~* as intrinsic MgC-L binding energies. Theoretical studies of ligated Mgw have been reported, primarily by Bauschlicher and co~orkers,&'~for singly-ligated MgLU cations with L = &, Nz, CO and many organic molecules and for multiply-ligated, MgL9 cations, with L = Hz0 and m. Gardner et al. have computed the ionic species Mgrand O( = H, F, Cl and OH)." The weakly-bound ligated Mg9-D2, Mgw-Nz, Mg9-02, MgM-H20, MgC--0 and Mg'-CO2 ions in which the bonding is primarily electrostatic in nature, as well as multiply CO2 ligated cations, have been examined experimentally usina photoexcitation and dissociation spectroscopy.15 Sequential bond energies for multiply- ligated MgC(H20). cations with n up to 4 have been determined from observed thresholds for collision-induced dissociation with xenon, proceeding within a guided-ion beam mas-spectrometer. l6 The photoabsorption and photodissociation of MgW(H2O). has been investigated experimentaliy for n = 1 and 2." The latter has been shown to lead 51 to water molecules being evapourated as weli as the production of M~oH?.'~Also. the dehydrosenation reaction (3.1) has been seen ro occur spontaneously upon hydration at room temperame for n > 4.18 MgW(H2O),,+ H20 + MgOHC(H20), + Ho (3-1) Relative Mg*-L ligand-bond energes can be deduced from measurements of reaction kinetics by establishing the pieferred directions of Ligand-switching reactions of types (3.2.1) and (3.2.2). Preliminary results of such investigations with M = M$ have been reported using the Smtechnique? ML++L+ML'++L' (3.2.1) ML&," + L + ML,-&+,"+ + L' (3.2.2) We report here the systematic SET investigations of the reactivity of Mg" towards inorganic molecules and of the changes in the Mg9 reactivity as the Mg cation becomes Ligated wiih one and two cyclopentadienyl radicals. Mg9, (c-CsHs)MgCand (c- CsH&MgW were generated from magnesocene by electron-impact ionization and were allowed to thermalize by collisional deactivation with He atoms prior to reaction. The reactivity of ground-state kIg* with inorganic gases was examined fust and then the change in reactivity was explored with the shgly- and doubly-iigated ions, (c-CsHs)Mgf and (c-C5H5)?~gC.It wiU be seen that the reactivity of ~g-changes dramaticdy with single and double ligation and that the inorganic ligands themselves exhibit a variety of degrees of coordination with (c-C5HS)MgCin sequentiai ligation reactions. The reactivities of Mgw, (c-C5Hs)Mgf and (C-C~H~)~M~*were assessed through measurements of rate coefficients for ligation at room temperature and at operating helium pressures that are sufficiently high to allow collisional stabilization of the ligated ions. Under such operating conditions ligations occur by reactions of type (3.3). Measured rate coefficients for sequential ligation provide a measure of inuinsic ML,f+LtHe+&+lf +He (3.3) coordination numbers since the rate coefficients for ligation reactions of type (3.3) are sensitive to the bond energy of the ligated species, D(M?-L,.,+& This is because the gas- phase Iigation proceeds in two steps, reactions (3.4.1) and (3.4.2), aod the lifetime of the ML++L tt (MLn+i3* (3.4.1) WL+iC)*+ He + ML,+lt (3.4.2) intermediate (ML.,,+i)* against dissociation back to reactants is dependent both on the degrees of freedom in (ML,+,?* effective in innamolecular energy redisposition in the transient intermediate and its attractive well depth, D(M+-L+~).'~Measurements such as these provide insight into the intrinsic efficiency of Iigation and provide a kinetic measure of int~siccoordination numbers for metai ions.-90.2 1 Coordination numbers are accessible because rate coefficients for ligation are sensitive to the bond ener9es of the ligated ions, D(m+-L). Furthemore, we report measurements of the intrinsic Ligation kinetics for the ligation of Mg* with ammonia. In principle, two types of ligation can occur when more than one ammonia molecule adds to a cation: the smmonia either cm bond directly to the core ion ("inner-sheil" lipationj or it may bond to an existing ammonia ligand by weak hydrogen bonding ("outer-shell" ligation). We explore such a transition more closely by cornparhg measured rates in the ligation of Mg" with ammonia with ligation energies computed for both direct bonding with MgC and hydrogen-bonding wirh an existing ammonia ligand. Our aim here is to ascertain whether a transition between the two types of ligation is again revealed in rneasured rate coefficients for sequential Iigation and whether: "outer-sheii ligation" can be shown to proceed before "inner-sheWYligation is complete. Evidence for the latter has recently been reported by Weinheirner and Lisy for methanol solvation of Cs+ using vibrational spectroscopy.~ We extend the intrinsic ligation studies to '"heterogeneous" ligation. The kinetics of sequential ligation of the metal ion already ligated with another ligand L' = c-CsHs is tracked as a function of the number of added ligands L = NH3. The generd ligation reaction is illustrated in reaction (3-5). L'&++L+H~+L'&+~'+H~ (3 .5) Such studies provide an opportunity to examine the role of degrees of-freedom in the transient intermediate on the measured coefficient of the rate of ligation. After their formation, the ligated ions LW&+ are subjected to coilision-induced dissociation to obtain insights into the structures of the ligated ions and into relative binding energies, and also to probe for the occurrence of intramolecular ligand-ligand interactions. 3.2. Results and Discussion Table 3.1 summarizes, in ordea of increasing molecular weight of the reactant, the rate coefficients measured for the reactions of Mg", (c-C5H5)Mg' and (c-CsH&MgC. All rate coefficients are apparent biniolecular rate coefficients at (294 k 3) K and a helium-buffer gas pressure of (0.35 f 0.01) Torr- Standard enthalpy changes referred to in the text were derived from the values found in the compilation of Lias et al.," unless indicated otherwise. 3.2.1. Mg* Reactions MgC was found to be unreactive with all molecules investigated, with the exception of ammonia. No ligand-association reactions, reac~on(3.6), or other reactions leading to bimolecular products, Mg* + L (+He) + no reaction (3-6) were observed between Mg" and H2, H20or 02,kl < 20-l4 cm3 molecule-l s-l, and Nz, CO,NO, CO2, N20 or NO2, kl < 10-l.~cm3 rnolecule-1 s-1 . However, ammonia did Ligate sequentially to forrn ammonia adduct-c according to reaction sequence (3.7) for n from O to 5.24 The rat$ coefficient for the ligation of bare Mc' with ammonia under our operating conditions was measured to be 4.1 (k1.2) x 10-" cm3 molecule-' s". CI Mg(NH3)n" + NH3 + MgWdwi (3-7) Rate coefficients for the ammonia ligation of M~(NH~)~"and Mg(NH&* could not be determined since the profiles for these ions were not suficiently developed to provide reliable rate coefficients by curve fittkng. The tabulated reaction-rate coefficients are 55 XXX -F. CC.- 0 ô CIv -v effective bimolecular rate coefficients. They were not investigated as a function of pressure because of the limited pressure range available with the SIFI' technique and so the molecularity of the ligation reaction could not 5e deterrnined experirnentdy. Nevertheless, reactions (3-4.1) and (3 -4.2) almost certainly proceed by association under our experimental opera~gconditions (with He acting as the third body) rather than by bimolecular (radiative) association. The multi-collision CID pro fües for Mg* ligated with deuterated ammonia observed at the relauvely hi& NDj flow of 2.0 x 10'' molecules sec-' are shown in Fiame 3.1. Deuterated ammonia was used to monitor for any hydrogen loss and none was observed- The CID profües indicate the successive elimination of single ammonia molecules nom Mg(ND3)3* and the elimination of one rnolecule of ammonia from Mg(NI&* to form Mg(ND3)3+0at relatively low collision energies. Also, the idection in the Mgm3)3Cprofile at about 40 V suggests the presence of two different states or isomers of Mg(ND3)3C. The M@lDj)5C signal was too srnall for a measurement of its dissociation. A plausible interpretation of these results is that the f~sttwo and some of the third ammonia molecules are bonded strongly and that the fourth and most of the third mo1ecuIes of ammonia are less strongly bound- It is interesting to note that ammonia was found not to cluster directly with Mg* in Sm studies in which ~g-was produced in a glow discharge' and these authors did report the clustering of CO2 with Mg'. For the reaction of Mg* with H20, both our results and those of Babcock et a17 appear to be inconsistent with the results of Castlernan et a1.I8 who report that water ligates Mg* in a flow-tube apparatus in He under similar CC. O d Ion Signal conditions of pressure and temperature (a reaction-rare coefficient was not reported). Also, hydrated Mg* ions were produced by Fuke et al.17 in a heiium buffer in a previous photodissociation study of rnagnesium-water ciuster ions. The one important difference between these vaious experiments is the mode of production of Mg* which is achieved by laser vaporization of the metal in the experiments of Fuke et al.I7 and Castlernan et daL8It is conceivable, as already has been pointed out by Babcock et al.,' that a large fraction of the MgC ions generated by laser vaporization is excited and that excited MC adds to water in the experiments of Fuke et aLL7and Castleman et al.'' It is also interesting to note that a slow addition reaction of Mg* with O2 has been observed to proceed in Ar buffer gas under operating conditions otherwise similar to ours with a third order rate coefficient of - 2.5 x 1050 cm6 molecule='-d3 Apparently argon is more effective than helium in stabilizing the transient intemediate (MgOT)*. The failure to observe bimolecular reaction products for the reactions with Mg" at the room temperature of our operating conditions can be attributed to endothennicity. For example, available standard enthalpies of formations indicate that oxidation reactions (3.7.1) and (3.7.2) are endotherrnic by more than 80 and 170 kcai mol-', respec tively. MgU + O2 + MgO* + O (3.7.1) -+ Mg0 + OC (3 -7.2) Also, a recent cornputation at MP2(FU)/6-3 11 G**f/MP2(FU)/6-3 11G** predicts an endothermicity for the hydrogenarion reaction (3.8) of 220 kcal mol-'. '' Mg" t Hz+ Mgr+ Hg (3-8) Insufficient thermochemical data is available for the standard enthalpy change for the analogous reaction between MgU and NZ. Oxidation reactions analogous to reaction (3.7.1) with Hfi, CO, NO, CO?,Na or NO2 and hydrogenation reactions analogous to reaction (3.8) with and H20 are also endothermic according to the available thermochemical data? However, the oxidation reacîion (3.9) with N20, which has a computed endothermicity of 5 f 8 kcal is a borderline case. Mg" + NzO + MgO" + N2 (3 -9) Failure to observe ligation with Mg* under SIFï operating conditions cm be attributed to a combination of low binding energy and low degrees of fieedom of the adduct ion since these are the parameters that determine the Metirne against unimolecular dissociation of the transient intermediate adduct ion whkh needs to be stabilized for ligation to cake place in the gas-phase.lg Ammonia has the largest number of atoms and degrees of freedom of aü the ligands investigated and, although not ail of the binding energies for the other molecules are known, probably also has the highest binding energy with Mg9. We have co~n~uted'~D, (Mg-NH3*) = 39.4 kcal mol-' (or Dzss (Mg-NH3') = 41.4 kcal mol-') at B3LYW6-3 l+G(d) and this value is larger than the binding energy with water which has a computed Do= 30.6 kcal mol-' at MCPF level using a TZ2P basis set'' and an experirnentally-determined value of 28.4 + 3.0 kcal mol'' at O KI6 33.2. (c-CsHs)Mgf Reactions In contrast to the chemistry of MgU, (c-C5H5)Mg+ was found to be generally reactive and to initiate higher-order ligation chemistry. The measured rate coefficients for the prirnary and higher order ligation reactions of (c-CsHs)M$ with Nz, CO, CO?, NO, NO?, &O, NI3, and H20 are summarized in Table 3.1. All of the observed reactions were addition reactions of the type (3.10). There was no evidence for the oxidation of magnesium to form (c-CsHs)MgOf. (c-csHs)Mg(L),-i' + L + (c-CsHs)Mg&),)= (3.10) No reactions were observed with Hz, k c 5 x 10-l3cm3 molecule-' s-', and Oz, k < 10-l3 cm3 molecule-l 5'. (c-CsHs)Mg+was observed to react with Nz only slowly, k = 3.8 x 10- " cm3 molecule-' s-', as indicted in Figure 3.2. A second addition was not observed in the flow range of the experimenrs. The experimental dissociation onset for (c- C5Hs)Mg(N$ in Fi,pre 3.2 is 12 V (lab energy). This smali vdue indicates that a weak bond is being fragrnented3 and this is consistent with the rather low value for the effective bimolecular rate coefficient for ligation. Figure 3.3 presents the obsened reaction profiles for the reactions of (c- C5Hs)Mgf with L = CO, NO, CO2 and &O. In each case two ligands were observed to add sequentially over the range in reactant flow employed, although there was also evidence for the addition of a third molecule of CO. The kinetics of Egation is similar for these four ligands in that a relatively fast addition of the fïrsï ligand is followed by an apparently much slower addition of the second ligand. An equilibriurn analysis of the Ion Signal Fi=,oure 3-3: Reaction profiles of (c-c,~,)~g(L),)'with L = CO,NO, CO,, N,O.- O 1 2 3 O 1 2 3 Reactant Neutra1 Flow, L / (1018molecule s") second Iigation step indicates that a number of these secondary Iigations achieve equilibrium in the flow range used. This is evident in Figure 3.4 which indicates the attainment of equilibrïum in the second Iigation step with CO and CO2 and the approach to equilibrium in the second iigation step with NO and ND. Table 3.2 provides a summary of the equiïbrium constants deduced fkom the plots shown in Figure 3.4 dong with the correspondhg values for the standard free-energy change for Ligation. The low values for ligation free energies for CO and CO2 and the higher values for NO and N20 are consistent with the results of collision-induced dissociation experiments shown in Figue 3.5 which indicate very low onset energies for the dissociation of (c- C~H~)M~(CO)~and (C-C~H~)M~(CO~)~+ and higher dissociation energies for (c- C5Hs)Mg(NO)c and (c-C~H~)M~(N~O)~~.It should ais0 be noted, however that the CID profde for (C-C~H~)M,P(NO)~+is unique in that it shows an inflection which suggests the presence of two isomen of (c-C~H~)M~(NO)~~.One isomer could contain a magnesium/oxy-gen bond and the other a magnesiudnitrogen bond. These two isomers would fra,.ment at different nose cone voltages and hence lead to an inflection in the CID profrie. The more strongly-bound isomer appears to be the more abundant. Figure 3.6 shows that three molecules of NO2 add sequentially to (c-C5Hs)Mgt under our operating conditions, and that the fmt addition is decidedly fastest, k = 7.4 x IO-'' cm3 molecule" s*'. The ligation reactions for the addition of the next two NOz molecules appeared to achieve equilibrium (see Table 3.2). The standard free energies of ligation deduced from the measured equilibrium constants are -8.5 and -9.4 kcal mol-' Figure 3.4: Equilibrïum plots for the reactions of (c-c,H~)M~(L)*with L = CO,NO, CO,,- N,O- - Reactant Neutra1 Flow, L 1 (10" molecules s-') 66 Table 3.2: Room temperature equilibrium constants (standard state = 1 atm.) and standard free energy changes (in kcal mol-') for the ligation reactions, + (c-CsHs)Mg(L),f + L - (c-CsHs)Mg(L)n+i Fi,oure 3.5: Mulri-CID'S for the reaction products of (C-C,H,)M~(L)'with L = CO, NO, CO,,- N,O.- Ion Signal for n = 2 and 3, respectively. These low free energïes of ligation are again consistent with the observed threshold for multi-coilision induced dissociation: (c-CsHs)Mg(NO~)3f exhibits a very early threshold for dissociation while the early onset of (c- C5H5)Mg(N02)+indicates an early threshold for the dissociation of (c-C5H5)Mg(N02)g which is present initialIy and is also produced by the dissociation of (C-C~H~)M~(NO~~+. The reaction profde of the ligation chemistry of (c-C5HS)Mg+with H20 is shown on the left hand side of Fiawe 3.7. Ligation with up to seven molecules was observed in the range of chosen addition of water vapour. A fit to the observed reaction profiles indicates that the ligation remains rapid al1 the way. Equilibrium analysis of ion ratios indicated that the ligation reactions were far rernoved fiom equilibrium up to the addition of at least three water molecules and this is reasonably consistent with the measured onsets for dissociation. For the higher additions, the ion profdes were not suficiently developed to provide reliable estimates of equilibrium constants and dissociation onsets could not be measured due to the low signal intensities. The onset observed for the dissociation of (c-CsH5)Mg(H20)j deserves further comment. The inflections in the dissocaiion curve of (c-C5HS)Mg(H20)< and the concomitant appearance curve of (c- CsH5)Mg(H20)< in Fi,me 3.7 indicate the presence of two States or isomers of (c- C5H5)Mg(H20)3t. We interpret the data in terms of the presence of two isomers of (c- C5H5)Mg(Hp)3+since the thud water ligand may coordinate directly to magnesium or hydrogen-bond to one of the directiy-coordinated water moIecules. The CID cuve for (C-C~H~)M~(H~O)~+suggests that the smaller fraction of these ions has a lower onset for Ion Signal dissociation. This fraction is attributed to the hydrogen-bonded isorners since the hydrogen bond will be weaker than the bond resulthg fiom direct coordination. The larger fraction of the ions is then more strongly bonded and this is consistent with the standard free energy of ligation estimated to be less than -10 kcal mol-' from an equilibrium ratio andysis of the kinetic data, Three ammonia molecules sequentially iigate to the half sandwich and do so rapidly with the rate coefficients of 1.2 x IO-', 1.4 x and 3.7 x IO-" cm3 molecule-' s-l, respectively. Typical reaction profües are shown in Figure 3.8. Ail three additions were far removed from equilibrium and there was no evidence for the addition of a fourth ammonia iigand. The CID curves in Figure 3.8 show the sequential removaï of ammonia ligands from (c-C~H~)M~(NH~)~+by collisional dissociation. The inflections in the dissociation cuve of (c-C5H5)Mg(NH3); and the concomitant appearance curve of (c- CSHS)Mg(NH3)2f in Fi,gre 3.8 suggest the presence of two isomers of (c- C5H5)Mg(NH3)3i,as was the case with (C-C~H=JM~(H~O)~+.However, the fraction of what presumably again is the more weakly bound hydropn-bonded isomer, is very srna. The total elecuonic energies and zero-point (ZPE) and thermal energies for ail the ligated ions are given in Table 3.3. Opùmized structures computed for Mg(NH3)n ions with n = 1 - 6 are given in Figure 3.9. For ions with n > 1, minima were found both for structures in which there is direct coordination of the ammonia molecule to the core magnesium atom and for the "solvated" ion structures in which the "solvating" amrnonia is hydrogen bonded to one of the hydrogen atoms of a coordinathg ammonia molecule. Ion Signal Table 3.3: Computed total electronic energies (in hartrees) at B3LYW6-3 l+G(d) and zero-point and thermal energies (in kcai mol-'). Ion Energies ZPE Thermal III -3 13.02586 48 .O 4.2 N -3 13.0039 1 47.7 4.1 I+NH3 -3 12.97599 45.7 4.0 Figue 3.9: Optimized structures for several isomers of ~g(NH3)c, where n = 1 - 6. + 2,193 Mg- NH, III Two minima were found on the Mm3" surface, structures 1 and II. Structure 1, where the bonding occurs through donation of the lone pair of electrons on nitrogen, is the global minimum. Strucnire II, the insertion cornplex, is 51.0 kcal mol-' above the global minimum at B3LYPfi-3 l+G(d) (with ZPE included) and has an energy higher than that of the isolated reagents by 10.7 kcal moï1. The Mg-N bond lenaoths in the direcrly coordinated ions increase as the nimber of ammonia adducts increases kom 2.193 A (for n = 1) to 2.229 A (for n = 3). This trend is reversed for the quadruply-Ligated tetrahedrd Mg(N&)?