Comprehensive Kinetics on the C₇H₇ Potential Energy Surface under Combustion Conditions

Abstract

The automated kinetics workflow code, KinBot, was used to explore and characterize the regions of the C 7 H 7 potential energy surface that are relevant to combustion environments and especially soot inception. We first explored the lowest-energy region, which includes the benzyl, fulvenallene + H, and cyclopentadienyl + acetylene entry points. We then expanded the model to include two higher-energy entry points, vinylpropargyl + acetylene and vinylacetylene + propargyl. The automated search was able to uncover the pathways from the literature. In addition, three important new routes were discovered: a lower-energy pathway connecting benzyl with vinylcyclopentadienyl, a decomposition mechanism from benzyl that results in side-chain hydrogen atom loss to produce fulvenallene + H, and shorter and lower energy routes to the dimethylene-cyclopentenyl intermediates. We systematically reduced the extended model to a chemically relevant domain composed of 63 wells, 10 bimolecular products, 87 barriers, and 1 barrierless channel and constructed a master equation using the CCSD(T)-F12a/cc-pVTZ//ωB97X-D/6-311++G(d,p) level of theory to provide rate coefficients for chemical modeling. Our calculated rate coefficients show excellent agreement with measured ones. We also simulated concentration profiles and calculated branching fractions from the important entry points to provide an interpretation of this important chemical landscape.

The Identity and Chemistry of C7H7 Radicals Observed during Soot Formation

“…They suggested a path to formation of vinylcyclopentadienyl via the reaction of cyclopentadienyl with acetylene or vinyl radical. The theoretical results of Marti ́et al 54 and Mao et al 56 indicate that, at temperatures above 1000 K, vinylcyclopentadienyl can be formed from the reaction of cyclopentadienyl with acetylene. At temperatures of 1500 K and above, the main products are fulvenallene + H via vinylcyclopentadienyl as an intermediate.…”

“…Minor channels include vinylacetylene + propargyl, benzyl, tropyl, and 1-or 2-ethynylcyclopentadiene + H. 54,58 All of these channels proceed through vinylcyclopentadienyl, 54,58 but 1-and 2-ethynylcyclopentadiene are only produced through the decomposition of benzyl. 54,74 Vinylcyclopentadienyl is also predicted to be produced from fulvenallene + H at temperatures of 1500 K and lower. 54 Jin et al 55 experimentally demonstrated a route to vinylcyclopentadienyl via the reaction of vinylacetylene with propargyl, which produces benzyl as a minor product.…”

“…Reilly et al identified 3-ethynylcyclopentenyl in toluene and 1,6-heptadiyne discharges. The theoretical results of Martí et al indicate that this isomer is unlikely to play a significant role in combustion.…”

“…Because of its high stability, benzyl is often presumed to be the dominant C 7 H 7 isomer in combustion and pyrolysis systems; however, some experimental investigations have detected other C 7 H 7 isomers. ,, We seek to understand the relative abundances of different C 7 H 7 isomers produced and the dependence of C 7 H 7 isomer distributions on the reactant and environment in which particles are formed. Using chemical kinetic models with an updated set of species and rate coefficients, produced by Martí et al for the C 7 H 7 reaction network, we investigate the possibility of previously overlooked C 7 H 7 isomers and assess model predictions of C 7 H 7 isomer distributions, relative to experimental estimates.…”

“…32,47,48 C 7 H 7 species have also been investigated as intermediates in the formation of larger PAHs in interstellar environments. 52,53 Marti ́et al 54 recently performed an extensive theoretical study of the isomers of C 7 H 7 and calculated rate coefficients for C 7 H 7 production, loss, and isomerization reactions. Figure 1 provides a summary of what appear to be the most important reactions, as suggested by the large breadth of experimental and theoretical work published previously.…”