TOLUENE CATION - NEARLY FREE ROTATION OF THE METHYL-GROUP

Threshold photoionization spectra of toluene+-h8 obtained by resonant two-photon ionization of toluene resolve internal rotor states of the CH3 group. The barrier to internal rotation derived from a model V6 potential is 16 +/- 3 cm-1. Thus CH3 rotation is nearly free in toluene+, as it is in S0 and S1 toluene. Spectral intensities indicate that the phase of the 6-fold symmetric torisional potential shifts by pi/6 from neutral to cation, with the minimum-energy conformation staggered in S1 and eclipsed in the cation. The natural resonance theory of Glendening and Weinhold provides new insight into why such V6 barriers are always small. We also measure adiabatic ionization potentials for the barc molecule and for van der Waals complexes to Ar as follows: IP(toluene) = 8.8276 +/- 0.0006 eV, IP(toluene.Ar) = 8.8070 +/- 0.0006 eV, and IP(toluene-Ar2) = 8.7869 +/- 0.0006 eV. The binding energy of Ar to toluene+ is approximately 400 cm-1, some eight times smaller than the binding energy of Ar to the atomic ion V+, due to delocalization of the charge.