The role of resonance-stabilized radical chain reactions in polycyclic aromatic hydrocarbon growth: Theoretical calculation and kinetic modeling

Resonance-stabilized radical chain (RSRC) reactions were recently proposed as alternative formation and growth pathways of polycyclic aromatic hydrocarbons (PAHs). In the present study, the growth of indene from the resonance-stabilized cyclopentadienyl radical following a series of RSRC sequences is investigated. Temperature- and pressure-dependent rate coefficients for acetylene addition reactions to the cyclopentadienyl, vinylcyclopentadienyl, cycloheptatrienyl, and benzyl radicals are determined based on high-level quantum chemistry and RRKM/master equation calculations. The acetylene addition to the resonance-stabilized cyclopentadienyl radical can generate resonance-stabilized vinylcyclopentadienyl, cycloheptatrienyl, and benzyl radicals. The formation reaction of cycloheptatrienyl has the highest rate constant compared to those of vinylcyclopentadienyl and benzyl. The acetylene addition to the above three resonance-stabilized C7H7 isomers forms indene. It is found that the indene formation reaction from C2H2 addition to benzyl has the highest rate constant, while that from C2H2 addition to cycloheptatrienyl is the lowest. The impacts of the investigated reactions on indene formation are then numerically explored in both counterflow diffusion and laminar premixed flame configurations. Results indicate that the RSRC pathways via C2H2 addition to vinylcyclopentadienyl and cycloheptatrienyl radicals have marginal contributions to indene formation.