Article pubs.acs.org/JPCA
Kinetic Study of the OH Radical Reaction with Phenylacetylene Ranjith Kumar Abhinavam Kailasanathan, Juddha Thapa, and Fabien Goulay* Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States ABSTRACT: The reaction of the OH radical with phenylacetylene is studied over the 298−423 K temperature range and 1−7.5 Torr pressure range in a quasi-static reaction cell. The OH radical is generated by 266 nm photolysis of hydrogen peroxide (H2O2) or 355 nm photolysis of nitrous acid (HONO), and its concentration monitored using laserinduced fluorescence. The measured reaction rates are found to strongly depend on laser fluence at 266 nm. The 266 nm absorption cross-section of phenylacetylene is measured to be 1.29 (±0.71) × 10−17 cm2, prohibiting any accurate kinetic measurements at this wavelength. The rates are independent of laser fluence at 355 nm with an average value of 8.75 (±0.73) × 10−11 cm3 s−1. The reaction exhibits no pressure or temperature dependence over the studied experimental conditions and is much faster than the estimated values presently used in combustion models. These results are consistent with the formation of a short lifetime intermediate that stabilizes by collisional quenching with the buffer gas. The structures of the most likely formed products are discussed based on both the computed energies for the OH-addition intermediates and previous theoretical investigations on similar chemical systems.
1. INTRODUCTION Aromatic molecules play a major role during the formation of large molecules and particles in high-energy and carbon-rich environments such as planetary atmosphere,1,2 interstellar environments,3,4 and combustion environments.5−7 In flames, they are believed to be the gas phase molecular precursors of soot particles through the formation of large polycyclic aromatic hydrocarbons (PAHs).8−10 Computational models used to predict particle formation based on such mechanisms6,11−14 are still limited by our understanding of the underlying chemistry and the lack of kinetic and thermodynamic data at relevant pressures and temperatures. The formation of benzene and its successive reactions with combustion relevant radicals are central to the PAH growth mechanism.15,16 The most widely accepted benzene formation mechanism is the propargyl (C3H3) recombination to form C6H6 intermediates that stabilize by collisional quenching with the surrounding gas.15,16 Other reaction pathways such as C2H + C4H6 have also been shown to be possible sources of benzene, especially at low temperatures and pressures.7,17 According to the hydrogen-abstraction acetylene-addition (HACA) mechanism, benzene may further react with atomic hydrogen to give the phenyl radical (C6H5) and molecular hydrogen.5,14,18 The reaction with acetylene followed by H-loss leads to the formation of phenylacetylene (C6H5CCH). Alternatively, benzene may react with the ethynyl radical (C2H) to form phenylacetylene through an addition/H-loss mechanism.19,20 Consecutive reactions of phenylacetylene with atomic hydrogen and acetylene may lead to the formation of the first PAH, naphthalene (C10H8).14,21 The accuracy of the models used to predict the formation of PAHs in combustion © XXXX American Chemical Society
environments is still limited by the lack of kinetic and mechanistic data of reactions involving aromatic intermediates such as phenylacetylene under relevant conditions of pressures and temperatures. The OH radical is one of the most abundant radical in combustion flames. Its reaction with benzene and substituted benzene molecules has been extensively studied both experimentally22−34 and theoretically.35,36 At room30,33 (295 K) and intermediate temperatures32 (
