Ab initio determination of the torsional and wagging FIR spectrum of methylamine

Abstract

The potential energy surface and the kinetic parameters for the torsion and wagging vibration modes in methylamine electronic ground state are determined by the RHF/MP2 approximation using a 6-31G+ + (3df,3pd) basis set. The numerical results are fitted, as a function of the torsional and wagging angles, to conveniently symmetry adapted functional forms. The Schrödinger equation for these two motions is solved by expanding the solutions onto a basis of products of trigonometric functions. From the energy levels and the torsional-wagging functions, the band locations and intensities are determined. The calculated FIR spectrum is compared with the available experimental data. It is found that whereas the torsional splitting and frequencies are reasonably well reproduced, the wagging frequencies appear to be too low. This disagreement is attributed to the interactions between the inversion and HNH bending modes. It is concluded that the HNH bending angle should be considered explicitly in a three-dimensional calculation, © 1996 Academic Press, Inc.