Effects of symmetry, methyl groups and serendipity on intramolecular vibrational energy dispersal

Abstract

We consider two key parameters that have been proposed to be important for vibrational energy delocalization, closely related to intramolecular vibrational redistribution (IVR), in molecules. These parameters are the symmetry of the molecule, and the presence of torsional (internal rotor) modes of a methyl group. We consider four para-disubstituted benzene molecules and examine their vibrational character. The molecules selected are para-difluorobenzene, para-chlorofluorobenzene, para-fluorotoluene, and para-xylene. This set of molecules allows the above parameters to be assessed in a systematic way. The probe we use is zero-electron-kinetic-energy (ZEKE) spectroscopy, which is employed in a resonant scheme, where the intermediate levels are selected vibrational levels of the S1 excited electronic state, with wavenumbers up to 1300 cm-1. We conclude that symmetry, and the presence of a methyl groups, do indeed have a profound effect on "restricted" IVR at low energies. This is underpinned by serendipitous coincidences in the energies of the levels, owing to small shifts in vibrational wavenumbers between molecules, so bringing levels into resonance. Additionally, methyl groups play an important role in opening up new routes for coupling between vibrations of different symmetry, and this is critical in the transition to "statistical" IVR at lower energies for molecules that contain them. Further, the presence of two methyl groups in the symmetrically-substituted p-xylene causes more widespread IVR than does the single methyl group in the asymmetrically-substituted p-fluorotoluene.

Complexity surrounding an apparently simple Fermi resonance in p-fluorotoluene revealed using two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy

“…Building on work by Parmenter and coworkers, 1,2,3,4 recent work by the Lawrance group and ourselves has identified that vibration-torsional ("vibtor") coupling is of key importance in molecules that contain methyl groups: toluene, 5,6,7,8,9,10,11, para-fluorotoluene (pFT) 12,13,14,15,16,17 and para-xylene (pXyl). 17,18,19 Both a combination of an increasing density of states (DOS) and symmetry-allowed vibtor coupling have been invoked to rationalize the rapid increase in interactions that occur in such molecules, 17 which drives energy dispersal through a molecule.…”

“…In previous work, we have studied the lower-wavenumber regions of the S1  S0 transition in parafluorotoluene (pFT) using resonance-enhanced multiphoton ionization (REMPI) and zero-kineticenergy (ZEKE) spectroscopy; 12,13,17,20 while both we 13,20 and the Lawrance group 16 have also employed the technique of two-dimensional laser-induced fluorescence (2D-LIF). 21 The low-wavenumber region has been shown to be rich in interactions, involving torsional, vibrational and vibrational-torsional levels.…”

Unravelling overlaps and torsion-facilitated coupling using two-dimensional laser-induced fluorescence

“…As a consequence, we conclude that each of the three states, 11 1 , 29 1 and 14 2 are inherently 'bright', again in agreement with conclusions we have made in other work on pFT and related molecules. 8,15,33,46 We have previously discussed in depth 29 a feature that appears at S1+845 cm -1 and the interactions there were isolated to the 18 2 m 1 level, which allowed couplings to vibtor combinations involving m = 2. In so-doing, the mechanism opens up couplings between vibrations of different symmetry; such is also included in the vibtor coupling of the three lowest-wavenumber vibrations included in the model of GL.…”

“…In Ref. 33 we have explicitly looked at the density of states, comparing pClFB, pFT, pDFB and pXyl. There we saw that there was a very erratic build-up in the density of states of 'pure' vibrational levels that did not differ too much for each of the molecules at < 1000 cm -1 ; however, there was a dramatic increase in the DOS once vibtor levels were included, but the build-up is still erratic at very low wavenumbers.…”

“…Overall, these vibtor interactions provide increased pathways for energy dissipation between the molecular motions and so can lead to the possibility of rapid intramolecular vibrational redistribution (IVR) at even quite low wavenumbers, and this has been compared and contrasted in recent papers focused on toluene, p-difluorobenzene (pDFB) and pFT, 6 as well as pDFB, p-chlorofluorobenzene (pClFB), pFT and pXyl. 33 Recently, 29 we showed that the vibtor levels at~845 cm -1 associated with the methyl torsion and the first overtone of an out-of-plane vibration in pFT were opening up new routes to interactions involving vibrations of different symmetry. This involved interactions between an excited torsional level of an overtone with vibtor levels formed by combinations with a set of levels at~400 cm -1 .…”