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-α-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 in toluene. 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-α-d₃ demonstrate complete isotopic scrambling and an energy dependent isotope effect. The H/D ratio stays constant below 11.1 eV, demonstrating that AE_{0 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 ∼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 ∼11 kcal/mol more stable than Bz⁺.