Conformational properties of 1-tert-butyl-1-silacyclohexane, C5H10SiH(t-Bu): gas-phase electron diffraction, temperature-dependent Raman spectroscopy, and quantum chemical calculations

Abstract

The conformational preference of the t-butyl group in 1-t-Bu-1-silacyclohexane was studied experimentally by means of gas-phase electron diffraction (GED), and temperature-dependent Raman spectroscopy as well as by quantum chemical calculations applying density functional theory and ab initio methods. According to the GED experiment at 283 K, the vapor of the title compound contains only the equatorial conformer. At 99.5 % level of confidence, up to 4 % of axial conformer cannot be completely excluded, however. The Raman spectroscopy experiment in the temperature range of 295–375 K of the neat liquid indicated that the equatorial conformer is favored over the axial one by 0.56 (15) kcal mol−1 (ΔH values). The experimental values are fairly well reproduced by the calculations. CCSD(T) calculations predict the equatorial conformer to have a 1.19 kcal mol−1 lower Gibbs free energy (corresponding to about 89 % equatorial preference) and a 0.99 kcal mol−1 lower enthalpy than the axial conformer at 300 K. According to natural bond orbital analysis, the equatorial conformer of the title compound is an example of a molecular stabilization, which is not favored by steric and conjugation effects but favored by electrostatic interactions. Results from dynamic NMR experiments were inconclusive.