Don’t forget the <i>trans</i>: double bond isomerism radical-acetylene growth reactions affect the primary stages of PAH and soot formation

In combustion, acetylene is a key species in molecular-weight growth reactions that form polycyclic aromatic hydrocarbons (PAHs) and ultimately soot particles. Radical addition to acetylene generates a vinyl radical intermediate, which has both <i>trans</i> and <i>cis</i> isomers. This isomerism can lead to profound changes in product distributions that are as yet insufficiently investigated. Herein, we explore acetylene addition to substituted <i>trans</i>-vinyl radicals, including potential rearrangement to <i>cis</i> structures, for eight combustion-related hydrocarbon radicals calculated using a composite method (G3X-K). Of these eight systems, the phenyl <i>trans</i>- and <i>cis</i>-Bittner-Howard HACA (hydrogen abstraction, C₂H₂ Addition) process, where acetylene successively adds to a phenyl radical <i>via</i> a β-styryl intermediate, is simulated using a unified Master Equation model. Including the <i>trans</i>-Bittner-Howard pathway changes the products significantly at all simulated temperatures (550-1800 K) and pressures (5.33 × 10²-10⁷ Pa), relative to a <i>cis</i>-only model. Typically, naphthalene remains the dominant product, but its abundance decreases at higher temperatures and pressures. For example, at 1200 K and 10⁵ Pa, its branching ratio decreases from 78.5% to 62.9% when the <i>trans</i> pathway is included. At higher temperatures this decrease corresponds to the formation of alternative C₁₀H₈ isomers, including the <i>cis</i> product benzofulvene with 8% maximum abundance at 1200 K and 5.33 × 10² Pa, and <i>E</i>-2-ethynyl-1-phenylethylene, a <i>trans</i> product with 26% maximum abundance at 1800 K, with little pressure dependence. At higher pressures, our model predicts a range of C₁₀H₉ radicals, including resonance-stabilised radicals (RSRs). The impact of <i>trans</i>-vinyl radical chemistry in reactive environments means that they are essential to accurately describe combustion reactions and inhibit soot formation.