“Expanded” local mode approach for XY2 (C2v) molecules

For XY₂ (C_2v symmetry) molecules, the local mode method is modified on the basis of a careful analysis of the transformation coefficients l_Nαλ and the properties of the intramolecular potential function, and an expanded local mode approach is derived. It is shown that relationships between known spectroscopic parameters, found in the spectroscopic literature, can be improved. New relationships between the centrifugal distortion coefficients are obtained. It is shown that knowledge of the centrifugal distortion coefficients of the ground vibrational state allows the possibility of predicting values of both the band centres and the spectroscopic parameters of deformational bands, even under conditions of a total absence of initial information on these deformational bands. The prediction possibility of the derived expanded local mode approach is analyzed.