THRESHOLD FORMATION OF BENZYLIUM (BZ+) AND TROPYLIUM (TR+) FROM TOLUENE - NONSTATISTICAL BEHAVIOR IN FRANCK-CONDON GAPS

Benzylium (Bz+) and tropylium (Tr+) ion formation from toluene-h8 and toluene-alpha-d3 were studied by time-resolved photoionization mass spectrometry (TPIMS). Bz+ was distinguished from Tr+ through its ion/molecule reaction with toluene, which converts it quantitatively to C8H9+. The appearance energies (AE's) at 0 K of C7H7+ without ion trapping (11.5 eV) and of Bz+ with ion trapping (11.1 eV) are in excellent agreement with predictions by time-resolved photodissociation (TRPD). The structure observed at photon energies below 11.1 eV in the Bz+ photoionization efficiency curve is ascribed to autoionizing Rydberg states converging to the third ionization energy intoluene. These states, which reside in a Franck-Condon gap, dissociate in competition with autoionization. This dissociation is a non-RRKM process forming Bz+, in preference to Tr+, and is made possible energetically by virtue of the thermal energy at the temperature of the experiment (298 K). H/D loss ratios for toluene-alpha-d3 demonstrate complete isotopic scrambling and an energy dependent isotope effect. The H/D ratio stays constant below 11.1 eV, demonstrating that AE0 K (Tr+) = 11.1 eV and that there is equality of the AE's of the two C7H7+ isomers within experimental error. The preferential, nonstatistical, formation of Bz+ over Tr+ below approximately 11.1 eV is given further proof by the observation of an increased direct CD2+ transfer probability from C6H5CD2+ to C6H5CD3. These results, combined with previously published ab initio calculations which demonstrated a reverse activation energy for the Tr+ exit channel, explain why there is no energy range in which there is pure Tr+ formation from toluene, under either photoionization or electron ionization conditions, although Tr+ is approximately 11 kcal/mol more stable than Bz+.