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

Abstract

Bromomethyl dimethyl fluorosilane (CH 2Br(CH 3) 2SiF) was synthesized for the first time. Raman spectra of the liquid were obtained at various temperatures between 295 and 163 K, and spectra of the amorphous and crystalline solids were recorded. The infrared spectra were recorded of the vapour, and of the amorphous and crystalline states at liquid nitrogen temperature. Additional IR spectra were obtained at 5 and 15 K, isolated in argon and nitrogen matrices. The compound exists as anti and gauche conformers. Five IR bands and eight Raman bands present in the fluid phases vanished upon crystallization, suggesting a large overlap of the vibrational bands between the conformers. Raman temperature studies in the liquid gave a ΔH o (anti — gauche) value of 0.4 ± 0.3 kJ mol −1, gauche being the low energy conformer which was also present in the crystal. The IR bands vanishing in the argon and nitrogen matrix spectra after annealing to about 25–28 K suggested that the anti conformer had a lower energy than the gauche one in both matrices, and the conformational barrier was 6–7 kJ mol −1. Ab initio calculations with the basis sets HF/3-21G∗ and HF/6-31G∗, HF/6-311G∗ and MP2/6-31G∗ gave optimized geometries, IR and Raman intensities, and scaled vibrational frequencies for the anti and gauche conformers. Reasonably good agreement was obtained between the experimental and calculated wavenumbers for the anti and gauche conformers.