Rapid soot inception via α-alkynyl substitution of polycyclic aromatic hydrocarbons

Soot particles alter global climate and dominate the origin and evolution of carbonaceous interstellar material. Convincing experimental evidence has linked polycyclic aromatic hydrocarbons (PAH) to soot inception under low-temperature astrochemistry and high-temperature combustion conditions. However, significant gaps still remain in the knowledge of PAH and soot formation mechanisms. Here, we report theoretical and experimental evidence for a soot inception and growth pathway driven by peri-condensed aromatic hydrocarbons (PCAH) with an alkynyl substitution. Initially, free radicals attack the α-alkynyl substitution of PCAHs to form covalently bound compounds yielding resonantly stabilized radicals (RSRs), which promote further clustering through repeated addition reactions with negligible energy barriers. The proposed pathway is shown to be competitive at temperatures relevant to astrochemistry, engine exhaust manifold and flames because it does not require H-abstraction reactions, the requisite reaction precursors are in abundance, and the reaction rate is high. Such addition reactions of PCAHs with α-alkyne substituents create covalently bound clusters from moderate-size PAHs that may otherwise be too small to coagulate.