Time- and Isomer-Resolved Measurements of Sequential Addition of Acetylene to the Propargyl Radical

Soot formation in combustion is a complex process in which polycyclic aromatic hydrocarbons (PAHs) are believed to play a critical role. Recent works concluded that three consecutive additions of acetylene (C₂H₂) to propargyl (C₃H₃) create a facile route to the PAH indene (C₉H₈). However, the isomeric forms of C₅H₅ and C₇H₇ intermediates in this reaction sequence are not known. We directly investigate these intermediates using time- and isomer-resolved experiments. Both the resonance stabilized vinylpropargyl (vp-C₅H₅) and 2,4-cyclopentadienyl (c-C₅H₅) radical isomers of C₅H₅ are produced, with substantially different intensities at 800 K vs 1000 K. In agreement with literature master equation calculations, we find that c-C₅H₅ + C₂H₂ produces only the tropyl isomer of C₇H₇ (tp-C₇H₇) below 1000 K, and that tp-C₇H₇ + C₂H₂ terminates the reaction sequence yielding C₉H₈ (indene) + H. This work demonstrates a pathway for PAH formation that does not proceed through benzene.