Variations in the torsion-vibration energy structure ofCH3OHfrom fundamental, overtone, and combination bands of theν7,ν8,andν11CH3rocking andCOstretching modes

Abstract

Torsion-vibration energy structures deduced from the ${\ensuremath{\nu}}_{7}$ ${(A}^{\ensuremath{'}}$ ${\mathrm{CH}}_{3}$ in-plane rocking mode), ${\ensuremath{\nu}}_{8}$ ${(A}^{\ensuremath{'}}$ CO stretching mode), ${\ensuremath{\nu}}_{11}$ ${(A}^{\ensuremath{''}}$ ${\mathrm{CH}}_{3}$ out-of-plane rocking mode), $2{\ensuremath{\nu}}_{7},$ $2{\ensuremath{\nu}}_{8},$ ${\ensuremath{\nu}}_{7}+{\ensuremath{\nu}}_{8},$ and ${\ensuremath{\nu}}_{8}+{\ensuremath{\nu}}_{11}$ fundamental, overtone, and combination bands of ${\mathrm{CH}}_{3}\mathrm{OH}$ are compared and contrasted to that of the ground vibrational state. The $2{\ensuremath{\nu}}_{7}$ in-plane ${\mathrm{CH}}_{3}$-rocking overtone and the ${\ensuremath{\nu}}_{8}+{\ensuremath{\nu}}_{11}$ 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 ${\ensuremath{\nu}}_{8}$ mode has little influence on the torsional structure, but the A-E torsional energy splittings are sharply reduced with excitation of ${\ensuremath{\nu}}_{7}$ and inverted with excitation of ${\ensuremath{\nu}}_{11}.$ 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 ${\ensuremath{\nu}}_{7}$ and ${\ensuremath{\nu}}_{11}$ supports Hougen's model but with some differences in detail. The values of the K-scaling parameter \ensuremath{\rho} 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 ${\mathrm{CH}}_{3}$ methyl top.