VARIATIONS IN TORSION-VIBRATION ENERGY STRUCTURE OF $CH_{3}$OH FROM FUNDAMENTAL, OVERTONE AND COMBINATION BANDS OF THE $CH_{3}$-ROCKING AND CO-STRETCHING MODES
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Date
2002
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
Torsion-vibration energy structures deduced from the $2\nu_{7} (A^{\prime} CH_{3}$ in-plane rock), $\nu_{8} (A^{\prime} CO$ stretch), $\nu_{11} (A^{\prime\prime} CH_{3}$ out-of-plane rock), $2\nu_{7},2\nu_{8}, \nu_{7} + \nu_{8}$ and $\nu_{8} + \nu_{11}$ fundamental, overtone and combination bands of $CH_{3}OH$ are compared and contrasted to that of the ground vibrational state. The $2v_{7}$ in-plane $CH_{3}$-rocking overtone and the $\nu_{8} + \nu_{11}$ 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 $\nu_{8}$ mode has little influence on the torsional structure, but the A - E torsional energy splittings are sharply reduced with excitation of $\nu_{7}$ and inverted with excitation of $\nu_{11}$. These changes are examined from the perspective of Hougen's torsion-vibration interaction model for states of degenerate E vibrational $parentage.^{a}$ The K = 0 substate energy pattern for the $v_{7}$ and $\nu_{8}$ states supports the model but with some differences in detail. The values of the K-scaling parameter $\rho$ 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 $CH_{3}$ methyl top.
Description
$^{a}$J.T. Hougen. J. Mol Spectrosc. 207. 60-65 (2001).
Author Institution: Department of Physical Sciences, University of New Brunswick; Physikalisch-Chemisches Institut, Justus Liebig University; Department of Physics, The Ohio State University
Author Institution: Department of Physical Sciences, University of New Brunswick; Physikalisch-Chemisches Institut, Justus Liebig University; Department of Physics, The Ohio State University