* in which the Mg-N bond lenath is the shortest at 2.156 A. Smcture III, Mg(NH3)ï, has a NMgN bond angle of 103.3", consistent with the value of 99.9" at SCF/TZ2P reported by Bauschlicher and CO-workersZ6 This angle is much smaller than expecred since the unpaired eIectron is formaUy located on the magnesium. According to these authors, the second ammonia Ligand does not bond on the opposite side of the magnesium from the first ligand because this is a region of high electron density; the magnesium 3s orbital is polarized away from the first ligand to enhance the bonding between Mg* and the 6nt ammonia. An alternative explanation is that bonding occurs through donation from nitrogen Into the vacant p-orbitals of magnesium and the expected angle of 90" is increased slightly by stenc interaction between the two ligands. This type of bonding, where the unpaired electron is essentially in the 3s orbital, cm be attributed to the larger s/p gap that occurs in the second full row of the periodic table. This type of bonding would also lead to the three coordinate ion havirg small NMgN angles and the angles in ion V are indeed slightly smaller than in the diadduct III. In Mg(NH3)4* there are formally nine valence electrons around the magnesium atom, eigbt fiom lone pairs donated fkom the four ammonia molecuies and the ninth orïgînating fiom MgC. However, the complex does not adopt either a triponal bypyramidal or a square pyramidal structure. Rather it prefers a tetrahedral arrangement. The unpaired eIectron is in an orbital of al-symrnetry to which the major contnbutor is the outer s-function on the magnesium. In terms of atomic orbitals on magnesium, this corresponds to the magnesium accepting electrons (fiom the nitrogen atoms) into sp3 hybrïd orbitals constructed from the 3s and 3p orbitals and the unpaired electron being in the 4s orbital. Structures IV, VI and VIII are solvated ions. The Ne-H solvatiizg distance is approximately twice that of the N-H bond distance in ammonia and increases as the number of amnia molecules is increased. The directly coordinated core of the solvated complex, structure VI, has similar geometrical parameters to the unsolvated diadduct, structure m. Likewise, the directly coordinated core of the solvated ion VIII has similar strucmal parameters to V. The geometries of the solvating NH3 molecuies in the solvated ions IV, VI and Vm have cornparabTe dimensions to that of an isolated ammonia molecule. AU these structural parameters indicate relatively srnall interactions between ions and a solvating ammonia molecule. Binding enthalpies for the directly bonded and the solvated ions are given in Table 3.4 at B3LYP/6-31+G(d). Our value for the Mg-NH3" binding enthalpy in the fmt adduct, 40.5 kcal mol-', is in agreement with the values of 38.7 f 5 and 40.0 kcal Table 3.4: Calculated bindiog enthalpies (in kcal mol-') at 298 K at B3LYP/6-3 l+G(d). Ion Bindino Enthal~v Other theoretical resuits" 1 III IV V VI VII VIII Ix X XI a. Binding energies at O K have been revised to bond enthalpies at 298 K. b. Reference 9 at MCPF(AN0). c. Reference 26 at SCF(TZP). mol-' calculated by Bauschlicher et al?"6 As the number of ammonia ligands increases, the binding enthalpies of the directly bonded ions as calculated at B3LYP/6-31+G(d) decreases from 40.5 kcal mol-' for Mg(NH3)- to 14.2 kcal mol-' for Mg(NH3)c. The calculated binding enthalpies for the solvated ions are aiways srnalier than those for the directly-bonded ions. They also decrease with the number of bonded amrnonia molecules and range between 154 and 8.4 kcal mol-' For structures 1, III, V, VII, X and XI the standard enthalpies of formation at 298 K and are given in Table 3.5. We estimate that the error in these enthalpies of formation fiom previous theoretical resdts is likely to be +10 kcal mol-'. There are no current literature values available for the standard enthalpies of formation of these ions, either theoretical or experimental. 3.23. (C-C~&)~M~+Reactions The full-sandwich (C-C~H~)~M~~cation was observed to react only with MI3 and H20. The observed reaction with these two molecules correspond to ligand substitution or switching as in reaction (3.1 1). (c-C~H~)~M~"+ L + (c-C5H5)MgLC+ (c-C~H~)' (3-1 1) Ligand switching was not observed with L = Hî,N2, CO, NO, OZ,COz, N20 or NOz, nor was simple addition. The measured upper limits to the rate coefficients for reaction of (c- C5H&MgC with these molecules were in the range l~-'~to 10-l3 cm3 molecule-l s-l. Figure 3.10 presents reaction profiles for the reactions of (c-C5H&MgC with NH3 and H20.The fmt step in the observed chernistry in both cases is a rapid switching with Table 3.5: Standard enthalpies of formation for adductsJ of Mg-),", as calcuiated at B 3LYP/6-3 l+G(d), LU^^^^^^ (in kcal mol-'). Ion B3LYP/6-3 l+G(d) 1 III v VFI X XI a. Ail NH3 molecules are coordinated directly to the mapesium atom. Ion Signal one of the cyclopentadienyl rings in (C-C~H~)~M~*.The ligand substitution is followed by the sequential addition of two ligands in the case of N& and more than two ligands in the case of H20, in a manner similar to that observed for the higher-order reaction of the haif-sandwich cation, and with similar rates. The switching reactions with NEand &O are not equally fast: the switching reaction with H20is about five thes slower than. that with NH3,-whichreacts at close to the collision rate (see Table 3.1). The rapid occurrence of switching implies that these two molecules ligate more strongly to (c-CsHs)Mgc than (c-C5H5) itself, viz. D298((~' C5H5)Mg+-m) and Dzgs((c-CsHs)Mg'-H20) > D~gs((c-C,Hs)Mgi-(c-C~H5)). Conversely, the failure to observe switching with ETt, N2, CO, NO, Oz,CO2, &O and NOÎ, implies that these molecules Ligate less strongly. 3.2.4. Variation in the Rate of Ligation with the Number of (c-C5H5)Ligands The results of the measurements reported here show a strong dependence of the rate of iigation on the number of (c-C5H5) ligands. We have seen that Mg* is unreactive except with amrnonia which ligates slowly, k = 4 x IO-" cm3 rnole~ule'~8'. Single ligation of Mg* with c-C5H5 substantially enhances reactivity: the initial ligation of the half-sandwich cation is rapid k > 5 x 10-ILcm3 molecule" s-', with ail ligands except Hz, N2 and 02. Double ligation with c-C5H5substantially reduces reactivity. No ligation was observed with the full-sandwich cation, although fast bimolecular ligand-switching reactions were observed to occur with NH3 and H20. We anribute the enhanced rates of ligation of the half-sandwich relative to the bare magnesiun ion to increased collisional stabilization of the intermediate Ligated complex. An increase in collisional stabilization results fiom an increase in lifetime of the intermediate complex that is expected fkom the increase in the number of degrees of freedom which may participate in energy redisposition. Mgn in the full-sandwich appears to be coodinatively saturated and we take this to account for the observed inertness of the fuU sandwich to Merlibation, even by longer-range electrostatic bonding. The observed non-reactivity of the full-sandwich cation also suggests that there is no strong bonding of any of the ligands investigated with c-CSHsring substituents themselves, as mi@ be expected. 3.2.5. Variation in the Rate of Ligation with the Number of Ligands, L The range in the rates of iigation of (c-C5HS)Mg+with different ligands and their dependence on the number of ligands is shown in Figure 3.11. Measured rate coefficients for ligation with a single rnolecule range from c 5 x 10"~ to 1.6 x cm3 molecule-' s-' and the number of ligands observed to add sequentially range from 1 to > 7. Only hydrogen and oxygen molecules were observed not to ligate to (c-CsH5)Mg+under our chosen operating conditions. The maPonitude of the rate of ligation of (c-C5H5)Mg' under our operathg conditions, presumed here to occur by collisional stabilization, is determined in part by the strength, and therefore the nature, of the bonding interaction between the ligand and (c-C5Hs)Mg of (c-C5H5)Mg(L)>. This interaction cm be covalent or purely electrostatic or, as is clearly the case in the observed water Ligation, may involve hydrogen bonding Figure 3.11: The rates of ligation of (c-c,H,)M~+ with a varïety of inorganic Ligands. Number of Ligands, n 85 with existhg ligands. (c-C5H5)MgC-Lbinding energies are generdy not known. But we can say that, for multipIe additions in which degrees of freedom continue to rise, a sharp drop in rate must reflect a sharp drop in ligand-binding energy under our operating conditions. This being the case, we can defme coordination number, viz. the nurnber of ligands that bond directly with the Mg in the (c-C5Hs)MgC cation, as we have done previously, as the number of ligands added sequentially before the occurrence of a sharp drop in the rate of Ligation.24 The def~tion,or "sharpne~s~~,of this coordination number of course will depend on the sharpness of the transition in the ligation energy, A transition between a strong coordinate bond to a weak electrostatic bond upon ligation should lead to a drop in the rate of ligation by two or more orders of rna,pitude." The measured multi-collision induced dissociation profiles provide additional information on relative bond strengths and so may be used in conjunction with the measured kinetics to better determine the coordination number "Outer-shell" figation to existing ligands bonded directly to Mg will mask the determination of primary coordination numbers. Accordingly, on the basis of the results shown in Fi,p.re 3.11, the following coordination numbers cm be assigned for coordination with (c-C5H5)Mg+:O for 02and Hz, 1 for Nz, 1 or 2 for CO and NO2, 2 for NO, N20and CO2, and 3 for NH3. The situation with water is unique since H20 may bond in an outer coordination sphere by weak hydrogen bonding to a water molecule coordinated directly to the metal. In this case we must distin,guish between a primary coordination number corresponding to ligation in an inner coordination sphere and a secondary coordination number corresponding to ligation in an outer coordination sphere as we have done recently in Our studies of the ligatioon of FeO' with water molecules." The break in the kinetics of ligation with H20 observed ac n = 2 in Figure 3.1 1 suggests an apparent primary coordination number for &û of 2 and a secondary coordination number of >S. The third water molecule apparently is bonded weakly compared to the first two and presumably by hydrogen bondinp. However the CID data indicate that the third hydrate is a mixture of directly coordinated and hydrogen bonded isomers which suggests a primary coordination number of 3. A transition from direct to hydrogen bonding has been shown to occur theoretically for the addition of water ro the bare Mgw ion. The fourth water molecule in Mg(HrO)4* is predicted to bond more strongly to the other water molecules by hydrogen bonding than to the meral itseKZ4 However, threshold CID measurements of the bond dissociation of Mg(H20)," with n = 1 to 4 showed no evidence of a break that would suggest the formation of a hydration she~.'~ 3.2.6. Stmcture and Bonding Our multiple-CID experiments indicate that the ligands that were observed to add sequentimy to Mg* and (c-CsHs)Mg+ are al1 rernoved sequentidy by collision-induced dissociation. There was no evidence for the occurrence of intramolecular ligand-ligand interactions leading to bond redisposition in the Lipated ions of the type we have reported previously for several hydrocarbon-ligated FeC ions." Production of MgLU was not observed over the available range in CID energies. Apparently the (c-CsH5)Mg-L+bond is the weakest bond in each case. Also, all the observed CID profdes, with the exception of those for (G-C&)M~(NO)~+and (C-C~H~)M~(H~O)~+,are consistent wirh the presence of only one isomer of the iigated ion. As far as we are aware, there are no previous detenninations, either experimental or theoretical, of the structures and bonding of coordinated (c-C5H5)M&,+ cations. Some plausible structures for these other ions are shown in Figure 3.12. The kinetics kom the experiment of (c-C5H5)Mpf/NW3show that three MI3- molecules are added sequentiaily and then removed sequentiaily in the CID experiment. There is no interaction between each of the Ligating ammonias and they attach to the magnesium through the lone pair on the nitrogen. The "end-on" structure of (c-C5HS)Mg(N$ proposed in Fi,pre 3.12 is based on what is known about Mg(&)+' and CH3- and HN~+." Also, results of a spectroscopie study of Mg(-)" produced in a molecular- beam environment usine a pulsed nozzleflaser vaporization cluster source support a hear ground-state "end-on" arrangement as in Mg+tN=N (CLv, 2Z3.L5Furthemore, ~auschlicher's~theoretical snidies support this arrangement for the ground state, yielding a small Do value of (Mg+.-&) = 2.4 kcal mol". Tachikawa et al? used ab initio molecular orbital theory to study this ion-molecule complex and found the same results previously reported by Bauschlicher in 1994. Based upon rhis, the haIf sandwich ligated by Nz would bond through the lone pair on one of the nitrogens forming a complex with a five-fold axis of symmetry. Studies by ~auschlicher'~on M~C/CO~systems detemiined that a linear ion- molecule cornplex is the result of the fist addition of CO2 to Mg* with the bonding Figure 3.12: Some plausible structures for (C-C~H~)M~(L)~+,where is an inorganic ligand. 1 o.. CO dominated by charge-quadrupole interactions. The second ligation of Mg* by CO2 gives a structure that has an OMgO angle of -90" and a MgOC angle -170". Based on this, rhe probable suucnire for (c-C5H5)Mg+ with two COz ligands is shown in Figure 3.12. CO/MgC has also been examined by Bauschlicher's group? Ligation through both sites of CO was investigation. MCPF calculations using an AN0 bais set showed that ligation through the carbon is preferred by 3.2 kcal mol-' over coordination through the oxygen and rhe stmcnue proposed in Fieme 3.12 for the half sandwich complex is based upon these results. Our NO/(c-C5H5)Mg+ CID experiments suggest that there are two possible isomers of (C-C~H~)M~(NO)~+and as preliminary computational snidies indicate that the oxygen of NO is the preferred site of ligation tu the closed shell ion M$', then we propose that in one isomer (c-CsHs)Mgf is coordinated b y both oxygen atoms while in the second isomer at least one NO is coordinated through nitrogen. Addition to the nitrogen of NO has only a slightly srnalier exothennicity, suggesting that both isorners cmbe present and cm give nse to the observed shape of the CID. As with the NO CID experiment, the H20 CID also suggests the presence of two isomers of (c- C5H5)Mg(H20)c. Two plausible isomers are shown in Figure 3.12, one with an inner shell containing two water molecules and the second with an inner shell of three water molecules. Examination of the rate constants for sequential addition of water molecules reveals a &op in the rate for the addition of the third Hz0 molecule. This suggests a diffèrent type of ligation occumng, with the thkd water molecule attaching to one of the water molecules rather than direct ligation to the magnesium core. In our experiments there are probably both of these isomers present for (c-C5H5)Mg(H20);. 3.3. Conclusions The experimental results reported here provide thermal rate coefficients for the ligation of MgC, (c-CsHs)Mg+ and (c-C&)~M~* with inorganic ligands at room temperature with a helium buffer .gas pressure of (0.35 +, 0.01) Torr. Our gas-phase measurements have allowed us to track both the kinetics and the energetics of sequentid ligation as a function of the number of ligands added. For the ligation of Mg" with ammonia, the measured rate coefficients show a dramatic increase in the rate of ligation after the addition of the first ammonia ligand followed by a fairly rapid decrease in the rate. The c-CsHsligand dramatically enhances the rate of ligation of the first ;immonia ligand, but a drop in the rate is again observed for higher degrees of ammonia ligation. We can qualitatively account for these rate enhancements and profrles in te- of the classical two-step ligation mechanism indicated by reactions (3 -4.1) and (3 -4.2). This mechanism predicts a rate coefficient directly proportional to the Iifetime of the transient intermediate (m+13*given by expression (3.12) for non-linear intermediates, where A is a r = A{D + ~RT)/~RT)"'~ (3.12) time corresponding to the period of one ~ibration.'