Rate constants of the fulvenallenyl recombination with propargyl and its role in PAH formation: a theoretical and kinetic modeling study†

The temperature- and pressure-dependent rate constants of the reaction of fulvenallenyl (C₇H₅) and propargyl (C₃H₃) radicals have been explored using advanced electronic structure methods and kinetics theories. The results show that the head-C₃H₃ + tail-C₇H₅ addition is the fastest, followed by the tail + tail addition, and that at typical combustion conditions of 1500 K and 1 atm, the reaction mostly results in collisional stabilization of entrance channel adducts without further cyclization, whereas the well-skipping pathway to fulvalene – a precursor to naphthalene, prevails at high temperatures. The formation of the aromatic two-ring isomers, naphthalene, methylene-indenes, and azulene, can be enhanced only at high temperatures and much lower pressures, when the collisional stabilization of the intermediate wells is not efficient. The computed phenomenological rate constants were consequently simplified using master equation-based lumping and the pseudo steady state approximation to reduce the size of the system. This post-processing confirmed fulvalene as the main two-ring aromatic product and thus, its contribution should be considered in kinetic models. While naphthalene is only a trace product of the C₇H₅ + C₃H₃ reaction, it can be formed from fulvalene via H-assisted isomerization. The impact of the updated thermochemistry and rate constants in CRECK and ITV kinetic models on mole fractions of relevant cyclic aromatic species was also analyzed. The results suggest that the relevance of the C₇H₅ + C₃H₃ reaction has been likely overestimated in the current literature and needs to be carefully reevaluated by revising the pathways involving the formation and consumption of C₇H₆ and C₇H₅ and improving predictions of their precursors, such as unsaturated C₄ species in flames.