C₁₄H₁₀ polycyclic aromatic hydrocarbon formation by acetylene addition to naphthalenyl radicals observed

The formation of polycyclic aromatic hydrocarbons (PAHs) during combustion has a substantial impact on environmental pollution and public health. The hydrogen-abstraction-acetylene-addition (HACA) mechanism is expected to be a significant source of larger PAHs containing more than two rings. In this study, the reactions of 1-naphthalenyl and 2-naphthalenyl radicals with acetylene (C2H2) are investigated using VUV photoionization time-of-flight mass spectrometry at 500 to 800 K, 15 to 50 torr, and reaction times up to 10 ms. Our experimental conditions allow us to probe the Bittner-Howard and modified Frenklach HACA routes, but not routes that require multiple radicals to drive the chemistry. The kinetic measurements are compared to a temperature-dependent kinetic model constructed using quantum chemistry calculations and accounting for chemical-activation and fall-off effects. We measure significant quantities of C14H10 (likely phenanthrene and anthracene), as well as 2-ethynylnaphthalene (C12H8), from the reaction of the 2-naphthalenyl radical with C2H2; these results are consistent with the predictions of the kinetic model and the HACA mechanism, but contradict a previous experimental study that indicated no C14H10 formation in the 2-naphthalenyl + C2H2 reaction. In the 1-naphthalenyl radical + C2H2 reaction system, the primary product measured is C12H8, consistent with the predicted formation of acenaphthylene via HACA. The present work provides direct experimental evidence that single-radical HACA can be an important mechanism for the formation of PAHs larger than naphthalene, validating a common assumption in combustion models.