Barriers to methyl torsion in 2-fluoro-6-chlorotoluene: Additivity of ortho-substituent effects in S-0, S-1, and D-0

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-1) and of 2-fluoro-6-chlorotoluene(+) (D-0, the cation ground state) for internally cold molecules in a pulsed nozzle expansion. The adiabatic ionization potential is 72 943 +/- 5 cm(-1) = 9.0438 +/- 0.0006 eV. We assign much of the low-frequency torsional and vibrational structure in both S-1 and D-0. Analysis yields the one-dimensional model torsional potential parameters V-3(') = 139.4 +/- 2.0 cm(-1) and V-6(') = -7 +/- 5 cm(-1) in Si and V-3(+) = 192.7 +/- 2.0 cm(-1) and V-6(+) = 20 +/- 5 cm(-1) in D-0. In earlier work, ab initio calculations at the HF/6-31G* level found V-3('') = 230 cm(-1) in S-0. An interlocking combination of experimental band intensities and nb 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. That is, in S-0, S-1, and D-0 the torsional potential of 2-fluoro-6-chlorotoluene is approximately equal to the algebraic sum of the potential for o-fluorotoluene plus the potential for o-chlorbtoluene shifted in phase by 180 degrees. This is true to high accuracy in S-0 and S-1; it is qualitatively true in D-0 as well. 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-1 and upon pi ionization to D-0 is unusual among 3-fold symmetric molecules. We attribute this to alignment of the pi molecular orbitals (essentially Huckel orbitals) so as to have approximate C-2 nu symmetry about the methyl rotor axis. In effect, fluorine and chlorine act as equivalent, weak pi donors. Thus, 2-fluoro-6-chlorotoluene has a substantial 3-fold barrier due to net steric repulsion, but behaves much like a 6-fold molecule with respect to effects of pi excitation or ionization.