Molecular symmetry group, selection rules and vibration-rotation band envelopes of molecules with planar frame and C3v type free internal rotation

Abstract

The effect of a low barrier C 3v-type internal rotation on the vibration-rotation spectra of nearly symmetric top molecules with C s frame has been investigated. A computer program has been developed for the computation of A, B and C type band envelopes for asymmetric top molecules in the presence of free internal rotation. The permutation-inversion group of such molecules has been studied and the selection rules are derived. The spin statistical weights have also been taken into account. For the purpose of computation we have used the vibration—internal rotation—rotation Hamiltonian derived by us recently. Numerical computation for a model case shows that the ratio [ A − 1/2( B + C)] / G plays an important role in the simulated band envelope, where G depends on the moment of inertia of the top, principal moments of inertia of the molecule and the orientation of the internal rotation axis with the principal axes; A, B and C are the rotationalconstants.