~ It is seen that this Lifetime is determined by the binding energy of ligation (i.e. the attractive well depth, D(M-L+IC)) and the number of degrees of ffeedom, s, effective in the redisposition of the excess energy of the activated intermediate. It is not possible to count the actual degrees of fkeedom effective in the energy redisposition simply by inspecting the structure of the intermediate and hence to calculate absolute values for lifetimes, but equation (3.12) certainly predicts a strong dependence of the lifetime on S. For example, wirh the maximum value for s = 3N - 6, equation (3.12) predicts an increase by about 107 in the rate of ligation from the hst to the second addition of an amnionia molecule to either Mg", ifs is the only vanable. The acmal predicted increase is expected to be rnuch less than 10' since the acmal degrees of &dom effective in energy redisposition will be siadcantIy lower than 3N - 6. Thus the addition of the first ammonia is slow because the effective degrees of fieedom or the binding energy of the intemediate, or both, are relatively small. The addition rate of the second ammonia is about 100 times faster because the effective degrees of freedoms predominate in this addition and increase the rate even though, in the case of Mg* iigation, the binding enthalpy of the second ligand is smaller than that of the fmt. Calculations using equation (3.12) with s = 11 and ou calculated ligation enthalpies indicate a drop in the intermediate lifetime in the ratio 10:2: 1:O. 1 for the addition of the first four ammonia ligands to ~g*.In addition s is likely not to be constant but may be nearly so afier the addition of the first ligand if only "local" vibrations are effective in energy redisposition upon ligation. Indeed, the variation on the rate coefficient for the sequentid ammonia ligation can be fit using computed bulding enthalpies and values of s between 9 and 13. The huge increase, by a factor of about 100, in the rate-coefficient for ammonia ligation upon the addition of c- CsH5 Ligand to Mgw cm be qualitatively attrïbuted to the sharp increase in the number of degrees of freedom in the transient intermediate (~-C~H~)M~(NH~)+*.There is no indication from the CID profiles that the Mg+-NH3 binding energies are significantly enhanced by the presence of the c-C5H5ligand. In the case of the ligation of MgU, we propose that the precipitous drop in the rate of ligation for the addition of the fourth ligand and the apparent emergenw of a weakly bonded population of Ligated ions in the CID specua for Mg(M13)3* and Mg(NH3)4+' may sig* a change in the nature of the bonding. The addition of the fourth ammonia and a fraction of the addition of a third ammonia could lead to hydrogen-bond formation with one of the ammonia ligands already attached to Mg* and so begin a second coordination sheU rather than bond directly to the magnesium. Outer-shell ligation may become preferred simply on steric erounds. The inflection in the M,o(NH~)~*dissociation profile shown in Figure 3.1 Y is consistent with the presence of two isomen and thus a transition from a stronger direct bonding with Mgw to weaker hydrogen bonding in an outer solvation sheil. The occurrence of a transition in the nature of the bonding of ammonia in the sequential ligation of (c-C5H5)Mg+is less clear. Since ligation enthalpies were not calculated for these systems, it was not possible to pursue a correlation of the type for the ammonia Ligation of Mg? However, it is worthy of note that the onset of (c- C5H5)Mg(NH3)c also shows two populations (see Figure 3.8) that again would be consistent with the occurrence of hydrogen bonding of the third ammonia Ligand in a fraction of the (c-C5H5)~g(M13); ions. A two-step mechanism, involving the formation and collisional stabilization of a Iigated transient intermediate, cm account qualitatively for the observed trends in the maadnide of the rate coefficient with increasing ligation. In the case of the sequential ligation of Mg" for which computed binding energies are avaiiable, the two-step mechanism also provides an interpretation of the observed precipitous drop in the rate of ligation of the fourth ligand in te* of a change in the nature of the bonding from direct Mg-NH3+*bonding to outer-shell N-He-N hydrogen bonding. Additional support for this hypothesis is found in a correlation of measured onset energies for multi-collision induced dissociation with computed Mg-NH3C and hydrogen-bond energïes and in the observed variation on the measured onset ener,oy with the number of ligands added. 3.4. References 1. Ferguson, E.E.; Fehsenfeld, F.C. J. Geophys. Res. 1968, 73,62 15. 2. Isotornin, V.G. Space Res. 1963,3,209. 3. Rowe, B.R.; Fahey, D.W.; Ferguson, E.E.; Fehsenfeld F.C. J. Chem. Phys. 1981, 75, 3325, 4. See for example, Baggaley, W.J.; Cummack, C.H. 3. Amos. Terr. Phys. 1974,26, 1759. 5. Kawaguchi, K; Kagi, E.; Hirano, T.; Takano, S.; Saito, S. Astrophys. J. 1993,406, L39. 6. See for example, Lyons, J.R. Science 1995,267,648. 7. Linder, C.B .; Dalton, AL.; B abcock, L.M.J. Proceedings of the 43rdASMS Conference of Mass Spectrornerry and ~lliedTopics, ~tla&a,Georgîa, May 2 l-26,19M- 8. Maitre, P.; Bauschlicher, C.W., Jr. Chem. Phys. Let. 1994,225,467. 9. Bauschlicher, C.W., Jr.; Pddge, H. Chem. Phys. Let. 1991,181, 129. 10. Sodupe, M.; Bauschlicher, C.W., Jr.; Partxidge, H. Chem Phys. Let 1992,192, 185. 11. Bauschlicher, C.W., Jr.; Partridge, H. J. Phys. J. Chem. 1991,95,3946. 12. Pamidge, H.; Bauschlicher, C.W., Jr. Phys. Chem. 1992,96,8827. 13. Bauschlicher, C.W., Jr.; Partridge, H. J. Phys. Chem. 1991,95,9694. 14. Gardener, P.J.; Preston, S.R.; Siertserna, R.; Steele, D. J. Comp. Chem. 1993,14, f 523. 15. Yeh, C.S .; Pila&, J.S.; Willey, K.F.; Robbins, D.L.; Duncan, M.A. Int. Rev. Phys. Chenz. 1994,13,23 1. 16. Daiieska. N.F.; Tjelta, B.L.; Armentrout, P.B. Phys. Chem. 1994,98,419 1. 17. Misaizu, F.; Sanekata, M.; Tsukamoto, K.; Fuke, K. Phys. Chem. 1992,96,8259. 18. H~s,AC.; Khanna, SN.;Chen, B.; Casdeman, A.W. J. Chem. Phys. 1994,100, 3540. 19. See for example, Todqa, R.; Roman, M.; Weisshaar, J-C. Phys. Chem. 1988,92, 92. 20. Baranov, V-1-;Javahery, G.; Hopkinson, A.C.; Bohme, D.K Am. Chem Soc. 1995, 117,12801. 21. Baranov, V.I.; Becker, H.; Bohme, D.K. J. Phys. Chem. A 1997,101,5137. 22. Weinheimer, C.J.; Lisy, J.M. Int. J. Mass Spectrom. Ion Processes 1996,159, 197. 23. Lias, S.G.; Barmiess, JE;Liebman, J.F.; Holmes, J.L.; Levin, R.D.; Mallard, W.G. J. Phys. Chem. Re$ Data 1988,17, 1 (Suppl. 1). 24. (a) Milburn, R.K.; Baranov, V.1.; Hopkinson, AC.; Bohme, D.K. J. Phys. Chem. 1998,102,9803. (b) Milbum, R.K.; Baranov, V.I.; Hopkinson, AC; Bohme, D.K. J. Phys. Chem. A 1999,103,6373. 25. Baranov, V.I.; Bohme. D.K. Int. J. Mass Spectrom. Ion Processes 1996,154,7 1. 26. Bauschlicher, C.W., Jr.; Sodupe, M.; Partridge, H. J. Phys. Chem. 1992,96,4453. 27. Cunje, A.; Rodriquez, CF.; Bohme, D.K.; Hopkinson, AC. Can. J. Chem. 1998, 76, 1138. 28. Tachikawa, H.; Yoshida, H. J. Mol. Strucr. (Theochem.) 1996,363,263. 29. Good, A. Trans. Faraday Soc. 1971,67,3495. 95 CHAPTER 4 SATURATED,UNSATURATED AND CYCLIC EYDROCARBONS 4.1. Introduction The binding energies of metai ions with organic ligands have been the topic of considerable interest over the past few years.14 Koowledge of the preferred sites of ligation and binding strengths of the metal ions to organic molecules is useful in evaluating the energetics of catdytic processes and the feasibility of proposed reaction pathways.5 A variety of experimental techniques have been used to establish binding energies and rate coefficients for formation of these organometallic ions. Experimental techniques such as ion cyclotron resonance (KR) spectroscopy have been used extensively to measure relative binding energies by 'bracke ting' . Freiser s group has used this technique, as well as collision induced dissociation, to establish the ligand binding energies of Mg* with a variety of organic ligands? They also have estabfished trends in the primary rate coefficients for iigation as a function of substituent size (or polarizability) for a variety of alcohol, aldehyde, ether and ketone ligands. This chapter will discuss the elucidation of similar trends in the primary rate coefficient for ligation with families of saturated and unsarurated hydrocarbons using the SPT apparatus. Theoretical methods are also used to determine metal-ligand binding energies and the results often are in good agreement with the experiment result~.~Work by B amchlicher 'A has focused on the mono-subs tituted rnagnesium ion with methane, ethane, ethylene and acetylene. His work has shown the ~g+'binding energies to be weak and he considers the bonds to be electrostatic associations. Structure optimization 96 endonligation studies show that M~*can Ligate saturated hydrocarbons in two places; through the end of the chah or at a slightly higher energy arrangement in which the M~*bdges two of the carbons asymmetricdly. The ~g interaction was found to be much stronger with the unsaturated hydrocarbons. Ln this case, M~+'interacts with the n-system of the hydrocarbon, resulting in a shorter ~g*-Lbond, as weii as a larger binding energy. Calcdations have dso shown that as the carbon chah is lengthened the M~*binding energy increases. This is also in agreement with previous experimental observations? 4.2. Results and Discussion Table 4.1 summarizes, in order of increasing molecular weight of the reactant, the rate coefficients measured for the reactions of Mg*, (c-C5H5)Mg' and (C-C~H~)~M~? AU rate coefficients are apparent bimolecular rate coefficients at (294 + 3) K and a helium-buffer gas pressure of (0.35 + 0.01) Tom. Room-temperature equilibrium constants, K, standard state = 1 atm., and fieeenergy changes. (in kcal mol-') for the Ligation reactions are given in Table 4.2. Additional reaction profiles and multi-CID'S that are not discussed in detail in this chapter are contained in the Appeadix section. 4.2.1. Saturated Hydrocarbons a. Reactions with Mg" A series of saturated open-chained hydrocarbons (containing up to seven carbons) were studied in the SIFT apparatus. The bare ion, Mg*, did not measurably iigate with this family until the carbon chah was greater than two (Fi-we 4.1). Methane and ethane did not have enough effective degrees of freedom present to create a long enough Lifetime for M&** to be stabilised by collision with the bmer gas. With larger akanes only one molecule was observed to ligate the bare ion. The rate coefficient for the ligation of C3& to Mcis recorded as a lower limit at 2 7.1 (S.1) x IO-" cm3 rn~lecde'~s-' because of the equilibrium established in the SDT tube. The equilibrium of ~g(c,H~)~/Mg+'has an equilibnum constant K, = 1.2 x106 and a AG0 = -8.3 kcal mol-'. Figure 4-2 shows that the rate coefficient for ligation increases as the degrees 0.f freedom in the hydrocarbon increases until it reaches the collision rate constant, kTc. This same trend Table 4.1: Meiisiind iak coefficieiils for ilie reüctioiis of gmiind stntes of ~g",(C-C~H~)M~' and (C-C~H~)~M~"witli selccted hydrocerbo~isligands piaceeding ai (294 + 3)K in a heliuni b~iffcigus ai n totiil pressure of (0.35 I 0.01) Torr. Reiiciionn und collisioii" rate coefficients are giveii iii units of cin~inolecule-'S.'. All (lie observed rcnctions, except those underlined, arc addition retictions (the iinderlined iwctions are ligand switcliing rei\ctions). Ligand saturated inet hane, CH4 1 A(f0.5) x 10''~) (1 .O x 1~~) NR, c IO-'" < 10"" 9.5(-12.9) x 1O"O (1. 1 x IO") NR, < IO-'" < 10'13 I .o(ko.o.3)x 1u9(1.1 x lu9) NR, < 10-1" < 10-l3 butane, C4H 10 1 .O(M.3) x 10'" (1.1 x 1 O*') NR, < IO*'" < IO-l3 l.l(k0.3)~10'~(1.3x W9) .. NR, < 10"" < 10-13 hexane, CJ-1 '4 1.3(&0,4)x 10'~ ((1.3x 10") NR, < 10"" < 10-l3 heptane, C7HIU 1.3(&0.4)x 10" (1.3 x w9) NR, iIO-" < 10-1" l 25.3(I1.6) x lo""(1.4 x 10") 9.3(&2.8)x 10"' (1 .O x IO-') NR, < 10.12 4.7(*1.4) x 10"~ >5,0(&2S)x 10'12 piapeiie, C3H6 4.3(fl.3)x10-~~(1.6~10'~88(I2.6)xl0-'~(1.2~~0-') NII, < 10'12 21.2(*0.4) x 10." $7.2(&2.2)x 10"~ Table 4.2: Room-temperature equilibrium constants (standard state = 1 am) and standard fiee-energy changes (in kcal mol") for the lïgation reaction Table 4.2: continued cyclic c-C& (switching) 0 21.7 x 10" 5 -15.3 Figure 4.1: Reaction profiles for the reactions of MJ with C3H8,C,H,,, C,Hl2 and C6HI,- Neutra1 Reactant Flow, ~/(1O l8 rnolecule s'') Figure 4.2: The primary rare coefficients for ligation of several saturated molecules vs. the degrees of fkeedorn in the Ligating molecule for ~g*and (c-c,H,)M~+. CC. E '3 w \ has been previously reported by Openi et al? in a study of Mg9 with a series of alcohols, aldehydes, ketones and ethers. Other metal cation studies have dso shown the same behaviour as that observed with M~*.~** The rnulti-CID'S of this group of ions show Iow energy onsets, which suggest a weak bond is being frapented to produce the bare ion (Fibwe4.3). The lower energy arrangement would be due to a single electrostatic interaction-between the metal and hydrocarbon. 6. Reacriuns with (c-CjHs)Mg+ The half sandwich, (c-C5Hs)Mg+,was ligated by aLl of the saturated open-chained hydrocarbons investigated (Figure 4.4). The priroary rate of ligation increases as the effective degrees of freedom are increased in the ligand reaching a maximum at the collision rate. The upper limit for the Ligation rate coefficient, kTCiis reach by the tirne the carbon chah is two in lena@, whereas Mg9 does not reach this point und the carbon chah lenodi is six. The effective degrees of freedom dictate the lifetime of the intermediate and so the rate of the formation of the product ion (discussed previously in chapter 3). The rate of ligation to the half sandwich will aiways be larger than to the bare ion because of the naber of effective degrees of freedom present in the half sandwich is larger than in the bare ion. The rnulti-CID smdy of the half sandwich with the saturated molecules shows a clear example of onset energies increasing as a function of the size of the product ion (Figure 4.5). Openi's snidJ previously showed that as the &y1 chah is lengthened the Figure 4.3: Multi-CID'S for the reaction products of M$- with C4H10* and C6Hl,- Fieme 4.4: Reaction profdes for the reactions of (c-c,H,)M~+with C2H6,C3% and c4H,0* 0.0 0.2 0.4 0.6 0.00 0.05 0.10 0.15 Reactant Neutral Flow, L/(~o'*molecule s-') 107 Fi,-e 4.5: Multi-CID'S for the reaction products of (C-C~H,)M~with bond energy increases. More energy is therefore required to fragment the Mg-alkane complex bond as the carbon chah is len,dened+ c Reactions with (C-C&)~M~+' The fidl sandwich ion, (C-C~H~)~M~,did not react with any allcanes by ligation or ligand swirching, k cm3 molecule-' s-l. The failure to observe switchinp suggests that the strength of the bond between c-C5H5'and (c-C5H5)M$ is greater than that with the saturated hydrocarbons. 4.2.2. Unsaturated Hydrocarbons a Reacrions with Mg+' and (c-C&Mg+ A larger number of unsaturated than saturated hydrocarbons was investigated. In contrast to the gas-phase chemistry of the saturated molecules, aii of the unsaturated molecules in this study reacted with Mg*. The ligations to Mg" with the unsaturated molecules ail esrablish an equilibrium state between MgLC and MgLzU (AG0 in the range of -7.9 to -9.5 kcal mol-'). The ody exception to this is the reaction of ethylene with Mg? The reaction of Mgwith ethylene developed an equilibnum state with the ligation of the fint emylene molecule. The reaction of Mg* with ethylene is also interesting because five adducts were observed. The Mg(C2&),,+' product presumably is a polymer, but no rnulti-CID was performed because the product ion signal intensities were too low. Earlier snidies with C(jODCand ethylene has demonstrated ethylene's ability to polymerize in the gas-phaseg and based upon this a possible polymer type structure of Mg(C?H&" is shown below. Previous studies have shown that as the size of the neutral moIecule is increased, the increase in the effective degrees of freedom affects the rate of the reaction.' The rate of ligation of the ion increases as a function of carbon chain fength of the ligand up to the collision rate. This is aiso seen with the saturated hydrocarbons. Figure 4.6 is a plot of the primary rate coefficient (with the double bond in the fust position) venus the number of degrees of freedom. The reactions of Mg* with unsaturated hydrocarbons reach the collision rate by the time the carbon chah len,ath is three, whereas for the saturated systems the coilision rate is not reached until the chah is six in lena@- This is a direct result of the stronger bond being formed between Mg* and the x-system in the unsaturated hydrocarbon compared with Mg* and the saturated hydrocarbon. This also becomes evident from the CID study- When comparing the onset energy for a like number of carbons in a iigand, that for the unsamated ligand is greater than for the saturated. For example the onset energy of Mg(~8~~)~is 10V compared to 20V for Mg(n-Ca8)" indicating a stronger bond is being broken in the unsaturated cornplex. This same trend has also been shown by the computational work of Bauschlicher er al.'-* The higher-order ligation reacrions are usuaiIy obsemed to occur at slower rates, reaction (4.1). Figure 4.6: The prirnary rate coefficients for Ligation of several unsaturated molecules vs. the degrees of keedom in the ligating molecule for MX+*and (c-c,H,)M$. -- Mg* kcap A slower rate is somewhat surpnsing in that the (c-C5H5)MgLf has a greater number of effective degrees of Geedom, but a large reduction in the binding energy for the higher ligation would explain the drop in the rate coefficient. The large drop in the rate coefficient may also be an indication of direct ligation to the metal ion becoming saturated and so the subsequent iigations are solvating in nature, Le. ion induced dipoles. The drop in ligation rates for higher order adwon was also observed in FTMS~and SIFTIO studies. The effects of isomers on the kinetic and rnulti-CID studies were investigated. The three isomers of n-hexene were used to see if the placement of the double bond would dictate the chemistry of the ligand. Fi,pres 4.7, 4.8, 4.9 and 4.10 show the kinetics and multi-CID results for three isomers of n-hexene reacting with Mgand (c- CSH5)M$). Within experimental uncenainty, the primary rate coefficients and onset energies show no dependence upon the ligating isomer. Similar results were also observed in the reactions of MgC, (C-C~H~)M~"and (C-C~H~)~M~~ with n-butene and isobutene. 