Toluene: Structure, dynamics, and barrier to methyl group rotation in its electonically exited state. A route to IVR

摘要

Rotationally resolved fluorescence excitation spectra of several torsionally active bands in the S —S~ electronic transitions of toluene and toluene-d3 have been recorded in the collision-free environment of a molecular beam. Analyses of these data provide accurate values of the internal rotor constants F: the barrier heights V6 the frame rotational constants A F the overall rotational constants B and C: and the torsion-rotation coupling constants Al-: inthemO andm±l levels of the S,~ state and the m0 il and 3+ levels of the 5, state. Comparison of the AF. B, aad.C values in the m =0 levels of the two states shows that S1 toluene is quinoidal in form, with shorter ring ‘parallel” C—C bonds than “perpendicular” ones~ unlike the 5o state. The preferred conformation of the methyl eroup is staggered in both states, but the V6 values are significantly different: V6tS)—4.874 and V6(S,)’~-26.376Cm’. Comparison of the F,AF, andA~ values in the different torsional levels of the 5, state shows that, below the barrier, the methyl group tilts and the rime bond lengths change with increasing displacements along the torsional coordinate. Above the barrier, the precessional motion of the CH3 is quenched but larger fling distortions are observed. Thus, the data are consistent with an enhanced hyperconjugative interaction between the benzene ring and the methyl group in the 5, state. This interaction is substantially modulated by the relative motion of the two attached groups, providing a facile route to IVR.