Nuclear Magnetic Shielding Tensors for the Carbon, Nitrogen, and Selenium Nuclei of Selenocyanates — a Combined Experimental and Theoretical Approach

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Nuclear magnetic shielding tensors for the carbon, nitrogen, and selenium nuclei of selenocyanates — a combined experimental and theoretical approach

Guy M. Bernard, Klaus Eichele, Gang Wu, Christopher W. Kirby, and Roderick E. Wasylishen

Abstract: The principal components of the carbon, nitrogen, and selenium chemical shift (CS) tensors for several solid selenocyanate salts have been determined by NMR measurements on stationary or slow magic-angle-spinning powder samples. Within experimental error, all three CS tensors are axially symmetric, consistent with the expected linear geometry of these anions. The spans (Ω) of the carbon and selenium CS tensors for the selenocyanate anion (SeCN–) are approximately 300 and 800 ppm, respectively, much less than the corresponding values for carbon diselenide (CSe2). This difference is a consequence of the difference in the CS tensor components perpendicular to the C∞ symmetry axes in these systems. Ab initio calculations show that the orbital symmetries of these compounds are a signifi- σ π cant factor in the shielding. For CSe2, efficient mixing of the and orbitals results in a large paramagnetic contribution to the total shielding of the chemical shielding tensor components perpendicular to the molecular axis. Such mixing is less efficient for the SeCN–, resulting in a smaller paramagnetic contribution and hence in greater shielding in directions perpendicular to the molecular axis. Key words: selenocyanates, solid-state NMR, carbon shielding tensors, nitrogen shielding tensors, selenium shielding tensors, ab initio calculations. Résumé : À partir de mesures RMN sur des échantillons de poudre stationnaires ou à rotation lente à l’angle magique de plusieurs sélénocyanates solides, on en a déterminé les principaux composant des tenseurs des déplacements chimiques (DC) du carbone, de l’azote et du sélénium. Dans les limites des erreurs expérimentales, les trois tenseurs des DC sont axialement symétriques, ce qui est en accord avec la géométrie linéaire attendue pour ces anions. Les plages, Ω, des tenseurs du carbone et du sélénium pour l’anion sélénocyanate (SeCN–), sont approximativement de 300 et de 800 ppm respectivement, des valeurs beaucoup plus faible que les valeurs correspondantes observées dans le disélénure de carbone (CSe2). Cette différence est une conséquence de la différence dans les composants du tenseur DC qui sont perpendiculaire aux axes de symétrie C2 dans ces systèmes. Des calculs ab initio montrent que les symétries des orbitales de ces composés sont un facteur important dans le blindage. Pour le CSe2, une combinaison efficace des orbitales σ et π conduit à une contribution paramagnétique importante au blindage total des composantes du tenseur de blindage chimique perpendiculaires à l’axe moléculaire. Une telle combinaison est moins efficace pour le SeCN–;ilen résulte que la contribution paramagnétique est plus faible et, en conséquence, que le blindage est plus grand dans les directions perpendiculaires à l’axe moléculaire. Mots clés : sélénocyanates, RMN à l’état solide, tenseurs de blindage du carbone, tenseurs de blindage de l’azote, tenseurs de blindage du sélénium, calculs ab initio.

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Introduction may coordinate to a metal through the nitrogen, the sele- – nium, or by various bridging modes. In the absence of dif- The selenocyanate ligand (SeCN ) has been of consider- fraction data, NMR spectroscopy may provide the most able interest because of its ambidentate nature (1). That is, it reliable information concerning the coordination and structure of these ligands. A review by Duddeck provides an Received January 27, 2000. Published on the NRC Research overview of the 77Se NMR literature including the results of Press website on May 12, 2000. several solution NMR studies of organic selenocyanate com- G.M. Bernard, K. Eichele, G. Wu, C.W. Kirby, and R.E. pounds (2). Carbon-13 and nitrogen-14 or -15 NMR results Wasylishen.1 Department of Chemistry, Dalhousie University, for selenocyanates have been discussed in several reviews Halifax, NS B3H 4J3, Canada. (3, 4). The coordination of metal atoms with several 1Author to whom correspondence may be addressed. selenocyanate anions has been investigated by solution Telephone: (902) 494-2564. Fax: (902) 494-1310. NMR (5, 6). An analogous study of the thiocyanate anion in e-mail: [email protected] copper(I) complexes was conducted using solid-state 13C

