Conformational stability, ab initio calculations, and r0 structural parameters of 3-methyl-1-butene and dimethylvinylsilane

Abstract

The infrared spectra (3500–400 cm −1) of 3-methyl-1-butene, CH 2CHCH(CH 3) 2, and dimethylvinylsilane, CH 2CHSiH(CH 3) 2, dissolved in liquid xenon and krypton, respectively, have been recorded. From temperature (−55 to −110 °C) dependent FT–IR spectra of xenon solutions, it is shown that the cis conformer (hydrogen atom eclipsing the double bond) of 3-methyl-1-butene is the more stable form with an enthalpy difference of 65±8 cm −1 (0.78±0.10 kJ/mol) which agrees with all of the theoretical predictions. Similar studies for dimethylvinylsilane in krypton solutions (−105 to −150 °C) give an enthalpy difference of 55±5 cm −1 (0.66±0.06 kJ/mol) with the gauche conformer (methyl group eclipsing the double bond) the more stable rotamer which is consistent with the ab initio calculations except with the largest basis set MP2/6-311+G(2d,2p) utilized. Both ab initio calculations with full electron correlation by the perturbation method to the second order (MP2) and density functional theory (DFT) calculations by the B3LYP method have been carried out with a variety of basis sets up to 6-311G(2d,2p) with and without diffuse functions for both molecules to predict the optimized geometries and conformation stabilities. Utilizing previously reported microwave rotational constants for both conformers of 3-methyl-1-butene along with ab initio predictions, adjusted r 0 parameters have been obtained. The r 0 Si–H bond distances of 1.489 and 1.490 Å have been obtained for the cis and gauche conformers, respectively, from the silicon-hydrogen stretching frequencies and the remaining parameters are estimated from the MP2/6-311+G(d,p) ab initio calculations.