The conformations of bromomethyl dimethyl silane and bromomethyl dimethyl silane-d1 as studied by vibrational spectroscopy and by ab initio calculations

Bromomethyl dimethyl silane, CH2Br(CH3)(2)SiH (BDS), and its deuterated analogue, CH2Br(CH3)(2)SiD (BDSD), were synthesised for the first time. Raman spectra of both liquids were recorded at various temperatures between 295 and 160 K and spectra of the amorphous and crystalline solids were obtained. Infrared spectra were recorded in the vapour phase and in the amorphous and crystalline states in the range 4000-60 cm(-1). Additional infrared spectra of the compounds isolated in argon and nitrogen matrices were recorded at 5 K before and after annealing to temperatures in the range 15-35 K. The compounds exist in anti and gauche conformers due to restricted rotation of the CH2Br group. Raman temperature studies gave an average Delta H-conf(0) (gauche-anti) of 0.6 +/- 0.3 kJ mol(-1), anti being the low energy conformer in the liquid and in the matrices. Two different crystals were observed for these molecules after careful annealing of the amorphous solid in the range 110-150 K: one crystal (I) containing molecules in the gauche conformation and one crystal (II) with molecules in the anti conformation. For the parent molecule, both of these crystals were repeatedly investigated in the infrared and in the Raman spectra, while crystallisation was much more difficult to achieve for the deuterated compound. However, two different crystals (I, III) both containing the gauche conformer, was observed in the Raman spectra and one with anti (II) in the infrared spectrum of the deuterated compound. Ab initio calculations at the HF/6-311G* level gave vibrational frequencies and Raman and infrared intensities for both conformers. Complete assignments were made for the anti and gauche conformer spectra of both molecules. (C) 1998 Elsevier Science B.V. All rights reserved.