Infrared and Raman spectra, conformational stability, barriers to internal rotation, normal-coordinate calculations and vibrational assignments for vinyl silyl bromide

Abstract

The infrared (3200–30 cm −1 spectra of gaseous and solid, the Raman spectra (3200–30 cm −1) of the liquid and solid vinyl silyl bromide, CH 2CHSiH 2Br, have been recorded. Additionally, quantitative depolarization values have been obtained. Both the gauche and cis conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature studies from 0 to −87°C of the Raman spectrum of the liquid was carried out. From these data, the enthalpy difference has been determined to be 22±6 cm −1 (0.26±0.08 kJ/mol), with the gauche conformer being the more stable form. The predictions from the ab initio calculations up to MP2/6-311++G(2d,2p) basis set favor the gauche as the more stable form. A complete vibrational assignment is proposed for both the gauche and cis conformers based on infrared band contours, relative intensities, depolarization values and group frequencies. The vibrational assignments are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations and the potential energy terms for the conformer interconversion have been obtained from the same calculations. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing a variety of basis sets up to 6-311++G(2d,2p) at levels of restricted Hartree-Fock (RHF) and/or Moller-Plesset (MP) to second order. The results are discussed and compared to those obtained for some similar molecules.