Vibrationally induced rotational axis switching: A novel mechanism for vibrational mode coupling

Abstract

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 vibrationtorsion 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 .

Evidence of Vibrational-Induced Rotational Axis Switching for HD12C16O: New High-Resolution Analysis of the ν5 and ν6 Bands and First Analysis of the ν4 Band (10-μm Region)

“…As a consequence, the VIRAS model that we describe below takes into account this fact. Actually, this mechanism has been proposed to explain enhanced densities of coupled states in 2-fluoroethanol (29).…”

The role of molecular flexibility in accelerating intramolecular vibrational relaxation

“…On the other hand, the high barrier in 1-pentyne (1088 cm-' for gauche+gauche (19)) is associated with fast relaxation of the methyl C-H. The relaxation in 1-pentyne might be faster because the three minima in the isomerization coordinate are inequivalent or because isomerization causes a significant reorientation of the rotational axes (20).…”

K-scrambling in a near-symmetric top molecule containing an excited noncoaxial internal rotor

“…We believe that K-scrambled states in excited internal rotor molecules can participate in the enhancement of essentially irreversible intramolecular vibrational redistribution ͑IVR͒ via a mechanism similar to the vibrationally induced rotational-axis-switching mechanism ͑VIRAS͒ proposed by Li, Ezra, and Philips 16 to explain vibrational mode coupling and enhancement of IVR in molecules containing internal rotor groups. Studies on the role of K scrambling in the enhancement of energy transfer in molecules without internal rotation were carried out by Nathanson and McClelland, who proposed 17 using the reduction of fluorescence polarization to characterize the transition from regular to statistical rotation and to explore the breakdown 18 of the rigid body description of rotational motion as a result of strong Coriolis interactions.…”

“…For other kinds of molecules other mechanisms have been proposed. Li, Ezra, and Philips 16 found that one source of coupling leading to enhanced IVR is vibrationally induced rotational axis switching ͑VIRAS͒. Their formalism is based on the physical picture that the rotation of body-fixed axes caused by vibrational excitation must be particularly important when large vibrational displacements are caused or stabilized by centrifugal distortion forces.…”

“…Their formalism is based on the physical picture that the rotation of body-fixed axes caused by vibrational excitation must be particularly important when large vibrational displacements are caused or stabilized by centrifugal distortion forces. The consequence, when their formalism is applied to CH 2 F-CH 2 OH, 16 is that rotational wave functions for different torsional states have very different K compositions. Purely anharmonic interaction terms in the potential energy surface are then able to couple one rotational level of a given zeroth-order vibration-torsion state with many rotational levels in some other vibration-torsion state.…”