The conformers of bromomethyl dimethyl chlorosilane studied by vibrational spectroscopy and ab initio methods

Abstract

Bromomethyl dimethyl chlorosilane (CH2Br(CH3)2SiCl) was synthesised and the infrared spectra of the vapour and of the amorphous and crystalline states at liquid nitrogen temperature were obtained. Additional spectra of the compound, isolated in argon, nitrogen and xenon matrices were recorded at 5 and 15K. Raman spectra of the liquid were obtained at various temperatures between 295 and 173K, and spectra of the amorphous and crystalline solids were recorded.The compound is present as anti and gauche conformers in the vapour and in the liquid states. Various infrared and Raman bands present in these phases vanished upon crystallisation. Raman temperature studies in the liquid gave a gauche–anti value of 1.0±0.3kJmol−1,anti was the low energy conformer and was also present in the crystal. The infrared bands diminishing in the argon, nitrogen and xenon matrix spectra after annealing to 28–60K suggested that the anti conformer also had a slightly lower energy than gauche in all the matrices. The conformational barrier was estimated to be 8–10kJmol−1.Ab initio calculations on different levels of approximation gave optimised geometries, infrared and Raman intensities and vibrational frequencies for the anti and gauche conformers. All calculations predicted anti as the low energy conformer. After scaling, a reasonably good agreement between the experimental and calculated wavenumbers for the two rotamers was obtained.