6. Reactions with (c-CsHsJtMg+' The reaction profiles for the reaction of the full sandwich with three of the hexene isomers are unusual, Figure 4.11. The profiles show a slow addition of one hexene molecule to the fuil sandwich without displacing a c-Cs& reaction 4.2. (c-C~H~)~M~+ L t;, (c-C5H5)2MgLv (4-2) Ion Signal Cr. O Ion Signal Ion Signal An equilibrium is established with a lower limit of the rate constants of 22.6 x 10-'~cm 3 rnolecule-' S-' (Keq between 7.0 x 106 to 4.2 x 107 and AG0 between -9.3 to -10.4 kcal mol-'). The equilibrium constant is determined by plotting the ratio of ion intensities ((c- CsHs)2MgL?(c-CsH5)zMg*)versus the flow rate of n-hexene. The dope of the plot is - AG0 is determined by ushg the relationship AG0 = -RTlnq. me smdl value of AG0 suggests the occurrence of solvation rather than ligation. A solvathg arrangement is also sugested by the multi-CD results which indicate a weak association due to the low energy onset observed for bagnemation into the reactants, (C-C~H~)~M~"and C6& (Fibwe 4.12). This is the first example of the (c-CsH&Mg9 being solvated by a molecule instead of undergoing a ligand switching reaction. A possible structure is shown below. Oa wO Om O O Tl- Om O O Tl- Om O 4.23. Cyclic Eydrocarbons a. Reacrions with (c-CrnzMg+' The full sandwich can react by huo pathways, ligand addition as previously seen with n-hexene or a switching reaction. Fiawe 4-13 shows the reaction profde and rnulti- CID of the reaction of (c-C~H~)~M~*with benzene, reaction 4.3. (c-C5H&Mg* t C-Ca-+ (c-c~H~)M~(c-c&&)++ C-CsHc (4.3) Unlike the slow solvation reactions observed with the hexene isomers, the benzene switching reaction is fast, k = 7.8 (e.3) x 10-'O cm3 molecule-' s-'. A second c-C5Hs Ligand displacement is not observed, nor is ligand switching ever obsemed in the half sandwich chemistry. The rnulti-CID shows a late onset indicatùig a relatively strong bond being broken to form (c-CsHs)Mg+ and c-C~.The CD's of the product ion, (c- C~H~)M~(C-C&)+,formed by ligand switching with the full sandwich or Ligand addition to the haif sandwich are identical, indicating that the product ion formed is the same. The ody cycLic molecule to react with the full sandwich was benzene. No reaction was observed with the saturated cyclic moIecules cyclopropane or cyclohexane. Benzene has an aromatic system with a z-system available to interact with Mg" there is therefore a stronger interaction between mapesium and benzene than between magnesium and cyclopropane or cyclohexane. b. Reactions wirh Mg+' Mg* is Ligated twice by aU the cyclic hydrocarbons investigated. The kinetic data of Mg* reactïng with cyclohexane is shown on the left side of Figure 4.14. The Fi-pre 4- 13: Reaction profile and rnulti-CID for the reaction of (c-C5H&Mgf' with benzene. 0.5 1 .O c-C,H, Flow /(1018molecule se') CC.O- Ion Signal equilibnum analysis of M~(C-C~H~~)~C/M~(C-C~H~~)~is shown in the right hand side of Figure 4.14. A similar equilibrium analysis was done with cyclopropane and benzene. The second ring addition is always in equüibrium with Mg(ring>", and the rate coefficients quoted are lower limits for these reactions. Mg(~g)" + ring - ~g(ring)~* (4) Cyclopropane was investigated due in part to the high ring suain present in the srnail cyclic system and in the hope that Mg* would insert into a C-C bond and open the highly suained ring. Mulri-CD results give no evidence to support or disprove that this reaction pathway is being followed as units of the same mass as cyclopropane are removed sequentially to produce MgU. c. Reactions with (c-Ca5)Mgf Cyclopropane ligated the half sandwich three hes, which is unusual. The second and third cyclopropane molecules Iigate very slowly and the CID removes them at very low energy onset voltages consistent with a solvated type arrangement. A possible structure is shown below which is consistent with the experimental observations. The second and third cyclopropane molecules would have weak induced ion- dipole interactions with the metal center. These weak interactions would only be possible due to steric hindrance iniroduced by ligation of the first cyclopropane molecule. 4.3. Structure and Bonding Bauschlicher et al. have performed theoretical studies of Mg* with various hydrocarbons. l4 Shown on the following page is a summary of their results, Figure 4.15. The sarurated systems have very small binding energies, Le. less than 10 kcal mol", which is consistent with the low-energy onsets fiom the CID experimentals. There is also an increase in the binding energies as the carbon chain increases, which is aIso consistent with experimental onset energies. The increase in the binding energy correlates directly with a shortening of the Mg-C bond lena&, indicating that the interaction is stren,@ening. The Mg(c2&)* potentid energy surface has two minima about 1 kcal mol-Ldifferent in energy with the end-on conformation the slightly better mangement. When the Mg+ is situated above wo carbons an asymrnetric arrangement is preferred. The unsatmted systems have larger binding energies and shorter Mg-C distances, due mainly to MgU interacting with the z-bonds. Stronger bonding was also seen in the experimental results; the CD's show that a higher onset energy is required to fragment the product ion for example when comparing Mg(C3Hs)- and Mg(C&)*. Figure 3-15:Optimized structures for adducts of various hydrocarbons molecules with M$'. c3v De= 6.55 kcal mol-' c3v Cs De= 9.94 kcal mol-' De = 8.84 kcal mol-' c2v De = 18.01 kcal rnof' 4.4. Variation in the Rate of Ligation with the Number of Ligands The SIFT conditions allow for sufficient collisional stabilization of the intemediate ligated ions to probe the full extent of l.igation,10 and coordination of Mg, (c-C5H5)Mg+ and (c-C&)~M~* with a variety of h ydrocarbon Ligands. A coordination number can be defined as the number of ligands added to the ion before the occurrence of a sharp drop in the rate of ligation. A sharp &op occurs when-the order of magnitude changes by two or more. MoIecuIes that ligate after such a drop in the rate of ligation are considered to be involved in a solvation-type arrangement. The coordination numtier determined for each ion is aven in Table 4.3 (in brackets the maximum number of ligations observed). The saturated hydrocarbons have a maximum ligation number and coordination number for Mg* and (c-CsHs)Mg+ of 1. The unsaturated hydrocarbons have a coordination number of 2 with Mgand a third molecule was observed, but can be considered solvating in the next sheil. The only exception to this statement is in the reaction with ethylene, where five molecules were observed to ligate to Mg*. As suggested before, this might be a polymerization reaction with only one or two ethylene molecules being directly coordinated to the metal ion. The unsaturated hydrocarbons have a coordination number of 2 with the half sandwich, with ethylene being the exception. Ethylene was observed to directly coordinate once to the balf sandwich, but two more molecules were observed to ligate. In this case the CID does not suggest any type of ligand-ligand interaction because ethylene molecules are removed sequentially. A plausible expianation is that ethylene is simply solvating the Table 4.3: The coordination number of MgU, (c-C5H5)Mg' and (c-c5H5)2MgC at (294 f 3) K in a helium buffer gas at a total pressure of (0.35 + 0.01) Torr for a variery of hydrocarbon Ligands, L. (The numbers given in parenthesis are the maximum numbers of ligations observed.) Ligand, L Mg9 (c-CsHs) Mgf (c-CsH5)?Mg* half sandwich in other positions around the c-C5H5 ring, The full sandwich is slowly solvated by ody the largest akene, hexene. Mg* was ligated twice by aii of the cyclic molecules investigated. Benzene and cyclohexane ligated the half sandwich once, while cyclopropane was observed to ligate the half sandwich three times. Bernene undergoes rapid ligand switching once with the full sandwich and the other two cyck systems were not observed to react with the fuLI sandwich. 45. Conclusions The ligations of MgC, (c-CsHs)Mgf and (c-CsH&MgW were observed with saturated, unsaturated and cyclic hydrocarbons. The primary rate coeff~cientsincrease to the collision rate as the number of carbon atoms in the chain is increased. This is consistent with expectations in terms of the degrees of freedom and lifetimes. There was no evidence of the nature of the isomers affecting the kinetics or binding energies (C6& and Ca8). The presence of one c-C5H5 attached to Mg+' substantially increases the rate of ligation. Attachment of two c-CsHs groups substantially reduces reactiviq of Mg*. When a reaction is observed with the saturated molecules, they have a coordination number of one. With the exception of ethyiene, the unsaturated molecules have a coordination number of two. The cyclic hydrocarbons also have a maximum coordination number of two- A slow solvation of the full sandwich was observed with the hexene isomers, but rapid ligand switching was observed with benzene. AU of the li,oands are removed sequentiaily by multi-CID and consequentiaUy there is no evidence of Ïntramolecular interaction between ligands. 4.6. References 1. Sodupe, M. Bauschkher, C.W. Jr. Chem, Phys. 1994,185, 163. 2. Bauschlicher, C.W. Jr. Chem. Phys. Len. 1993,214,489. 3. Partridge, H. Bauschlicher, C.W. Jr. Phys. Chern. 1992,96,8827. 4. Bauschlicher, C.W. 3r-; Partridge, H. Phys. Chem 1991,95,3946. 5. Operti, L.; Tews, E.C.; Freiser, B.S. J. Am. Chem. Soc. 1988,110,3847- 6. (a) Murad, E.; Swider, W.; Benson, S.W. Nattrre (Londm) 1981, 289, 273. (b) Murad, E. J. Chern. Phys. 1981, 75,4080. 7. (a) Uppal, J.S.; Staley, R.H. J. Am. Chem.. Soc. 1982, 104, 1235. (b) Uppal, J.S.; Staley, R.H. J. Am. Chem Soc- 1982,104, 1238. (c) Jones, R.W.; Staley, RH Am. Chem. Soc. 1982,194,2296, + 8. Corderman, R.R.; Beauchamp, J.L. J. Am. Chem. Soc. 1976,98,3998. 9. Wang, J.; Javahery, G.; Baranov, V.; Bohme, D.K. Tetrahedron, 1996,52,5 19 1. 10. Baranov, V.; Becker, H,; Bohme, D.K. J. Phys. Chem. A 1997,101,5137. CHAPTER 5 CYANLDE AND ISOCYANIDE MOLECULES 5.1. Introduction Cyanoacetylene, cyanopolyynes, methylcyanoacetylene and many other related cyanides and isocyanides have been identified in several interstellar and atmosphenc medias. 1-3 The Voyager mission to Samrevealed that cyanoacetylene is the third most abundant compound in Titan's atmosphere, afier Nz and HCN. A proposed role of HQN on Titan is to carry nitrogen into the at~nos~here.~~HCaN is thought to be generated in Titan's atmosphere hmthe series of steps shown in reactions 5.1 to ~-3.~~ HCN- H+CN (5-1) 2CH4 ->_ C2H2+ 3H2 (5-2) 2CN + C2H2 + C2H4 - 2HC3N + 2H2 (5-3) The precursor molecules are produced by dissociation of HCN to produce CN and the photolysis of C& to produce CtH2. Cyanoacetylene is the expected product fkom the reaction of CN with C2Hz and Cfi. The dissociation of HC3N is not as weU understood, but it is thought to follow one of two pathways shown in reactions 5.1 and 5.5. Theoretical results suggest that reaction 5.5 is the preferred dissociation channel of cyanoacety~ene? The solution chemistry of *es with metal catalysts has provided unique and unexpected products. The original work by lXeppes has shown that acetylene polyrnerizes under mild conditions in the presence of Ni(CN)2 to give cyclooctatetraene (yield >708), with iittle benzene being formed despite its greater stability (reaction 5.6). Other metals have demonsuated this unique ability ro catalyse the formation of cyclooctatetraene as Ni(CN),/THF 4 HC=CH ~=80-120°,P=15 bar There has been extensive work done in an effort to deduce the rnechanism for the reaction shown aboveS7 A variety of mechanisrns have been put forward including a concerted process, stepwise growth at one of more metal centers or processes which contain either cyclobutadiene or benzene compounds as intermediates.* It is now believed that the concerted process does not take place, but that a number of steps are involved in the formation of cyclooctatetraene. A lot of evidence has been presented in suppoa of the cyclobutadiene-metal complex as the intermediate for the mechanism~but still more evidence has been put forth to discount this mechanism in favour of cyclooctatetraene being formed by ring enlargement of heterocyclic intemediates such as those shown below.1° Other work with metallocyclic 'molecules have shown that under reflux with phenylmagnesium bromide in xylene,ll diphenylacetylene produces octaphenylcyclooctatetraene (reaction 5.7). It has been suggested that tetraphenylcyclobutadiene complexes of magnesium are involved as intemediates in these reactions, but no supporthg evidence has been found.12 No reaction was observed under the SIFI' conditions with Mg*/acetylene, but cyanoacetylene has a larger number of degrees of freedom plus the alkyne linkage that could allow oligomerization to occur in the gas-phase using Mg+*as the metal catalyst. Another molecule with the possibility of polymerization 5s acrylonitrile. Polyacryloniuile is a common solution-phase addition polyrner used in Orlon and Acrilan fibers. Acrylonitrile undergoes chain-growth polymerization when rreated with smd amounts of suitable initiators. The chain-growth mechanism involves addition of the reactive end of the growing chah across the double bond of the monomer, acrylonitrile. However, acrylonitrile is a poor monomer for cationic polymerization because this mecharüsm requires formation of a relatively stable carbocation inte~mediate.'~ Magnesium-containing ions will be used as the initiators in this study in an effort to polymerize acrylonimle in the gas-phase. This chapter focuses on the rate coeffrcients and the product distributions for the reactions of Mg*, (c-C5Hs)Mgf and (c-CsH&MgU with several cyano and isocyano ligands, RCN and EWC. As weIl as deterrnining kinetic information, multi-CID experiments with the product ions allows us access to relative bond strengths and geometrïes based on patterns of fragmentation. 5.2. Results and Discussion Table 5.1 surnmarizes, in order of increasing molecular weight of the reactant, rate coefficients measured for the reactions of Mg*, (c-C5H5)Mg+and (c-C5H&MgC- All rare coefficients are apparent bimolecular rate coefficients at (294 f 3) K and a helium-buffer gas pressure of (0.35 t0.01) Torr. The values of kK are not quoted for the reactions involving CH3NC, HC3N and H2C=CHCN because no Iiterature values are available for the polarizabilities of these molecules. The experimental results for the cul reactions of HCN, CH3CN, CH3NC and H2C=CHCN with MgC, (c-CsHs)Mg4 and (c- C5H&Mg* can be found in the Appendix. 5.2-1-Reactions with HCN No reaction was observed between ~g*and HCN, k < IO-" cm3 molecule-' s-'. Mg* + HCN + No reaction (5-8) In the gas-phase there are two factors that determine the rate of an addition reaction: the effective degrees of freedom and the binding strength of the intermediate cornplex. The binding energy at 298 K of hydrogen cyanide to Mg* at B3LYP/6-3 l+G(d) is 3 1.7 kcal mol-'. From my theoretical resulis, the mg-Kr]* intermediate has a binding energy that is comparable to that of the other cyanides investigated, but this intermediate contains the fewest degrees of freedom. Consequently, when [Mg-NCrl* is initially formed, it quickly dissociates into Mg* and HCN because there are too few effective degrees of freedom to distribute the excess energy and the intermediate does not live long enough to have some energy removed by collisions with the helium buffer gas. The half sandwich, (c-C5H5)M& and the full sandwich, (C-C~H~)~M~*reacted with HCN. The effective bimolecuiar rate coefficient for the primary addition reaction of the half sandwich is 2.3 (M.7) x IO-' cm3 molecule-l s-', and is equal, within experimental errors, to the collision rate constant. An equilïbrium is established between (oCsHs)Mg(HCN)t'/(c-C~)~g~~~3+at a high HCN flow and, because of the equilibrium condition, a lower limit has been quoted for the addition of the third HCN molecule. A plot of the ion signal ratio of (~-c~H~)M~(HcN)~'~(~-C~H~)M~(HCN)~~ 137 against HCN flow, similar to Fi,gure 34, provides a standard fiee-energy change, AG0, of -10 kcal mol-'. The CID, produced at a high flow rate of 2.0 x 10'' m01ecules s-', shows that (C-C~H~)M~(HCN)~+loses HCN molecules sequentiaiiy. The reaction of the full sandwich with HCN behaves similarly to that with the half sandwich. HCN switches with for one of the cyclopentadienyl rings to produce, (c- C5H5)Mg(HCN)+with a rate coefficient of 3.0(+1.0) x IO-' cm3moleculël s-l, which is slightly faster than for the direct addition of HCN to the half sandwich. After the switching reaction, the full sandwich chemistry is identical to that of the half sandwich. 