RAMAN AND INFRARED SPECTRA, CONFORMATIONAL STABILITY, AB INITIO CALCULATIONS AND VIBRATIONAL ASSIGNMENTS FOR CHLOROMETHYL METHYL SILANE

Abstract

The infrared (3500 to $30 cm^{-1}$) spectra of gaseous and solid and the Raman (3500 to $10 cm^{-1}$) spectra of the liquid with quantitative depolarization ratios and solid chloromethyl methyl silane, $ClCH_{2}SiH_{2}CH_{3}$, have been recorded. Similar data have also been recorded for the $Si-d_{2}$ isotopomer. These data indicate that two conformers are present in the fluid states but only one conformer is present in the annealed crystalline state. The mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (-100 to $-70^{\circ}C$) have been recorded. Utilizing conformer pairs at 738 (gauche), 685 (gauche), and 700 (trans) $cm^{-1}$ the enthalpy difference has been determined to be $180 \pm 18 cm^{-1} (515 \pm 51 cal/mol)$ with the gauche conformer the more stable species. However, in the spectrum of the solid, the trans conformer is the stable rotamer remaining after the sample is well annealed. Utilizing the Si-H stretching frequencies from the infrared spectrum of the $ClCH_{2}SiHDCH_{3}$ isotopomer, the two Si-H bond distances are calculated to be 1.482 and 1.487{AA} for the gauche conformer. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies are reported for both conformers from $RHF/6-31G^{\ast}$ and/or $MP2/6-31G^{ast}$ ab initio calculations. The gauche conformer is predicted to be the more stable rotamer from both ab initio calculations in agreement with the experimental results. The other calculated quantities are compared to the experimentally determined values where applicable as well as with some corresponding results for some similar molecules.