A strong interaction between torsion and vibration in S 0 and S 1 m -fluorotoluene

Abstract

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 S 0 and 350 cm -1 in S 1 . Evidence is presented for interactions between torsion and low frequency vibrations in both S 0 and S 1 , 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 S 0 (20 cm -1 ) and modest in S 1 (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 (V 3 and V 6 ), and rotational constants (F) for the methyl torsion in S 0 and S 1 . The inclusion of torsion-vibration coupling primarily affects V 6 and F: |V 6 | is reduced and F increased, as was seen previously for the G 12 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 CH 3 wag mode are almost the same in S 0 and S 1 (15.5 cm -1 and 14.0 cm -1 , respectively).