Forming a Two-Ring Polycyclic Aromatic Hydrocarbon without a Benzene Intermediate: the Reaction of Propargyl with Acetylene

The reaction of acetylene (HCCH) with a resonance-stabilized free radical is a commonly invoked mechanism for the generation of polycyclic aromatic hydrocarbons (PAH), which are likely precursors of soot particles in combustion. In this work, we examine the sequential addition of acetylene to the propargyl radical (H$_{\mathrm{2}}$CCCH) at temperatures of 800 and 1000 K. Using time-resolved multiplexed photoionization mass spectrometry with tunable ionizing radiation, we identified the isomeric forms of the C$_{\mathrm{5}}$H$_{\mathrm{5}}$ and C$_{\mathrm{7}}$H$_{\mathrm{7}}$ intermediates in this reaction sequence, and confirmed that the final C$_{\mathrm{9}}$H$_{\mathrm{8}}$ product is the two-ring aromatic compound indene. We identified two different resonance-stabilized C$_{\mathrm{5}}$H$_{\mathrm{5}}$ intermediates, with different temperature dependencies. Furthermore, the C$_{\mathrm{7}}$H$_{\mathrm{7}}$ intermediate is the tropyl radical (c-C$_{\mathrm{7}}$H$_{\mathrm{7}})$, not the benzyl radical (C$_{\mathrm{6}}$H$_{\mathrm{5}}$CH$_{\mathrm{2}})$, as is usually assumed in combustion environments. These experimental results are in general agreement with the latest electronic structure / master equation results of da Silva et al. This work shows a pathway for PAH formation that bypasses benzene / benzyl intermediates.