PERTURBATIONS IN THE VIBRATION-ROTATION-TORSION ENERGY-LEVELS OF AN ETHANE MOLECULE EXHIBITING INTERNAL-ROTATION SPLITTINGS

Various examples of perturbations in the vibration-rotation-torsion energy levels of an ethane molecule exhibiting internal rotation splittings are discussed, both from the point of view of the point group D3d, appropriate when internal rotation tunneling effects cannot be observed, and from the point of view of the group G36†, appropriate when internal rotation tunneling in ethane leads to observable splittings in the spectrum. It is found, for perturbations allowed in both D3d and G36†, that each of the two torsional components of the perturbed D3d vibration-rotation level can in principle interact with a corresponding torsional component in the perturbing vibration-rotation level. It is found for perturbations forbidden in D3d but allowed in G36†, which all occur between D3d vibrational levels of different g, u parity, that only one of the two torsional components of the perturbed D 3d vibration-rotation level can interact with a corresponding torsional component in the perturbing vibration-rotation level. Some of the perturbations examined give intensity to otherwise forbidden transitions in such a way that perturbation-induced transitions can be used in conjunction with normally allowed transitions to determine the sum of the internal rotation splittings for two rotational levels differing in K by three units. Copyright © 1980 by Academic Press, Inc.