7 - Effects of Centrifugal Distortions

Abstract

Moments and products of inertia, as well as the elements of the inverse tensor of inertia which occur in the vibration-rotation Hamiltonian, are affected by molecular rotation as an effect of centrifugal distortions. This generates an effective dependence of the molecular rotational constants on the rotational quantum numbers, which can be expressed, in general, by a power expansion. The non-vanishing terms in this power expansion are determined by the molecular symmetry, and are identified following the procedure of J.K.G. Watson. Further simplifications are obtained by appropriate contact transformations. In asymmetric top molecules, two different reduced forms of the centrifugal distortion Hamiltonian terms, due to Watson, are commonly used. With the reduction A the form of the rotational matrix is close to that for symmetric tops, but some parameters become ill determined if the symmetric top limit is approached. In this case, an alternative transformation, called Reduction S, is preferable.