THE CONFORMATIONS OF BROMOMETHYL DIMETHYL SILANE AND BROMOMETHYL DIMETHYL SILANE-D(1) AS STUDIED BY VIBRATIONAL SPECTROSCOPY AND BY AB-INITIO CALCULATIONS

Bromomethyl dimethyl silane, CH2Br(CH3)(2)SiH (BDS), and its deuterate d analogue, CH2Br(CH3)(2)SiD (BDSD), were synthesised for the first ti me. Raman spectra of both liquids were recorded at various temperature s between 295 and 160 K and spectra of the amorphous and crystalline s olids were obtained. Infrared spectra were recorded in the vapour phas e and in the amorphous and crystalline states in the range 4000-60 cm( -1). Additional infrared spectra of the compounds isolated in argon an d 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 ga uche conformers due to restricted rotation of the CH2Br group. Raman t emperature 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 liqui d 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 conforma tion and one crystal (II) with molecules in the anti conformation. For the parent molecule, both of these crystals were repeatedly investiga ted in the infrared and in the Raman spectra, while crystallisation wa s 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 infra red 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 th e anti and gauche conformer spectra of both molecules. (C) 1998 Elsevi er Science B.V. All rights reserved.