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

Rotationally resolved fluorescence excitation spectra of several torsionall y active bands in the S-1-S-0 electronic transitions of toluene and toluene -d(3) have been recorded in the collision-free environment of a molecular b eam. Analyses of these data provide accurate values of the internal rotor c onstants F; the barrier heights V-6; the frame rotational constants A(F); t he overall rotational constants B and C; and the torsion-rotation coupling constants A(F)'; in the m = 0 and m = +/- 1 levels of the S-0 state and the m = 0, +/- 1, and 3+ levels of the S-1 state. Comparison of the A(F), B, a nd C values in the m = 0 levels of the two states shows that S-1 toluene is quinoidal in form, with shorter ring "parallel" C-C bonds than "perpendicu lar" ones, unlike the S-0 state. The preferred conformation of the methyl g roup is staggered in both states, but the V-6 values are significantly diff erent; V-6(S-0) = -4.874 and V-6(S-1) = -26.376 cm(-1). Comparison of the F , A(F), and A(F)' values in the different torsional levels of the S-1 state shows that, below the barrier, the methyl group tilts and the ring bond le ngths change with increasing displacements along the torsional coordinate. Above the barrier, the precessional motion of the CH3 is quenched but large r ring distortions are observed. Thus, the data are consistent with an enha nced hyperconjugative interaction between the benzene ring and the methyl g roup in the S-1 state. This interaction is substantially modulated by the r elative motion of the two attached groups, providing a facile route to IVR. (C) 2000 American Institute of Physics. [S0021-9606(00)00133-1].