INFRARED AND RAMAN SPECTRA, CONFORMATIONS AND AB INITIO CALCULATIONS OF ETHYL BROMOSILANE AND ETHYL DIBROMOSILANE

Abstract Ethyl bromosilane (CH 3 CH 2 SiH 2 Br) and ethyl dibromosilane (CH 3 CH 2 SiHBr 2 ) were synthesized and their infrared and Raman spectra determined in vapour (IR), liquid (Raman), amorphous solid, and crystalline states. Additional infrared spectra of the compounds isolated in argon and nitrogen matrices at 5 K were recorded before and after annealing to temperatures in the range of 15–35 K. Raman spectra of the liquid were recorded at various temperatures between 295 and 153 K. Both the spectra showed the existence of two conformers – anti and gauche – present in the fluid and amorphous phases. The crystals contained the gauche conformer for bromide and anti conformer for the dibromide in accordance with the results for the chloro and iodo homologues. The enthalpy differences in the liquids measured in Raman gave: Δ H (anti-gauche) = 1.8 ± 0.3 and Δ H (gauche-anti) = 2.1 ± 0.3 kJ mol −1 , respectively. Ab initio calculations were performed using the Gaussian 94 program with the HF/6-311G* basis set and gave optimized geometries, infrared and Raman intensities and scaled vibrational frequencies for the anti and gauche conformers. The conformational energies were calculated to be 0.02 kJ mol −1 for ethyl bromosilane and 0.3 kJ mol −1 for ethyl dibromosilane with the anti and gauche conformers having the lower energy, respectively.