The CH₃+C₅H₅ reaction: A potential source of benene at high temperatures

The potential energy surface for the reaction of CH₃ with C₅H₅ has been investigated using the ab initio G2M (ree, MP2) and B3LYP/6-311G(d,p) methods. The most favorable channel is the elimination of the hydrogen atom from the ring carbon of the C₅H₅CH₃ intermediate, C₅H₅+CH₃→C₅H₅CH₃→C₅H₄CH₃+H. Neither step has a barrier, and the overall reaction endothermicity is 7.2 kcal/mol. The calculated strengths of the out-of-ring C-C bond in C₅H₅CH₃ and the C-H bond in C₅H₆ are 72.5 and 83.4 kcal/mol, respectively. The C-H bond in C₅H₆ is predicted to be significantly stronger than the previous experimental estimates. The elimination of H from the methyl group of C₅H₅CH₃ requires an activation energy by 22.9 kcal/mol higher than the elimination of H from the ring carbon. Subsequent hydrogen elimination in the C₅H₄CH₃ and C₅H₅CH₂ leads to the formation of fulvene. Fulvene can also be formed directly from C₅H₅CH₃ by the 1,2-H₂ elimination or by the 1,1-H₂ elimination, followed by the 1,2-H shift. However, these channels cannot compete with the splitting of the hydrogen atom because of high barriers. In addition to these unimolecular processes, C₅H₅CH₃ can be converted to fulvene by bimolecular reactions with atoms or radicals. Fulvene can isomerize to benzene under high-temperature combustion conditions, making C₅H₅+CH₃ a potential source of aromatics.