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-fie ld ionization (PFI) were used to measure the absorption spectra of 2-f luoro-6-chlorotoluene (S-1) and of 2-fluoro-6-chlorotoluene(+) (D-0, t he cation ground state) for internally cold molecules in a pulsed nozz le expansion. The adiabatic ionization potential is 72 943 +/- 5 cm(-1 ) = 9.0438 +/- 0.0006 eV. We assign much of the low-frequency torsiona l and vibrational structure in both S-1 and D-0. Analysis yields the o ne-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 in itio 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, th e minimum energy conformation places one methyl CH bond in the plane o f 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 heig hts 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-chlorotol uene is approximately equal to the algebraic sum of the potential for o-fluorotoluene plus the potential for o-chlorbtoluene shifted in phas e 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 contribu tion to the barrier in all three states is steric repulsion between th e 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 up on 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 approxi mate C-2 nu symmetry about the methyl rotor axis. In effect, fluorine and chlorine act as equivalent, weak pi donors. Thus, 2-fluoro-6-chlor otoluene 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.