Threshold Formation of Benzylium (Bz+) and Tropylium (Tr+) from Toluene. Nonstatistical Behavior in Franck-Condon Gaps

Article abstract of DOI:10.1021/j100149a032

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 ca. 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 ca. 11 kcal/mol more stable than Bz⁺.