Torsional splittings in small-amplitude vibrational fundamental states of methanol-type molecules

lGroup-theoretical methods are used to show that inverted torsional splittings in fundamental levels of small-amplitude vibrations of methanol-like molecules can be parameterized and understood in terms of the energy level patterns induced when a pair of high-barrier torsionally split components of given v(t) and (t)A + E-t symmetry species in the molecular symmetry group G(6) is allowed to interact with small-amplitude vibrational modes of symmetry E-v. Such doubly degenerate E-v vibrational modes arise rather naturally in G(6) (isomorphic with the point-group C-3v) for those methyl-group vibrations in print-group-C-s asymmetric tops such as CH3-CHO that are analogs of the degenerate methyl-group stretch, bend, and rocking vibrations in point-group-C-3v symmetric tops such as CH3-C=C-H. The present group-theoretical treatment is somewhat different from, 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. (C) 2001 Academic Press.