Propargyl (∙C3H3) and butadienyl (∙<i>i</i>-C4H5) radical–radical reactions well-skipping to vinylcyclopentadienyl radical and toluene: A theoretical and kinetic modeling study

Propargyl radical (•C3H3) and butadienyl radical (•i-C4H5) are two crucial intermediates in combustion and astrochemistry, particularly in the formation of C7H8 aromatics such as toluene. However, the precise formation mechanisms of the first-ring aromatics through C3 + C4 reactions have remained ambiguous. This study explores the detailed potential energy surface (PES) of C7H8 at the •C3H3 + •i-C4H5 entrance reaction channel, alongside conducting kinetic calculations and modeling. The PES reveals distinct mechanistic pathways that depend on the resonance configurations of •C3H3 (propyne-3-yl and allenyl-1-yl). Key C7H8 isomers, including 5-ethylidenecyclopenta-1,3-diene, cycloheptatriene, and norcaradiene, are preferentially formed via the allenyl-1-yl configuration, underlining the significant influence of π electron delocalization of propargyl. Kinetic analysis using the phase space theory and the RRKM/ME method identifies well-skipping reactions, leading to larger resonance-stabilized •C7H7 radical and hydrogen atom through the less dominant allenyl-1-yl configuration reacting with •i-C4H5. Rate constants for •C3H3 + •i-C4H5 reaction yielding toluene and vinylcyclopentadienyl (vinylCPDyl) + H are determined. Subsequent kinetic modeling indicates that the formation pathway •C3H3 + •i-C4H5 → toluene predominates at low temperatures and pressure, contrasting with other toluene formations via benzyl + H and phenyl + CH3 reactions. •C3H3 + •i-C4H5 reaction is also notably significant for generating vinylCPDyl at temperatures exceeding 1050 K at 760 Torr. Although polycyclic aromatic hydrocarbons (PAHs) typically form in high-temperature scenarios, this research suggests viable low-temperature pathways for toluene, which are important in cooling zones of engines, thereby influencing PAH and soot production via resonance stabilized radical chain reactions.