Raman and infrared spectra, quantum chemical calculations, conformations and spectral assignments of 1-chloro-1-methyl-1-silacyclohexane

Abstract

A 1,1-disubstituted silacyclohexane, 1-chloro-1-methyl-1-silacyclohexane, C5H10SiClCH3, was synthesized and studied by infrared and Raman spectroscopy. The infrared spectra of the vapor and liquid were recorded at ambient temperature and the solid sample investigated at 78K. Negligible spectral changes ocurred at 78K compared with the fluid state, but after annealing to ca. 165K an apparent crystal was formed.Raman spectra of the liquid were recorded at 293K and depolarization data obtained. Additional Raman spectra were recorded at various temperatures between 293 and 143K. A supercooled liquid appeared after slow cooling, but an amorphous solid phase was observed after shock freezing to 78K. After annealing, a plastic phase was observed and an anisotropic crystal appeared after further annealing. In the crystalline phase spectral shifts and some vanishing bands were observed and only the a (Cl) conformer remained.The compound exists in two conformers, equatorial (Cl) (e) and axial (Cl)(a) in the liquid, amorphous and plastic phases, but only the a-conformer was present in the crystal. The experimental results suggest that the a-conformer has 1.5±0.5kJmol−1 lower enthalpy than e in the liquid.B3LYP calculations with various basis sets and the G3 model chemistry gave conformational energy difference ΔE (a–e) in the range 3.2–2.4kJmol−1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the two conformers were calculated. The 32A′ and 25A″ modes for the e(Cl) and a(Cl) conformations were assigned, supported by anharmonic B3LYP/cc-pVTZ calculations.