D.C. McKean

New light on the stretching vibrations, lengths and strengths of CH, SiH and GeH bonds

Abstract Recent developments in the field of ‘isolated’ MH stretching frequencies are reviewed. A new r O versus ν is CH correlation plot is described. The ‘local mode’, high overtone, approach to CH bond strengths is compared with the ν is CH one. 2ν is CH studies are described which also lead to anharmonicities of individual bonds. In current work on ν is SiH and ν is GeH , conformational effects of halogens, lone pairs and methyl groups are found similar to those in the carbon analogues. These create problems in assessing substituent effects. New correlations with Σσ * for alkyl groups are very different from previous ones.

The ground state structures of disilane, methyl silane and the silyl halides, and an SiH bond length correlation with stretching frequency

Abstract Infrared spectra of the molecules Si 2 HD 5 , SiHD 2 CH 3 , SiH 3 CHD 2 , SiHD 2 X (X = F, Cl, Br, I) have been recorded at 0.05 cm −1 resolution. Analyses of “perpendicular” fundamentals of these very slightly asymmetric top molecules permit the accurate determination of their ground state ( A 0 — B 0 ) constants, from which the A 0 rotational constants may be obtained. When combined with all known Raman and microwave constants, this enables the ground state structures to be determined with precision, allowance being made for the shortening of SiH(CH) bonds on deuteration. For disilane this work represents the first spectroscopic determination of a complete structure, while for the silyl halides it demonstrates overestimates in the SiH bond length of nearly 1% by previous workers. In the case of methyl silane, comparison with a recent ab initio structure calculation reveals a considerable overestimate of the CSi bond length in the latter. A correlation between isolated SiH stretching frequency and ground state bond length over 15 molecules reveals a somewhat reduced sensitivity compared with the corresponding CH correlation. Nevertheless, a predictive capability of better than ±0.003 A seems possible from the isolated SiH stretching frequency, at least for molecules with fully saturated bonding.

CH bond length variations due to the intramolecular environment: a comparison of the results obtained by the method of isolated CH stretching frequencies and by ab initio gradient calculations

Abstract Two approaches, one experimental and the other theoretical, to the elucidation of CH bond length variation due to the intramolecular environment are compared. The experimental method is based on the concept of isolated CH stretching frequencies while the theoretical method involves the ab initio gradient procedure. Provided molecules of not too dissimilar structure are compared agreement between the two methods is encouragingly satisfactory. The methods are to some extent complementary, the ν is CH method being especially applicable to compounds with heavy atoms (e.g. Br, Ge, etc.) and the ab initio method in cases where isotopic frequency data are lacking. The potential applications of this dual technique approach include assessment of the quality of microwave-based geometries, exploration of link between structure and reactivity and prediction of the structure of these basic units in much larger molecules.

The harmonic force field of methyl acetylene

Abstract Samples of methyl acetylene with 13C isotopic substitution at all three C positions have been prepared and their IR spectra studied. The resulting vibration frequencies yield accurate frequency shift data, which are of great value in increasing the precision with which the harmonic potential function for the molecule is determined. A total of 23 of the 30 force constant parameters can be estimated. The remaining constants, which link basis symmetry coordinates separated by at least one bond or one non-common atom, are assumed to be zero. In the course of the spectroscopic study, a Fermi resonance between the ue5fcCH stretch vibration v1, and (v3 + 2vO9) was discovered, which becomes of first-order importance in the 13C substituted species. The effects of this resonance have been elucidated.

Isolated CH stretching frequencies, bond properties and fermi resonances in some CH3-N compounds

Abstract CHD 2 - substituted derivatives have been prepared and their IR spectra recorded for the following compounds: CH 3 NH 2 , (CH 3 ) 2 ,NH, (CH 3 ) 3 N, (CH 3 ) 2 NSiH 3 ,(CH 3 ) 2 NSiF 3 and [(CH 3 ) 2 N] 2 SiF 2 . The isolated CH stretching frequency for the CH bond trans to the nitrogen unshared pair varies markedly with substitution and can be correlated with the first ionization potential. It appears to offer a useful indication of the extent of electron withdrawal by silicon; the CH stretchin spectrum for (CHD 2 ) 2 NSiF 3 is consistent with a planar C 2 NSi skeleton. “3 × 3” refinement calculations for the CH 3 , CD 3 and CHD 2 species in the amines enable a quantitative description to be given of the “Bohlmann” bands near 2800 cm −1 . These are fundamentals, displaced by Fermi resonances to the extent of about 30, 20 and 10 cm −1 , respectively, for CH 3 NH 2 (CH 3 ) 2 NH and (CH 3 ) (CD 3 ) 2 N, which are primarily motions involving stretching of the weak CH bond. D 0 298 values predicted for the two CH bonds in each methyl group of trimethylamine differ by about 13 kcal mole −1