5.2.2. Reactions with CHICN and C&NC a. Reactions with CH.CN The primary ion, Mc, reacts with CH3CN to form Mg(CH3CN)' with a rate coefficient of 7.9 (32.4)x lû1° cm3 molecule-l se', and, in total, five CH3CN molecules were observed to ligate Mg*. The half sandwich was observed to be ligated sequentidly by three CHFN molecules. The CD profiles for M~(cH~CN)~"and (c- C5H5)Mg(CH3CN)3+both show sequential removal of single units of CH3CN. Ligand switching is the fxst step in the reaction of the fuli sandwich with CH3CN and the subsequent chemistry of (c-C5H5)Mg(CH3CN)+foilows that of the half sandwich. b. Reactions with CH$K In the case of methyl isocyanide, CH3NC, six ligands were observed to sequentiaiiy add to MgU under normal SIET condiùons, with an equilibrium established between the fourth and frfth adducts. In the CID mode, methyl isocyanide uni& are removed sequentiaiiy. Three CHaC molecules were observed to sequentially Ligate the half sandwich and they were then removed sequentialiy under CID conditions. The full sandwich did not react with methyl isocyanide under the SIFT conditions; in this respect methy l isocyanide reacts differentl y than methyl cyanide. Methyl cyanide reacted faster than methyl isocyanide with dl the ions investigated Mgw, (c-CsHs)Mgf cd(C-C~H~)~M~"; for both methyl cyanide and methyl isocyanide multi-CID results showed no intramolecular interactions. The nonreactivity of the full sandwich with methyl isocyanide was the only notable difference in the chemistries of these isomers. Bauschlicher and Partridge have done an extensive 14.15 theoretical study of the binding energies of Mg- with a variery of ligands. Although they have not looked at either of these IWO ligands specifically, in general, ligands that bind through the nitrogen to magnesium have laqer binding energies than those that bind to magnesium through carbon. The caiculated binding energy of methylisocyanide is only slighrly smaller than the methylcyanide at B3LYW6-3 l+G(d). The binding energy at 298K of methyl isocyanide to MgU is 39.4 kcal mol-' compared to methyl cyanide to MgU of 41.8 kcal mol-'. Even this slight energy difference could be enough to account for methyl isocyanide not being able to displace one of the CsHs rings of the full sandwich. 5.23. Reactions with H2C=CECN a- Reaction with MgC' The profie for the reaction of Mg+ wiih acrylonitrile shows seven adducts, with no evidence of periodicity in the kinetics. The cuves for the sixth and seventh adducts are not fitted. Acrylonitrile polymerizes easily in solution and could behave similarly in the gas-phase, but it is difficult to tell. The CID profile (acrylonitrile flow of 1.2 x 1017 molecules s-I) is very complicated. It indicates that there is some ligandlligand interaction, because loss of single units of acrylonitrile did not occur under the CID conditions. The Mh adduct fragments to lose some unis containhg three molecules of acrylonitrile and others containïng a single unit As the nose cone voltage is further increased, the fourth adduct loses units of one and two acrylonitrile molecules. b- Reaction with (c-CsZ%JMgf Three acrylonitrile units are obsemed to add sequentially to the half sandwich, a pattern resembhg the behaviour seen with other ligands like ammonia, methylcyanide and methylisocyanide. The (c-CsHs)MgC decays with a rate coefficient of 4.6 (k1.4) x IO-' cm3 molecule s-1 to form (c-C5H5)Mg(H2C=CHCN)tand under CID conditions the sequential removal of acrylonitrile is observed. c. Reacrion wirh (~-c&)~Mg+' The fxst step in this reaction is ligand switching of C-C5H5*for acryloniflle, as shown in reaction 5.9- (c-C~H~)~M~*+ H2C=CHCN + (c-C5H5)Mg(H2C=CHCN)++ c-C~H~'(5.9) The chemistry after the first step is anaiogous to that of the half sandwich with acrylonitrile. Two more acrylonitrile molecdes were observed to ligate (c- C5H5)Mg(HK=CHCN)' and under CID conditions the acrylonitrile units were removed sequentidiy- 5.2.4. Reactions with HCfl Cyanoacetylene is very simirar to HCN in that they both have a large dipole moment and are iinear rnolecuies. This suggests that the experimental results obtained with HC3N could be anaiogous to those obtained wiih HCN- One substantial difference that could have an auence on the experimental results is that HC3N has more degrees of fteedom than HCN. a. Reacfiort with MgC' No reaction was seen in the case of MgW/HCN,but cyanoacetylene was seen to add seven times to Mg*', Fiagure 5-1-This can be amibuted to more effective degrees of freedom available in HC3N. Figure 5.1 also shows that up to seven HC3N molecules add sequentially. The addition of the seventh HC* molecule is very slow, 3.7 (i1.9)x IO-" cm3 molecule-1 s-1 ,suggesting that the HC3N rnolecule interacts through solvation. From a semilogarithmic plot of rate coefficients versus the number of ligands, Figure 5.2, an alternating pattern emerges. The second, fourth and sixth cyanoacetylene adducts are formed quickly and react more slowly than the odd numbered adducts. This suggest some sort of special stability is associated with the second, fouah and sixth adducts; maybe a ring-like system is being formed. The idea of a special stability is further Ion Simal Fi,we 55.: A semilogarithmic plot of the rate coefficients vs. the number of ligands for the reaction of Mg* with HC3N. suggested by the CID results. On the rïght side of Figure 5.1, the CID profile is very complicated- The following reactions summarize the observed fragmentation charnels. Various sized HC* units are removed in the CID experiment suggesting some sort of LigandlLigand interaction. As evident fiom the CID profile, Mg* coordinated by four cyanoacetylene ligands is unique and very stable. Another interesting feature to note is that apparendy a neutral cyanoacetyiene tetramer is Iost from Mg(HC3N)6Cat a very low onset ener,oy. b. Reaction with (c-C&)Mg+ The halfiandwicWcyanoacetylene chemistry is very straightforward in comparison with that of MC. Three cyanoacetylene molecules add fairly quickly to the haif sandwich, Figure 5.3, while a fourth molecule is seen to add very slowly, perhaps in a solvating manner. A sernilogarithmic plot of rate coefficients versus the number of ligands (Fi,gure 5-41does not show the altemation that was seen with Mg*/HCfl. On the right side of Fi,gure 5.3, the CID shows that the successively added HC3N units are removed sequentially as the nose cone voltage is increased, mer suggesùng no ligand/ligand interactions. Ion Signal Figure 5.4: A semilogarithmic plot of the rate coefficients vs. the number of ligands for the reaction of (c-c,H~)M~~with HC3N. c Reaction with (c-CsH&Mg+' In the full sandwich chemistry, the btstep is the displacement of one c-Cs& ring by cyanoacetylene (k = 3.2 (k1.0) x 10" cm3 molecule-l s-l). Afier the ligand switching, the observed chemistry behaves in a rnanner analogous to that initiated by the half sandwich, Fiapre 5.5, by adding four cyanoacetylene molecu~esthat are sequentially removed in the CID experiment. - 53. Structure and Bonding of Mg+*/HCa The unique kinetic penodicity and complicated CID profile seen in the reaction of MgW/HC3Nprompted a mertheoreticai investigation to probe various structures that might account for the experimental results. The geometnc parameters of the Mg@K3N)c structures ar B3LYP/6-3 1+G(d) are shown in Fi-pres 5.6 to 5.12. Table 5.2 contains the total energies (in hartrees) and unscaled zero-point and thermal corrections (in kcal mol-') fkom the optimizations at B3LYP/6-3 1tG(d). Three critical points were found for the fust addition of cyanoacetylene to Mg? The Sest arrangement is structure A, attack by the lone pair on nitrogen of the cyanoacetylene creating a linear 41c structure with a Mg-N bond lenath of 2.130A. The bond energy of structure A is 37 kcal mol-'. Two x-type structures were investigated, one with the. magnesium interacting with the NC 7c-system and the other with the magnesium interacting with the CC x-system. When allowed to interact with the NC x- system the magnesium migrated to the lone pair on the nitrogen, collapsing to structure 147 Figure 5.5: Reaction profile for the reaction of (C-C,H~)@< with HC,N O 1 2 HC,N Flow /loL7(molecule s-') 148 Table 5.2: Total energies (in hartrees), zero-point vibration energies and thermal corrections (in kcd mol-'). The relative energies (in kcai mol-') are given in brackets. Molecde B3LYP/6-3 l+G(d) ZPE Thermal Energy MgC -199.79555 -- -- A -369.43743 (O) : 18-3 - 3-4 B -369.38694 (3 1.7) 17-1 3 -7 C -369.308 17 (8 1.1) 13.3 3.9 A + HCCCN -539.01864 (27.8) D -539.06292 (O) E -539.05335 (6.0) F -539.05069 (7.7) G -539.04975 (8 -3) EI -539.04680 (10.1) 1 -539.030 14 (20.6) J -539.01947 (27.3) E + HCCCN -708.63456 (5 1.1) - - K -708.7 1603 (0) 56.7 8.2 L -708.70806 (5.0) 56.3 8-5 M -708.69074 (15.9) 53.7 9.6 N -708.68048 (22.3) 56.1 8 -5 O -878.40404 (O) 79.7 8 -8 P -878.38693 (1 0.7 ) 79.8 8 -9 Q -878.33986 (40.3) 74.5 11-7 R* -878.30057 (64.9) 74.0 11.5 S* -878,29358 (69.3) 74.0 11.5 T -878.28941 (7 1.9) 70.4 13.2 U -878.22749 (1 10.8) 78.2 9 -5 *These calculations are incomplete and the quoted total energies, DE,thermal ener9es and geometries are pro ected values. I estimate by following this procedure to be no greater than 1 kcal mol' i. Fi,.ure 5.6: Optimized geometrical parameters for structures at several cnticical points on the M~(Hc~N),*surface fiom B3LYP/6-3 l+G(d) calculations. + 2.130 Mo- Figure 5-7: Optimized geometrical,+ paramerers for structures at several critical points on the Mg(HC,N) surface &orn B3LYPf 6-3 1+G(d) calculations. bond angles for structure F: B3LYP/6-3 l+G(d) 1 123.1 2 103.7 3 147.4 4 109 -5 5 116.2 6 -120.3 Figure 5.8: Opumized geometrical parameters for structures at several critical points on the M~(HC,N),* surface fiom B3LYP/6-3 l+G(d) calculations. Fi,we 59: Optimized geometrïcal parameters for structures at several cntical points on the M~(Hc~N)~surface fiom B 3LYP/6-3 1+G(d) calculations. Figure 5.10: OptimÏzedM~(Hc,N),+ pometrical parameters for structures at several critical points on the surface i?om B3LYP/6-3 l+G(d) calculations. 1.489 bond angles for structure Figure 5.11: Optimïzed geomenical parameters for structures at several critical points on the Mg(HC3N),+-surface fkom B3LYP/6-3 l+G(d) calculations. 180.0 Figure 5.12: Optimized geometrical parameters for structures at several critical points on the M~(HC,N),~*surface from B3LYPl6-3 l+G(d) calculations. A. The n-cornplex formed over the CC n-system, structure B, resulted in a structure with a long MpC bond len,gth of 2.514 A and an energy that is 32 kcal mol-' above structure A. The insertion of MgC into the C-H bond was also investigated (structure C)and this is a high energy isomer that is more than 40 kcal mol-' above the dissociation products. McLigated by two HC3N molecules apparently has a unique stability, as evident from the hetic plot, Figure 5-2- The global miiiimum on the Mg(HC3N)2" surface is structure D. It is an 8ic-system with two cyanoacetylene ligands attached to the magnesium Ehrough the lone pain on the nitrogens (Mg-N bond lena@ of 2.125A and a NMgN bond angle of 97.8"). A similar NMgN bond angle has aiso been seen with Mgc/ammonia and MgTwater. The energy required to break a Mg-N bond and form structure A is 26 kcal mol-'. There are two arrangements on this surface that are planar cyclic systems with two Mg-N bonds and Czv symmetry, srnichues E and G (6.0 and 8.3 kcal mol'[ above structure D, respectively). Structures E and G have similar geometrical parameters, with the only notable difference being a weak CC bond (1.596 A) present in structure E. Another distinguishing feature is the n-orbital populations of these two structures. Stmchre E is a 9ir-system, whereas structure G is an 8x-system and therefore these smctures cannot interconvert without changing the x-population. Structure F is the only criticai point found that lies below the dissociation limit that has a nitrogen center in the ring. It is a planar structure with a six-membered ring and the magnesium is ligated through the other nitrogen in the system. Structure H is a four carbon, planar, ring system that is 10.1 kcal mol-' above structure D. Structure I is 20 kcal mol-' above the global minimum and contains two different rnagnesium bonds, Mg-N and LM~-C. Structure J is a polymer-like chain and is also at a critical point on this surface. Shown below are several other dimer ions, but they are high energy species and were only calculated at HH3-2 1G(d). On the Mg(HC3W3" surface four arrangements were investigated (structures IZ- -W. Unlike on the dimer surface, the structures on the Mg(HC3N);* surface that have 8 membered Rngs are the most stable. Structures K and L are the best arrangements and are only 5 kcal moïLdifferent in energy. The ring system of structure K is similar to structure E on the dimer surface and the ring system of structure L is sirnilar to structure G. As was seen on the dirner surface, the only difference between these two structures is a long C-C bond present in structure K, but not found in structure L. The TC-populations of these two structures are also different, structure K has 13n; electrons, whereas strucnire L only has 12x electrons. The energy required to fra,pent one MpN bond as shown in 158 reaction 5.9 and form structure E and HCCCN is Dzs8 = 49.3 kcal mol-'. The energy required to fragment the same bond in structure L and form structure G and HCCCN is DZg8= 46.5 k~d Structure M is the only arrangement thai involves no ligandlligand interactions. Structure M is also planar and benefits f?om delocalizing the ir-system over the entire structure. The analogous ammor~ia,~~(~~(bE4~)~") and ~ater'~"' (M~(H~o),~) ions are pyramidal with the unpaired electron located in a p-type orbital on the magnesium. Ligands like ammonia and water do not have x-systems through which they can delocalize and therefore adopt a pyramidal conformation in order to reduce the lone pair interactions." The bond energy of one Mg-N in structure M is D298= 27.3 kcal mol-! Structure N is more than 20 kcd mol-' higher in energy than structure K. Structure N contains a dimer unit analogous to structure H found on the M~(Hc~~*dace. An interesting feature of this arrangement is the short Mg-N bond of 1.866A. This is the shortest Mg-N bond found in any of the calculations in this thesis. The endothermicity for reaction S. 1O is D298= 30.9 kcd mol-'. The CID experiment indicates that M~(Hc~N)~*has some speciai stability because even at the highest nose-cone voltages it was not bgnented. The special stability nrst suggested a structure with a cyclam type arrangement with M~ sihiated in the rniddle of a ring like structure U. Structure U is a critical point with long magnesiurdnitrogen interactions of 2.3 14 A. However, the large unsaturation and ring sîrain present in this arrangement makes this a hi&-energy conformation, which is more than 100 kcal mol-' above the best structures on this swface. Structure O is the best structure on the M~(Hc~N)~*surface. The magnesium center is situated on top of a 1,2,5,6-teimcyano- l,3,5,7-c yclooctatetr frzunework. This arrangement ailows the magnesium center to interact with al1 four nitrogens (Mg-N bond length equal to 2.1 92A) and requires more than 160 kcal mol-' to remove Mg£?om the arrangement, reaction Structure P is only slightly higher in rnergy than structure 0. lt is also a cyclooctatetraene, but with the cynno-groups located in the 1,2,3,4 positions. The magnesium center can still interact with al1 nitrogens in this arrangement (1Mg-N bond length of 2.192A). Structure T also contains four Mg-N bonds of 2.052 A. The HC3N ligands in this structure are non-interacting, but even though there is no ring strain present in this arrangement it lies weli above the best structure on the surface. The endothemiicity of reaction 5.12 is bg8= L 1.8 kcd mol? P Stiucture S contains two individual rings, analogous to structure G. that are occupying planes that are perpendicular to each other. Smcture S is almost 70 kcal mol-' above the besc structure on the tetramer surface. Structure Q is a better arrangement than structure S. It contains only one planar ring system and two non-interacting HC3N ligands. Structure R is more than 60 kcal mol-' above the cycloctatetraene arrangements. This structure contains two dimer components as previously seen in structure H and a NMgN bond angle of 180". 5.4. Possible Mechanism for the Formation of CycIaoctatetraene A possible mechanism for the formation of 1,2,5,6-tetracyano- 1,3,5,7- cyclooctatetraene-magnesium ion in the sequential reaction of four cyanoacetylene molecules to Mgw is shown in Fi-we 5.13. The first adduct, Mg(HC3hvC, is formed by nucleophilic attack of on Mg* by the lone pair of electrons on the nitrogen of the incoming HC3N molecule, This step is relatively slow due iq part to the degrees of freedom present in the intermediate, wg(HC3N)"]*. Upon bond formation, the charge becomes delocalized &ou@ the n-system; one of the resonance structures localizes the charge on the terminai carbon of the cyanoacetylene Ligand. The next cyanoacetylene molecule is involved in a 212 cycloaddîtion that requires the n-systems to be coplanar in order to be able to form a four carbon ring with some of the positive charge Iocated on the magnesium center. The third adduct of Mg(HCN3" is formed by nucleophilic attack of the lone pair on the nitrogen of the incomhg cyanoacetylene molecule, analogous to the first step in this mechanism. The fourth adduct is formed via another 24cycloaddition and forms another four carbon ring with the cyano-groups in the tram conformation. This is an intermediate structure that ailows the two individual cyanoacetylene ~g dimers to be able to freely rotate. Cyclooctatetraene cm then be formed through a ring closure step that requires the two four-membered ring systems to be face to face with each other. The periodicity obsemed in the kinetics is consistent with this mechaaism and calculations have shown that the cyclooctatetraene conformation is the best arrangement on the surface by more than 40 kcal mol? The Figure 5-13: A possible mechanism for the formation of 1,2,5,6-tetracyano- 1,3,5,7-cycIooctatetraenernagnesiurn ion in the gas-phase. ]Hmc-c=N= >'Mg *Mg 2 \ %+ N' NC \C \ /" C-C 4" C=C \\ I I - I l /'=""H /'=" 'H &c N N ** @c *O calculations have also shown that if the 1,2,5,6-tetracyano- 1,s ,5,7-cyclooctatetraene- magnesium ion is formed then it wiii require a lot of energy to fraDpent, which is consistent with the CID experiments. Fiagure 5.14 is an overview of only a few of the possible thermochemical pathways that could be accessible of Mgw when it is reacted with HC3N. The green boxes indicate the pathway most ljkely followed on route to the formation of 1,2,5,6- tetracyano- l,3,5,7-c yclooctatetraene-magnesium ion. This pathway is aiso consistent with the alternation in rate constant values seen in the kinetics and CD experiments. Calculations have only be attempted for structures containing up to four cyanoacetylene molecules, but based upon the results f?om the Mg(HC3N)r calculations the next two HC3N molecules could fom another dimer arrangement that is directly ligated to the metal center. A sevenrh cyanoacetylene molecule was observed to add slowly and could be interacting with Mg(HCsN)6U in a solvating rnanner. 