Identification of the Elusive Methyl-Loss Channel in the Crossed Molecular Beam Study of Gas-Phase Reaction of Dicarbon Molecules (C2; X1Σg+/a3Πu) with 2-Methyl-1,3-butadiene (C5H8; X1A').

Abstract

The crossed molecular beam technique was utilized to explore the reaction of dicarbon C₂ (X¹Σ_g⁺/a³Π_u) with 2-methyl-1,3-butadiene (isoprene, CH₂C(CH₃)CHCH₂; X¹A') at a collision energy of 28 ± 1 kJ mol^-1 using a supersonic dicarbon beam generated via photolysis (248 nm) of helium-seeded tetrachloroethylene (C₂Cl₄). Experimental data combined with previous ab initio calculations provide evidence of the detection of the hitherto elusive methyl elimination channels leading to acyclic resonantly stabilized hexatetraenyl radicals: 1,2,4,5-hexatetraen-3-yl (CH₂CC^•CHCCH₂) and/or 1,3,4,5-hexatetraen-3-yl (CH₂CHC^•CCCH₂). These pathways are exclusive to the singlet potential energy surface, with the reaction initiated by the barrierless addition of a dicarbon to one of the carbon-carbon double bonds in the diene. In combustion systems, both hexatetraenyl radicals can isomerize to the phenyl radical (C₆H₅) through a hydrogen atom-assisted isomerization─the crucial reaction intermediate and molecular mass growth species step toward the formation of polycyclic aromatic hydrocarbons and soot.