Vibration-inversion-internal rotation-rotation Hamiltonian for methylamine

Abstract

A new vibration-inversion-internal rotation-rotation Hamiltonian for methylamine has been developed in which the inversion and internal rotation motions are removed from the vibrational problem by allowing the molecular reference configuration to be the function of inversion (ρ) and internal rotation (τ) coordinates. The treatment is based on the idea of Hougen, Bunker, and Johns [ J. Mol. Spectrosc., 34, 136 (1970)] who have studied the large amplitude bending motion of triatomic molecules. We have used the inversion-internal rotation potential function V( ρ, τ) = aρ 4 − bρ 2 + cρ 3cos 3 τ in which the coupling between the two large amplitude motions is taken into account. The inversion-internal rotation energy levels of CH 3NH 2 have been calculated with the use of the zeroth-order Hamiltonian for the zero value of the total angular momentum quantum number J. The electric dipole selection rules resulting from the symmetry classification of states have also been discussed.