Spectra and structure of silicon containing compounds. XXXI. Raman and infrared spectra, conformational stability, ab initio calculations, and vibrational assignment of ethyl bromosilane and ethyl bromosilane-Si-d 2

Abstract

The infrared spectra of ethyl bromosilane, CH 3CH 2SiH 2Br and the Si-d 2 isotopomer, were recorded in the vapor, amorphous and the crystalline solid phases in the range 4000–400 cm −1. The mid-infrared spectra of the ‘light’ compound, isolated at 4.8 K in the argon and the nitrogen matrices were also observed. Raman spectra of the liquids were recorded at various temperatures between 298 and 218 K, and the spectrum of the crystal was obtained from a sample sealed in a capillary at 210 K. Additional spectra of the amorphous and annealed crystals, which were deposited on a copper finger cooled with liquid nitrogen, were recorded. These vibrational spectra show that two conformers— anti and gauche—are present in the vapor and in the liquid, but only the gauche conformer remains in the crystalline solid. Six conformer pairs in the liquid phase were used to obtain the enthalpy difference which gave an average of 140±25 cm −1 (1.67±0.3 kJ/mol) with gauche form lower in energy. Additionally variable temperature (−120 to −155 °C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out and the enthalpy difference has been determined to be 130±13 cm −1 (1.56±0.16 kJ/mol), again with the gauche conformer the more stable form. At ambient temperature it is estimated that there is 22±2% of the anti conformer present. The optimized geometries, infrared and Raman intensities, and scaled vibrational frequencies for the anti and gauche conformers were obtained from ab initio MP2/6-31G(d) calculations. The conformational energy difference was also obtained from ab initio MP2/6-311+G(d, p) calculations which gave a predicted energy difference of 129 cm −1 with the gauche form the lowest energy. From the isolated Si–H stretch frequencies from the Si-d 1 isotopomer the Si–H distances were determined to be 1.481 and 1.480 Å for the gauche and anti conformers, respectively. The spectroscopic and theoretical results are discussed and compared to the corresponding quantities for some similar molecules.