Analysis of the Reactivity on the C7H6 Potential Energy Surface

The reactivity and decomposition kinetics on the C7H6 potential energy surface (PES) were investigated, determining structures of stationary points at the B3LYP/6-31+G(d,p) level and energies at the CCSD(T)/cc-pVTZ level with extension to the complete basis set limit. For the reactions characterized by a significant multireference character, the energies were calculated at the CASPT2/cc-pVTZ level. The portion of the PES investigated consisted of 27 wells connected by 39 saddle points. Of the 27 wells, 16 can be accessed through transition states having activation energies smaller than the dissociation threshold. In agreement with previous theoretical studies, it was found that the main interconversion channel takes place on the singlet PES and connects phenylcarbene, cycloheptatetrane, spiroheptatriene, fulvenallene, and three ethynylcyclopentadiene isomers. Two new mechanisms are proposed for the formation of 5-ethynylcyclopentadiene and for the conversion of spiroheptatriene to fulvenallene. The unimolecular decomposition kinetics was thoroughly investigated. It was found that the fastest high pressure decomposition channel, at the temperatures at which C7H6 undergoes unimolecular decomposition (1500-2000 K), leads to the formation of cyclopentadienylidene and acetylene. The rate of crossing from the singlet to the triplet PES may affect considerably this reaction channel, as it is formally spin forbidden. The alternative pathway, which is the decomposition to fulvenallenyl, is however only a factor of 2-3 slower and significantly less activated (82 vs 96 kcal/mol).