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

We have examined the S₁←S₀ transition of toluene in the region from the 0 0 0 band to ∼210 cm^-1 above it. The spectrum reveals methyl rotor levels of 0⁰ toluene up to m = 6 and of the lowest frequency vibration, 20¹, up to m = 4. The rotor levels of both 20¹ and 0⁰ are perturbed by torsion-vibration coupling. The inclusion of torsion-vibration coupling leads to the S₁ torsional barrier, V₆, 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 - X20¹ torsion-vibration coupling; so, this interaction permeates the vib-rotor manifold, providing a mechanism to enhance intramolecular vibrational energy redistribution.