Dissociative Photoionization of the C7H8 Isomers Cycloheptatriene and Toluene: Looking at Two Sides of the Same Coin Simultaneously.

Abstract

The dissociation of energy-selected 1,3,5-cycloheptatriene (CHT) and toluene (Tol) cations was investigated by imaging photoelectron photoion coincidence spectroscopy. In the measured energy ranges of 10.30-11.75 eV for CHT and 11.45-12.55 eV for Tol, only the hydrogen atom loss channels open up, leading to C7H7+ from both molecular ions, which are both metastable at the H-loss threshold. Quantum chemical calculations showed that an interconversion of the molecular ions happens below the dissociation threshold. Therefore, a single statistical model was constructed to describe both systems simultaneously. We determined 0 K appearance energies for the tropylium (Tr+) and benzyl (Bz+) fragment ions from CHT to be 9.520 ± 0.060 and 9.738 ± 0.082 eV and that from Tol to be 10.978 ± 0.063 and 11.196 ± 0.080 eV, respectively. Using the experimentally determined benzyl ion appearance energy, its 0 K heat of formation was calculated to be 937.9 ± 7.7 kJ mol-1. Finally, on the basis of this value and the recently determined benzyl ionization energy, we point out discrepancies concerning the benzyl radical thermochemistry.