Donald C. McKean

Scaled quantum chemical force fields for 1,1-difluorocyclopropane and the influence of vibrational anharmonicity.

Potential functions and harmonic (omega(i)) and anharmonic (nu(i)) fundamental frequencies have been calculated for 1,1-difluorocyclopropane (DFCP) and its d4 and d2 isotopomers using the program Gaussian 03. B3LYP and MP2 models were employed, each with the bases 6-311++G** and cc-pVTZ. Anharmonicity corrections Delta(i) = omega(i) - nu(i) are listed and shown to be different for symmetric and antisymmetric CH stretching modes in situations where Fermi resonance appears to be absent. The same effect is missing in C2H4, for which similar calculations were made. The quadratic force fields for DFCP have been scaled in symmetry coordinate space with 15 scale factors both to observed frequencies nu(obsd)and also to omega (obsd), where omega(obsd) = nu(obsd) + Delta. With nu(obsd) especially, different scale factors are needed for the symmetric and antisymmetric CH stretching force constants due to their differing anharmonicities. The source of the latter in the quartic and cubic force field is explored. MP2 calculations of valence interaction force constants involving the stretching of bonds on a common carbon atom are preferred to those from a B3LYP model. In either model, scaling to omega(obsd) rather than to nu(obsd) does not remove the necessity of varying scale factors for differing types of motion in the same group. Theoretical values of the five quartic centrifugal distortion constants are listed for the normal species and compared with new experimental data. The predictions are sufficiently good to be useful in fitting pure rotational transitions. A weakness is identified in the current Gaussian 03 code for the calculation of vibration-rotation quantities, and limitations are noted in the manner in which Fermi resonance is handled.

Ab initio coupled cluster determination of the equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene.

The equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene, C(2)H(2)F(2), have been determined with high-level coupled cluster techniques combined with large basis sets, explicit consideration of core/valence, and scalar relativistic and higher order correlation effects. Excellent agreement was found with new semiexperimental structures, increasing the level of confidence in both approaches. Differences in bond lengths among ethylene and the fluoroethylenes are discussed.

Infrared studies of CH and CD stretching anharmonicity

Progress is reported in the analysis of CH and CD stretching overtones and their accompanying Fermi resonances in C2H4, C2H3D, CH2CD2 and C2HD3. CH stretching overtone data have been obtained for a number of partially deuterated alkanes, halogenated alkanes, MeCN, MeC2H, MeNH2, Me2NH, MeOH and Me2O. Values of anharmonicity constants xii obtained from these are compared with the results of local-mode studies. A provisional Fermi resonance analysis of CH(CD3)3 is reported. The possiblity of a relationship between xii, ωi and CH dissociation energy is explored.

EXPERIMENTAL AND AB INITIO INFRARED INTENSITIES IN DIMETHYLETHER : ATOMIC POLAR TENSORS AND ATOMIC CHARGES

Abstract Infrared intensities have been recorded in (CH 3 ) 2 O over the range 100–3300 cm −1 and ab initio calculations of geometry, frequency and infrared intensity made at the MP2/6–31G ∗ level. The experimental results are compatible with previous data for (CHD 2 ) 2 O and support the earlier results for (CH 3 ) 2 O of Blom et al. The more recent data of Rogers appear to be too high. Additivity of νCH infrared intensity, per CH bond, is found in both the experimental and ab initio studies. Overall agreement between the two approaches is quite good. Ab initio based atomic polar tensors and their invariants are listed for representative atoms in dimethyl ether, methane and ethane and their Mulliken and King effective atomic charges compared. Dipole derivatives for the two types of CH bond present are markedly different and are directed away from the bond directions by 14–19°. Rotation of the permanent dipole moment makes a significant contribution to the νCH stretching intensities in the d 5 species.

Ab initio and density functional studies of the structure, vibrational spectra and force field of trimethylsilane

Abstract Ab initio and density functional Q–M treatments have been carried out on trimethylsilane, SiHMe3. The harmonic force fields so obtained are scaled to frequencies selected from four isotopic species, SiH(CH3)3, SiD(CH3)3, SiH(CD3)3 and SiD(CD3)3, some new infrared data for the d0 species being employed. The region 250–200 cm−1 is reinterpreted in terms of the skeletal bending modes ν8(a1) and ν23(e), with ν8>ν23. Microwave information regarding the positions of the methyl torsions apparently conflicts with evidence both from the Q–M calculations and from infrared combination bands, which favour values less than 160 cm−1. All the scaled force fields give a poor fit to frequencies in the E symmetry species and nine force constants there are further refined. The spread of scale factors narrows when a larger basis set or density functional theory is employed, but widens slightly when electron correlation is introduced through an MP2 calculation. Valence interaction constants are reported for bond stretching motions and their significance assessed. Coriolis and centrifugal distortion constants are listed for the preferred B3LYP/6-311G** based force field. Electrical properties of the atoms and bonds are discussed.

Infrared and Raman spectra of 1,1-dichlorodisilane species: a scaled ab initio force field, atomic polar tensors, dipole derivatives and atomic charges

Abstract Infrared and Raman spectra are reported for 1,1-Si 2 H 4 Cl 2 and 1,1-Si 2 D 4 Cl 2 . All fundamentals except the silyl torsion are firmly assigned. A scaled ab initio force field is calculated at the HF/6–31G∗ level, embodying eleven scale factors. Calculated infrared and Raman intensities, atomic polar tensors, King effective and Mulliken atomic charges are all listed. These confirm the α and β effects on SiH bonds of chlorine substitution found earlier in Si 2 H 5 Cl. SiH bonds gauche to chlorine become more polar (SiH − ), while those in trans positions remain unaffected. Torsional structure observed in the v SiH region may permit the analysis of torsional levels and provide an estimate of barrier height.

