Methyl rotor dependent vibrational interactions in toluene

Jason Gascooke, Warren Lawrance

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    28 Citations (Scopus)

    Abstract

    The methyl rotor dependence of a three state Fermi resonance in S 1 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 [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 Δm ±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 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.

    Original languageEnglish
    Article number134302
    Pages (from-to)134302-1-134302-17
    Number of pages17
    JournalJournal of Chemical Physics
    Volume138
    Issue number13
    DOIs
    Publication statusPublished - 7 Apr 2013

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