FORBIDDEN TORSIONAL BAND INTENSITIES IN $S_{1}-S_{0}$ SPECTRA OF SUBSTITUTED TOLUENE MOLECULES WITH 6-FOLD SYMMETRY
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Date
1995
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Ohio State University
Abstract
We have developed a model to explain the relative intensities of low energy, pure torsional bands observed in $S_{1}-S_{0}$ spectra of toluene-like molecules having six-fold internal $symmetry.^{1}$ Within the Born Oppenheimer approximation, the transitions ${m^{2}}_{1}, {m^{3}}_{0}$, and ${m^{4}}_{1}$ are Franck-Condon forbidden. Nevertheless, these bands always appear in $S_{1}-S_{0}$ absorption spectra with similar relative intensities. We report relative intensity ratios for five molecules: toluene, $d_{3}-toluene, \emph{p}$-fluorotoluene, Phenylsilane, and 2,6-difluorotoluen, We also show how expanding the $S_{1}-S_{0}$ transition dipole moment as a Fourier series in the torsional coordinate $\alpha$ predicts relative intensities in quantitative agreement with experiment. The model fixes the symmetry of the upper state of the stronger ${m^{3}}_{0}$ transition as $a_{1}$''. Securing this assignment means the preferred $S_{1}$ conformation is staggered for all molecules studied thus far. This $(m^{3+}}_{0}$ assignment also plays a crucial role when interpreting conformational preferences in $S_{0}$ and $D_{0}.$
Description
1. R. A. Walker, E. C. Richard, K.-T. Lu, E. L. Sibert III, J. C. Weisshaar, J. Chem, Phys., in press.
Author Institution: University of Wisconsin-Madison, Madison, WI, 53706.; NOAA, Boulder, CO, 80303."; UC-Berkeley, Berkeley, CA, 94720.
Author Institution: University of Wisconsin-Madison, Madison, WI, 53706.; NOAA, Boulder, CO, 80303."; UC-Berkeley, Berkeley, CA, 94720.