INTENSITIES OF FORBIDDEN PURE TORSIONAL BANDS IN S-1-S-0 SPECTRA OF TOLUENES

In toluenelike molecules with sixfold internal rotation symmetry, three weak, Franck-Condon forbidden, pure torsional transitions m1 2, m03 and m14 invariably appear in S1 - S0 absorption spectra. The intensities are typically 1% - 5% of the allowed bands, m00 and m 11. Determination of the preferred conformer in S 1 as staggered or eclipsed relies on proper assignment of the upper state of the m03 band as 3a1″ or 3a 2″ under the molecular symmetry group G12. In addition, inferences of the preferred conformer in S0 and D 0 (ground state cation) from band intensities of fluorescence and threshold photoionization spectra have also relied on the same assignment. For a set of six molecules having sixfold rotor potentials, including both -CH 3 and -CD3 rotors, we present experimental relative intensities for the three forbidden S1-S0 absorption bands. Within an adiabatic electronic representation, we show how a Fourier expansion of the three components of the S1-S0 electric dipole transition moment predicts relative forbidden band intensities in quantitative agreement with experiment. This fixes the assignment of the upper state of the key m03 transition as 3a″1, establishes the preferred S1 conformer as staggered for all six molecules, and places earlier inferences of the preferred conformers in S 0 and D0 on solid ground. © 1995 American Institute of Physics.