Resolving C6H6 Isomers from the Propargyl Self-Reaction at Combustion-Relevant Temperatures by PEPICO Spectroscopy

The recombination of the resonance-stabilized propargyl (C3H3) radical is a key step in the formation of the first aromatic ring in combustion environments. In addition to benzene, several other C6H6 isomers can also form, as Miller and Klippenstein [J. Phys. Chem. A 2003, 107 (39), 7783-7799] theoretically demonstrated by mapping the complex potential energy surface of the C3H3 + C3H3 system. In this work, we investigated the propargyl self-reaction experimentally by flash pyrolysis in a resistively heated silicon carbide microreactor at combustion-relevant temperatures of 1000 and 1190 K and at pressures near 10-20 mbar. Using photoion mass-selected threshold photoelectron spectra (ms-TPES) obtained with photoelectron photoion coincidence (PEPICO) techniques, we identified six C6H6 species: 2-ethynyl-1,3-butadiene, benzene, fulvene, cis- and trans-1,3-hexadien-5-yne, and 3,4-dimethylenecyclobutene. We further determined their branching ratios by least-squares fitting a linear combination of individual reference spectra to the experimental mass-selected photoelectron spectrum of m/z 78. In agreement with theoretical predictions, we found 2-ethynyl-1,3-butadiene to be the most abundant C6H6 isomer, followed by benzene and fulvene.