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)].