Spectra and structure of silicon containing compounds: XXIX. Conformational studies of methyl vinyl difluorosilane from temperature dependent FT-IR spectra of xenon and krypton solutions

Abstract

Variable temperature (−55 to −150°C) studies of the infrared spectra (3500–400 cm −1) of methyl vinyl difluorosilane, CH 2CHSiF 2CH 3, dissolved in liquid xenon and krypton have been recorded. Utilizing three sets of conformer doublets due to the cis and gauche rotamers from the krypton solution and two pairs from the xenon solution, the enthalpy difference has been determined to be 67±7 cm −1 (0.80±0.09 kJ/mol) and 83±11 cm −1 (0.99±0.14 kJ/mol), respectively, with the gauche conformer the more stable form. However, in the crystalline solid only the cis conformer is present. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G(2d,2p) with full electron correlation by the perturbation method from which the conformational stabilities have been determined. With the largest basis set MP2/6-311+G(2d,2p), the cis conformer is predicted to be the more stable conformer by 10 cm −1 which is inconsistent with the experimental results; however, this value is so small that the ab initio prediction cannot be relied on to give the correct conformer stability. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.