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

Abstract

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 0(0) toluene up to m = 6 and of the lowest frequency vibration, 20(1), up to m = 4. The rotor levels of both 20(1) and 0(0) 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 - X20(1) torsion-vibration coupling; so, this interaction permeates the vib-rotor manifold, providing a mechanism to enhance intramolecular vibrational energy redistribution.

Torsion and vibration-torsion levels of the S1 and ground cation electronic states of para-fluorotoluene

“…The reassignment of the 137 cm -1 band to a vibtor level is much more in line with the new assignments for toluene 14 and also fits in with the assignments of the other ZEKE spectra here. This means the wavenumber associated with this band and also the other assignments in ref.…”

“…38 What is clear from the results here is that the vibration-torsional coupling is pervasive and involves different vibrations, with the symmetry of the vibration dictating which m levels can interact. Further, subsequent interactions between the vibtor levels and vibrational states can lead to eigenstates that are admixtures of these, and hence provide generalized routes for energy flow between torsional and vibrational motions and hence, as discussed by (amongst others) Parmenter et al 1,23,24,25,31,35 , Reid and Davies 28 and Gascooke et al, 14 accelerated IVR in methyl-group-containing molecules. We also deduce that the vibration-torsional coupling is subtly different for each of the vibrational levels (fundamentals, combinations and overtones), as expected.…”

“…Gascooke et al 14 Other weaker features in the spectrum are also straightforward to assign and these are given in the figure. We particularly note the band at 45 cm -1 is assignable as a Interestingly, we also see a ZEKE band corresponding to 2D20 and two associated vibtor levels, and this could be explained by mixing between the totally symmetric 2D20 and D20 m = 3(-) levels in the S1 (or D0 + ) state, or simply by Franck-Condon activity.…”

Molecular symmetry group analysis of the low-wavenumber torsions and vibration-torsions in the S1 state and ground state cation of p-xylene: An investigation using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy

“…3,4 In previous work, our group has studied toluene, using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy, 5,6 as has the group of Lawrance, 7 using two-dimensional laser-induced fluorescence (2D-LIF). Recently, this work was furthered by Lawrance and coworkers, 8,9 with the role of methyl groups in coupling to vibrational levels being emphasised. This is related to the concept of intramolecular vibrational redistribution (IVR), or more generally intramolecular energy redistribution.…”

“…9, vibration-torsional interactions will pervade the higher energy levels of each electronic state and so combination levels involving such vibtor levels must be considered when assigning the rest of the spectrum. In an accompanying paper, we shall consider the 350-600 cm -1 region of the spectrum of pXyl, where it will be seen that, although dominated by vibrational bands, vibtor levels also appear in this region.…”