Direct observation of methyl rotor and vib-rotor states of S0 toluene: a revised torsional barrier due to torsion-vibration coupling

Jason Gascooke, Edwina Virgo, Warren Lawrance

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    Abstract

    We report a two dimensional, laser induced fluorescence study of the lowest 345 cm-1 region of S 0 toluene. Methyl rotor levels of 0 0 up to m = 6 and of 20 1 up to m = 4 are observed. The rotor levels of 0 0 and 20 1 have quite different energy spacings that are well fit by a model that includes strong torsion-vibration coupling between them. The model requires that the rotor barrier height be revised from -4.84 cm-1 (methyl hydrogens in a staggered conformation) to +1.57 cm-1 (eclipsed conformation). However, the 3a 2 ″ state lies below the 3a 1 ″ state as expected for a staggered conformation due to energy shifts associated with the torsion-vibration coupling. It is shown that the rotor wave-functions exhibit little localization at the torsional energy minima. The variation in the m = 0 wavefunction probability distribution with torsional angle is shown to be very similar for the previously accepted negative V 6 value and the torsion-vibration coupling model as this coupling shifts the phase of the wavefunction by 30° compared with its phase for V 6 alone. The presence of a strong Δυ = ± 1 torsion-vibration coupling involving the lowest frequency vibrational mode provides a potential pathway for rapid intramolecular vibrational energy redistribution at higher energies.

    Original languageEnglish
    Article number024315
    Pages (from-to)Article: 024315
    Number of pages13
    JournalJournal of Chemical Physics
    Volume142
    Issue number2
    DOIs
    Publication statusPublished - 14 Jan 2015

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