Kinetics of the CH3 + C5H5 Reaction: A Theoretical Study.

Formation of fulvene and benzene through the reaction of cyclopentadienyl (C5H5) with methyl radical (CH3) and consequent dissociation of its primary C6H7 products has been studied using ab initio and theoretical kinetics calculations. The potential energies and geometries of all involved species have been computed at the CCSD(T)-F12/cc-pVTZ-f12//B2PLYPD3/aug-cc-pVDZ level theory. Multichannel/multiwell RRKM-Master Equation calculations have been utilized to produce phenomenological pressure- and temperature-dependent absolute and individual-channel rate constants for various reactions at the C6H8 and C6H7 potential energy surfaces. The kinetic scheme combining the primary and secondary reactions has been used to generate the overall rate constants for the production of fulvene and benzene and their branching ratios. Analyses of the kinetic data revealed that at low pressures (0.01 atm) benzene formation prevails, with branching ratios exceeding 60%, whereas at the highest pressure (100 atm) fulvene formation is prevalent, with the branching ratio of benzene being lower than 40%. At intermediate pressures (1 and 10 atm) the two product channels compete and fulvene formation is preferable at temperatures above 1600 K. The results demonstrate that a five-member ring can be efficiently transformed into an aromatic six-member ring by methylation and corroborate the potentially important role of the methyl radical in the mechanism of PAH growth where CH3 additions alternate with H abstractions and acetylene additions.