Spectra and structure of silicon containing compounds. XXXII. Raman and infrared spectra, conformational stability, vibrational assignment and ab initio calculations of n-propylsilane-d 0 and Si-d 3

Abstract

The infrared (3100–40 cm −1) and Raman (3100–20 cm −1) spectra of gaseous and solid n-propylsilane, CH 3CH 2CH 2SiH 3 and the Si-d 3 isotopomer, CH 3CH 2CH 2SiD 3, have been recorded. Additionally, the Raman spectra of the liquids have been recorded and qualitative depolarization values obtained. Both the anti and gauche conformers have been identified in the fluid phases but only the anti conformer remains in the solid. Variable temperature (−105 to −150 °C) studies of the infrared spectra of n-propylsilane dissolved in liquid krypton have been recorded and the enthalpy difference has been determined to be 220±22 cm −1 (2.63±0.26 kJ mol −1) with the anti conformer the more stable form. A similar value of 234±23 cm −1 (2.80±0.28 kJ mol −1) was obtained for Δ H for the Si-d 3 isotopomer. At ambient temperature it is estimated that there is 30±2% of the gauche conformer present. The potential function governing the conformation interchange has been estimated from the far infrared spectral data, the enthalpy difference, and the dihedral angle of the gauche conformer, which is compared to the one predicted from ab initio MP2/6-31G(d) calculations. The barriers to conformational interchange are: 942, 970 and 716 cm −1 for the anti to gauche, gauche to gauche, and gauche to anti conformers, respectively. Relatively complete vibrational assignments are proposed for both the n-propylsilane-d 0 and Si-d 3 molecules based on the relative infrared and Raman spectral intensities, infrared band contours, depolarization ratios, and normal coordinate calculations. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities and depolarization ratios, and energy differences have been obtained for the anti and gauche conformers from ab initio MP2/6-31G(d) calculations. Structural parameters and energy differences have also been obtained utilizing the larger 6-311+G(d,p) and 6-311+G(2d,2p) basis sets. From the isolated SiH stretching frequency from the Si-d 2 isotopomer the r 0 distances of 1.484 and 1.485 Å have been determined for the SiH s and SiH a bonds, respectively, for the anti conformer, and 1.486 Å for the SiH bond for the gauche conformer. Utilizing previously reported microwave rotational constants for the anti conformer and the determined SiH distances along with ab initio predicted parameters ‘adjusted r 0’ parameters have been obtained for the anti conformer. The results are discussed and compared to those obtained for some similar molecules.