Raman and infrared studies supported by ab initio calculations for the determination of conformational stability, silyl rotational barrier and structural parameters of cyclohexyl silane

Abstract

The Raman spectra (4000–100 cm −1) of liquid and solid and infrared spectra (4000–400 cm −1) of gaseous, liquid and solid cyclohexyl silane, c-C 6H 11SiH 3, have been recorded and assigned. These spectral data indicate the presence of two conformers in the fluid states. Variable temperature (21 to −71 °C) Raman spectra of the liquid were also recorded and by utilizing three conformer pairs, an enthalpy difference of 520±70 cm −1 (6.22±0.84 kJ/mol) was obtained with the chair-equatorial form the more stable conformer. It is calculated that at ambient temperature there is only 7.5±2.4% of the chair-axial form present in the liquid phase. Comparison with the spectra of the polycrystalline solid phase shows that the chair-equatorial conformer is the only form remaining in the solid. MP2 and DFT calculated conformational energy differences are slightly larger but in reasonable agreement with the experimental value. In addition, force constants, infrared intensities, Raman activities, depolarization ratios, scaled vibrational frequencies and potential energy distributions have been calculated from the MP2(full)/6–31G(d) results for both chair forms. These data support the complete vibrational assignment for the chair-equatorial form as well as the assignments for several of the fundamentals of the chair-axial form. By utilizing a series of sum and difference bands on the SiH stretching modes, the barrier to SiH 3 internal rotation has been determined to be 684±10 cm −1 (8.18±0.12 kJ/mol) for the chair-equatorial form. Estimated r 0 structural parameters have been obtained for both conformers by adjusting MP2(full)/6–311+G(d,p) structural predictions. Three additional twist forms (equatorial, axial and form III) were also predicted to be local minima since all calculated vibrational frequencies are real. However, all three twist forms are much higher in energy (2000–3000 cm −1) than the chair forms. Effects of electronegativity and steric effect on the conformational stability are compared among a series of mono-substituted cyclohexanes by NBO analyses of the donor-acceptor delocalization interactions.