Torsional Splittings in Small-Amplitude Vibrational Fundamental States of Methanol-Type Molecules

Abstract

Group-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 vt 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 C3v) for those methyl-group vibrations in point-group-Cs asymmetric tops such as CH3–CHO that are analogs of the degenerate methyl-group stretch, bend, and rocking vibrations in point-group-C3v 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.