Spectra and structure of silicon containing compounds. XLII. Conformational stability, ab initio calculations, r0 structural parameters, and vibrational assignments for ethylmethylsilane and ethylmethylsilane-Si-d 2

Abstract

The infrared (3100–40 cm −1) spectra of gaseous and solid and Raman (3200–20 cm −1) spectra of liquid with qualitative depolarization values of ethylmethylsilane, CH 3CH 2SiH 2CH 3 and the Si-d 2 isotopomer have been recorded. Both the trans and gauche conformers have been identified in the fluid phases with the trans conformer the only rotamer remaining in the solid. Variable temperature (−105 to −150 °C) studies of the infrared spectra of the samples dissolved in liquid krypton have been recorded and the enthalpy difference has been determined to be 20±5 cm −1 (0.24±0.06 kJ/mol) with the trans conformer the more stable form. At ambient temperature it is estimated that there is 64±2% of the gauche conformer present. Relatively complete vibrational assignments are proposed for both conformers of both isotopomers based on the relative infrared and Raman spectral intensities, infrared band contours, and depolarization ratios which are supported by normal coordinate calculations. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and energies have been obtained for the trans and gauche conformers from ab initio MP2/631G(d) calculations. Structural parameters and energies have also been obtained utilizing some larger basis sets up to 6-311+G(2df,2pd). The MP2 calculations with even the largest basis set predict the gauche conformer the more stable form by 11 cm −1 whereas all of the B3LYP calculations predict the trans conformer the more stable form by 55 cm −1 or more. From the isolated SiH frequency from the CH 3CH 2SiHDCH 3 isotopomer the r 0 SiH distances were determined to be 1.489 and 1.490 Å for the trans and gauche conformers, respectively. By utilizing the previously reported microwave rotational constants for two isotopomers of the trans conformer along with ab initio predicted structural values, r 0 parameters have been obtained for ethylmethylsilane. These ‘adjusted r 0’ parameters have been compared to those previously estimated from the limited microwave data as well as the recently reported electron diffraction values. The results from these vibrational and theoretical studies are discussed and compared to those obtained for some similar molecules.