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NMR (7). In addition to providing insight into the coordina- −µ e2 1 l [3] σ p = 0 ∑∑[ 00ki nnl tion mode of the cation in these complexes, this study ii π 2 − 3 ki 42mEE> n 0 rk showed that conclusions about coordination to the metal n 0 k l drawn from solution NMR studies cannot always be ex- + 00∑∑l nn ki ] ki 3 tended to solids. k r r k With four NMR-active nuclei, 13C, 14N, 15N, and 77Se, µ much information may be gained from solid-state NMR In the above equations, 0 is the permeability of free space, studies of SeCN–. For example, internuclear distances may e and m are the electron charge and rest mass, respectively, be determined from dipolar interactions, and information about while +0| and +n| represent ground and excited electronic – the bonding modes of SeCN is available from isotropic states of the molecule with electronic energy E0 and En,re- chemical shifts and spin–spin coupling constants. Also, ex- spectively. The position vector for electron k is defined by rk amination of stationary powder samples provides the princi- and lki is the angular momentum operator. Equations 2 and 3 pal components of the chemical shift (CS) tensors, which describe the shielding in the directions of the principal com- potentially contain a wealth of structural information. Before ponents of the chemical shielding tensor and are valid only embarking on a solid-state NMR study of selenocyanate–metal if the nucleus of interest is chosen as the gauge origin. Ex- complexes, it is important to understand the shielding ten- pressions for the off-diagonal elements of the chemical sors for the uncoordinated ligand. Towards this end, we pres- shielding tensor and for different gauge origins have been ent the results of solid-state NMR studies of the carbon, derived (14, 15). nitrogen, and selenium CS tensors for selenocyanate salts The contribution to shielding from σ d (eq. [2]) is always + – M [SeCN] ,M=K,NH4,orNMe4. Also presented are the positive, leading to increased shielding relative to the bare preliminary results of a solid-state 13C NMR study of a re- nucleus. Because σ d depends only on the ground electronic lated compound, potassium cyanate. The interpretation of state of the molecule, it can be calculated accurately at rela- the observed magnetic shielding tensors is aided by high- tively low levels of theory and hence is expected to be essen- level ab initio calculations. tially invariant to basis set size or to the effects of electron Selenocyanate anions are thought to be linear; the correlation. Gierke and Flygare (16) have shown that σ d selenocyanate ion of a copper complex which had been re- may be approximated if the molecular structure is known. ported as bent (8) has recently been shown to be linear (9). For example, the shielding for nucleus A in the x-direction Linear systems are ideal for a study of nuclear magnetic may be approximated by: shielding since, in the absence of intermolecular effects, the µ e2 Z orientation of the axially symmetric CS tensor is known ex- [4] σσd ≈+d (free atom) + 0B∑ ()yz22 xxav π 3 B B actly and the unique component of the CS tensor, along the 42mrBA≠ AB C∞ symmetry axis, is dependent only on the diamagnetic σ d term of the shielding (vide infra). The relative simplicity of where av(free atom) is the shielding for the “free” atom. these systems allows one to identify the specific molecular The summation is carried out over all other atoms in the orbitals (MOs) responsible for the paramagnetic shielding. molecule, ZB is the atomic number of the atom being Although carbon and nitrogen shielding tensors are avail- summed, yB and zB are the Cartesian coordinates of the able for most functional groups, corresponding data for sele- atoms which are a distance rAB from the origin (the site of nium are not as widespread (10). The first comprehensive the observed nucleus). The remaining tensor components may experimental study of selenium CS tensors was that of Col- be approximated by cyclic permutations of eq. [4]. In linear lins et al. (11). A surprising feature of the selenocyanate lig- molecules, the component of the chemical shielding tensor σ ands is that the spans of the carbon and selenium CS tensors along the C∞ axis is labelled 2 while the two components σ σ p are much smaller than those of CSe2 (12). With high-level perpendicular to this axis are labelled z. Since || = 0 for – ab initio calculations on SeCN and CSe2, the MOs respon- the nuclei of linear molecules, it follows from eq. [4] that: sible for the different magnetic shielding in these closely re- [5] σσ=≈d σd (free atom) lated compounds were identified. || || av The paramagnetic term (eq. [3]) is a second-order elec- Background theory tronic property which depends on the mixing of ground and excited electronic states of the molecule. These states corre- It is convenient to discuss the nuclear magnetic shielding spond approximately to the occupied and unoccupied MOs σ ( ) of nuclei in linear molecules or ions using Ramsey’s the- (17). The paramagnetic term is usually negative and is in- ory (13). In this model, the magnetic shielding is expressed versely proportional to the difference in the electronic en- σd σp as the sum of diamagnetic ( ) and paramagnetic ( ) com- ergy of the orbitals undergoing mixing. Contributions to σ p ponents (14): arise from induced electron motion in the plane perpendicu- [1] σσσ=+d p lar to the direction of the applied magnetic field such that ii ii ii charge appears to rotate (17). This can be understood by where considering the action of the angular momentum operator in eq. [3]. For example, consider lz|px,; the operator lz rotates px µ e222rr− [2] σ d = 0 00∑ kki by 90° about the z-axis. Thus, there is an effective mixing ii π 3 42m k r k between px and py. Mixing of MOs can only occur between occupied and unoccupied MOs with magnetic-dipole al- and lowed symmetry. This is shown graphically in Fig. 1.

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Fig. 1. Examples of magnetic-dipole allowed (A) and forbidden ods by using an absolute shielding scale for the nucleus of (B) mixing of orbitals for a hypothetical diatomic molecule with interest (19). Absolute shielding scales for carbon (20), ni- an applied magnetic field perpendicular to the page. trogen (21), and selenium (22) have been established by Jameson and coworkers. It is convenient to discuss the NMR line shape of static Ω σ σ ≈δ δ powder samples in terms of the span, = 33 – 11 11 – 33, κ σ σ Ω δ δ Ω and the skew, =3( iso – 22)/ =3(22 – iso)/ (23). With a few exceptions (24), σ2 > σz and κ = +1.0 for linear molecules with perfect axial symmetry.

