The Vibration-Rotation Energy Levels of Polyatomic Molecules II. Perturbations Due to Nearby Vibrational States

The use of the Wang secular equation for the interpretation of the rotational structure of a vibration-rotation band of an asymmetrical top molecule is not justified if either of the vibrational states involved in producing the band is perturbed by another vibrational state of nearly the same energy. The present paper discusses the conditions under which such a perturbation is to be expected and gives the secular equation which should be used instead of the Wang determinant in case the perturbation is important. This equation assumes a somewhat simpler form when applied to orthorhombic molecules, in which case it involves only one parameter in addition to the three moments of inertia.