Methyl rotor dependent vibrational interactions in toluene
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The methyl rotor dependence of a three state Fermi resonance in S1 toluene at ∼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. 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 Δm = ±3n (n = 1, 2, …) coupling selection rule associated with torsion-rotation coupling in combination with the m2 scaling of the rotor energies, which changes the ΔE 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.