Experimental Ammonium selenocyanate (25) and tetramethylammonium selenocyanate (26) were prepared according to literature methods. Potassium cyanate and potassium selenocyanate were pur- chased from Aldrich and used without further purification. Natural abundance 13C, 15N, and 77Se NMR spectra were obtained on Bruker MSL-200 and AMX-400 NMR spectro- meters (B0 = 4.7 and 9.4 T, respectively). Samples were packed into 7 and 4 mm o.d. zirconium oxide rotors. Cross polarization (CP) and high-power proton decoupling were employed in acquiring all NMR spectra, except for KSeCN, where spectra were acquired after a single pulse. Unless oth- π σ ∗ erwise noted, recycle delays were 5 s. Carbon chemical shifts Mixing of u and u MOs (Fig. 1A) is magnetic-dipole al- π π∗ were referenced to tetramethylsilane by setting the high- lowed, but mixing of u and g MOs (Fig. 1B) is forbidden frequency isotropic peak of solid adamantane to 38.56 ppm by symmetry. Hence, significant deshielding is expected in (27). Nitrogen chemical shifts were referenced to NH3(ᐉ, 293 directions perpendicular to the symmetry planes of occupied K) by setting the NMR peak of the ammonium ion in solid and virtual MOs that have the proper symmetry for mixing, 15 NH4NO3 to +23.8 ppm. The chemical shift for CH3NO2(ᐉ), and that are separated by a small energy difference. It is of another common nitrogen chemical shift reference, is interest to point out that a related discussion of nuclear 380.20 ppm with respect to NH (ᐉ, 293 K) (28). Selenium shielding in terms of electronic current density conveniently 3 chemical shifts were referenced to Se(CH3)2(ᐉ) by setting the leads to equivalent partitioning of the diamagnetic and para- 77 Se NMR peak of solid (NH4)2SeO4 to +1040.2 ppm (11). magnetic terms (eqs. [1]–[3]) (18). NMR spectra of MAS samples were analyzed by the In eqs. [1]–[5], σ denotes the magnetic shielding relative σ ≥σ ≥σ method of Herzfeld and Berger (29) using the program HBA. to the bare nucleus such that 33 22 11. While ab initio Spectra of stationary samples were analyzed with the program methods calculate shielding, experimentally one measures a δ WSOLIDS, which incorporates the POWDER algorithm of chemical shift ( ii) relative to a given reference with the con- δ ≥δ ≥δ Alderman et al. (30). Both programs were developed in this vention 11 22 33. The two are related by: laboratory and run on IBM compatible microprocessors. νν− σσ− Ab initio calculations of the carbon, nitrogen, and selenium [6] δ = iiref ()106 = ref ii ii ν − σ chemical shielding tensors were performed with the Gaussian ref 1 ref 98 suite of programs2 mounted on an IBM RS/6000 com- ν ν where ii and ref are the resonance frequencies of the princi- puter. The chemical shielding tensors for all compounds were pal tensor components for the nucleus of interest and for the calculated at the Hartree–Fock (HF) level of theory using the σ ≈ δ ≈σ σ reference sample, respectively. Since 1 – ref 1, ii ref – ii. gauge-independent atomic orbitals (GIAO) method (31) with In the ensuing sections the experimental results will be dis- the 6–311+G* basis set. The basis-set dependence of these re- cussed in terms of chemical shifts and the theoretical results sults was determined by calculating the chemical shielding – 3 will be discussed in terms of chemical shielding. The experi- tensors for CSe2 and SeCN with basis sets ranging from mental data are related to those calculated by ab initio meth- 3–21G to cc-pVQZ (32). The effects of electron correlation

2 M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery, Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Ciolowski, J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, and J.A. Pople. Gaussian 98, Revision A.4. Gaussian, Inc. Pittsburgh, Pa. 1998. 3 Basis sets were obtained from the Extensible Computational Chemistry Environment Basis Set Database, Version 1.0, as developed and dis- tributed by Molecular Science Computing Facility, Environmental and Molecular Sciences Laboratory which is part of the Pacific North- west Laboratory, P.O. Box 999, Richland, Washington 99352, USA, and funded by the U.S. Department of Energy. The Pacific Northwest Laboratory is a multi-program laboratory operated by the Batelle Memorial Institute for the U.S. Department of Energy under contract DE- AC06–76RLO 1830. Contact David Feller or Karen Schuchardt for further information.

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Table 1. Experimental carbon, nitrogen, and selenium chemical shift tensors for some solid cyanate salts.a δ b δ δ δ Ωc κd Compound Nucleus iso 11 22 33 KOCN 13C 130.2 — — — — — e 13 NH4SCN C 133.1 240 240 –81 321 1.00 13 f NH4SeCN C 125.7 228 226 –77 305 0.99 13 g NMe4SeCN C 119.1 215 212 –70 285 0.98 e 15 NH4SCN N 211.0 349 349 –66 415 1.00 15 h NMe4SeCN N 255 401 401 –38 439 1.00 KSeCN 77Se –282.6i 25 –23 –849 874 0.89 77 NH4SeCN Se –255.9 26 –15 –778 804 0.9 77 NMe4SeCN Se –299.4 –40 –40 –814 854 1.00 a δ All chemical shift values are in ppm. Uncertainties are estimated to be less than 0.5 ppm for iso; those of the individual components are estimated to be on the order of 2–3% of the total span of the tensor. bδ iso may not be exactly equal to the average of the individual tensor components since the values of the former were obtained from the isotropic peak of an MAS sample. c Ω δ δ Span = 11 – 33. d κ δ δ Ω Skew =3( 22 – iso)/ . eRef. 39. fFrom MAS spectra: χ(14N) = –2.6 ± 0.3 MHz; 1J(14N, 13C)=15±3Hz. g χ 14 1 77 13 δ From MAS spectra: ( N) = –2.6 ± 0.3 MHz; J( Se, C) = 260 ± 30 Hz; methyl groups: iso = 56.1 ppm. h δ Tetramethylammonium: iso = 47.3 ppm. iSurface solvated: –297.5 ppm.

