Vibration-internal rotation interaction and normal modes in nonrigid asymmetric top molecules: Application to trans-methylnitrite

Abstract

The kinetic energy part of the vibrational Hamiltonian of an asymmetric top molecule containing a nearly free internal rotor is formulated. The vibration-internal rotation interaction terms are expressed in terms of torsional Coriolis coupling, the torsional angle derivatives of normal coordinates, and the normal coordinate derivatives of the moment of inertia of the internal rotor. Perturbation calculations have been carried out up to second order to yield corrections to the frequencies of the fundamentals and hot bands. Numerical calculations are presented for trans-methylnitrite which has a nearly free internal rotation of the methyl top. It is found that the normal modes involving top atoms are strongly affected by the torsional Coriolis coupling. The relative effects of the different interaction terms are discussed. The small effect of vibration-internal rotation interaction terms on the “hot” band spacing shows that the kinetic energy corrections are not important for the “hot” bands.