Unimolecular reactions of the resonance-stabilized cyclopentadienyl radicals and their role in the polycyclic aromatic hydrocarbon formation

Resonance-stabilized cyclopentadienyl radicals are important intermediate species in the combustion of transportation fuels as it serves as precursors for PAH formation and also involves in the formation of fundamental PAH precursors, e.g., propargyl and acetylene. The unimolecular reactions of the cyclopentadienyl radicals were theoretically studied based on high level quantum chemistry and RRKM/master equation calculations. Stationary points on the potential energy surface (PES) were calculated at the CCSD(T)/CBS//M06–2X/6–311 + +(d,p) level of theory. The branching ratios of unimolecular reactions of the cyclopentadienyl radicals were analyzed for a broad temperature range from 500 K to 2500 K and pressures from 0.01 to 100 atm. The isomerization reaction of the cyclopentadienyl radical via 1,2-hydrogen transfer dominated at low temperatures and high pressures, while the well-skipping decomposition reaction which forms propargyl and acetylene is important at high temperatures and low pressures. Both the decomposition reaction of the cyclopentadienyl radicals and its reverse reaction showed pronounced pressure dependence, and their reaction rate constants were compared against available low-pressure experimental measurements and theoretical studies. The temperature- and pressure-dependent rate coefficients for important reactions involved on the C₅H₅ PES were calculated and updated in a chemical kinetic model.