on these two compounds were investigated by calculating the Fig. 2. Isotropic region of the experimental (top) and calculated chemical shielding tensors at the second-order Møller–Plesset 13 (bottom) C NMR spectra of NH4SeCN acquired at 4.7 T with (MP2) level of theory (33). The geometries used in these cal- a spinning frequency of 3320 Hz; 7455 transients were acquired. culations are from a high-level ab initio geometry optimiza- – tion of SeCN (34), a neutron diffraction study of NH4SCN (35), a microwave study of SeCO (36), and an IR study of CSe2 (37). To facilitate comparison, experimental chemical shifts (δ) were converted to chemical shielding (σ) according to σ(exp) = σ(ref) – δ(exp). The carbon shielding in TMS(ᐉ, 300 K) was taken to be 188.1 ppm (20), that for nitrogen in NH3(ᐉ, 293 K) was taken to be 264.5 ppm (21) and that for the selenium in Se(CH3)2(ᐉ) was taken to be 1769 ppm (22). The latter value is an approximate non-relativistic value. That is, in deriving the absolute shielding scale for selenium, it was assumed that σ(free 77Se atom) = 2998 ppm (38).

Results and discussion Observed chemical shift and spin–spin coupling data determined from stationary and MAS powder samples of solid selenocyanate salts investigated in this study are collected in Table 1. Included for comparison are data obtained from a previous 13C and 15N NMR study of ammonium thiocyanate (39).

Carbon-13 NMR spectra The 13C NMR spectrum of an MAS sample of solid ammonium selenocyanate is shown in Fig. 2. The spinning sidebands of the experimental spectrum have been added to sors from spinning sidebands because the presence of the the isotropic peak to produce the spectrum expected at the dipolar interaction with 14N nuclei must be considered (40). high-spinning limit. The asymmetric doublets arise because The 13C NMR spectrum of a stationary powder sample of 14 the carbon is directly bonded to the quadrupolar N nucleus NH4SeCN is shown in Fig. 3. As well as being perturbed by (I = 1, natural abundance = 99.64%). Although the isotropic anisotropic shielding, the 13C NMR line shape is modified 13 14 13 C chemical shift can be accurately determined from the by the N, C dipolar interaction. The splittings at δz and δ 14 13 spectrum of an MAS sample, it is more difficult to extract 2 are RDD and 2RDD, respectively (41). The N, C direct the principal components of the carbon chemical shift ten- dipolar coupling constant (RDD) is defined as

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Fig. 3. Experimental (top) and calculated (bottom) 13CNMR value is in excellent agreement with the value reported for – 14 spectra of a stationary powder sample of NH4SeCN at 4.7 T; the NCS anion, |CQ( N)| = 2.3 MHz (43). 20549 transients were acquired. The splitting of the parallel and The 13C NMR spectrum of an MAS sample of perpendicular components arise from the dipolar interaction with NMe4SeCN (not shown) contains some smaller peaks, attrib- 14N. uted to 13C nuclei bonded to the 77Se nucleus (natural abun- 1 77 13 dance = 7.58%, I = 1/2). The value of Jiso( Se, C) is estimated to be 260 ± 30 Hz. These satellite peaks are not resolved in the spectrum of NH4SeCN (Fig. 2), a consequence of the broader lines. The line shape of the asymmetric doublet in the 13C NMR spectrum of an MAS sample of NMe4SeCN cannot be simulated as well as that of NH4SeCN (Fig. 2); this may be caused by the onset of self- decoupling of the 14N (44). The 13C NMR spectrum of a stationary sample of NMe4SeCN is dominated by the contribution from the four methyl groups, but the splittings due to 14 13 RDD( N, C) in the high-frequency region of the spectrum are well resolved. With these features and with the isotropic chemical shift obtained from the spectra of MAS samples, analysis of the spectrum is possible, yielding the principal components of the carbon CS tensors summarized in Ta- ble 1. From the 14N, 13C dipolar splittings observed in the high-frequency region, RDD = 1180 ± 30 Hz, corresponding to rCN = 1.23 ± 0.01 Å. This value is slightly longer than the value of 1.205 Å obtained for NH4SeCN. While the difference is only slightly greater than the experimental error, the longer value obtained for NMe4SeCN is probably a consequence of greater librational motion of the selenocyanate anion of the latter salt. Note that motion of the N—C bond will 14 13 result in partial averaging of RDD( N, C) and an apparent lengthening of rCN. For the 13C MAS NMR spectrum of KOCN, recycle delays in excess of one hour were used, but the signal-to-noise ratio was still very low, precluding any analysis for the compound at natural abundance. Hence only the isotropic chemi- δ cal shift was determined. The value of iso, 130.2 ppm, is within experimental error of the value determined for KOCN dissolved in D2O, 129.7 ppm (45). µ h = 0 γγ13 14 −3 Nitrogen-15 NMR spectra [7] RrDD ()()CNCN 15 42ππ The N MAS NMR spectrum of NMe4SeCN (Fig. 4) + consists of an intense peak at 47.3 ppm, due to NMe4 , and a where +rCN, is the motionally averaged C—N bond length. weaker, spinning rate invariant peak at 255.0 ppm due to – The splittings due to RDD for crystallites oriented such that SeCN . The spinning sidebands of the latter peak were ana- δ is parallel to B are clearly seen at high frequency in lyzed by the Herzfeld–Berger method to obtain the span of z 0 Ω Fig. 3; although it is not as well resolved, the effect of RDD the nitrogen CS tensor, = 439 ± 10 ppm. This value is on the spectra of crystallites oriented such that δ2 is parallel within experimental error of the value obtained for to B0 are also observed at low frequency. From the fit of the NH4SCN, 415 ± 15 ppm (39). The average nitrogen shield- – NMR spectrum of a stationary powder sample, RDD = 1250 ing for SeCN in solid NMe4SeCN is 44 ppm less than the – ± 20 Hz, corresponding to rCN = 1.205 ± 0.007 Å. This is value for SCN of solid NH4SCN. In aqueous solutions, the – significantly longer than the value of rCN determined from shielding of the nitrogen nuclei of SeCN dissolved in D2O an early X-ray diffraction study of KSeCN (42), 1.117 Å, (46) decreases by 30 ppm compared to that of the nitrogen – but is in agreement with the results of an ab initio study nuclei of SCN dissolved in H2O (47). (34). In principle, the quadrupolar interaction with 14N should also be considered in the fit, but it has been shown Selenium-77 NMR spectra (39–41) that this interaction has only minor effects on the Selenium-77 NMR spectra of MAS samples of 77 positions of the critical frequencies in spectra of stationary NMe4SeCN are shown in Fig. 5. The isotropic Se chemical samples acquired at the external magnetic fields used in this shift is comparable to that of KSeCN measured in solution study. From the dipolar coupling, and assuming that the larg- (6). The experimental spectra of the selenocyanates were est component of the 14N EFG tensor is along the C,N bond simulated via a non-linear least-square implementation of with an asymmetry of zero, analysis of the spectrum of an the method of Herzfeld and Berger (29); the resulting data 14 MAS sample (Fig. 2) yields CQ( N) = –2.6 ± 0.3 MHz. This are summarized in Table 1. These simulations indicate large

