title: Mystery of 1-Vinylpropargyl Formation from Acetylene Addition to the Propargyl Radical: An Open-and-Shut Case.
abstract: The addition of acetylene (C2H2) to the propargyl radical (C3H3) initiates a cascade of molecular weight growth reactions that result in the production of polycyclic aromatic hydrocarbons (PAHs) in flames. Although it is well-established that the first reaction step produces the cyclic C5H5radical cyclopentadienyl (c-C5H5), recent studies have also detected significant quantities of the open-chain form, 1-vinylpropargyl (l-C5H5). This work presents a mechanism for the C3H3+ C2H2reaction from ab initio calculations, which includes pathways for the formation of both the open and shut isomers as well as for their interconversion. Formation of both isomers proceeds from the initial HCCCH2CHCH•reaction adduct with similar barriers, both well below the entrance channel energy. Subsequent isomerization of l-C5H5with c-C5H5also transpires at below the energy of the reactants, although this process connects two deep wells (being resonance stabilized radicals), and must compete with collisional energy transfer. An RRKM theory/master equation model is developed for the reported C5H5reaction mechanism. Master equation simulations suggest that both cyclic and open-chain isomers are expected to form from the C3H3+ C2H2reaction across a range of temperatures, although the lifetime of l-C5H5is relatively short for rearrangement to c-C5H5.
authors.affiliation: Department of Chemical and Biomolecular Engineering, The University of Melbourne , Melbourne, Victoria 3010, Australia.
authors.forename: Gabriel
authors.identifier: http://orcid.org/0000-0003-4284-4474
authors.initials: G
authors.lastname: da Silva