A strong interaction between torsion and vibration in S0 and S1m-fluorotoluene.

We report results of a two dimensional laser induced fluorescence study of torsional states, low frequency vibrations, and combinations of torsion with low frequency vibration in m-fluorotoluene up to 560 cm-1 in S0 and 350 cm-1 in S1. Evidence is presented for interactions between torsion and low frequency vibrations in both S0 and S1, demonstrating that the coupling of torsion and vibration observed previously in toluene and p-fluorotoluene extends to a molecule with a threefold torsional barrier. This barrier is low in S0 (20 cm-1) and modest in S1 (116 cm-1). The methyl torsion-vibration interaction is much larger for the mode involving out-of-plane wagging of the methyl group with respect to the planar frame compared with the analogous out-of-plane fluorine atom motion. Methyl group out-of-plane modes were found to be most important for torsion-vibration interactions in toluene and p-fluorotoluene, and the evidence is accumulating that this motion is fundamental in torsion-vibration interactions. Fits of the experimental band positions yield torsion-vibration coupling constants, torsional potential terms (V3 and V6), and rotational constants (F) for the methyl torsion in S0 and S1. The inclusion of torsion-vibration coupling primarily affects V6 and F: |V6| is reduced and F increased, as was seen previously for the G12 molecules, toluene and p-fluorotoluene. The torsional barrier height does not appear to influence the magnitude of the torsion-vibration interaction: the coupling constants for the out-of-plane CH3 wag mode are almost the same in S0 and S1 (15.5 cm-1 and 14.0 cm-1, respectively).

Pervasive interactions between methyl torsion and low frequency vibrations in S0 and S1p-fluorotoluene.

We report two dimensional laser induced fluorescence spectral images exploring the lower torsion-vibration manifolds in S0 (E < 560 cm-1) and S1 (E < 420 cm-1) p-fluorotoluene. Analysis of the images reveals strong torsion-vibration interactions and provides an extensive set of torsion-vibration state energies in both electronic states (estimated uncertainty ±0.2 cm-1), which are fit to determine key constants including barrier heights, torsional constants, and torsion-vibration interaction constants. The dominant interactions in both electronic states are between methyl torsion (internal rotation) and the lowest frequency out-of-plane modes, D20 and D19, both of which involve a methyl wagging motion. This is the second aromatic (following toluene) for which a significant interaction between torsion and methyl out-of-plane wagging vibrations has been quantified. Given the generic nature of this motion in substituted toluenes and similar molecules, this mechanism for torsion-vibration coupling may be common in these types of molecules. The inclusion of torsion-vibration coupling affects key molecular constants such as barrier heights and torsional (and rotational) constants, and the possibility of such an interaction should thus be considered in spectral analyses when determining parameters in these types of molecules. p-Fluorotoluene is the first molecule in which the role of methyl torsion in promoting intramolecular vibrational energy redistribution (IVR) was established and the observed torsion-vibration coupling provides one conduit for the state mixing that is a precursor to IVR, as originally proposed by Moss et al. [J. Chem. Phys. 86, 51 (1987)].