THE CONFORMERS OF BROMOMETHYL DIMETHYL FLUOROSILANE STUDIED BY VIBRATIONAL SPECTROSCOPY AND AB-INITIO METHODS

Bromomethyl dimethyl fluorosilane (CH2Br-(CH3)(2)SIF) was synthesized for the first time. Raman spectra of the liquid were obtained at vario us temperatures between 295 and 163 K, and spectra of the amorphous an d 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 ant i and gauche conformers. Five IR bands and eight Raman bands present i n the fluid phases vanished upon crystallization, suggesting a large o verlap of the vibrational bands between the conformers. Raman temperat ure studies in the liquid gave a Delta H degrees(anti-gauche) value of 0.4 +/- 0.3 kJ mol(-1), gauche being the low energy conformer which w as also present in the crystal. The IR bands vanishing in the argon an d nitrogen matrix spectra after annealing to about 25-28 K suggested t hat 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, an d scaled vibrational frequencies for the anti and gauche conformers. R easonably good agreement was obtained between the experimental and cal culated wavenumbers for the anti and gauche conformers. (C) 1997 Elsev ier Science B.V.