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

The infrared (3200-30 cm(-1) spectra of gaseous and solid, the Raman spectr a (3200-30 cm(-1)) of the liquid and solid vinyl silyl bromide, CH2CHSiH2Br , 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. Variab le temperature studies from 0 to - 87 degrees C of the Raman spectrum of th e 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 confo rmer being the more stable form. The predictions from the ab initio calcula tions up to MP2/6-311 + G(2d,2p) basis set favor the gauche as the more sta ble 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 ar e supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations and the potential energy terms fo r the conformer interconversion have been obtained from the same calculatio ns. 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 obtai ned for some similar molecules. (C) 2000 Elsevier Science B.V. All rights r eserved.