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15 77 Fig. 4. Experimental N NMR spectrum of solid NMe4SeCN ac- Fig. 5. Experimental and calculated Se MAS NMR spectra of quired at 9.4 T with a spinning frequency of 3040 Hz. 10 000 solid NMe4SeCN at 9.4 T and a spinning frequency of 1500 Hz. transients were acquired, with a recycle time of 30 s. Top: regular cross-polarization experiment with 3000 scans and 10 ms contact time. Middle: variable-amplitude cross-polarization experiment with 2532 scans and 15 ms contact time. Bottom: δ δ δ calculated spectrum with 11 = 22 = –40 ppm, 33 = –814 ppm. The isotropic peaks are indicated with an asterisk.

anisotropies in the 77Se CS tensors, making the observation of spectra of stationary powder samples difficult. Non- axially symmetric CS tensors are not precluded by the linear SeCN– anion, since the positions of the cations in the crystal lattice may affect the shielding. For example, the crystal structure of KSeCN shows that the potassium atoms are not arranged symmetrically about the selenium (42). Chemical shift tensors determined from the analysis of spinning sideband intensities in spectra of slow-spinning MAS samples are subject to significant errors if the tensor is approximately axially symmetric (48). In light of the above and considering the errors associated with the experimental data, the values quoted in Table 1 do not conclusively determine the symmetry of the selenium CS tensor. In cases where cross- polarization was used to obtain the spectra, further artifacts could be introduced in the spinning sideband intensities from anisotropy in the cross-relaxation stage of the experiment (49). For example, clear differences were observed for NMe4SeCN in regular CP experiments (Fig. 5, top) and experiments employing variable-amplitude cross-polarization (50) (see Fig. 5, middle). In the spectrum obtained with regular CP, the intensities of the spinning sidebands in the For comparison, the chemical shift values of the neighbourhood of the isotropic peak (indicated with an as- experimental data have been converted to chemical shielding terisk) are much stronger relative to the higher-order spin- (see experimental section). While the observed spans for the carbon CS tensors of SCN– and SeCN– are similar (321 to ning sidebands about the position of δz and δ2 than are those obtained with VACP. The latter spectrum agrees much 285 ppm), the span is significantly greater for SeCO better with the calculated spectrum. This effect is probably (399 ppm) and CSe2 (506 ppm). These experimental trends due to the offset dependence of the Hartmann–Hahn match- are reproduced by the highest level ab initio computations ing condition (50). In spite of the possible errors and arti- that we can presently apply to all three molecules, HF/6– facts, it is clear that the selenium CS tensors do not deviate 311+G*. Likewise, ab initio calculations correctly predict a larger span for the nitrogen nucleus of the SeCN– anion than significantly from axial symmetry (the maximum deviation – in κ is estimated to be 10%). In the ensuing discussion, we for that of the SCN anion. The calculated isotropic carbon chemical shielding for will assume that the carbon, nitrogen, and selenium tensors – are axially symmetric, consistent with our experimental data. SeCN plotted against basis set size is shown in Fig. 6 (top). This figure indicates that the calculated isotropic shielding is Ab initio calculations converging to a value which is approximately 15 ppm less than the experimental value. Similar results were obtained Carbon and nitrogen shielding tensors for SeCN– and for the calculated shielding of the nitrogen of SeCN– (Ta- related molecules ble 3) and the carbon of CSe2 (Table 2). The calculated carbon and nitrogen chemical shielding The calculated carbon chemical shielding tensor compo- tensor components are listed in Tables 2 and 3, respectively. nents parallel to the bond are invariant to basis set size or to

