Conformational equilibrium in dimethyl vinyl fluorosilane studied by infrared and Raman spectroscopy

The Raman spectra (3500-20 cm(-1)) of liquid with depolarization values and solid, as well as the infrared spectra of the gas, the sample isolated in argon and nitrogen matrices at ca. 5 K and solid dimethyl vinyl fluorosilan e, CH2=CHSi(CH3)(2)F, have been recorded. Both gauche and syn rotamers have been identified in the fluid phases but only the syn conformer remains in the solid. Variable temperature (- 55 to - 150 degreesC) studies of the inf rared spectra (4000 and 400 cm(-1)) of dimethyl vinyl fluorosilane dissolve d in liquid xenon and krypton have been recorded. From the xenon and krypto n data, the enthalpy differences have been determined to be 53 +/- 9 cm(-1) (0.64 +/- 0.10 kJ/mol) and 44 +/- 7 cm(-1) (0.53 +/- 0.09 kJ/mol), respect ively, with the gauche conformer being the more stable form. The intensity variations with temperature of the Raman spectrum of the liquid gave an ent halpy difference of 25 +/- 15 cm(-1) (0.30 +/- 0.18 kJ/mol) also with the g auche conformer being the more stable form. Vibrational assignments are pro vided for both conformers. Complete equilibrium geometries have been determ ined for both rotamers using ab initio calculations employing the 6-31G(d), 6-311+G(d,p) and 6-311+G(2d,2p) basis sets at the levels of restricted Har tree-Fock (RHF) and/or with full electron correlation by the perturbation m ethod, Moller-Plesset (MP), to second order. The syn conformer is predicted to be the more stable conformer from all ab initio calculations except tho se of MP2/6-31(d) which predict the gauche form being the more stable confo rmer by 54 cm(-1) (0.65 kJ/mol) although the values favoring the syn form a re all very small. These results are compared to the corresponding quantiti es of some similar molecules. (C) 2000 Elsevier Science B.V. All rights res erved.