Infrared and Raman spectra, conformations, ab initio calculations and spectral assignments of 1-fluoro-1-silacyclohexane

Raman spectra of 1-fluoro-1-silacyclohexane as a liquid were recorded at 293 K and polarization data obtained. Additional Raman spectra were recorded at various temperatures between 293 and 143 K, and intensity changes of certain bands with temperature were investigated. An apparently plastic phase was observed around 170 K, but no definite crystallization was ever obtained on cooling. The infrared spectra have been studied of the vapor, of an amorphous solid at 78 K and of the liquid in the range 600–100 cm−1. No infrared bands present in the vapor or liquid vanished upon cooling.The compound exists a priori in two conformers, equatorial (e) and axial (a), and the experimental results suggest an equilibrium in which the a-conformer has 1.2 kJ mol−1 lower enthalpy than the e-conformer in the liquid, leading to 60% a-conformer at ambient temperature.B3LYP calculations with various basis sets and the G3 model chemistry gave conformational enthalpy difference ΔH(e − a) in the range 0.6 and 1.8 kJ mol−1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the e and a conformers were calculated. The wavenumbers of the vibrational modes were derived in the anharmonic approximation in B3LYP/cc-pVTZ calculations. An average relative deviation of ca. 1% between the observed and calculated wavenumbers for the 48 modes of the e and a conformers was found.

► Infrared and Raman spectra were recorded in different phases. ► The enthalpy difference ΔH(e − a) was observed by Raman spectroscopy. ► DFT calculations made in the anharmonic approximation (B3LYP/cc-pVTZ). ► The vibrational modes of the e and a conformers were observed and calculated. ► 1-Fluoro-1-silacyclohexane was prepared and purified.