The gas-phase infrared spectra of nitromethane and methyl boron difluoride; fine structure caused by internal rotation

Abstract

The gas-phase infrared spectra of nitromethane and methyl boron difluoride have been analysed in some detail. The various skeletal modes of vibration, and the vibrations of the methyl group with dipole changes parallel to the carbon–nitrogen and carbon–boron axes respectively, have vibration–rotation band contours which are of the expected type as calculated from the moments of inertia for overall rotation of the molecules. The perpendicular vibrations of the methyl groups all have complex contours, and in a number of cases widely-spaced fine structure lines are present. These can only be accounted for in terms of the internal rotation degrees of freedom. This is as expected because in classical terms the internal rotation frequencies modulate the oscillating vibrational dipole moment of these (and only these) methyl vibrations; in quantum-mechanical terms this leads to addi­tional transitions involving changes in the quantum number for internal rotation. These perpendicular methyl vibration bands have complex rotational structure because of interaction of the internal rotation degree of freedom with the overall rotations of these asymmetric top molecules. Nevertheless their main Q -branch features have been rather successfully analysed in terms of a theoretical model in which it is assumed that the internal rotation is free, and that the degeneracies of these perpendicular modes are retained. The former is expected to be a good approximation because of the known very low barriers to internal rotation. Some unresolved complexities, particularly towards the centres of the bands, may be caused by deviations from these simplifying assumptions. Analyses of the bands in this manner leads to information about the band origins and to reasonable values for the Coriolis coupling constants of the degenerate vibrations.