5.5. Conchsions MgC has essentially no effect on the geometncal parameters of the ligands. On the dirner surface the best structure contains two non-interacting ligand units and a NMgN bond angle of -98'. The MgL" structures containing ring dimers become likely candidates for formation when n 2 3. MgL3+ prefen non-interacting ligands in a planar arrangement to benefit fiom the delocalization available through the n-system. The formation of a cyclooctatetrene with the four -CN groups interacting with the magnesium center is the best arrangement on the Mgh" surface. The periodicity in the kinetics of Figure 5.14: A few possible +@ thennochernical pathways for the reaction of Mg+. with HC3N. * Calculation not complete. m298in kcal mol-' Mg"/HC3N can be amibuted to the formation of the dimer rings such as those found in structures E, G or H. The special stability found in the rnulh-CID measurements could be associated with the ~g(Hc3N)kstructure could be amibuted to structures O or P. 5.6. References Bohme, D.K; Raksit, AB. Mon. No?. R. Astr. Soc. 1985,213,717- Fox, A.; Raksit, A.B.; Dheandhanoo, S.; Bohme, D.K. Can. .L Chem. 1986,64,399- Raksit, A.B .; Bohme, D.K. Can. J. Chem 1985,4,854. (a) Francisco, J.S.; Richardson, S.L. J. Chem. Phys. 1994, 101, 7707. (b) Kunde, V.G.; Aikin, AC; Hanel, R.A.; Jennings, D.E.; &Mapire,WC; Samuelson, R.E. Narure, 1981,292,686. Reppe, W.; Schlichting, O.; Klager, K; Toepel, T. Jusrw Liebigs Ann Chem. 1948, 1,560. (a) Maitiis, PM. Pure Appl. Chem. 1972, 30, 427. @) Maitlis, P.M- Pure Appl. Chem 1973,33,489- (a) Jolly, P.W.; WiUce, G. The Organic Chemistry of Nickel, Academic Press, New York, 1975 @) Diercks, R.; Stamp, L.; Dieck. H. Chem. Ber. 1984, 117, 1913. (c) Simons, L.H.; Lagowski, JJ. Fundam. Res. Homogeneous Catal. 1978, 2, 73. (d) Cope, A.C.; Campbell, H.C. J. Am Chem Soc. 1952, 74, 179. (e) Cope, AC; Rugen, D.F. J. Am. Chem. Soc. 1953, 75,3215. Colborn, R.E.; Volthardt, K.P.C. J. Am- Chern. Soc. 1986,108,5470. (a) Longuet-Higogins, H.C.; Orgel, LE. J. Chem Soc. 1956, 1969. @) Criegee, R.; Schfider, G. Justus Liebigs Am.Chem. 1959,623, 1. (c) Hogberg, H.; Frohlich, C. Angew. Chem., Int. Ed. Engl. 1980,19, 145. 10. (a)-schrauzer, G.N.; ~ichlër,S. Ber. 1962, 95, 550. (b) Schrauzer, G.N.; Eichler, S. Angew. Chem. 1961,73,546. 11. Tsutsui, M.; Zeiss, H. J. Am. Chem Soc. 1969,82,6355- Cava, M.P. ; Mitchell, M. J. Cyclobutadiene and Relared Compoundr, Academic Press, New York, 1967. Wade, L.G., Jr. Organic Chemisrry, znd ed., Prentice Hail Inc., Toronto, Canada, 1987. (a) Bauschlicher, C.W., Ir.; Partridge, H. Chem. Phys. Lens 1991, 95, 3946. (b) Bauschlicher, C.W., Jr.; Partridge, H. J. Phys. Chern. 1991,95,9694. (c)Glendening, E.D.; Feller, D. J. Phys. Chem 1995,99,3060. 15. Partridge, H.; Bauschlicher, C.W., Jr. J. Phys. Chem 1992,96,8827. 16. Milbum, R.&; Baranov, V.I.; Hopkinson, A.C.; Bohme, D.K. J. Phys. Chem. A 1998,102,9803. 17. Bernardi, F.; Epiotis, N. Applications of MO Theory in Organic Chemistry 1977, 2, 47. CHAPTER 6 THERMOCHEMZCAL PROPERTIES OF R/l&NPHmAND MgO& 6.1. Introduction Magnesium-containing compounds have recently attracted considerable attention in the astrochemical fieId. Magnesium-containing moIecules are as abundant as sillcon- containing compounds in the cosm~s.~Several Si-containing compounds that have been characterized by radio or IR observations include: SiO, Sis, Sic, SN, Sic?, SiCa and Sa. By contrast, the only magnesiurn-containing molecules identified by radioastronomy are the radicals MgNC and M~cN.'.' The chemistry of magnesium in flames has been the reoent focus of an experimental investigationO3 The chemistry of magnesium ions in bel-rich, Hz-0rN2 flames at atmospheric pressure in the temperature range 1820-2400 K was investigated by sampling the fiames doped with magnesium through a nozzIe into a mass spectrometer. The smdy focused on determinhg the proton affiinities of MgO, MgOH and Mg(OE&, estimating the rate coefficient for chemi-ionization of Mg + OH and/or Mg0 + H, and the rneasuring of a global electron-ion recombination coefficient for magnesium ions with e-. Here the theoretical details of this paper are given. They provided insights into geometrïcal structures as well as infonnatian regarding the thermochemical properties of ions that cm not be measured under these experimental conditions. This chapter WU discuss the results of a theoretical snidy into the structural and thermochemical trends of the magnesium-nitrogen-hydrogen and the analogous 170 mapesium-oxygen-hydrogen systems using molecular orbital theory- A review of this system of molecules and cations revealed that there are either no current literature vaiues, or in some cases large uncertainties, associated with the experimental values. This theoretical study establishes some thennochernical properties for quantities not yet measured experimentay and attempts to solidify experimentai values that have large uncertainties . 6.2. Results and Discussion Table 6.1 contains the total energies, zero-point energies and thermal corrections from structural optimizations. 6*2*1.Structural Detaiis Calculations fkom the B3LYP (top numbers) and the MP2 (botmm numbers) optimizations provide the geornetrical parameters given in Figures 6.1 and 6.2, (except for MgNH+). There are no literature values available for this family of ions and molecules with which to compare these structural parameters. It is noteworthy that both MgOH and MgOr are linear structures and that the removal of an electron fiom MgOH results in a considerable shortening of the Mg-O distance @y 0.074 A at MP2) but a smd lergghening of the O-Hdistance @y O-OO~Aat MP2). Conversely, protonation of MgOH on O results in elongation of the Mg-O distance @y 0.286 A at MP2), indicating a much weaker bond. The dihydroxide Mg(OE& has a OMgO bond angle that is close to hear, but bas a MgOH bond mgle that is non-linear, and the overall structure belongs to the C2 point group with a dihedral angle between the two terminal H atoms of 118.4". At MP2, protonation of Mg(OH)2 on O 171 Table 6.1: Total eiiergies, zero-point and thermal energies from structural optiinizations". Molecule B~LYP~ M P2' ZPE~ Thermal QCIc- CCSD' (1i:irirees) (Iiarirccs) (kclil niol") (kcal iiiol") (tiarirccs) (Iinrtrccs) -275,22597 -274.79448 1.3 1.5 -274,855 1 1 -274.85244 Table 6.1 :Coiitiiiued. Molecule B 3LY Pl' M P2' ZPE" Thermal QCl" CCSD' (hnrirccs) (I~nrirces) (kcal mol") (kcal mol*') (tiarirccs) -76.42257 -76.29395 12.8 1.8 -76.34422 -56.55699 -56.43468 20.6 1.8 -56.484 12 -200.07958 - 199.79555 -54.58777 -75.06760 -0.50027 -1.17548 Total energies, in liarti.ees, for tlie sinaller atoms and molecules used in calculaiing the atoiiiization energies at B3LY Ph31 1++G(2df,p) and CCSD(T)(full)/6-31 1++G(2df,p), respectively, are M~NH+(~L)-255.103 13, -254.70253; H(~s)-0.50226, -0.49982; H~('z'',)- 1.17957, 1.16838; N(~P)-54.60072, -54.53 132; M~('s)-200.09326, - 199.77364; M$(*s) - 199.80927, - 199.49594. Optimization at B3LYPI6-3 1+G(d). Optiinization at MP2(fu11)/6-3 1 1++G(d,p). Zero-point energies are scaled by 0.94. Single-point at QCISD(T)(îuII)/6-31 1 ++G(2df,p)//MP2(full)/6-3 1 1++G(d,p). Single-poiiit at CCSD(T)(full)/G-31 1 +tG(2df,p)//MP2(ful1)16-31 1 ++G(d,p). ' Optiiiiizat ions ai B3LYPl6-3 1 1++G(2df,p) and ai CCSD(T)(full)l6-31 1++G(Zdf,p). Figure 6.1 :Optimized geometxkal parameters nom B3LYP/6-3 1-d) (top numb ers) and MP2(fuU)16-3 1 lf+G(d,p) (bottom numbers) caiculations for several molecules and cations ofMflnEE,- Figure 6.2: Optixuized geometrical parameters fkom B3LYP/6-3 l+G(d) (top numbers) and MP2(full) /63 1lttG(d,p) (bottom numbers) calculations for several molecules and cations ofMw&. results in the lengthening of the Mg0 distance involving the protonated oxygen by 0.192 A, while the other Mg0distance decreases by 0.058 A. At the same level of theory the calculations show a structure that has a similar Mg-O distances to those in MgOr and in MgOH2'. There is a second minimum found for HOMgOH2+, solvation by water in a second shell, represented by MgOF. ..(OH2). The MP2 calculation yields the solvated species 42.2 kcal moï' higher in energy th- the directly coordinated structure, HOMgOH2+. The structure of singlet MgNH is shown below, with a double bond bemeen magnesium and nitrogen and a lone pair of electrons in the plane on the nitrogen. When the neutral is ionized the electron is removed Erom the lone pair on the nitrogen and as a result the MgNH bond angle opens up dramatically from 105.8" to become hear. The Mg-N bond length in the neutral shortens upon forming the ion. The energy difference between the singlet-triplet ground states of MgNH is smalf. At B3LYP the triplet is lower in energy than the singlet by about 7 kcal mol"; at the MP2 level the energy gap closes to less than 3 kcal mol-'. In the CCSD calculation, the energy gap still favours the triplet, but by less than 0.5 kcal mol? The singlet state of MgNH becornes the sound state at QCI. The triplet state of MgNH has one unpaired elecuon located on the rnagnesium in a O-orbital and the other in the X-system. The geometry is very different than the optimized structure found for the ground state singlet. The singlet state has a Mmangle of 105.8' at MP2 whereas the mpiet is linear. Also the Mg-N distance of 1-832 A in the singlet is much s horter than that in the uiplet (1-9 1 1 A), consistent with one fewer n-bcnding electrons in the triplet. Mmrequired a more extensive treatment because of the States very close in energy and an unequally populated n-system. The structure of-the optimized doublet, Mgm, is linear with a Mg-N bond lena@ at B3LYP and CCSD(T)(FuU), using a basis set of 6-3 1 l++G(2df,p), of 1.8 15 and 1.8 12 A, respectively. Mmis a 3n electron system with the unpaired electron localized in plane on the nitrogen center and the pair of R-electrons in the out-of-plane system, formbg a x bond between magnesium and nitrogen. When an elecvon is rernoved from singlelet MgNH or nom MgNH?, the Mg-N bond shortens slightly. In Mm2,the bonding can be descnbed as a single Mg-N bond with a lone pair on the N and the unpaired electron is localized on the Mg atom; it is this unpaùed electron that is removed to form the cation and the resultïng positive charge is prirnarily localized on the magnesium center. O* + mœ *Mg- N - Mg- N + e \ H \ H Mg(NI& is isoelectronic with allene and similarly belongs to the Dx point group, with the hydrogens in a staggered conformation. Looking dong the NMgN axis 177 the moiecule wodd appear as shown below. The Mg-N distances at MP2 are slightly shorter than that in Mm2(1.914A compared with 1-931A) but are much longer than the Mg-N double bond in MgNH ( 1-832A). Another interesthg geometrical feature occurs when magnesium is coordinated directly by two ammonia or water ligands. The angle NMgN of 99.5" or OMgO of 88.8" is much smaller than a typical sp2 hybridized atom. ~auschlicher~attributes this to elecuon density and polarization of the 3s orbital. An alternative exphnation is that bonding occurs through donation from the nitrogen into the vacant p-orbitds of magnesium and steric interaction then causes the angle to open up fkom 900.~When a second ammonia is introduced into the system the Mg-N bond 1eqg.h increases slïghtly; this also happens when a second water is introduced but to a lesser extent because water is not as buky a ligand. In generd, the addition of a hydrogen atom to a molecule or ion causes an increase in the Mg-N bond lena&, The most dramatic example of this is seen when Mm2+is compared to MgNH3+. The Mg-N bond lena@ increases by over 0.2A. In sharp contrast, when Mg0 is protonated the Mg0 bond length shortens by 0.064A, but when MgNH is protonated the MgN bond length increases by 0.056A. 6.2.2. Thennochernical Properties 6.2.2-1. Enthalpies of Formation The energy required to cornpletely dissociate a molecule, AHrt, (reaction 6.1 and Figure 6.3)ïnto its component atoms is called the atomization enerav. AHn+A+nH The enthdpy of formation of the molecule, A&, is given by equation 6.3. mfaO(AHn)= mf.oO(A)+ nA&oON - ZDoO(AHn) (6 -2) The total atomization energy, EDO0 (AHn), was calculated from molecular orbital calculations using an iso,*c reaction involving H atoms and H2 to balance the spins6 and then compensating for the addition of H atoms by using the experimental Defor ~2.~ We illustrate the procedure using the example Mg0 ('~9-A common problem when calculating dissociation energies is the dissimilar number of unpaired elecnons in the products and the reactants and this results in the correlation energies being different for each side of the equation. Calculation of accurate atomisation energies then requires accurate assessments of the correlation energy, and since it is not computauonally possible at this rime, the problem is overcome by using isogyric equations as seen in equation 6.3. An iso,+c equation has the same number of unpaired spins on each side of the reaction. MgO+2H+Mg+O+H2 (6-3) Subsequent removal of 2H from the reactants and Hzfrom the products by using the ZD, of Hz (0.17447 hartrees8) and the zero-point energy of MgO, unscaled, (1.3 kcal mol") Schematic illustrating the determination of the enthalpy of formation of MgO(g) fkom the total atomisation energy of Mg0 and the enthalpy of formation of the individual atoms. gives a value for the atomisation energy, EDO,of 67.4 kcai mol-' for reaction 6.1. AHJo" for Mg0 was calculated by subtractïng ZDo from the sum of the experimental enthalpies of formation of the atoms ~BJo(aroms)= 34.87 1 (for Mg) + 58.984 (for O) = 93.855 kcal mol-']; i.e. AHJo" = 93.855 - 67.368 = 26.487 kcal mol? The heat capacity of MgO, calculated fiom the theoretical thermal energy plus the work tenn (PV), was added to AHJO0, and the experimental heat capacitiesg of the elements mg and %O1] were subtracted to gïve mJ2980= 26.3 kcal mol-'. Ions were treated in an identical way but with an electron being added into the atomisation reaction. The stationary electron convention was used. Standard enthalpies of formation at 298 K are given in Table 6.2 in kcal mol-'. Previous work in this group ha shown that enthalpies of formation using QCISD(T)(FUU)/6-3 1li-+G(2df,p) are within +3 kcal mol-' of the accepted experimental values.'@'3 From previous experience, the MP2 calculations are too low level to give reliable values for enthalpies of formation. For ail the molecules and ions investigated in this study single-point QCI and CCSD calculations are in excellent agreement with the largest difference being for Mg0 (1.7 kcal mol-'). There are three primary literature sources considered: the fint mo are the standard evaluated compilations by Lias et alL4 and the JANAF ab les", and the third is a paper by Freiser's gro~~'~with which our calculations show a fair measure of agreement. Some of our calculated values disagree with the values in the two compilations. Table 6.2: Standard Eiiilialpies of Formation ai 298 K, AH? in kcal mol". Molecule B3LYP" MP~" 0Clc ccsD" Ex~erime~itnl M~O'(*il) MgO+ (2~) MgOH (*L) M~OH+(lx) M~OH; (*z) ~p@H2)2+(*V Mg(OW2 H20~g0Ht M~OH'. .. (OH2) Table 6.2: Continued. Molecule B3LYPn MP~' 8ClC CCS~ Exrierimental Optimiziition at B3LYP16-3 1+G(d). Optimization nt MP2(fu11)/6-3 1 1 ++G(d,p). Single-point at QCISD(T)(full)/G-3 1 1++G(2df7p)//MP2(fu11)/6-31 l++G(d,p). Si ngle-point at CCSD(T)(fuII)/G-3 1 1++G(2df,p)//MP2(full)/6-3 1 1++G(d,p). Optimizat ions at B3LY P/6-3 1 I+tG(2df,p) and at CCSD(T)(full)/6-3 1 1++G(2df7p). Lias, S,G,;Bartmess, J.E.; Liebman, J.F.; Molmes, J.L.; Levin, R.D.;Mallard, W.G. J. Phys. Cliein. Ref. Data, 1988, 17 (Suppl. 1) Cliase, M.W.,Jr.; Davies, C.A.; Downey, J.R., Jr.; Frurip, D.J.;McDonald, R.A.; Syveriid, A.N. JANAP Thermocliemical Tables, 3".' ed., -1. Phys. Cliem. Re& Dota, 1985, Data 14 (Suppl. 1); Murad, E. J. Pllys. Clier~r.1981, 75,4080. 6.2.2.2, Proton Affiities The proton affinity of a molecule B is defmed as the enthalpy change for the reaction shown in equation 6.4. Protonation reactions have the same number of electrons on both sides of the equation and therefore inclusion of electron correiation is expected to be of less importance in the calculation of proton affîties. The proton af5uLities quoted in this text are derived directly from the eiectronic energies, but they can aiso be obtained from the calculated enthalpies of formations. BH++B+P (6-4) Table 6.3 contains the proton affinities at 298 K in kcal mol" for Mm,MgNH2, Mg(NH2)2, MgO, MgOH and Mg(OH)? at all levels of theory investipated. The calculated proton affinities are similar at ail levels of theory with the MP2 value always being the largest and the order being MP2 > B3LYP. For this property the DFT results agree fairly well with those from the more sophisticated methods employed in this study, QCI and CCSD. Both the rnasesiurn-nitrogen and magnesium-oxygen molecules have larger proton affïnities than both ammonia (204 kcal and water (165.2 kcal moi- 1 17 ) The magnesium-nitrogen molecules have larger proton affinities than the analogous magnesium-oxygen molecules. The calculated proton afnnities of MgOH and Mg(0H)z are 215.7 and 204.3 kcal mol-', respectively, at CCSD. These are more than 20 kcal mol' 1 less than the analogous magnesium-nitrogen molecules, MgNH2 and Mg(NH2)2. The increased proton affinity is a result of magnesium being more electropositive than hydropn, which in mm makes the electrons on the oxygen and nitmgen more available for protonation. Table 6.3: Proton nffinity at 298 K in kcal mol". MgNH (IA') 26 1.3 263.3 258.0 258.9 4M-h(*AI) 239,5 240q7 238,l 2382 Mg(NH2)2 229.3 230.6 229.4 229.4 a. Optiinization at B3LYPl6-3 1 +G(d). b. Optiniization at MP2(fu11)/6-3 1 1 ++G(d,p). c. Single-point at QCISD(T)(full)/6-3 1 1 +tG(2df,p)/lMP2(full)/G-3 1 l++G(d,p). d. Single-point at CCSD(T)(fiill)/G-31 I++G(2df,p)//MP2(full)/G-3 1 l++G(d,p). l-' 00 e. Hunter, E.P.;Lias, S.G. NET Standard Datu Base Numbei 69-March 1998 ielease, Vi tittp://webbook.nist.gov/cl~e~nistrylp~t~n/ f. Operti, L.; Tews, E.C.; MacMalion, T.J.; Fseiser, B.S. J. Ani. Chern. Soc. 1989, Ill, 9 152. g. Miirad, E. J. Cliein. Pliys. 