Spectra and structure of silicon containing compounds. XL. Infrared and Raman spectra, conformational stability, vibrational assignment and ab initio calculations of ethylmethylsilyl difluoride

Abstract

The infrared (3100–40 cm−1) spectra of gaseous and solid and Raman (3100–20 cm−1) spectrum of liquid ethylmethylsilyl difluoride, CH3CH2SiF2CH3, have been recorded. Both the trans and gauche conformers have been identified in the fluid phases but only the trans conformer remains in the solid. Variable temperature (−105 to −150 °C) studies of the infrared spectra of ethylmethylsilyl difluoride dissolved in liquid krypton have been recorded and the enthalpy difference has been determined to be 20±4 cm−1 (0.24±0.05 kJ/mol) with the trans conformer the more stable form. At ambient temperature it is estimated that there is 64±1% of the gauche conformer present. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities and depolarization ratios, and energy differences have been obtained for the trans and gauche conformers from ab initio MP2/631G(d) calculations. Relatively complete vibrational assignments are proposed for both conformers based on the relative infrared and Raman spectral intensities, infrared band contours, depolarization ratios, and normal coordinate calculations. The potential functions governing the conformation interchange have been predicted with barriers of 509 and 497 cm−1 from ab initio MP2/6-31G(d) calculations and 581 and 500 cm−1 from MP2/6-311+G(d,p) for the trans to gauche and gauche to gauche transitions, respectively. Structural parameters and energy differences have also been obtained by utilizing the larger 6-311+G(d,p) and 6-311+G(2d,2p) basis sets. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.