Methyl group rotational dynamics in o-, m-, and p-xylene cations from pulsed field ionization zero-kinetic-energy spectroscopy

The pulsed field ionization zero-kinetic-energy (PFI-ZEKE) spectra of o-, m - and p-xylene have been recorded and analyzed. These clearly demonstrate n eighboring methyl groups to be interacting and further apart groups to be n oninteracting. The spectra show well-resolved features, typically within 15 0 cm(-1) of the S-1 --> ion origins, which are assigned to transitions asso ciated with the internal rotations of the ring methyl groups. The observed transitions are fit using a one-dimensional periodic potential with a free rotor basis. The results of the analysis show that the potential barrier to methyl rotation in the xylene ion increases in the order para ( meta < ort ho and that the barriers differ from those determined for the So and S1 neu tral state in significant ways. This is particularly true for the interacti ng groups of o-xylene. The two methyl groups of p- and m-xylene are determi ned to be noninteracting whereas the methyl groups of o-xylene interact str ongly. The ionization potentials for o-, m-, and p-xylene are also derived from the PFI-ZEKE spectra.