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

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 degrees compared with its phase for V-6 alone. The presence of a strong Delta upsilon = +/- 1 torsion-vibration coupling involving the lowest frequency vibrational mode provides a potential pathway for rapid intramolecular vibrational energy redistribution at higher energies. (C) 2015 AIP Publishing LLC.