Carbon–hydrogen bond properties and alkyl group geometries in dichloro(η5-cyclopentadienyl)-methyltitanium(IV) and dichloro(η5-cyclopentadienyl)-ethyltitanium(IV)[TiR(η5-C5H5)Cl2](R = Me or Et)

Infrared spectra have been recorded for various isotopomers of [TiMe(cp)Cl2](cp = cyclopentadienyl, η5-C5H5; Me = CH3, or CHD2) and [TiEt(cp)Cl2](Et = CH3CH2, CD3CH2, CH3CD2 or CHD2CD2) and assignments proposed for the alkyl group vibrations. A method was elaborated for the calculation of Fermi-resonance shifts on CH and CD stretching modes in methyl groups with C3v and Cs symmetry. Estimates of CH bond lengths, bond strengths and HCH angles were derived from the resonance-corrected frequency data. The results show the methyl group in [TiMe(cp)Cl2] to be markedly asymmetric, with the CH bond trans to the cyclopentadienyl ligand being ca. 0.005 A longer and 15 kJ mol–1 weaker than those trans to chlorine. In the ethyl compound, the terminal methyl group is similarly asymmetric, again with one weak bond and two stronger bonds. The νCH2 and νCD2 frequencies for the methylene group are anomalous and can only be satisfactorily interpreted in terms of a model in which the two methylene CH bonds are inequivalent. The estimated bond lengths are 1.1005 and 1.1059A, and the corresponding bond dissociation energies are 403 and 383 kJ mol–1, respectively. These results appear to point to a direct α-interaction between at least one of the methylene CH bonds and the titanium atom. A similar effect may also occur in [TiMe(cp)Cl2].

Isolated CH stretching frequencies, methyl group geometry, and methyl CH bond lengths and strengths in tricarbonyl(η5-cyclopentadienyl)methyl-chromium(II), -molybdenum(II), and -tungsten(II)

‘Isolated’ methyl CH stretching frequencies, ν(CHis), are reported for [M(CHD2)(η5-C5H5)(CO)3](M = Cr, Mo, or W) and are used to predict CH bond lengths, bond-dissociation energies, and mean HCH angles. The methyl groups are subject to appreciable internal rotation energy barriers and each contain two different types of CH bond, with non-equivalent orientations in the molecule and with significantly different bond lengths and strengths. The CH bond strengths decrease in the order Cr > Mo > W, reflecting a corresponding increase in metal–carbon bond strength estimated to be of the order of 55–70% from Cr to W. The methyl groups in [Cr(CHD2)(η5-C5H5)(CO)3] and [Cr(CD3)(η5-C5H5)(CO)3] are affected by extensive H/D exchange, probably with the 1,2-dimethoxyethane solvent used in the preparative reaction.

Infrared and quantum-chemical studies of the structure and vibrations of trisilylamine.

New infrared spectra are reported for variously labelled trisilylamines. Quantum-chemical (QC) calculations of structure and force field have been made at HF, MP2 and B3LYP levels, each with the 6-31G* and 6-311G** basis sets. At each level, a minimum in the potential surface occurs at the C3h configuration. No evidence was found for a significant variation in SiH bond length with orientation. The appearance of two bands in the infrared spectrum of N(SiH3)3 in the 2nuSiH region is explained by local mode theory in terms of transitions to (200) and (110) levels. In the gas phase, signal averaging appears to occur in the nuSiH region in the infrared spectrum, but not in the Raman. In solid films, both IR and Raman spectra indicate the presence of a range of SiH bond strengths, corresponding to an absence of any site symmetry. Each complete QC calculated force field was fitted to the frequencies observed for N(SiH3)3 and N(SiD3)3, using nine independent scale factors. An interaction force constant between nu(as)NSi3 and delta(s)SiH3 motions required further adjustment. Unobserved frequencies in the d0 and d9 species are predicted. The out-of-plane skeletal bending mode is expected to lie between 170 and 200 cm(-1). Unscaled SiH3 torsional frequencies vary from 64 cm(-1) upwards. The effect of the presence of three internal rotors on the spectra throughout calls for theoretical study.

Infra-red spectra of 13CD3GeH3 and DFT studies of force constants and electrical properties in methylsilane and methylgermane

Abstract New infra-red data are reported for some fundamental bands of 13 CD 3 GeH 3 in the gas phase. Triple-ζ DFT calculations of geometry and force field have been made for CH 4 , SiH 4 , GeH 4 , CH 3 SiH 3 and CH 3 GeH 3 . The DFT force fields for methylsilane and methylgermane are scaled (SDFT) on all available data, including harmonised frequencies and frequency shifts, Coriolis constants and centrifugal distortion coefficients. Comparison of scale factors reveals considerable variation. Empirical force fields (ESDFT) are determined, a number of off-diagonal constants being constrained to SDFT values. Problems remaining in the observed data are identified. Atomic polar tensors from the DFT calculations are used to obtain King effective atomic charges x and bond dipole derivatives d μ d r with respect to terminal bond stretching. Comparison between silanes and germanes reveals common trends in bending and stretching dipole moments together with the inductive effect of a methyl group.