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 be en recorded for the Si-d(2) isotopomer. These data indicate that two c onformers are present in the fluid states, but that only one conformer is present in the annealed crystalline state. The mid-infrared spectr a of the sample dissolved in liquified xenon as a function of temperat ure (-100 to -60 degrees C) have been recorded. Utilizing conformer pa irs 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 spe cies. However, in the spectrum of the solid, the gauche conformer is t he 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 Cl2HCSiH DCH3 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 correspondin g results for some similar molecules. (C) 1998 Elsevier Science B.V. A ll rights reserved.