title: Direct observation of methyl rotor and vib-rotor states of S0 toluene: a revised torsional barrier due to torsion-vibration coupling.
abstract: We report a two dimensional, laser induced fluorescence study of the lowest 345 cm(-1) region of S0 toluene. Methyl rotor levels of 00 up to m = 6 and of 201 up to m = 4 are observed. The rotor levels of 00 and 201 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 3a2″ state lies below the 3a1″ 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 V6 value and the torsion-vibration coupling model as this coupling shifts the phase of the wavefunction by 30° compared with its phase for V6 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.
authors.affiliation: School of Chemical and Physical Sciences, Flinders University, GPO Box 2100, Adelaide, South Australia 5001, Australia.
authors.forename: Jason R
authors.initials: JR
authors.lastname: Gascooke
authors.affiliation: School of Chemical and Physical Sciences, Flinders University, GPO Box 2100, Adelaide, South Australia 5001, Australia.
authors.forename: Edwina A
authors.initials: EA
authors.lastname: Virgo
authors.affiliation: School of Chemical and Physical Sciences, Flinders University, GPO Box 2100, Adelaide, South Australia 5001, Australia.
authors.forename: Warren D
authors.initials: WD
authors.lastname: Lawrance