1981, 75,4080. 6.2.2.3. Ionization Energies The adiabatic ionization energy for a molecule is defmed as the energy required to remove an electron £rom a neutral molecule in the gas-phase at O K. Isoawc equations are used in a similar rnanner as with enthalpies of formation to remove drfferences in the correlation. The ionization energies are shown in Table 6.4 in eV. The ionization energy of M@H2 at QCI and CCSD is calculated to be 7.041 and 7.044 eV, respectively. In the neutral, the unpaired electron is localized primarily on the magnesium, and this is removed to produce a closed shell ion. In the ion the magnesium carries the majom of the positive charge; ionization of Mmthen takes reIatively less energy when compared to rernoving an electron from MgNH. Neutral, Mm,is a closed sheii system and the electron being removed is cornes fkom the n-system. The ion being formed in this situation also does not have as many centers over which to delocalize the charge when compared to Mm2+and hence requires more energy to make such an ion. As the number of centers are increased in the ion, the ionization energy decreases because of the system's ability to delocalize the charge over more centers. From a study of magnesium- oxygen molecules and ions,3 the ionization energy of MgOH was 7.337 eV at CCSD. Comparably, Mm2bas an ionization energy of 7.044 eV at the same level of theory. The smailer energy required couid be amibuted to two factors; first MgNHz has more centers over which to delocalize the positive charge and second the electron is more readily available in M@W2 than in MgOH. 5n-ea - CC- 6.2.2.4. Binding Energies Table 6.5 contains binding energies for ammonia or water to some magnesium- containhg ions, cdculated from the total energies given in Table 6.1. The values given are very similar at all levels investigated. For comparative purposes MgOHZChas a binding enthatpy of 33.2 kcal mol-' at CCSD versus 39.9 kcal mol-' for MgNH3+, Le. a Mg-O bond strength is sirniiar in magnitude to a Mg-N bond. For MgNHz+, ammonia prefers to coordinate directly to Mg rather than act as a solvahg molecule and hydrogen bond through an NHy*NH3 interaction. At MP2 the directly bonded confiawation, HD:-tMgNHz is favoured by almost 48 kcal mol". Water also prefers to directly coordinate to the mapesium in MgOr by about 42 kcal mol-' at MP2. As the number of centers in an ion is increased the charge delocalites more and as a result the lone pair of the ammonia would be expected to be less strongly bound to the magnesium. This trend is not followed if we look at MgNHzCor Mgop. A reason could be because removd of the lone electron from the magnesium center in MgNH2 or MgOH, leaves a vacant 0-orbital into which the lone pair on the nitrogen of ammonia or on the oxygen of water donates. The Mulliken charge dismbution shows that the magnesium in MgNHzf carries a +1 -1 point charge, which would be more attractive than the Mg, which carries a point charge of +1.0. .. CI L -- Nb: +i 3 N m M C? m m 3 Cc, N* M CJ 'Z: O 4'E 6.3. Conclusions The calculations were used to determine the standard enthalpies of formation and other thermochemical properties for a range of MgN,H, and MgOnHmbath neutrd and ionic species. There is good agreement between CCSD and QCI calculations for di thermochemical properties investigated in this study. To resolve some of the discrepancies rhat surround some of the thermochemicd properties in the magnesium- oxygen-hydrogen family this study has proposed values that we deem the "best" values. The benefit of using experimental and theoretical techniques to attack a chernical problem was demonstrated in a recent paper by Chen et aZ? 6.4. References Kawaguchi, K.; Kagi, E.; Hirona, T.; Takano, S .; Shuji, S. Astrophys. Journal 1993, 406, L39. Ziurys, LM.; Apponi, A. J.; Guélin, M.; Cernicharo, J. Astrophys. Journal 1995,445, L47. Chen, Q. ; Milburn, R.K.; Hopkinson, A.C.; Bohme, D.K.; Goodings, J-M. 3. Mass Specrrorn. Ion Processes 1999,184, 153. Bauschlicher, C.W.; Partridge, H. Chem Phys. Len 1991,181, 129. Milburn, R.K.; Baranov, V.I.; Hopkinson, AC; Bohme, D.K. J. Phys. Chenz. A 1998, 102,9803. (a) Pople, I.A.; Luke, B.T.;Frisch, M.J.; ~inklcy,J.S. J. Phys: Chem. 1985,89,2 198. (b) Curtiss, L.; Pople. J.A. J. Phys. Chem. 1987,91, 155. (c) Pople, J.A.; Curtiss, L.A. .L Phys. Chem 1987,91,3637. (d) Curiiss, LA.; Ra~havachari,K.; Pople, I.A. Chem. Phys. Lett. 1993,214, 183. Kolos, W.; Wolniewiecz, L. J. Chem. Phys. 1968,49,404. Lias, S.G.; Barmiess, J.E.; Liebman, J.F.; Levin, R.D.; Mallard, W.G. J. Phys. Chem. Re$ Data 1988,17,Suppl. 1. (a) Schlegel, H.B. J. Phys. Chem. 1988,92,3075. @) Schlegel, H.B. Phys. Chem. 1986,84,4530. 10. Rodnquez, C.F.; Hopkinson, A.C. Cm. Chem. 1992, 70,2234. 11. Rodriquez, C.F.; Bohme, D.K.; Hopkinson, A.C. J. Am. Chem. Soc. 1993,115,3263. 12. Rodriquez, C.F.; Hopkinson, AC;Bohme, D.K J. Phys. Chem. 1996,100,2942. 13. Ketvirtis, A.E.; Bohme, D.K.; Hopkinson, AC. Phys. Chem. 1995,99, 16121. 14. Lias, S.G.; Bartmess, LE.; Liebrnan, J.F.; Hohes, J.L.; Levin, RD.; Mailard, W.G. 5. Phys. Chem. Ref. Data, 1988,17 (Suppl- 1). 15. Chase, M.W.; Jr.; Davies, C.A.; Downey, J.R., Jr.; Frurip, D.J.; McDonald, R.A.; Syverud, A.N. JANAF Thermochemical Tables, 3rded., .TJ. Phys. Chem. Re$ Data, 1985, Data 14 (Suppl. 1). 16. Murad, E. J. Phys. Chem. 1981,75,4080. 17. Hunter, E.P.;Lias, S.G. NIST Standard Data Base Number 69-March 1998 Release, http://webbook.nist.gov/chemistry/paser.htm/ CH.R7 DIMER CATIONS OF CYANOACETYLENE: A THEORETICAL STUDY 7.1. Introduction A large number of studies of fullerene chemistry have been undenaken following the discoveryl and synthesis2 of these ail-carbon cage molecules. CeOand its cations present a well-defmed carbonaceous surface to incoming molecules. Singiy-ionized - Merenes such as C6()*react with very few neutral molecules, whereas the dication c~~'~ is highly reactive towards many neutral &olecules, such as unsaturated hydrocarbons3, amines and ammonia4, wate4, alcohols4 and ether~.~ Many nitriles5 undergo nucleophilic reactions with Cson+in the gas-phase, as shown below. The reaction of cyanoacetylene with results in charge-separated products, a dimer ion of cyanoacetylene and CGC,shown below. In this chapter we present the results of the reactivity of HC3N with itself in isolation and in the presence of C& This dimer ion was previously produced fiom the direct association of HCSwith HC3N. The CID experiments on dimers formed in the presence and absence of cso2+produce very interesting and dramaticaily different results. MoIecuIar orbital calculations were used to investigate the potential energy surface for the dimer ions. Several isomers were identifïed: a weakly bound-linear dimer, a strongly bonded non-linear dimer and several four-membered cyclic structures. Some of these structures are consistent with the experimental observations of two distinctly different isomers of (Hc3N)2*- 7.2. Results and Discussion 7.2.1. Review of Previous Experimental Results The ions, Cso2+ and HC3w were produced in a low-pressure ion source by electron-impact ionization of Ca and HC3N vapour at electron energies between 60-100 and 25-45 eV, respectively. The cyanoacetylene was prepared kom propiolic acid (98% Aldrich Chernical Company) by converting the acid into an ester, and then into an amide and anid ide.^ The CID spectra of the product ions were taken in the manner reported by Baranov et al? using 26% argon in heliurn as the bufferkollision gas. The reaction profile for C~()'+/HC~as well as both the CID proffies were performed by Dr. Jing Sun. The reaction profile for (HC3N")/(HC3N) was produced by A. Fox et aL8 Table 7.1 sumarizes the products and rate coefficients measured for the reaction of c(jO2+with HC3N at the standard SIFT conditions. No reaction was observed between Table 7.1: Effective birnoiecular rate coefficients ( in units of cm3 molecule" s-') for the reaction of cfjo2+with HCfl at (294 t 3) K in a heiium buffer gas pressure of (0.35 f 0.0 1) Torr. Reactant Ions Product Ions kucp TC' c~~~ c60(~~3~'+ 7.3 (s.4) x 10-l~ 2-6 x 10" c~~(Hc~N)" +- (HC~N)?+- 7.7 (f2.6) x IO' CsO+-/(HC3N)2*- not observed 1 x i0-13 a. Su, T.; Bower, M.T. ht. J. Mass Spectrom, Ion Phys. 1973,12,347. Table 7.2: Effective bimolecular rate coefficients ( in units of cm3 molecule-l s-') for the reaction of HC3wwith HCaat 298 K in a helium buffer gas pressure of 0.3 19 Torr. Reactant Ions Product Ions kexp TC' HC3N'- GGNh+- 1.2 (9.4) 10-~ 3.5 x 10'' wc3N))- (HC3N)3+- not measured a. Su, T.; Bower, M-T. Int. J. Mass Spectrorn. Ion Phys. 1973,12,347. Cm+ and HC3N, lGxp < 1 x 10-13. The reaction profie for cso2+LHC3~is shown in Figure 7.l(left side). The effective bimolecular rate coefficient, k, for the primary addition reaction is 7.3 (IT2.4)x IO-'' cm3 molecule-' s-', shown in reactions (7.1) and (7.2). cm2'+ HC3N -t [c~~(Hc~N)'~* (7-1) [c~~(Hc&~*t He + C~~(HCSJ)" + He* (7-2) No charge transfer was observed to-occur betweed cs2+(LE. of G60Cis 11.39 f 0.05 eV)' and HC3N (LE. = 11.64 f 0.01 eV)'' as would be expected based upon the ionization energes. The Qation of the second HC3N molecule occurs at the collision rate and results in a separation of the charge, equation (7.3). c~(Hc~~~~* t HC3N + CmU + (HC3N)2+' (7-3) Such a pathway is not unique to this system, and has been pieviously reported for the reactions between cm" and other nitriles, RCN (where R = C2H3, C2Hs, CH2CN and CN).[' Table 7.2 summarizes the product and rate coefficient measured for the reaction of (HC3N)" with HC* at 298 K and in a helium buffer gas pressure of 0.3 19 Torr. The reaction profile for (HC3N)/(HC3N)* system is also shown in Figure 7.1 (right side). The formation of the cyanoacetylene dimer is very rapid and was measured to be almost collision rate at 1.2 (kO.4) x cm3 molecule-' 8'. The collision rate, k~~,for this reaction is so fast because the dipole moment of cyanoacetylene is large. The proton transfer channel was observed to be aimost absem6 A second HCsN molecule was observed to add to the dimer, (HCSrl)T, at an immeasurably slow rate. 195 Figure 7.1: (Right side) Reaction profde for the reaction of c6O2+ with HC3N. (Left side) Reaction profile for the reaction of HBN" with HC3N. The CID-spectrum of the dimer formed in the presence of CZis shown in Figure 7.2 on the lefi side at a cyanoacetylene flow of 5.0 x loL' molecules s-'. This CID shows the loss of H-atoms and CN-groups at a nose cone voltage of greater than 70V. In sharp contrast, the CID spectntm of the dimer formed from HCsw and HC3N is shown on the right side of Figure 7.2. The dimer (NC3H)r formed in rhis environment be=+s to fragment at very low nose-cone voltages and:breaks to fom-the HC3pion which fuaher fragments to CzK! 7.2.2. Theoretical Results Table 7.3 lists the total energies (in hamees) fiom the geometry optimizations at B3LYP/6-3 1+G (d), unscaled zero-point energies (in kcal mol-') and the relative energies (in kcal mol-'). Table 7.4 contains the total energies (in hartrees) and relative energies (in kcal mol") from the structural optimization at B3LYP/6-3 1lttG(2df,p). The (HC3W2* structures that have been optirnized at B3LYP/6-31+G(d) (top numbers) and B3LYP/6- 3 1l++G(2df,p) (bottom numbers) are shown in Fiove 7.3. The bond parameters at the minima on the potential energy surface (PES) for (HC3N)2* are shown in Figure 7-4 for both levels of theory investigated (top number at B 3LYP/6-3 l+G(d) and bottom numbers at B3LYW6-3 1ltrG(2df.p)). Other dimer structures investigated at B3LYP/3-2 1G(d), are shown below, but are high energy structures on the potential energy surface and were Figure 7.2: (Right side) Mulri-CID of dirner of (HC;N)2" produced from the reaction of csa"with HC;N. (Left side) Mulri-CID of dimer of (HC3N)?* produced from the reaction of HC3N" with HC3N. Table 7.3: Total energies (in hartrees). unscaled zero-point vibrational energies and thermal energies (in kcal mol-') and relative energies (at 298 K in kcal mol-'). Structure B3LYP/6-3 l+G(d) Relative Energies ZPE Thermal Energies Energies 1 -338.859 18 37.3 4-4 O Table 7.4: Total Energïes (in hartrees) and relative energies (in kcal mol-'). Structure B3LYP/6-3 1 1+tG(2df,p) Relative Energies Energies 1 -338.93241 O II -338.93 149 0-6 III -338.92604 4-0 IV -338.87070 38-7 V -338.85423 49-0 VI -338.8 1622 72-9 Figure 7.3 : Opti mized structural parameters of several dimers of cyanoacetylene at B 3LYP /63 1 +G(d) (top numbers) and B3 LYP/6-3 1 1*G(2df,p)(bottom numbers). 1.069 1.373 1,082 1.340 1.064 1.369 f -078 1.335 + H-C-CCN H-C-CCN 1.211 1.167 1.242 1.191 1.204 1.159 1236 1-183 Lv not investigated at the higher levels of theory. At B3LYP/6-3 1+G(d), the dissociation products, HCsN + HC3r,are 69.7 kcal . . mol-' above the global minimum. The ion at the global m~nfmumis structure 1, a planar sc-radical containing a four-membered ring system with cyano groups cis to each other. At B3LYW6-31+G(d) the calculated ionization energy for smicnire I is 8.83 eV. Removing a x-electron from the ring of the neutrd gives Neto structure 1, which has eighteen resonance structures (9 equivalent pairs) that conmbute to the stability of the dimer by delocalkation of the positive charge over six of the heavy centers. Note that the charge and unpaired spin are never on the C of the CN group. The OC-C(CN) bond of 1-391 A indicates a bond intermediate between a double and a single bond. The (NC)C-C(CN) bond of 1.542 A is long due in part to ligand- ligand repulsion between the cis CN-groups. The PES at B3LYP/6-3 11trG(2df9p) predicts the relative order of stability of the dimers 1-VI to be the same, but the magnitude of the energy clifferences are slightly smaller. The dissociation products are only 64.7 kcal mol-1 above the global minimum, structure 1. The relative energy difference between structures 1 and II is small but has decreased by a factor of almost two. With a larger bais set, the optimized geomeîries have slightly shorter bond len,&s, but in general the structures remain essentially unchanged from one basis set to the next. The planar Dans substituted cyclobutadienyl cation-structure II is only slighdy higher on the PES at 1.1 kcal mol-! At B3LYP/6-31+G(d) the calculated ionization energy for structure II is 8.88 eV. Structure II has the dipoles of the cyanoacetylene monomers correctly aligned so as to minirnize the dipole and has four types of 202 contributing resonance structures with a total of 16 resonance structures. The positive charge has been delocahed over six of the centers in the dimer. The unsymmetric bond lenaas in the carbon ring system of smicture II, (1.507A and 1.389&, have also been seen in the unsubstituted parent ion, c-C4H4f, for calculations done at MP21Lb The lowest non-cyclic dimer on the PES is structure III, only 7.4 kcal mol-' above the global minimum. In the gas-phase a possible,pathway to the formation of this dimer is shown in Figure 7.5. The lone pair of electrons on the nitrogen attacks the empty p- orbital on the carbon in the cyanoacetylene cation and forms a dimer with the positive charge formally located on the nitrogen (structure ma). This is in resonance with structures IIIb and IIIc. A bonding pair of K-electrons can be moved ont0 the nitrogen carrying the positive charge and the charge is now formally located on the adjacent carbon. The x-system of structure III also allows the dimer to delocalize the positive charge to the end of the chah and we aitribute the stability of this ion to delocalization of the charge ont0 these three centers. Structure IV is a wealdy bound linear dimer of (HC3N)y and is 43.8 kcal moï1 above the global minimum on the B3LYP/6-3 1+G(d) PES, (38.7 kcal mold1above the global minimum on the B3LYP/6-3 L ItsG(Zdf,p) PES). The geometrical parameters of structure IV resemble the individual monomers, HC3rand HC3N with a long solvating Ne-H bond of 2.01 1 A, i.e. the (HC3hv)*..(HC3N) interaction leaves the fragment structures essentially unchanged. Figure 7.5: A possible pathway to the formation of dimer cation III of cyanoacetylene. NEC-C IIIb Structures V and VI have four-membered rings containing nitrogen. Structure V is pIanar and delocalizes the positive charge over four carbon atoms and two nitrogen atoms in the dimer hework, Stnicture VI is above the dissociation products, but is at a minimum on the (HC3w2* PES at both level of theory. It has a puckered ring and only has a two-fold axis of rotation. If structure VI were formed in the gas-phase, it would likely dissociate into a stable Nz molecule and a linear H~-M-c=cH~+ ion. The reaction is exotherrnic by more than 50 kcal mol-' at B3LYP/6-3 l+G(d). Structure VI, C2 On the PES surface, structures 1, II and III are very close in energy and because of this we are unable to predict which isomer would be forrned in the presence of CfjO2+ . The next step of the investigation was to detennine which structure 1, II or III wodd be the likely candidate, based upon the dissociation fkagmentation pattern of the CID experiment. The dimer formed in the presence of c~~~~resulted in the CID fragmentation pattern of CN molecule and H-atom loss at very high nose-cone voltages. There are three possible pathways of fragmentation, CCCW or K foss for each of the parent ions (shown in Tables 7.5, 7.6 and 7.7 dong with the energy associated with each charnel quoted at B3LYPl6-3 l+G(d), the upper numbers and B3LYP/6-3 11-G(2df,p), the hwer numbers. Table 7.8 contains the calculated energies for each of these structures as well 205 Table 75: The calculated bond energies at 298 K fiom the parent ion, structure 1, at B 3LYP/6-3 l+G(d) and B3LW6-3I I ++G(2df;p). ~02'~ Dissociation Products (kcal mol-') H, CN CN' + / c-c, H r-Ysinglet Table 7.6: The calcuiated bond energies at 298 K fiom ttie parent ion, structure II, at B3LYP/6-3 l+G(d) and B3LYP/6-311 ++G(2dJtp). ~02'~ Dissociation Products (kcal mol") /c==c, NC triplet Table 7.7: The calculated bond energies at 298 K fkom the parent ion, structure III, at B 3LYPf6-3 l+G(d) and B.XYP/6-311 ++G(2dfTp). Dissociation Products + H' + NC-C-C-mCC2H singlet HO + NC-C- singlet-< + Nqc: Table 78: The total energies of the dissociation products at B3LYP/6-31+G(d) and B3LYPl6-3 1 lt-rG(Zdf,p) in hartrees. The unscaled ZPE and thermal energïes are given at B3LYEV6-3 1+G(d) in kcal (Xnie energy quoted is obtained frorn a single-point calculation at B3LYP16-3 l+G(d)//B3LYP/DZVP and frequency calculations were done at B3LYP/DZVP.) Dissociation Products B3LYP/6-3 1-d) B3LYl?