VIBRATIONALLY INDUCED ROTATIONAL AXIS SWITCHING - A NOVEL MECHANISM FOR VIBRATIONAL-MODE COUPLING

High resolution IR spectra of small- to medium-sized molecules such as 2-fluoroethanol (2FE) show that the effective density of coupled states is often greater than that obtained by a direct count of vibrational states. A novel mechanism for rotation-vibration interaction, vibrationally induced rotational axis switching (VIRAS), is proposed as a possible explanation for these discrepancies. VIRAS has its origin in centrifugal distortion, and is physically distinct from Coriolis coupling. In the case of 2FE, we explicitly treat the coupling of overall rotation with large-amplitude internal rotation about the C-C bond. Assuming a uniform coupling of all dark vibration-torsion states to the bright state, we predict a density of coupled states in good agreement with that observed in the C-H stretching region at 2980 cm-1.