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Crossed molecular beam experiments and electronic structure calculations on the reaction of the meta-tolyl radical with vinylacetylene were conducted to probe the formation of methyl-substituted naphthalene isomers. We present the compelling evidence that under single collision conditions 1- and 2-methylnaphthalene can be formed without an entrance barrier via indirect scattering dynamics through a bimolecular collision of two non-PAH reactants: the meta-tolyl radical and vinylacetylene. The electronic structure calculations, conducted at the UCCSD(T)-F12b/cc-pVDZ//UM06-2x/cc-pVTZ + ZPE(UM06-2x/cc-pVTZ) level of theory, reveal that this reaction is initiated by the barrierless addition of the meta-tolyl radical to the terminal vinyl carbon (C1) of vinylacetylene, viamore » a van-der-Waals complex implying that this mechanism can play a key role in forming methyl-substituted PAHs in low temperature extreme environments such as the low temperature interstellar medium and hydrocarbon-rich atmospheres of planets and their moons in the outer solar system. The reaction mechanism, proposed from the C₁₁H₁₁ potential energy surface, involves a sequence of isomerizations involving hydrogen transfer and ring closure, followed by hydrogen dissociation, which eventually leads to 1- and 2-methylnaphthalene in an overall exoergic process.« less

We introduce for the very first time single collision experimental evidence that a methyl-substituted polycyclic aromatic hydrocarbon (PAH) – 2-methylnaphthalene – can be formed without an entrance barrier via indirect scattering dynamics through a bimolecular collision of two non-PAH reactants: the para-tolyl radical and vinylacetylene. Theory reflects that this reaction is initiated by the addition of the para-tolyl radical to either the terminal acetylene carbon (C⁴) or a vinyl carbon (C¹) leading eventually to two distinct radical intermediates. Importantly, addition at C¹ was found to be barrierless via a van-der-Waals complex implying this mechanism can play a key role inmore » forming methyl substituted PAHs in low temperature extreme environments such as the interstellar medium and hydrocarbon-rich atmospheres of planets and their moons in the outer Solar System. Both reaction pathways involve a sequence of isomerizations via hydrogen transfer, ring closure, and ring opening and final hydrogen dissociation through tight exit transition states to form 2-methylnaphthalene in overall exoergic processes. Less favorable pathways leading to monocyclic products are also found. Our report predict that reactions of substituted aromatic radicals can mechanistically deliver odd-numbered PAHs that are formed in significant quantities in the combustion of fossil fuels.« less