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. It not only serves as precursors for polycyclic aromatic hydrocarbon (PAH) formation, but also involves in the formation of fundamental PAH precursors such as propargyl and acetylene. In this work, the unimolecular reactions of the cyclopentadienyl radicals are theoretically studied based on high-level quantum chemistry and RRKM/master equation calculations. Stationary points on the potential energy surface (PES) are 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 are analyzed for a broad temperature range from 500 to 2500 K and pressures from 0.01 to 100 atm. It is found that the isomerization reaction of the cyclopentadienyl radical via 1,2-hydrogen transfer dominates 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 show pronounced pressure dependence, and their reaction rate constants are 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 are calculated and updated in a chemical kinetic model. Impacts of the unimolecular reactions of the cyclopentadienyl radicals on the PAH formation are explored by the numerical modeling of a low-pressure cyclopentene counterflow diffusion flame.