The role of radical-radical chain-propagating pathways in the phenyl + propargyl reaction

Well-skipping radical-radical reactions can provide a chain-propagating pathway for formation of polycyclic radicals implicated in soot inception. Here we use controlled pyrolysis in a microreactor to isolate and examine the role of well-skipping channels in the phenyl (C6H5) + propargyl (C3H3) radical-radical reaction at temperatures of 800–1600 K and pressures near 25 Torr. The temperature and concentration dependence of the closed-shell (C9H8) and radical (C9H7) products are observed using electron-ionization mass spectrometry. The flow in the reactor is simulated using a boundary layer model employing a chemical mechanism based on recent rate coefficient calculations. Comparison between simulation and experiment shows reasonable agreement, within a factor of 3, while suggesting possible improvements to the model. In contrast, eliminating the well-skipping reactions from the chemistry mechanism causes a much larger discrepancy between simulation and experiment in the temperature dependence of the radical concentration, revealing that the well-skipping pathways, especially to form indenyl radical, are significant at temperatures of 1200 K and higher. While most C9H7 forms by well-skipping at 25 Torr, an additional simulation indicates that the well-skipping channels only contribute around 3% of the C9Hx yield at atmospheric pressure, thus indicating a negligible role of the well-skipping pathways at atmospheric and higher pressures.