Torsion-vibration coupling in S1 toluene: Implications for IVR, the torsional barrier height, and rotational constants

We have examined the S-1 <- S-0 transition of toluene in the region from the O-0(0) band to similar to 210 cm(-1) above it. The spectrum reveals methyl rotor levels of O-0 toluene up to m = 6 and of the lowest frequency vibration, 201, up to m = 4. The rotor levels of both 201 and O-0 are perturbed by torsion-vibration coupling. The inclusion of torsion-vibration coupling leads to the S1 torsional barrier, V-6, being revised from -26.376 cm(-1) to -5.59 cm(-1). The torsion-vibration coupling constant is determined to be 21.1 cm(-1). This situation is the S1 analogue of that recently reported for S0 toluene [Gascooke et al., J. Chem. Phys. 142, 024315 (2015)]. Torsion-vibration coupling alters both the rotor band positions and the rotational contours, which particularly affects the rotational constants associated with motion around the a-axis, about which the methyl group rotates. Every vibrational state (indicated generically by X) will be involved in the corresponding X-X20(1) torsion-vibration coupling; so, this interaction permeates the vib-rotor manifold, providing a mechanism to enhance intramolecular vibrational energy redistribution. (C) 2015 AIP Publishing LLC.