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a Table 2. Carbon chemical shielding tensors for CSe2, SeCO, and for some cyanate salts. σ σ σ Ω Compound Method iso z 2 KOCN Exp 57.9 — — — OCN– HF/6–311+G* 58.7 –57 290 347 b NH4SCN Exp 55.0 –52 269 321 SCN– HF/6–311+G* 42.7 –81 290 371 NH4SeCN Exp 62.4 –40 265 305 NMe4SeCN Exp 69.0 –27 258 285 SeCN– HF/6–311+G* 26.7 –106 292 398 MP2/6–311+G* 65 –49 293 342 SeCO Expc 34 –99 300 399 HF/6–311+G* –4 –151 289 440 d CSe2 Exp –21.8 –190 316 506 HF/6–311+G* –121 –329 295 624 MP2/6–311+G* 8 –137 298 435 aSee the footnotes to Table 1 for details on experimental data. bFrom reference 39. Values reported in Table 1 of this reference are slightly different from those shown here, since the values were based on an earlier absolute shielding scale for carbon. c σ From reference 56(a), Vol. 25, p.48. A comparable value of iso, 31.5 ppm, has also been reported from a solution-state study (reference 57). d σ Ω σ The individual tensor components were calculated from the experimental values of iso (reference 57) and (reference 12) according to z = σ Ω σ σ Ω iso – (1/3) and 2 = iso + (2/3) .

Fig. 6. Isotropic carbon and selenium chemical shielding, calcu- Table 3. Nitrogen chemical shielding tensors for some cyanate lated at the HF level, plotted against the basis set used for the salts.a calculation. The scale for the abscissa is qualitative. σ σ σ Ω Compound Method iso z 2 OCN– HF/6–311+G* 223.4 162 347 185 b NH4SCN Exp 53.5 –85 331 416 SCN– HF/6–311+G* 19.3 –144 346 490 NMe4SeCN Exp 9.5 –137 303 440 SeCN– HF/6–311+G* –32.3 –221 345 566 MP2/6–311+G* 32.0 –125 346 471 aSee the footnotes to Table 1 for details on experimental data. bFrom reference 39.

would be instructive but are not practical at this time given the computational requirements. In addition to intermolecular effects, vibrational effects are a factor, although these are expected to be fairly small for carbon and nitrogen (51). For example, Lounila et al. (52) calculated a vibrational cor- σ rection of iso = –2.4 ppm for the carbon of CSe2. For molecules containing large atoms, relativistic effects (53) may become significant (vide infra), even for the calculated shielding of neighbouring nuclei (54). The chemical shielding tensors for the carbon and nitro- – gen nuclei of SeCN as well as that for the carbon of CSe2 were calculated at the MP2 level (Tables 2 and 3). In all cases, the isotropic shielding values calculated at the MP2 level are significantly greater than the corresponding HF values, and are generally greater than the experimental values, reflecting the fact that the effects of electron correlation are not considered in HF calculations but are often overesti- the effects of electron correlation, reflecting the fact that mated by MP2 (55). A particularly large electron correlation there is no paramagnetic contribution to this component of effect is expected for the carbon of CSe2 because of its mul- the chemical shielding tensor. However, the experimental tiple bonding (56). The large deshielding (57) and span (12) σ values of 2 are approximately 30 ppm less than the calcu- reported for the carbon of CSe2 are in agreement with our lated values, perhaps a consequence of intermolecular ef- calculations, as well as with those of an earlier computa- fects. Calculations on a larger portion of the crystal lattice tional study (52).

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Bernard et al. 621

a Table 4. Selenium chemical shielding tensors for CSe2, SeCO, and for some selenocyanate salts. σ σ σ Ω Compound Method iso z 2

