A b i n i t i o dipole surfaces, vibrationally averaged dipole moments, and infrared transition intensities for KCN and LiCN

Abstract

Using the rovibrational wave functions which have been obtained from ab initio potentials and making analytic fits to the ab initio calculated dipole surfaces, we have evaluated the dipole (transition) moments of KCN and LiCN for several vibrational states (for J=0 and 1). The ‘‘exact’’ rotational transition intensities of the a-type transitions in (triangular) KCN and those of (linear) LiNC and LiCN can be rather accurately reproduced by a rigid rotor model with the vibrationally averaged dipole moments; for the (weak) b-type transitions in KCN this model breaks down, however. Although the bending motions in these cyanides have large amplitudes, the vibrational transition intensities in LiNC and LiCN conform to the harmonic oscillator model. In KCN, where the large amplitudes of the bending modes and their coupling with the K–CN stretch leads to significant anharmonic shifts in the transition frequencies, the harmonic intensity pattern is perturbed by Fermi resonances, for instance, between the stretch fundamental and the second bending overtone. We have also calculated the lifetime for radiative decay of (metastable) LiCN into LiNC: 24 s for the ground state, much less for vibrationally excited states.