SPECTRA AND STRUCTURE OF SILICON-CONTAINING COMPOUNDS .18. RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, VIBRATIONAL ASSIGNMENT, BARRIER TO INTERNAL-ROTATION AND AB-INITIO CALCULATIONS OF ETHYLDICHLOROSILANE

The Raman (3200-10 cm-1) and infrared (3200-30 cm-1) spectra were recorded for gaseous and solid phases of ethyldichlorosilane, CH3CH2SiHCl2. Additionally, the Raman spectrum of the liquid was recorded and quantitative depolarization values were obtained. In the spectra of the gas and liquid, both the trans and gauche confomers are present with the gauche conformer being the dominant rotamer at ambient temperature, but at -126-degrees-C the trans conformer dominates the spectrum. Only the trans form is present in the vibrational spectrum of the solid. In the Raman spectrum of the liquid at ambient temperature, the amount of the gauche conformer is significantly reduced compared with that present in the gas. From the relative intensities of two pairs of Raman lines of the liquid at 192/180 and 661/709 cm-1 as a function of temperature, the enthalpy difference is found to be 143 +/- 17 cm-1 (408 +/- 48 cal mol-1) with the trans rotamer the more stable conformer. Similar variable-temperature studies of the infrared spectrum with the sample dissolved in liquid krypton were carried out and the value of the enthalpy difference was determined to be 61 +/- 10 cm-1 (172 +/- 30 cal mol-1). This value should be near the DELTAH value for the Gas. The asymmetric torsion is observed at 71 cm-1 for the gauche conformer and the potential function governing the conformational interchange is estimated. The fundamenal modes of both conformers were assigned utilizing band contours, depolarization values, group frequencies and normal coordinate calculations. The Si-H bond distance was determined to have values of 1.475 and 1.476 angstrom for the trans and gauche conformers, respectively, from the stretching wavenumbers. All of these data are compared with the corresponding quantities obtained from ab initio gradient calculations employing the RHF/3-21G, RHF/3-21G*, RHF/6-31G*, MP2/6-31G*, and/or MP2/6-31 + G* basis sets. Complete equilibrium geometries were calculated for both conformers. The results are compared with corresponding quantities obtained for some similar molecules.