Barriers to methyl torsion in 2-fluoro-6-chlorotoluene: Additivity of ortho-substituent effects in S{sub 0}, S{sub 1} and D{sub 0}
- Univ. of Wisconsin, Madison, WI (United States)
The techniques of resonant two-photon ionization (R2PI) and pulsed-field ionization (PFI) were used to measure the absorption spectra of 2-fluoro-6-chlorotoluene (S{sub 1}) and of 2-fluoro-6-chlorotoluene{sup +} (D{sub 0}, the cation ground state) for internally cold molecules in a pulsed nozzle expansion. We assign much of the low-frequency torsional and vibrational structure in both S{sub 1} and D{sub 0}. Analysis yields the one-dimensional model torsional potential parameters. An interlocking combination of experimental band intensities and ab initio calculations shows that, in all three electronic states, the minimum energy conformation places one methyl CH bond in the plane of the ring pseudo-trans to the chlorine substituent, i.e., on the same side of the ring as the fluorine substituent. The 3-fold barrier heights exhibit a kind of additivity of ortho-substituent effects. We argue that the dominant contribution to the barrier in all three states is steric repulsion between the methyl group and a lone pair on each halogen, with the weaker methyl-fluorine interaction partially canceling the stronger methyl-chlorine interaction. The approximate invariance of the torsional potential upon {pi} excitation to S{sub 1} and upon {pi} ionization to D{sub 0} is unusual among 3-fold symmetric molecules. We attribute this to alignment of the {pi} molecular orbitals (essentially Hueckel orbitals) so as to have approximate C{sub 2v}, symmetry about the methyl rotor axis. 37 refs., 9 figs., 7 tabs.
- OSTI ID:
- 252929
- Journal Information:
- Journal of Physical Chemistry, Vol. 100, Issue 18; Other Information: PBD: 2 May 1996
- Country of Publication:
- United States
- Language:
- English
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