Infrared and Raman spectra, conformations and ab initio calculations of dichloromethylmethyl dichlorosilane

The infrared (IR) spectra of dichloromethylmethyl dichlorosilane (Cl2CHCH3SiCl2) were recorded in the vapor, amorphous and partly crystalline phases between 4000 and 50 cm−1. IR spectra (4000–450 cm−1) of the compound were also obtained when isolated in argon, nitrogen and krypton matrices at 4.8 K before and after annealing to temperatures in the range 15–50 K. Raman spectra of the liquid were recorded at various temperatures between 295 and 147 K. Spectra of the amorphous and annealed crystal deposited on a copper finger at 78 K were obtained. The spectra showed the existence of two conformers — anti and gauche — in the vapor and in the liquid. Approximately 12 IR bands were reduced in intensity and 6 Raman bands present in the fluid phases vanished upon crystallization. From the intensity variations of two band pairs with temperature, a ΔH0 value of 0.4±0.3 kJ mol−1 between the conformers was obtained in the liquid, anti being the low-energy conformer. Small increases and decreases in the IR band intensities were observed in the matrix spectra after annealing to 32 (nitrogen), 36 (argon) and 50 K (krypton). The enthalpy difference between the conformers is low in the matrices, but the anti conformer had lower energy than the gauche. Ab initio calculations were performed using the Gaussian -94 program with the RHF/6-311G∗ basis function and gave optimized geometries, IR and Raman intensities and scaled vibrational frequencies for the anti and gauche conformers. The conformational energy derived was 4.4 kJ mol−1 with anti being the low-energy conformer. The dipole moments were calculated to be 1.2 and 3.1 debye for the anti and gauche conformers, respectively. Correlation between the observed and calculated wavenumbers of both conformers revealed that anti was present in the crystal, and complete assignments of the spectra have been carried out.