Unexpected Chemistry from the Reaction of Naphthyl and Acetylene at Combustion-Like Temperatures

The hydrogen abstraction/acetylene addition HACA) mechanism has long been viewed as a key route to aromatic ring growth of polycyclic aromatic hydrocarbons PAHs) in combustion systems. However, doubt has been drawn on the ubiquity of the mechanism by recent electronic structure calculations which predict that the HACA mechanism starting from the naphthyl radical preferentially forms acenaphthylene, thereby blocking cyclization to a third sixmembered ring. Here, by probing the products formed in the reaction of 1-and 2-naphthyl radicals in excess acetylene under combustion-like conditions with the help of photoionization mass spectrometry, we provide experimental evidence that this reaction produces 1-and 2-ethynylnaphthalenes C12H8), acenaphthylene C12H8) and diethynylnaphthalenes C14H8). Importantly, neither phenanthrene nor anthracene C14H10) was found, which indicates that the HACA mechanism does not lead to cyclization of the third aromatic ring as expected but rather undergoes ethynyl substitution reactions instead.