Lawrence Berkeley National Laboratory

The formation of the simplest polycyclic aromatic hydrocarbon (PAH), naphthalene (C₁₀H₈), was explored in a high-temperature chemical reactor under combustion-like conditions in the phenyl (C₆H₅)-vinylacetylene (C₄H₄) system. The products were probed utilizing tunable vacuum ultraviolet light by scanning the photoionization efficiency (PIE) curve at a mass-to-charge m/ z = 128 (C₁₀H₈⁺) of molecules entrained in a molecular beam. The data fitting with PIE reference curves of naphthalene, 4-phenylvinylacetylene (C₆H₅CCC₂H₃), and trans-1-phenylvinylacetylene (C₆H₅CHCHCCH) indicates that the isomers were generated with branching ratios of 43.5±9.0 : 6.5±1.0 : 50.0±10.0%. Kinetics simulations agree nicely with the experimental findings with naphthalene synthesized via the hydrogen abstraction-vinylacetylene addition (HAVA) pathway and through hydrogen-assisted isomerization of phenylvinylacetylenes. The HAVA route to naphthalene at elevated temperatures represents an alternative pathway to the hydrogen abstraction-acetylene addition (HACA) forming naphthalene in flames and circumstellar envelopes, whereas in cold molecular clouds, HAVA synthesizes naphthalene via a barrierless bimolecular route.