Products of the Propargyl Self-Reaction at High Temperatures Investigated by IR/UV Ion Dip Spectroscopy

The propargyl radical is considered to be of key importance in the formation of the first aromatic ring in combustion processes. Here we study the bimolecular (self-) reactions of propargyl in a high-temperature pyrolysis flow reactor. The aromatic reaction products are identified by IR/UV ion dip spectroscopy, using the free electron laser FELIX as mid-infrared source. This technique combines mass selectivity with structural sensitivity. We identified several aromatic reaction products based on their infrared spectra, among them benzene, naphthalene, phenanthrene, indene, biphenyl, and surprisingly a number of aromatic compounds with acetylenic (ethynyl) side chains. The observation of benzene confirms that propargyl is involved in the formation of the first aromatic ring. The observation of compounds with acetylenic side chains shows that, in addition to a propargyl- and phenyl-based mechanism, the HACA (hydrogen abstraction C₂H₂ addition) mechanism of polycyclic aromatic hydrocarbons formation is present, although no acetylene was used as a reactant. On the basis of the experimental results we suggest a mechanism that connects the two pathways.