Don't forget the trans : double bond isomerism radical-acetylene growth reactions affect the primary stages of PAH and soot formation

• Published in: Physical chemistry chemical physics : PCCP 2024-12, Vol.27 (1), p.83-95
• Main Authors: Kelly, Patricia D, Turner, Jack A, Shiels, Oisin J, da Silva, Gabriel, Blanksby, Stephen J, Poad, Berwyck L J, Trevitt, Adam J
• Format: Article
• Language: English
• Subjects: Abundance; Acetylene; Chemical bonds; Combustion; Isomers; Naphthalene; Polycyclic aromatic hydrocarbons; Pressure dependence; Reactive environments; Soot; Vinyl radical
• ISSN: 1463-9076; 1463-9084; 1463-9084
• DOI: 10.1039/d4cp03554b

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 and isomers. This isomerism can lead to profound changes in product distributions that are as yet insufficiently investigated. Herein, we explore acetylene addition to substituted -vinyl radicals, including potential rearrangement to structures, for eight combustion-related hydrocarbon radicals calculated using a composite method (G3X-K). Of these eight systems, the phenyl - and -Bittner-Howard HACA (hydrogen abstraction, C H Addition) process, where acetylene successively adds to a phenyl radical a β-styryl intermediate, is simulated using a unified Master Equation model. Including the -Bittner-Howard pathway changes the products significantly at all simulated temperatures (550-1800 K) and pressures (5.33 × 10 -10 Pa), relative to a -only model. Typically, naphthalene remains the dominant product, but its abundance decreases at higher temperatures and pressures. For example, at 1200 K and 10 Pa, its branching ratio decreases from 78.5% to 62.9% when the pathway is included. At higher temperatures this decrease corresponds to the formation of alternative C H isomers, including the product benzofulvene with 8% maximum abundance at 1200 K and 5.33 × 10 Pa, and -2-ethynyl-1-phenylethylene, a product with 26% maximum abundance at 1800 K, with little pressure dependence. At higher pressures, our model predicts a range of C H radicals, including resonance-stabilised radicals (RSRs). The impact of -vinyl radical chemistry in reactive environments means that they are essential to accurately describe combustion reactions and inhibit soot formation.