Infrared and Raman spectra, conformations, quantum chemical calculations and spectral assignments of 1-methyl-1-silacyclohexane

Raman spectra of of 1-methyl-1-silacyclohexane as a liquid were recorded at 293 K and depolarization data obtained. Additional Raman spectra were recorded at various temperatures between 293 and 143 K, and intensity changes with temperature of certain Raman bands were detected. A supercooled liquid appeared after slow cooling, but an amorphous phase was observed after shock freezing to 78 K. After annealing, first a plastic phase and subsequently a crystal were observed. In the crystalline phase spectral shifts and some vanishing bands were observed.The infrared spectra of the vapor and liquid were studied at ambient temperature and the solid sample investigated at 78 K. Negligible spectral changes ocurred at 78 K compared with the fluid state, but after annealing to ca. 170 K an apparent crystal was formed and a few bands vanished and others were shifted.The compound exists in two conformers, equatorial (e) and axial (a) in the liquid, amorphous and plastic phases, but only the e-conformer was present in the crystal. The experimental results suggest that the e-conformer has 0.6 kJ mol−1 lower energy than a in the liquid.B3LYP calculations with various basis sets gave a conformational enthalpy difference ΔH (a–e) around 2.4 kJ mol−1 while G3 model chemistry gave 0.6 kJ mol−1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the e and a conformers were calculated. The fundamental wavenumbers were also derived in the anharmonic approximation in B3LYP/cc-pVTZ calculations. A relative deviation of 0.94% and 1.31% between the observed and calculated wave numbers for the 57 modes of the e-and a-conformers, respectively, was obtained.

► Infrared and Raman spectra of 1-methyl-1-silacyclohexane. ► Attribution of equatorial (e) and axial (a) conformers from spectra of the crystal. ► Assignments of the A′ and A″ modes of the e and a conformers. ► Determination of the enthalpy difference ΔH between the e and a conformers from variable temperature Raman spectra. ► DFT calculations of vibrational modes in the anharmonic approximation.