Reaction of cyclopentadienyl and methyl radicals

Geometries and potential energies of reagents, products, and intermediate states for the reaction between cyclopentadienyl (C5H5) and methyl (CH3) radicals are found by means of ab initio quantum mechanical methods CCSD(T)/cc-pVTZ-f12, B2PLYPD3/AUG-CC-PVDZ and B3LYP/6-311G. Basing on the analysis of the found energy, structural and kinetic characteristics of the compounds involved, the reaction paths leading to the formation of fulvene and benzene, the simplest aromatic compound, are determined. The reaction path begins from the formation of the intermediate compound, methylcyclopentadiene, followed by tearing-off a hydrogen atom from it: C5H5 + CH3 → C5H5CH3 → C5H4CH3 + H. The subsequent monomolecular transformations of C5H4CH3 are closed by the formation of either fulvene (via the loss of one hydrogen atom from the methyl group) or benzene (via the stages of transforming the pentamerous ring into a hexamerous one and tearing-off a hydrogen atom). The rate constants found in the paper using the software package MESS show that the rate of benzene formation is always higher than that for fulvene within the temperature interval 500-2250 K. Since fulvene can also isomerize into benzene, the reaction C5H5 + CH3 is an important supplier of the initial bricks for building polycyclic aromatic hydrocarbons dangerous for living systems.