11 - Floppy Molecules

Abstract

An outline of peculiar features in the spectra of molecules with more interconvertible geometries is given. We discuss molecular inversion and the inversion doubling in ammonia, and internal rotation. The level splitting caused by the hindered internal rotation in the spectra of molecules like acetaldehyde or methanol, and ethane-like molecules, is also discussed. Special attention is given to the effects of the coupling of the angular momenta generated by the vibrational motion and the internal rotation (torsional Coriolis coupling), and the interplay with the coupling of the internally rotating molecular moieties (hindering barrier and its dependence on molecular deformations). Typical states in limit coupling states are defined by the methods of group theory.