Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignments for dichloromethyl methyl silane

For dichloromethyl methyl silane, Cl2HCSiH2CH3, the infrared (3500 to 30 cm(-1)) spectra in the gaseous and solid phase, and the Raman (3500 to 10 cm(-1)) spectra of the liquid with quantitative depolarization ratios and of the solid, 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 that 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 -60 degrees C) have been recorded. Utilizing conformer pairs at 486 (gauche), 513 (trans), 638 (trans) and 660 (gauche) cm(-1), the enthalpy difference has been determined to be 120 +/- 18 cm(-1) (1.44 +/- 0.22 kJ mol(-1)) with the trans conformer the more stable species. However, in the spectrum of the solid, the gauche conformer is the stable rotamer remaining after the sample is well annealed. The Si-H distances have been obtained from the frequencies of the isolated Si-H stretching modes from the infrared spectrum of the gaseous Cl2HCSiHDCH3 isotopomer. This parameter has a value of 1.481 Angstrom for the trans conformer and 1.479 and 1.482 Angstrom 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* and/or MP2/6-31G*, and MP2/6-3 11 + G** ab initio calculations. The trans conformer is predicted to be the more stable rotamer from all ab initio calculations, in agreement with the experimental results. The other calculated quantities are compared with the experimentally determined values where applicable, as well as with some corresponding results for some similar molecules. (C) 1998 Elsevier Science B.V. All rights reserved.