title: Reaction dynamics in astrochemistry: low-temperature pathways to polycyclic aromatic hydrocarbons in the interstellar medium.
abstract: Bimolecular reactions of phenyl-type radicals with the C4 and C5 hydrocarbons vinylacetylene and (methyl-substituted) 1,3-butadiene have been found to synthesize polycyclic aromatic hydrocarbons (PAHs) with naphthalene and 1,4-dihydronaphthalene cores in exoergic and entrance barrierless reactions under single-collision conditions. The reaction mechanism involves the initial formation of a van der Waals complex and addition of a phenyl-type radical to the C1 position of a vinyl-type group through a submerged barrier. Investigations suggest that in the hydrocarbon reactant, the vinyl-type group must be in conjugation with a -C≡CH or -HC=CH2 group to form a resonantly stabilized free radical intermediate, which eventually isomerizes to a cyclic intermediate followed by hydrogen loss and aromatization (PAH formation). The vinylacetylene-mediated formation of PAHs might be expanded to more complex PAHs, such as anthracene and phenanthrene, in cold molecular clouds via barrierless reactions involving phenyl-type radicals, such as naphthyl, which cannot be accounted for by the classical hydrogen abstraction-acetylene addition mechanism.
authors.affiliation: Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822; email: ralfk@hawaii.edu.
authors.forename: Ralf I
authors.initials: RI
authors.lastname: Kaiser
authors.forename: Dorian S N
authors.initials: DS
authors.lastname: Parker
authors.forename: Alexander M
authors.initials: AM
authors.lastname: Mebel
keywords: ISM; aromaticity; cold molecular clouds; neutral-neutral reactions; nonequilibrium chemistry; resonantly stabilized free radical