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

Variable temperature (-55 to -150 degreesC) studies of the infrared spectra (3500-400 cm(-1)) of methyl vinyl difluorosilane, CH2CHSiF2CH3, dissolved in liquid xenon and krypton have been recorded. Utilizing three sets of con former doublets due to the cis and gauche rotamers from the krypton solutio n and two pairs from the xenon solution, the enthalpy difference has been d etermined 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 s table form. However, in the crystalline solid only the cis conformer is pre sent. Ab initio calculations have been carried out with several different b asis sets up to MP2/6-311 + G(2d,2p) with full electron correlation by the perturbation method from which the conformational stabilities have been det ermined. 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 inconsi stent with the experimental results; however, this value is so small that t he ah initio prediction cannot be relied on to give the correct conformer s tability. The spectroscopic and theoretical results are compared to the cor responding quantities for some similar molecules. (C) 2001 Elsevier Science B.V. All rights reserved.