Spectra and structure of silicon‐containing compounds. XXXVI—Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignment of ethyldibromosilane

The Raman spectrum of liquid ethyldibromosilane, CH 3 CH 2 SiHBr 2 , was recorded at various temperatures between 298 and 218 K, and the spectrum of the crystal was obtained at 210 K from a sample sealed in a capillary. Additional spectra of the amorphous and annealed crystals, which were deposited on a copper finger cooled with boiling liquid nitrogen, were recorded. The infrared spectra were recorded of the vapor and amorphous and crystalline solid in the range 4000–50 cm −1 and mid‐infrared spectra isolated at 4.8 K in argon and nitrogen matrices were also observed. These vibrational spectra show that two conformers, anti and gauche , are present in the vapor and in the liquid, but only the anti conformer remains in the crystalline solid. Three conformer pairs in the Raman spectrum of the liquid phase were used to obtain the enthalpy difference, which gave an average value of 128 ± 17 cm −1 (1.53 ± 0.2 kJ mol −1 ) with the anti form lower in energy. At ambient temperature it is estimated that there is 52 ± 2% of the gauche conformer present in the liquid. The optimized geometries, infrared and Raman intensities, and scaled vibrational wavenumbers for the anti and gauche conformers were obtained from ab initio MP2/6–31G(d) calculations with full electron correlation. The conformational energy difference was also obtained from ab initio MP2/6–311 + G(d,p) calculations which gave a predicted energy difference of 97 cm −1 with the anti form the conformer of lower energy. These spectroscopic and theoretical results are discussed and compared with the corresponding quantities for some similar molecules. Copyright © 2003 John Wiley & Sons, Ltd.