NH4SeCN Exp 2025 1764 2547 783 NMe4SeCN Exp 2067 1809 2583 774 SeCN– HF/6–311+G* 2343 2009 3011 1002 MP2/6–311+G* 2443 2160 3010 850 SeCO Expb 2347 2018 3006 988 Expc 2216 1887 2875 988 HF/6–311+G* 2327 1986 3008 1022 d e CSe2 Exp 1438 701 2911 2210 HF/6–311+G* 1547 816 3009 2193 MP2/6–311+G* 1816 1220 3007 1787 aSee the footnotes to Table 1 for details on the experimental data. bFrom reference 56(a), Vol. 25, p. 48. cThe isotropic value is from a solution-state study (reference 57). The chemical shielding tensor components were calculated from this value and the Ω σ σ Ω σ σ Ω experimental value of (reference 56(a), Vol. 25, p. 48) according to z = iso – (1/3) and 2 = iso + (2/3) . d σ Ω Components of the chemical shielding tensor are calculated from the experimental values of iso (reference 57) and (reference 12); see footnote c above. e A value of σ z = 1068 ppm, determined from a solid state NMR study at –90°C, has also been reported (reference 63). Selenium chemical shielding tensors for SeCN– and related Møller–Plesset calculated shielding may be approximated by molecules (64): The existence of an absolute shielding scale for selenium σσσ=+1 2 (22) combined with experimental data on simple molecules [8] iso HF MP2 has prompted several theoretical groups to investigate sele- 3 3 nium chemical shielding tensors. For example, the selenium Substituting our calculated values for CSe2 (Table 4), we ob- shielding for CSe2 has been calculated at the CHF (12), σ tain a value of iso = 1726 ppm, close to the value calculated HF/IGLO (55), MP2/GIAO (55, 58), CCD (58), and DFT with the CAS method, 1755.2 ppm (52). (59) levels of theory. Recently Lounila et al. reported the re- In the absence of intermolecular effects σ ≈σd (free sults of a detailed computational study of this molecule, 2 av atom) (eq. [5]). Our calculated values of σ (Table 4) are which included DFT and CAS calculations (52). Such calcu- 2 comparable to the non-relativistic value of σ d (free Se), lations are challenging, a consequence of the large chemical av 2998 ppm (38), reflecting the fact that first-order molecular shielding range of selenium (60) and of the fact that sele- properties can usually be calculated accurately. Experimental nium shielding is subject to relativistic effects (53). The values of σ for the selenocyanate salts are significantly progress of selenium shielding calculations has been re- 2 smaller than the calculated values. This difference is attrib- viewed recently (56). uted to intermolecular effects. A paramagnetic contribution The calculated selenium chemical shielding tensors are to σ 2 in the selenocyanate salts arises from the fact that, listed in Table 4. The calculated results are more dependent with the neighbouring nuclei such as potassium in KSeCN, on basis set size than are those of carbon and nitrogen the selenium atoms are not in a perfectly linear environment (Fig. 6), reflecting the greater chemical shift range of sele- σ p and hence the assumption that || = 0 is no longer strictly nium compared to those of carbon (3) or nitrogen (61). Fig- valid. ure 6 shows that the calculated isotropic selenium chemical – shielding for both SeCN and CSe2 are converging to values A comparison of the selenium chemical shielding tensors – which are significantly shielded compared to experimental for SeCN and CSe2. values. Ab initio calculations are performed on isolated mol- One might expect the selenium chemical shielding tensors ecules, which may result in significant discrepancies be- for Se=C=N– and Se=C=Se to be similar, but this is not the tween calculated and experimental results, since selenium is case (Table 4). Particularly surprising is the large difference known to be susceptible to medium effects. For example, the in the spans — that of CSe2 is almost three times greater isotropic shielding for gaseous H2Se is 126.6 ppm greater than the spans of the selenocyanates. This is remarkable than that for this compound in the liquid state, which in turn given that selenium is directly bonded to carbon in both is shielded by 11.4 ppm compared to its value in the solid cases. The difference cannot be attributed to the charge on – (62). For CSe2, the gas-phase shielding is 74.7 ppm greater SeCN , since the span of SeCO (65) is comparable to those than that for the same molecule in its liquid phase (63). of the selenocyanate salts. These large differences are repro- An electron correlation effect of 100 ppm is predicted for duced qualitatively by the calculations. In particular, we note SeCN–, based on the results of calculations at the MP2 level. that agreement between the experimental and calculated σ The effect is more significant for CSe2; iso calculated at the spans of SeCO is very good. Since the experimental span of MP2 level is 269 ppm greater than the value calculated at SeCO was determined from spin-rotation data (65), it is that the HF level (Table 4). Chesnut has noted that isotropic of an isolated molecule and is thus ideal for comparison chemical shielding constants calculated at various orders of with calculated values. To a first approximation, the span of Møller–Plesset theory converge such that the infinite-order the chemical shielding tensor is invariant to the effects of

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622 Can. J. Chem. Vol. 78, 2000

Fig. 7. Molecular orbital energy levels (in hartrees) for CSe2 and Fig. 8. MOs 33 and 38 (or 39) for CSe2, illustrating the mag- – σ →π∗ SeCN , calculated at the HF/cc-pVQZ level. Molecular orbitals netic-dipole allowed u u mixing for an applied magnetic below the dashed line are occupied. Some of the MOs have been field perpendicular to the page. labelled for reference.

