15min:

R. M. LEES, M. MOLLABASHI, LI-HONG XU, Department of Physical Sciences, University of New Brunswick, Saint John, NB, Canada E2L 4L5; MICHAEL LOCK, Physikalisch-Chemisches Institut, Justus Liebig University, D-35392 Giessen, Germany; B. P. WINNEWISSER, Department of Physics, The Ohio State University, 174 W. 18th Avenue, Columbus, OH 43210.

Torsion-vibration energy structures deduced from the ₇ (A' CH₃ in-plane rock), ₈ (A' CO stretch), ₁₁ (A'' CH₃ out-of-plane rock), 2 ₇, 2 ₈, ₇ + ₈ and ₈ + ₁₁ fundamental, overtone and combination bands of CH₃OH are compared and contrasted to that of the ground vibrational state. The 2 ₇ in-plane CH₃-rocking overtone and the ₈ + ₁₁ CO-stretch/out-of-plane-rock combination bands have only recently been identified in the high-resolution Fourier transform spectrum, and point to systematic trends in the excited-state torsional behaviour for methanol. Torsion-vibration substate origins for the 8 states have been fitted to a 5-parameter Fourier model to characterize the energy patterns. Excitation of the ₈ mode has little influence on the torsional structure, but the A-E torsional energy splittings are sharply reduced with excitation of ₇ and inverted with excitation of ₁₁. These changes are examined from the perspective of Hougen's torsion-vibration interaction model for states of degenerate E vibrational parentage. The K = 0 substate energy pattern for the v₇ and v₁₁ states supports the model but with some differences in detail. The values of the K-scaling parameter 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.