Vibrational spectra, conformations, quantum chemical calculations and spectral assignments of 1-chloro-1-silacyclohexane

Abstract

The infrared spectra of 1-chloro-1-silacyclohexane have been studied as a vapour and liquid at ambient temperature and as amorphous and annealed crystalline solids at 78K. Various infrared bands present in the vapour and liquid states vanished in the crystalline state upon cooling. Raman spectra of the liquid were recorded at 293K and polarization data obtained. Additional Raman spectra were recorded at various temperatures between 293 and 163K, and intensity changes with temperature of certain Raman bands were detected. A crystalline phase was observed around 140K, leading to spectral shifts and a number of vanishing bands.The compound exists in two conformers, equatorial (e) and axial (a) in the fluid phases, but only the a-conformer was present in the crystal. The experimental results suggest that the a-conformer has 2.8kJmol−1 lower enthalpy than e in the liquid, leading to 69% of the a conformer at 293K.B3LYP calculations with various basis sets and the G3 model chemistry gave conformational energy difference ΔE (e−a) in the range 0.4–1.4kJmol−1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the e and a conformers were calculated. The fundamental wavenumbers were also derived in the anharmonic approximation in B3LYP/cc-pVTZ calculations, a relative deviation of less than 2% between the observed and calculated wave numbers for the 48 modes of the e- and a-conformers was obtained.