Theoretical Study on the Self‐ and Cross‐Recombination of n ‐ C 4 H 3 and i ‐ C 4 H 3 Radicals to Form Initial Aromatic Rings as Soot Precursors

ABSTRACT The formation of initial aromatic rings as soot precursors is a core scientific issue in combustion chemistry. In this work, quantum chemical calculations and transition state theory (TST) with Eckart tunneling corrections were adopted to systematically investigate the isomerization and recombination reactions of n ‐C 4 H 3 and i ‐C 4 H 3 radicals, revealing the microscopic mechanisms and forward/reverse kinetic behaviors governing the formation of initial aromatic species. Potential energy surfaces (PES) were established at the M06‐2X/6–311++G(d,p) and ωB97X‐D/def2‐QZVPP levels, and rate constants of all forward ring‐closure and reverse ring‐opening elementary reactions were calculated from 300 to 4000 K. The results show that i ‐C 4 H 3 is thermodynamically favored in the n ‐C 4 H 3 / i ‐C 4 H 3 isomerization equilibrium. For nearly all cyclization pathways across the three recombination systems, reverse ring‐opening rate constants are larger than forward ring‐closure values over most temperatures; only the four‐membered ring pathway TS16 has a critical transition at 800 K, where forward cyclization prevails below this temperature. For the n ‐C 4 H 3 + n ‐C 4 H 3 self‐recombination system, the direct cyclization of open‐chain intermediate IM2 to form five‐membered ring species possesses an ultralow barrier of 10.21 kcal/mol, which is the kinetically dominant channel producing phenylacetylene and cyclopentadienyl derivatives. In the i ‐C 4 H 3 + i ‐C 4 H 3 system, ring‐closure rate constants follow the order k 4‐membered > k 5‐membered > k 6‐membered , yet the secondary cyclization of four‐membered intermediate IM17 into fused rings requires a high barrier of 64.85 kcal/mol, yielding styrenyl radicals and fused four/six‐membered ring intermediates. Two barrierless addition channels exist in the n ‐C 4 H 3 + i ‐C 4 H 3 cross‐recombination system: for the C1‐C2 coupling pathway, the six‐membered ring route dominates below 3200 K, while entropic effects accelerate five‐membered ring formation at ultrahigh temperatures; for the C1–C4 coupling pathway, the six‐membered ring channel exceeds the seven‐membered ring pathway by over 20 orders of magnitude, generating multiple critical precursors including phenylacetylene, styrenyl radicals and seven‐membered ring species. Despite the universally favorable reverse ring dissociation, successive hydrogen migration produces thermally stable cyclic aromatics and restrains ring‐opening dissociation, enabling effective accumulation of soot precursors. Three‐parameter Arrhenius formulas were only fitted for all forward elementary reactions, providing fundamental kinetic data for modeling soot precursor formation under combustion conditions.