Spectra and structure of silicon containing compounds. XXXIX. Raman and infrared spectra, conformational stability, vibrational assignment and ab initio calculations of n-propyltrifluorosilane

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 and solid n-propyltrifluorosilane, CH3CH2CH2SiF3, have been recorded. Additionally the infrared spectra of the sample in nitrogen and argon matrices have been recorded. 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 the sample dissolved in liquid krypton have been recorded and the enthalpy difference has been determined to be 135±14 cm−1 (1.62±0.17 kJ mol−1) with the anti conformer the more stable form. At ambient temperature it is estimated that there is 51±2% of the gauche conformer present. Also the enthalpy difference in the liquid was obtained from variable temperature studies of the Raman spectra and from three conformer pairs an average value of 179±18 cm−1 (2.14±0.22 kJ mol−1) was obtained again with the anti form the more stable conformer. Relatively complete vibrational assignments are proposed for both conformers based on the relative infrared and Raman spectral intensities, infrared band contours, depolarization ratios which are supported by normal coordinate calculations. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities, 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. By utilizing the previously reported microwave rotational constants for five isotopomers of CH3SiF3 along with ab initio predicted structural values, r0 parameters have been obtained for methyltrifluorosilane. Similarly, from the ab initio predicted parameters ‘adjusted r0’ parameters have been estimated for both conformers of n-propyltrifluorosilane. The results are discussed and compared with those obtained for some similar molecules.