15min:

JON T. HOUGEN, Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441; RONALD M. LEES, Department of Physical Sciences, University of New Brunswick, Saint John, NB E2L 4L5, Canada.

Group-theoretical methods are used to show that inverted torsional splittings in fundamental levels of small-amplitude vibrations of methanol-like molecules can be parametrized and understood in terms of the energy level patterns induced when a pair of high-barrier torsionally split components of given v_t and ^tA + ^tE symmetry species in the molecular symmetry group G₆ is allowed to interact with small-amplitude vibrational modes of symmetry ^vE. Such doubly degenerate ^vE vibrational modes arise rather naturally in G₆ (isomorphic with the point-group C_3v) for those methyl-group vibrations in point-group-C_s asymmetric tops like CH₃-CHO that are analogs of the degenerate methyl-group stretch, bend, and rocking vibrations in point-group-C_3v symmetric tops like CH₃-CC-H. The present group-theoretical treatment is somewhat different than, but (as a comparison of model parameters shows) still fundamentally similar to, the recent local mode explanation of inverted torsional splittings in the C-H stretching fundamental region in methanol. The formalism has been applied to new torsional splitting data for the CH₃-rocking modes, and is found to give moderate agreement.