An experimental and theoretical study on the formation of 2-methylnaphthalene (C11H10/C11H3D7) in the reactions of the para-tolyl (C7H7) and para-tolyl-d7 (C7D7) with vinylacetylene (C4H4).

We present 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 shows that this reaction is initiated by the addition of the para-tolyl radical to either the terminal acetylene carbon (C(4)) or a vinyl carbon (C(1)) leading eventually to two distinct radical intermediates. Importantly, addition at C(1) was found to be barrierless via a van der Waals complex implying this mechanism can play a key role in 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, ring-opening and final hydrogen dissociation through tight exit transition states to form 2-methylnaphthalene in an overall exoergic process. Less favorable pathways leading to monocyclic products are also found. Our studies predict that reactions of substituted aromatic radicals can mechanistically deliver odd-numbered PAHs which are formed in significant quantities in the combustion of fossil fuels.