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).