Evidence for widespread torsion–vibration interaction in substituted toluenes

The torsional constant (F) is a parameter extracted from spectroscopic analyses of molecules possessing a methyl group. Its value depends primarily on the methyl structure. Widely varying F values have been reported for substituted toluenes in their ground electronic state, first excited singlet electronic state or the ground electronic state of the cation. Conventionally, this variability is assumed to indicate significant changes in the methyl structure with substituent, its position on the ring and the electronic state. However, when the large amplitude methyl torsion interacts with other, small amplitude vibrations, this interpretation is misleading as the torsional states are shifted to lower energy, resulting in a reduced, ‘effective’ F being determined. We have observed coupling between methyl torsion and the low frequency, methyl group out-of-plane wag vibration in toluene, p-fluorotoluene, m-fluorotoluene and N-methylpyrrole, leading us to postulate that, since such motion will be present whenever the methyl group is attached to a planar frame, this type of interaction is widespread. This is tested for a series of substituted toluenes by comparing the methyl group structure calculated by quantum chemistry with the experimental torsional constants. The quantum chemistry calculations predict little variation in the methyl structure across a wide range of substituents, ring positions and electronic state. The wide variation in F values observed in experimental analyses is attributed to the torsion–vibration interaction affecting the torsional band structure, so that measured F values become ‘effective constants’. Comparisons between calculated and experimental torsional constants need to be cognisant of this effect.