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

The infrared (3200-30 cm(-1)) spectra of gaseous and solid and the Raman sp ectra of liquid (3200 - 30 cm(-1)), with quantitative depolarization values , and solid vinyldichlorosilane, CH2=CHSiHCl2, have been recorded. Both the gauche and the cis conformers have been identified in the fluid phases. Va riable temperature (105-150 degrees C)studies of the infrared spectra of th e sample dissolved in liquid krypton have been carried out. From these data the enthalpy difference has been determined to be 20 +/- 5 cm(-1) (235 +/- 59 J mol(-1)) with the gauche conformer the more stable rotamer. Pt was no t possible to obtain a single conformer in the solid even with repeated ann ealing of the sample. The experimental enthalpy difference is in agreement with the prediction from MP2/6-311 + G(2d,2p) ab initio calculations with f ull electron correlation. However, when smaller basis sets, i.e. 6-31G(d) a nd 6-311 + G(d,p) were utilized the cis conformer was predicted to be the m ore stable form. Complete vibrational assignments are proposed for both con formers based on infrared contours, relative infrared and Raman intensities , depolarization values and group frequencies, which are supported by norma l coordinate calculations utilizing the force constants from ab initio MP2/ 6-31G(d) calculations. From the frequencies of the Si-H stretches, the Si-H bond distance of 1.474 Angstrom has been determined for both the gauche an d the cis conformers. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G(d), 6-311 G(d,p) and 6-311 + (2d,2p) basis sets at level of Hartree-Fock (RHF) and/o r Moller-Plesset to the second order (MP2) with full electron correlation. The potential energy terms for the conformer interconversion have been obta ined from the MP2/6-31G(d) calculations. The results are discussed and comp ared with those obtained for some similar molecules. (C) 2000 Elsevier Scie nce B.V. All rights reserved.