Identification of the Elusive Methyl-Loss Channel in the Crossed Molecular Beam Study of Gas-Phase Reaction of Dicarbon Molecules (C₂; X¹Σ_g⁺/a³Π<

The crossed molecular beams 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). Here, 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 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 (PAHs) and soot.. Authors: Medvedkov, Iakov A. [University of Hawai‘i at Manoa, Honolulu, HI (United States)] (ORCID:0000000306722090); Yang, Zhenghai [University of Hawai‘i at Manoa, Honolulu, HI (United States)]; Goettl, Shane J. [University of Hawai‘i at Manoa, Honolulu, HI (United States)] (ORCID:0000000317965725); Kaiser, Ralf I. [University of Hawai‘i at Manoa, Honolulu, HI (United States)] (ORCID:0000000272337206). DOE Contract: FG02-03ER15411. Subjects: 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Alkyls; Carbon; Chemical reactions; Energy; Scattering