The infrared and Raman spectra, ab initio calculations and spectral assignments of cyclopropylmethyl dichlorosilane (c-C 3H 5)SiCl 2CH 3

Abstract

Raman spectra of cyclopropylmethyl dichlorosilane (c-C 3H 5)SiCl 2CH 3 as a liquid were recorded at 293 K and polarization data were obtained. Additional Raman spectra were recorded at various temperatures between 293 and 163 K, and intensity changes of certain bands with temperature were detected. No crystallization was ever obtained in the Raman cryostat in spite of extensive annealing. The infrared spectra have been studied as a vapour, as an amorphous solid at 78 K and as a liquid in the range 600–100 cm −1. No infrared bands present in the vapour or liquid seemed to vanish upon cooling, and the sample never formed crystals on the CsI window of an infrared cryostat. The compound exists a priori in two conformers, syn and gauche, and the experimental results suggest an equilibrium in which the gauche conformer has 1.64 kJ mol −1 lower enthalpy than syn in the liquid, leading to 20% syn at ambient temperature. Most of the syn bands were situated close to the corresponding gauche bands and it was difficult to obtain reliable Δ H values. B3LYP calculations with various basis sets and the CBS-QB3 and G2 and G3 models were employed, yielding the conformational enthalpy difference Δ H ( syn– gauche) between 2.6 and 3.4 kJ mol −1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the syn and gauche conformers were calculated. Instead of scaling the calculated wavenumbers in the harmonic approximation, calculations from B3LYP/cc-pVTZ were derived in the anharmonic approximation. In most cases these values were in good agreement with the experimental results for 38 observed modes of the gauche and 8 modes of the syn conformer with a deviation of ca. 1%.