Variations in the torsion-vibration energy structure of CH3OH from fundamental, overtone, and combination bands of the ν7, ν8, and ν11 CH3 rocking and CO stretching modes

Torsion-vibration energy structures deduced from the ν7 (A' CH3 in-plane rocking mode), ν8 (A' CO stretching mode), ν11 (A'' CH3 out-of-plane rocking mode), 2ν7, 2ν8, ν7+ν8, and ν8+ν11 fundamental, overtone, and combination bands of CH3OH are compared and contrasted to that of the ground vibrational state. The 2ν7 in-plane CH3-rocking overtone and the ν8+ν11 CO-stretching and out-of-plane rocking combination bands are identified in the high-resolution Fourier transform spectrum, and point to systematic trends in the excited-state torsional behavior for methanol. Torsion-vibration substate origins have been determined for the eight modes considered, and fitted to a five-parameter Fourier model to characterize the energy patterns. Excitation of the ν8 mode has little influence on the torsional structure, but the A-E torsional energy splittings are sharply reduced with excitation of ν7 and inverted with excitation of ν11. These changes are examined from the perspective of a recent torsion-vibration interaction model for states of degenerate E vibrational parentage [J. T. Hougen, J. Mol. Spectrosc. 207, 60 (2001)]; the K=0 substate energy pattern for ν7 and ν11 supports Hougen's model but with some differences in detail. The values of the K-scaling parameter ρ determining the periodicity of the torsional energies appear to vary almost linearly with the number of quanta of vibrational excitation. The changes are suggestive of substantial zero-point effects on the axial moments of inertia, with implications for the structural determination of the CH3 methyl top