ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Rotationally resolved fluorescence excitation spectra of several torsionally active bands in the S1–S0 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 AF; the overall rotational constants B and C; and the torsion-rotation coupling constants AF′; in the m=0 and m=±1 levels of the S0 state and the m=0, ±1, and 3+ levels of the S1 state. Comparison of the AF, B, and 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 S0 state. The preferred conformation of the methyl group is staggered in both states, but the V6 values are significantly different; V6(S0)=−4.874 and V6(S1)=−26.376 cm−1. Comparison of the F, AF, and AF′ values in the different torsional levels of the S1 state shows that, below the barrier, the methyl group tilts and the ring bond lengths change with increasing displacements along the torsional coordinate. Above the barrier, the precessional motion of the CH3 is quenched but larger ring distortions are observed. Thus, the data are consistent with an enhanced hyperconjugative interaction between the benzene ring and the methyl group in the S1 state. This interaction is substantially modulated by the relative motion of the two attached groups, providing a facile route to IVR. © 2000 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.1287392
