Methyl rotor dependent vibrational interactions in toluene

The methyl rotor dependence of a three state Fermi resonance in S-1 toluene at similar to 460 cm(-1) has been investigated using two-dimensional laser induced fluorescence. An earlier time-resolved study has shown the Fermi resonance levels to have different energy spacings at the two lowest methyl rotor states, m = 0 and 1 [J. A. Davies, A. M. Green, and K. L. Reid, Phys. Chem. Chem. Phys. 12, 9872 (2010)]. The overlapped m = 0 and 1 spectral features have been separated to provide direct spectral evidence for the m dependence of the resonance. The resonance has been probed at m = 3a(1)('') for the first time and found to be absent, providing further evidence for a large change in the interaction with m. Deperturbing the resonance at m = 0 and 1 reveals that the m dependence arises through differences in the separations of the zero-order, locally coupled states. It is shown that this is the result of the local zero-order states being perturbed by long-range torsion-vibration coupling that shifts their energy by small amounts. The m dependence of the shifts arises from the Delta m = +/- 3n (n = 1, 2,...) coupling selection rule associated with torsion-rotation coupling in combination with the m(2) scaling of the rotor energies, which changes the Delta E for the interaction for each m. There is also an increase in the number of states that can couple to m = 1 compared with m = 0. Consideration of the magnitude of reported torsion-rotation coupling constants suggests that this effect is likely to be pervasive in molecules with methyl rotors. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4795439]