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').

The crossed molecular beam technique was utilized to explore the reaction of dicarbon C2 (X1Σg+/a3Πu) 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.