* Se Se

σ Figure 8 illustrates that MO 33 with u symmetry can mix π∗ with degenerate MOs 38 or 39, with u symmetry. Because of the favourable symmetry, these MOs are expected to be a major factor in the deshielding observed in the direction per- relativity (52), and thus may be the best gauge of the accu- pendicular to the C∞ axis of CSe2. In Fig. 9, MO 36 (or de- π racy of non-relativistic calculations. generate MO 37) of CSe2, with g symmetry, is shown along σ ∗ The large difference in the spans of the selenium chemical with MO 40, which has g symmetry. Figures 8 and 9 illus- shielding tensors for CSe and SeCN– is a consequence of a trate the importance of symmetry in considering contribu- 2 tions to σ p. Although MOs 36 and 37 are only separated by large difference in their respective σz values because σ2 does not vary greatly amongst linear molecules. The contribution 0.46 hartrees from MO 40, compared to 0.57 hartrees sepa- to the span from σ d can be estimated from eq. [4]. For rating MO 33 from MOs 38 and 39, the shape of the latter is – σ d σ d much more favourable for mixing. SeCN , || = 2998 ppm and z = 3078 ppm, giving a span in σ d of 80 ppm. Similarly, a span of 151 ppm is predicted Figure 10 demonstrates why the selenium chemical σ d – for of CSe2. These numbers are much smaller and oppo- shielding tensor for SeCN has a large span which neverthe- p site in sign to those observed for σ . Thus, to understand the less is much smaller than that for CSe2. The illustrated MOs, – π large difference between the spans of CSe2 and SeCN , one occupied MO 23 (or degenerate MO 24) and unoccupied p σ* must consider σz. MO 25, have a relatively small difference in energy, 0.42 p One of the factors contributing to σz is the number of hartrees, but the symmetry of the MOs clearly is not as fa- MOs undergoing mixing, since eq. [3] is a summation. Fig- vourable for mixing here. Hence, these MOs are expected to ure 7 shows the energies for the highest occupied and lowest be a significant factor in the deshielding perpendicular to the – unoccupied MOs of SeCN and CSe2, calculated at the HF C∞ axis of the anion, but the lower symmetry makes mixing level using the cc-pVQZ basis set. Although the energy dif- much less favourable than in the case of CSe2. ference between the highest occupied and lowest unoccupied In summary, the smaller span of the chemical shielding – MOs are comparable for the two compounds, there are more tensor of SeCN compared to that of CSe2 may be attributed – low-lying unoccupied MOs for CSe2 than for SeCN .How- to two factors. The selenocyanate anion has fewer low-lying ever, we must also consider the symmetry of these MOs, unoccupied MOs which can mix with occupied MOs and p since mixing will only occur between MOs with magnetic- contribute to σz, and the mixing of most of those MOs sepa- dipole allowed symmetry. These are indicated with arrows in rated by a small energy difference are not magnetic-dipole Fig. 7. allowed. The latter may be attributed to the lower symmetry

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Bernard et al. 623

Fig. 9. MOs 36 (or 37) and 40 of CSe2, illustrating the mag- Fig. 10. MOs 23 (or 24) and 25 of CSe2, illustrating the mag- π →σ∗ π→ σ* netic-dipole allowed g g mixing for an applied magnetic netic-dipole allowed mixing for an applied magnetic field field perpendicular to the page. perpendicular to the page.

* Se Se

* NC Se

Se C Se

– of SeCN (C∞v), compared to that of CSe2 (D∞h). This con- NC Se clusion is supported by the observation that the chemical shielding tensor for SeCO, which also has C∞v symmetry, is comparable to that of SeCN–. Finally we note that, although we have not explicitly discussed the spans of the carbon – shielding tensors for SeCN and CSe2 (Table 2), the different spans of these shielding tensors may also be explained as above. For example, the mixing of MOs 33 with MOs 38 σ p Acknowledgments and 39 of CSe2, which contributes to of the selenium chemical shielding tensor, will also be an important factor in We are grateful to Tenkir Asfaw for help in the prepara- the paramagnetic component of the carbon shielding perpen- tion of a sample of ammonium selenocyanate. We thank dicular to the C∞ symmetry axis. Martine Ziliox for her help in initially setting up the VACP experiments on our Bruker AMX-400. We are grateful to members of the solid-state NMR group at Dalhousie Univer- sity for many helpful suggestions and to Andrew D. Phillips Conclusions for his help with the preparation of the molecular orbital dia- grams. The financial assistance of the Natural Sciences and The carbon, nitrogen, and selenium chemical shift tensors Engineering Research Council (NSERC) of Canada is grate- for several solid selenocyanate salts have been characterized fully acknowledged. GMB acknowledges NSERC, the Izaak by NMR. Within experimental error, the chemical shift ten- Walton Killam Trust, and the Walter C. Sumner Foundation sors are axially symmetric. A notable result of this study is for postgraduate scholarships. Spectra were acquired at the the observation that the spans of the carbon and selenium CS Atlantic Region Magnetic Resonance Centre, which is also – supported by NSERC. tensors for SeCN are much smaller than those for CSe2,a consequence of the smaller paramagnetic term in the shield- – ing perpendicular to the C∞ axis of SeCN compared to that References of CSe . High-level ab initio calculations show that the 2 1. (a) J.L. Burmeister. Coord. Chem. Rev. 1, 205 (1966); (b)J.L. smaller paramagnetic term results from the lower symmetry Burmeister. Coord. Chem. Rev. 3, 225 (1968); (c) J.L. of the MOs of SeCN– and from the fact that there are fewer – Burmeister. Coord. Chem. Rev. 105, 77 (1990). low-lying unoccupied MOs for SeCN compared to CSe2. – 2. H. Duddeck. Prog. Nucl. Magn. Reson. Spectrosc. 27,1 The shielding parallel to the C∞ axis of SeCN is less than (1995). expected for linear molecules, suggesting that this compo- 3. H.-O. Kalinowski, S. Berger, and S. Braun. Carbon-13 NMR nent is sensitive to intermolecular effects. This work has spectroscopy. John Wiley and Sons, Chichester. 1988. shown that multinuclear magnetic resonance studies of sol- 4. (a) N. Logan. In Applications of 14N NMR data in the study of ids can offer much information about solid selenocyanate inorganic molecules. Edited by M. Witanowski and G.A. salts; it is hoped that this study will encourage further stud- Webb. Plenum Press, N.Y. 1973. Ch. 6; (b) M. Witanowski, L. ies of these and related ligands. Stefaniak, and G.A. Webb. Annu. Rep. NMR Spectrosc. 11b,1

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