Evidence for widespread torsion–vibration interaction in substituted toluenes

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