Ab initio vibrational-rotational spectrum of potassium cyanide: KCN. II. Large amplitude motions and rovibrational coupling

The ten lowest vibrational states of KCN have been obtained ab initio using the close-coupling method of Le Roy and Van Kranendonk. Fundamental vibrations are found to lie at 114.7 (bending) and 293.0 cm−1 (K–CN stretch). Comparison is made with previous results obtained using the Watson Hamiltonian for nonlinear molecules and separating the rotations and vibrations. It is found that this model breaks down for the higher librations near and above the barrier to inversion. For these states the average geometry of the KCN molecule shifts towards the linear isocyanide (KNC) structure. Properties of the vibrational states and the rotational spectrum are analyzed.