Infrared and Raman spectra, ab initio calculations and conformational equilibria of chloromethyl methyl dichlorosilane

Chloromethyl methyl dichlorosilane (ClCH2CH3SiCl2) was synthesized and the infrared spectra of its vapour, and of the amorphous and crystalline states , cooled to the temperature of liquid nitrogen were recorded. Additional mi d IR spectra of the compound, isolated in both argon and nitrogen matrices, were taken at 5 K. Raman spectra of the liquid were recorded in a capillar y at various temperatures between 298 and 223 K. The crystal was also studi ed in a capillary at 210 K and deposited onto a copper finger cooled with l iquid nitrogen. The compound occurs as anti and gauche conformers in its vapour and liquid states. Approximately five Raman bands, some of them weak, present in the f luid phases vanished upon crystallization. An additional eight IR bands dis appeared or were reduced in intensity after crystallization. Two band pairs in the Raman spectra were used to calculate the enthalpy difference betwee n the conformers in the liquid phase giving a value: DeltaH((liq))(anti-gau che) of 0.0(+/-0.4 kJ mol(-1). In the argon and nitrogen matrices, various small intensity variations in the IR bands were observed after annealing to ca. 36 K (argon) or 32 K (nitrogen). The gauche bands generally increased and the anti bands decreased in intensity, suggesting that gauche has a low er energy in the matrices. The conformational barrier was estimated to be 8 -10 kJ mol(-1). The optimized geometries, infrared and Raman intensities, and scaled vibrat ional frequencies for the anti and gauche conformers were derived from ab i nitio calculations at the RHF/6-311G*level. The derived conformational ener gy difference was 7.2 kJ mol(-1) , gauche being the conformer of lowest ene rgy. The calculated dipole moments of the anti and gauche conformers were 4 .2 and 2.0 D, respectively. Correlation between the observed and calculated wave numbers of both conformers revealed that gauche was present in the cr ystal, and assignments of the spectra have been carried out. (C) 2001 Elsev ier Science B.V. All rights reserved.