Ab Initio Determination of Band Structures of Vibrational Spectra of Non-rigid Molecules. Applications to Methylamine and Dimethylamine

The ab ini tio determination of the band structures of infrared spec tra is described giving as examples the methyl torsion and amine wagging in methylamine and the double torsion in dimethylamine. In addition, the influence of the amine hydrogen symmetric bending and the CNC skeleton symmetric bending is considered in methylamine and dimethylamine, respectively. For this purpose, the potential energy sufaces and kinetic parameters are determined at the RHF /MP2 levels with large basis sets. The numerical results are fitted as a function of the vibrational angles to conveniently symmetry adapted functional forms. The Schrodinger equations for the nuclear motions solved by expanding the solutions into products of trigonometric functions. From the energy levels, the vibrational functions and the electric dipole moment variations the bands locations and intensities are determined. The calculated spectra are compared with the available experimental data. In the case of methylamine, the torsional splittings and frequencies are relatively well reproduced, whereas the wagging frequencies appear to be slightly too high. In the case, of dimethylamine, the symmetric and antisymmmetric torsion modes are very well reproduced, whereas the CNC bending frequencies appear also to be too high. List of