Spectra and structure of silicon containing compounds. XXII. Raman and infrared spectra, conformational stability, vibrational assignment and ab initio calculations of methylvinyldichlorosilane

Abstract

The infrared (3500–40 cm −1) and Raman (3500–20 cm −1) spectra of gaseous and solid methylvinyldichlorosilane, CH 2CHSiCl 2CH 3, have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values obtained. Both the cis and gauche conformers have been identified in the fluid phases but only the cis conformer remains in the solid. Variable temperature (−60 to −100°C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 77 ± 8 cm −1 (220 ± 23 cal mol −1), with the cis conformer being the more stable rotamer. A complete vibrational assignment is proposed for the cis conformer based on infrared band contours, relative intensities, depolarization values and group frequencies. Several of the fundamentals for the gauche conformer have also been identified. The vibrational assignments are supported by normal coordinate calculations. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 3–21G∗ and 6–31G∗ basis sets at the levels of restricted Hartree-Fock (RHF) and Møller-Plesset (MP) to second order. The frequencies for the normal modes and the potential energy terms for the conformer interconversion have been obtained from the RHF 6–31 G∗ calculation. The results are discussed and compared to those obtained for some similar molecules.