Microwave and infrared spectra of deuterium-substituted germyl halides and the structures of the germyl halides

Abstract We have obtained microwave spectra of fully and partially deuterated germyl halides GeD3X and GeHD2X (X = F, Cl, Br, I) and analysed the infrared spectra in the GeH stretching region of the asymmetric species. The resulting Ao and Bo values have been combined with existing data on GeH3X to give improved structures for the germyl halides. The values of the HGeX angles obtained show a correlation with GeH stretching frequencies, as in methyl compounds, but there appears to be no such correlation between stretching frequencies and GeH bond lengths.

Diborane: Infrared spectra of gaseous and crystalline phases, and a complete vibrational assignment

Abstract A thorough investigation of the infrared spectra in the gas phase and polycrystalline films of normal and 10 B enriched samples of B 2 H 6 and B 2 D 6 has been made with a resolution of ∼0.3 cm −1 . The data from this study, in conjunction with those from previous Raman studies, enable a complete and unambiguous assignment of all 18 fundamental vibrations of B 2 H 6 and B 2 D 6 to be made. All previously uncertain or unassigned fundamentals are located by direct observation in the crystal phase and by means of one or more infrared or Raman active combination bands. Boron isotopic vibration frequency shifts are used to analyze a number of important Fermi resonances. Good product rule fits then result in all symmetry species. Assignments in the regions 1900-1800 and 1500-1400 cm −1 respectively of B 2 H 6 and B 2 D 6 are clarified. Force field calculations predict a value of | ζ 9,14 a | close to zero, which removes an earlier difficulty in the assignment of the ν 9 infrared active rocking fundamental. The absence and presence, respectively, of a center of symmetry in the α and β crystal phases are demonstrated.

The infrared spectrum of chd2cn and the ground state geometry of methyl cyanide

Abstract Analysis of the rovibration spectrum of CHD 2 CN enables the rotation parameter ( A o —B o ) to be determined, from which the value of A o may be found using the microwave B o and C o constants. The use of A o (CHD 2 CN) in conjunction with all other available B o and C o constants for isotopes of methyl cyanide enables the ground state geometry to be defined within fairly narrow limits. The CH bond length is in agreement with the value predicted from the CH vibration frequency of CHD 2 CN, and with the value calculated from the results of a recent analysis of the infrared spectrum of CD 3 CN.

Vibrational anharmonicity and Fermi resonances in CHF2Cl

Abstract The infrared spectrum of CHF 2 Cl has been recorded between 15 000 and 350 cm −1 . The Fermi resonance between levels involving ν 4 and 2ν 6 is analysed in bands extending from 800 cm −1 to 7000 cm −1 leading to a best value of k 466 = ± 14.98 cm −1 . In conjunction with the recent results of Amrein, Dubal and Quack, Molec. Phys. 56,727 (1985); estimates are reported for 38 out of 45 possible x ij constants. A variation in the relative intensity of the two Q branches associated with ν 1 , on cooling the gas cell, indicates that a hot band contributes to the upper branch at 3024.55 cm −1 . However, other evidence suggests that the latter arises also from the combination ν 2 + ν 7 + ν 9 , in a very weak, close resonance with ν 1 at 3021.27 cm − . A number of anomalous band contours are reported.

Vibrational spectra of cis and trans but-2-enes: assignments, isolated CH stretching frequencies and CH bond lengths

Abstract Infrared and Raman spectra have been obtained from 11 isotopomers of cis and trans but-2-enes in various phases ( c - and t -CH 3 CHCHCH 3 , CH 3 CHCDCH 3 , CD 3 CHCHCD 3 , CD 3 CHCDCD 3 , CD 3 CDCDCD 3 ; c -CH 3 CDCDCH 3 ). Nearly all the fundamentals are securely assigned. Isolated CH stretching frequencies ν is CH are observed or deduced from the spectra and used to predict CH bond lengths and dissociation energies. The olefinic CH and methyl CH s bonds are stronger in the cis compound than those in the trans , in keeping with substituent effects previously observed in propene, and with molecular mechanics predictions. Comparison of ν is CH values with local mode fifth overtone spectra for cis -CH 3 ue5f8CHue5fbCHue5f8CH 3 indicates that the part of the latter associated with the CH a bond cannot be readily interpreted.