/ energy ZPE themalenergy 6-311*C(2df~) -338.17268 37.3 4 -338.93241 "rfm.g-< -4 singlet CN ,c-q. H singlet - CN "'ri?',c-c.. H ,t=< NC singlct H Table 7.8: continuai. Dissociation Products B 3 LYP/6-3 1+G(d) B3LYPf energy ZPE thermal energy 6-31 I+KQdf,p) as the thermal and the zero-point energy corrections). When the parent dimer homolyticaily fragments a bond to lose Cr or H?,the larger fkaagment would retain the positive charge and become initially a triplet state, but CN+ loss is also a possibiliw and would yield a radical fragment on the larger component. The more centers a positive charge has avaüable to delocalize over the more stable the structure will be. As a result, the products in which the positive-charge rernains on the CN will be at higher energy than those where the positive charge is on the larger fragment. The barrier to Crloss for each structure is at least 150 kcal mol" higher than any other dissociation pathway at both levels of theory. Loss of CN+ fkom structures 1, II or III is a high energy pathway because of the large ionization energy of CN (experiment value of 13.598 evL3).The CID profile of the dimers formed in the presence of the carbon surface shows radical fragmentation of H and CN have almost identical onsets and the calculations mirror this resdt for the parent structures 1 and II, while structure III is dramatically different. Structure III has two possible sources to H loss that were not present in structures I and II and these differ in onset energies by more than 30 kcal moï1. The lower energy channel to H loss in structure XII, is an upper limit because of convergence problems in the calculation, but this dissociation pathway is still a lot less than the calculated value for CN loss at 102.3 kcal mol-'. This is not consistent with experimental values and based upon this structure III can be eliminated as a possible dimer formed in the presence of a C60 surface. For each of the dissociations investigated, the puckered singIet state is energeticdy preferred over the planar triplet state by at least 10 kcal mol*' at both B3LYP/6-3 l+G(d) and B3LYW6-3 11 ++G(Zdf,p). Previous theoretical studies have determined that cyano groups can function either as a z-donor or as a x-acceptor depending upon the environment, although when it is adjacent to a cationic centre it generally functions as a x-donor.'' x-Donation fiom the cyano groups to the 4- membered ring increases the x-population and, as the singlet is a Zn-homoaromatic system, this interaction destroys the aromaticity and is destabibing. In the case of the triplet, any increase over the 3x-electrons that are formally in the ring is also potentially destabilizîng as it makes the system closer to having a 4x-antiaromatic occupancy. The fact that substitution by CN favourç the triplet relative to the singlet indicates that this destabilization is smaller in the triplet than in the singlet. 7.3. ConcIusions Experimental evidence shows structures are different in the presence and absence of and it therefore fouows that Cao has an infhence on the formation of the dimer. The structures of (HCsN)y formed under different reaction conditions are different. The presence of a Cao surface influences the structure of the dimer formed: chemical bond formation is preferred over solvation as in structures 1, II and III. A possible mechanism to account for the dramatically different dimer ions is shown in Figure 7.6. The first adduct, G~(HC&+, is formed by nucleophilic attack on c~O'+ by the lone pair of electrons on the nitrogen. The charge is delocalized through the n-system; one of the resonance structures delocahes the charge to the terminal carbon on the cyanoacetylene ligand. There are two plausible pathways by which the second Figure 7.6: A possible mechanism for the formation of the frmvcyclic and linear dimers of cyanoacetylene, nucleophilic attack of &N can lead to dimer formation. Pathway A shows the formation of the open chah dimer of (HC3N)2+, structure III. The second cyanoacetylene attacks through the lone pair on the nitrogen at the terminal carbon resulting in the positive charge being fomaily on the attacking nitrogen. Homolytic bond.cleavage of the Go-nitrogen bond results in the formation of the charge separated product ions. Pathway B is a 2+2 cycloaddition producing and a cyck dimer, structure II. The n-system of the second cyanoacetylene must be coplanor with the x- system of the coordinated cyanoacetylene to follow this pathway. There are two possible ali,onments, cis and tram; calculations indicate that these have almost identical energies. The tram isomer has been selected arbitrarily in Figure 7.6. The homolytic bond cfeavage at the fuiierene surface results in the formation of Csc and dicyanocyclobutadiene cation. In contras, the formation of dimer in the absence of c6Pis a weak solvation interaction between the lone pair of electrons on the nitrogen and the positive end of the cyanoacetylene cation, shown below. The formation of dimer in the absence of ~~0%is a weak association between the lone pair of electrons on the nitrogen and the positive end of the cyanoacetylene cation. The dimer (NC3H)2* formed in this enviromnt begins to fragment at very low nose-cone voltages and the cyanoacetylene dimer fragments via the pathway shown in equation (7.4). The monomer then subsequently fragments again into HC2+and CN pieces. 214 (NC3H)2w + NC3H? + NC3H (7-4) NC3T-+ HCC* t CN (7-5) Based upon thermochemical data from the JANAF tablesg, reactions (7.6) and (7.7) are the two possible fia,pentation pathways of NCsK" at 298 K. Pathway 2 is the lower energy channel and therefore will be the preferred pathway to dissociation of the parent ion. Both computational levels of theory predict the same orders of relative stabilities for the dimers investigated. The most stable structures in the (HC3N)?* PES are foür carbon aromatic ring systems, that benefit £Yom the delocalization of the positive charge through the ic-system, but have large ring straïns. Previous studies" on the PES of C4Hs' show that the best conformations are srnall ring systems that overcome the large ring strairi by delocalization of a positive charge through the ic-system as well as benefiting from homoaromaticity. Structure 111 is the lowest energy noncyclic dimer and also takes advantage of the delocalization of the positive charge. The niuogen-containing rings appear high on the PES and cannot be considered as likely candidates to be formed in the gas-phase. The Iooseiy bound dimer that is a minimum, structure IV, on the PES, cm be considered as a solvated ion. From the experhental data, the CID obtauled in the presence of c60suggests that the (HC3N)2 product fragments into CN- and H-atoms. This idea is also supported by the theory because structures 1, II and III, the most stable dimers on the surface, cm lose these pieces. The calculated dissociation onsets for structures 1, II and III, eliminates structure IU as a possible dimer formed because H loss occurs at a very low energy threshold and therefore the cyanoacetylene dimer formed in the presence of cso2+is a four membered carbon ring, structures 1 or II. The (HC~N)J)~* produced in the absence of cso2+,is a weakIy bound dimer, like smcnues IV and V on the PES, that £ka=ents at low nosecone voltages. Structure V can be eliminated on the grounds that it would not be possible to lose a (HCrnC' without breaking two bonds whereas structure TV cm easily lose this fragment by breaking a long N-*H solvahg bond. 7.4. References Kroto, H.W.; Heath, J.R.; O'Brien, SC.; Carl, R.F.; SmaHey, R.E. Nature 1985,318, 162. Kriitschmer, W.; Lamb, L.D.; Fostiropoulos, K.; Huffman, D.R. Narure 1990, 347, 354, Petrie, S.; Javahery, G.; Wang, J.; Bohme, D.K. J. Am. Chem. Soc. 1992,114,9 177- (a) Petrie, S.; Bohme, D.K. Cun. J. Chem. 1994, 72, 577. (b) Wang, 5.; Javahery, G.; Petrie, S.; Wincel, H.; Bohme, D.K. J. Am. Chem. Soc. 1992,114,9665. (c)Wang, J.; Baranov, V.I.; Bohme, D.K J. Am. Soc. Mass Spectrom. 1996, 7, 261. (d) Javaheq, G; Petrie, S.; Wang, J.; Wincel, H.; Bohme, J2.K J. Am. Chem. Soc. 1993,II5,6295. (e) Wang, J.; Javahery, G.; pime, S.; ~incël,H.; Hopkinfon, AC; Bohme, D.K. Agnew. Chem. In?. Ed. Er@. 1994, 33, 206. (0 Petrie, S.; Javahery, G.; Wincel, H.; Bohme, D.K. Int. .L Mass Spectrom. Ion Processes 1994,138, 187. Javahery, G.; Peme, S.; Wang, J.; Wincel, H.; Bohme, D.K. J. Am Chem. Soc. 1993, 115,9701. Murahasi, S.; Takizawa, T.; Kurioka, S.; Maekawa, S. J- Chem- Soc. Japan Pure Chem. Sect. 1956, 77, 1689. Baranov, V.1.; Bohme, DKTnt. J. Mass Specrrom- Ion. Proc. 1996,154,71. FOX,A.; Raksit, A.B.; Dheandhanoo, S.; Bohme, D.K. Can. J. Chem. 1986,64,399. Lias, S.G.; Bartmess, LE.; Liebmaa, J.F.; Hoimes, J.L.; Levin, R.D.; Mailard, W.Q. J. Phys. Chem. Ref. Data 1998,17, Suppl. 1. 10. The value for ionization energy of C6Ôis derived by using ionization energy of C60 = 7.61 t 0.02 eV (Lichtenberger, D.L.; Jareko, M.E.; Bebesny, KW.; Ray, C.D.; Huffman, D.D.; Lamb, L.D. Mater Res. Soc. Symp. Proc. 1991, 206, 673) and the ionization energy for the direct double ionization of Cso= 19.00 + 0.03 eV (Steger, H.; de Vries, J.; Kramke, W.; Drewello, T. Chem. Phys. krt. 1992,194,452). 11. Javahery, G.; Petrie, S.; Wang, J.; Wincel, H.; Bohme, D.K. J. Am. Chem. Soc. 1993, 115,9701. 12. (a) Kohn, D.W.; Chen, P. J. Am. Chem Soc. 1993, 115, 2844. (b) Roeselova, M.; Bally, T.; Junwirth, P.; Cdrsky, P. Chem Phys. Le= 1995,234, 395. 13. NIST tables, 1998. 14. Hopkinson, AC;Lien, M.H. Can. 3. Chem. 1985,63,3582. 15. Cunje, A.; Rodriquez, CF.; Lien, M.H.; Hopkinson, AC. Org. Chem. 1996, 61, 5212. CHAPTER 8 FUTURE WORK The work contained in this dissertation has begun the chemical investigation of the gas phase chernistries of MgU, (C-C~H~)M$and (c-C5H&MgCusing experimentai and theoretical methods. An extension to this work could inchde studying the chemical reactivities of molecules that contain amines, -des, alcohols, ketones and/or sulfur groups. Within the saturated hydrocarbons, hexane, the longest carbon chah investigated, showed evidence of two isomers present in the rnulti-CID. The suggestion is that the carbon chain is long enough to ligate twice to the metal ion. To further this idea of multiple bonds to the ion by a ligand, the chemistry of longer chain molecules couid be snidied. Possible ion-molecule reactions involving Mg*, (c-C5H5)MgCand (c- C5H5)2Mg* i~ only Mted by the abilis of the neutral molecule to be introduced into the Sm,and imagination. Another interesting extension to this work would be to see if the solution chemistry of Grignard reagents is mirrored in the gas-phase. In solution chemistry, organomagnesiurn haiides, with the empirical formula RMgX, are used widely in syntheses. Grignard reagents are made in the presence of ethen; with R being any organic goup and are interesting because they have nucleopWc carbon atoms, in contrast to the electrophilic carbon atoms of akyl halides. RX + Mg + RMgX It will be interesting to see whether Mginserts into an R-Cl bond and, if so, to snidy the bas-phase chernistry of the positively c harged Grignard reagent . 218 Gas-phase cationic polymerization has long been a popula. topic in our research group, using Fef and c~~+~as the initiators. A similar smdy could be undertaken using Mgas the initiator with such common solution-phase monomers as styrene. Mg* is a small enough system and will dow for computationd modeling to be done on the first few adducts in the polper chah In an earlier study,' the g~-phasereactions of cso3-with several nitriles were investigated. The reactions of C~Pwith C2H3CN or C3H5CN behaved in a sirnilar rnanner as seen previously with ~~~'~/c~anoacet~lene.Through a minor channel, a charge separation reaction produces ~6~'~and (RCN)? (proposed mechanism shown below). At that time the CID method was not available, but now we can use this procedure to give us some insight into the structures of the dimers formed in the presence of c603* and then study the dimer structures using theoretical methods. + +- WC-R - kc-R 1 (R = CN, C&,, C3H,, CH2CN) 8.2. References 1. Wade, L.G. Organic Chemisny. Prentice-Hail Inc.: New Jersey, U.S.A., 1991. 2. Javahery, G.;Petrie, S.; Wang, J.; Wincel, H.; Bobe, D.K. J. Am- Chenz. Soc. 1993, 115,9701. CHAPTER 9 APPENDIX List of Illustrations and Tables Saturated, Unsaturatecl and Cyclic Hydrocarbons Page Reaction profile and multi-CID for the reaction of (c-CsHs)Mg' with C5Hir- A-3 Reaction profile and rnulti-CID for the reaction of (c-C5H5)Mgfwith CsHl+ A-4 Reaction profdes for the reactions of Mg(L)* with L = C2&, ~3~6,n-Ca8 A-5 and iso-Cas. Muiti-Cm's for the reaction products of Mg&)" with L = C3&, n-C4H8 and iso-Ca8- Reaction profiles for the reactions of (c-CsH5)Mg(L)' with L = C2&, C3H6, n-Caand is&C4H8. Multi-CID' s for the reaction products of (c-C5H5)Mg(L)' wiih L = CI&, C3H6,n'Ca8 and iso-C4Hs. Reaction profdes for the reactions of Mg&)" with L = c-C&, C-C~HII and c-C6&- Multi-CID'S for the reaction products of Mg&)* with L = c-QI& c-Ca12 and c-C6H6. Reaction profiles for the reactions of (c-cSH5)Mg(L)+with L = C-C3H6, A-I 1 c-C6H12and c-C6H6. Multi-CID'S for the reacrion products of (c-c5H5)Mg(L)' with L = c-Ci&, C-C6H 2 and c-C6H6. Measured rate coefficients for the reactions of ground States of MgC, (c-C5Hs)Mg+,(c-CsH&Mg* with selected hydrocarbon ligands. Reaction profile for the reaction of (c-C5H5)M$ with C2H2. Reaction profde and multi-CID for the reaction of Mg" with C3& Reaction profile and multi-CID for the reaction of (c-CsHs)Mg with C3&. Reaction profile and multi-CID for the reaction of Mg* with Ca?. Reaction profde and multi-CID for the reaction of (c-C5H5)M$ with Ca?. Reaction profde and multi-CID for the reaction of Mg" with C3&. Reaction profile and multi-CID for the reaction of (c-CsHs)M$ with C3&. Reaction profile and multi-CID for .the reaction of:MgU with C4K6. Reaction profde and mulri-CID for the reaction of (C-C~H~)M~+with C4&. Reaction profde for the reaction of (C-C~H~)~M~*with Ca&. Reaction profile and multi-CID for the reaction of Mg* with C7HL6- Reaction profile and rnulri-CID for the reaction of (C-C~H~)M~+with C7H16- Cyanide and Isocyanide Moiecules Reaction profile and mulri-CID for the reaction of (c-C5H5)Mg"with HCN. Reaction profile for the reaction of (C-C~H~)~M~+'with HCN. Reaction profile and multi-CID for ths reaction of Mg+*with CH3CN. Reaction profile and rnulh-CID for the reaction of (c-CsH5)Mg+with CH3CN. Reaction profde for the reaction of (c-CsH&Mpw with CH3CN. Reaction profile and multi-CID for the reaction of Mcwith CH3NC. Reaction profde and multi-CID for the reaction of (C-C~H~)M~+with CH3NC. Reaction profde and multi-CID for the reaction of Mg" with H?C=CHCN. Reaction profile and multi-CID for the reaction of (C-C~H~)M~+with H?C=CHCN. Reaction profile for the reaction of (C-C~H~)~M%with H2C=CHCN. Ion Signal Ion Signal Fi,we A-5: Reaction profile for the reactions of M@)" with L = C2H4, C,H,, 0.0 0.5 1.0 1.5 0.2 0.4 Neutra1 Reactant Flow, ~/(10l8 molecules s-') A-5 Fi,pre A-6: Multi-CD's for the reaction products of M~(L)*with L = C,H,, n-C4H, and iso-C4H,. Figure A-7: Reaction profde for the reactions of (c-c,H,)M~(L)+ with L = C,H,,- C,H6, n-C4H8 and iso-C4H,. Neutra1 Reactant Flow, L/(~O'~molecules s") Figure A-8: Multi-CD's for the reaction products of (c-C,HS)Mg(L)' with L = C,H,,- C3H6,n-CA and iso-C&. Figure A-9: Reaction profles for the reactions of M~(L)+'with L = c-C,H,, C-C6H,, and C-C6H6. Neutra1 Reactant Flow, ~/(1018 molecules 8') A-9 Fiove A-10: Muhi-CID'Sfor the reaction products of MO(L)" with L = c-C3H6, Fi,oure A-1 1: Reactions profiles for the reactions of (c-C,%)Mg(L)* with L = c-C,H,, Neutra1 Reactant Flow, ~/(10'*molecules s-') A-1 l Figure A-12: Multi-CD's for the reaction products of (c-c,H,)M~(L)+with L = c-C,H,, c-C,H,, and c-C,H6- Appendix 4.1: Meiisured riite coefficients foi tlic renctions of ground siates of Mgt', (c-CsHs)Mgt and (~-CsI-is)zMg"witli selcctcd Iiydrocaibons ligaiids pioceeding at (294 f 3)K in a hclium buffer gas nt a ioiul pressure of (0.35 f 0.01) Torr. Reuciionn cind collisionh rate coefficients are given in units of cm~nolecule''se'. 6.7(I2.0) x IO'" (9.6 x 10'~) NR, < 1 O"* < 10"" 1.4(î-0.4)x 1 0'' ( (1.4 x 1u9) NR, < 10"" 1 .9(f0.6)x JO"' S.O(f 1.5) x 1 0'12 8.9(f 3.8) x 10'" (?) NR, < 10'12 1.2(fO.4) x 10.'~ < 10"' 1 .0(10.3) x 10'" 1 .1 x 1o'~) NR,< 10"" l.4(îO.4)x 1 0"' < 10''~ 9.7(&2.9)x 10"O (1.2 x 10.') 3,0(f0.9) x 10''~(1.1 x 10'~) 23.l(f0.9) x 10"' 1 < 10"" a. The relative uncertainty in al1 reaction rate coefficients does not cxcccd IO%, however the übsolute error may be as IiigIi ns 30%, b. Collision rate coefficients are given in brackets and calculaied using TC theory. (refcretice: SLI,T,; Bowers, M,T, Irlr, J, Mms Spectrotii. lori Pliys, 1973, 12,347,) c. NR denotes no reaction. d. No polnrizabiliiy is rivailable iii ille literature, Figure A44Reaction profile for the reaction of (C-C~H~)M~+with CzHz. C,H, Flow /(lol*rnolecule s-') CC. O Ion Signai A-15 Ion Signal Ion Signal Ion Signal Ion Signal A- 19 Ion Signal Ion Signal Ion Signal Figure A-23: Reaction profile for the reaction of (c-c,H,),M~+ with C,H,. CH,CHCHCH, - Flow /(IO l7 molecule 8) Ion Signal Ion Signal Ion Signal Fi-oure A-27: Reaction profÏie for the reaction of (c-C,H5)Mgt' with HCN. CC. O œ Ion Signal Ion Signal Fiame A-30: Reaction profde for the reaction of (c-C,H,),M$ with CqCN. CH,CN Flow /(1018 molecule s-') A-30 Ion Signal Ion Signal Ion Sipal Ion Signal Figure A-35: Reaction profile for the reaction of (c-c5&),MgC. with H,C=CHCN.- 2 4 H$=CHCN Flow 410'' molecule s-~)