Conformational stabilities of CH2=CHSi(CH3)(n)X3-n (X = F and Cl) from variable temperature FT-IR spectra of rare gas solutions

Variable temperature (-105 to - 150 degreesC) studies of the infrared spect ra (3500-400 cm(-1)) of methylvinyldifluorosilane, CH2CHSiF2CH3, dimethvlvi nylfluorosilane CH2CHSiF(CH)(2) and methylvinyldichlorosilane, CH2CHSiCl2CH 3, dissolved in liquid krypton have been recorded. The enthalpy difference of methylvinyldifluorosilane has been determined to be 53 +/- 11 cm(-1) (0. 64 +/- 0.13 kJ/mol) with the gauche conformer the more stable form. However in the crystalline solid only the cis conformer is present. For the corres ponding dichlorosilane the enthalpy difference has been determined to be 46 +/- 20 cm(-1) (0.55 +/- 0.23 kJ/mol) with the cis conformer more stable. F or dimethylvinylfluorosilane the enthalpy difference has been determined to be 32 +/- 7 cm(-1) (0.38 +/- 0.08 kJ/mol) with the gauche conformer the mo re stable form, Ab initio calculations have been carried out with several d ifferent basis sets up to MP2/6-311 + G(2d,2p) with full electron correlati on by the perturbation method for all three of the silanes from which the c onformational stabilities have been determined. (C) 2001 Elsevier Science B .V. All rights reserved.