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

Abstract

The Raman spectra (3500–20cm−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. 5K and solid dimethyl vinyl fluorosilane, CH2CHSi(CH3)2F, 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°C) studies of the infrared spectra (4000 and 400cm−1) of dimethyl vinyl fluorosilane dissolved in liquid xenon and krypton have been recorded. From the xenon and krypton data, the enthalpy differences have been determined to be 53±9cm−1 (0.64±0.10kJ/mol) and 44±7cm−1 (0.53±0.09kJ/mol), respectively, with the gauche conformer being the more stable form. The intensity variations with temperature of the Raman spectrum of the liquid gave an enthalpy difference of 25±15cm−1 (0.30±0.18kJ/mol) also with the gauche conformer being the more stable form. Vibrational assignments are provided for both conformers. Complete equilibrium geometries have been determined 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 Hartree–Fock (RHF) and/or with full electron correlation by the perturbation method, Moller–Plesset (MP), to second order. The syn conformer is predicted to be the more stable conformer from all ab initio calculations except those of MP2/6-31(d) which predict the gauche form being the more stable conformer by 54cm−1 (0.65kJ/mol) although the values favoring the syn form are all very small. These results are compared to the corresponding quantities of some similar molecules.