Infrared and Raman spectra, ab initio calculations and conformational equilibria of chloromethyl methyl dichlorosilane
Abstract
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 mid 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 capillary at various temperatures between 298 and 223 K. The crystal was also studied in a capillary at 210 K and deposited onto a copper finger cooled with liquid 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 fluid phases vanished upon crystallization. An additional eight IR bands disappeared or were reduced in intensity after crystallization. Two band pairs in the Raman spectra were used to calculate the enthalpy difference between the conformers in the liquid phase giving a value: ΔH(liq)(anti-gauche) 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 lower 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 vibrational frequencies for the anti and gauche conformers were derived from ab initio calculations at the RHF/6-311G∗level. The derived conformational energy difference was 7.2 kJ mol-1, gauche being the conformer of lowest energy. 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 crystal, and assignments of the spectra have been carried out.- Publication:
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Journal of Molecular Structure
- Pub Date:
- October 2001
- DOI:
- 10.1016/S0022-2860(01)00590-7
- Bibcode:
- 2001JMoSt.597..137K
- Keywords:
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- Conformations;
- Vibrational spectra;
- Halosilanes;
- Ab initio calculations