Vibration—Rotation Interactions and the Choice of Rotating Axes for Polyatomic Molecules

A Hamiltonian for vibration and rotation in polyatomic molecules is described which is general for large and small amplitude motion and most choices of rotating axes. An analysis of the effect of a change in the definition of the rotating axes is presented and the convergence properties of perturbation expansions of the Hamiltonian are discussed. Some rotation—vibration terms converge slowly in an arbitrary set of axes, and the effect of these terms is shown to be equivalent to rotation from the original set of axes to a new set of axes in which the coriolis terms are minimized. Ways to find this new set of axes are discussed, and application of the results of this work to the microwave spectra of several molecules is presented.