Elucidating the recombination kinetics of 2-naphthyl radicals with phenylacetylene

Abstract

Polycyclic aromatic hydrocarbons (PAHs) serve as critical precursors in soot formation, and understanding their formation mechanisms is fundamental for controlling pollutant emissions during the combustion of carbonaceous fuels, such as petrochemical derivatives. This study combines experimental and theoretical techniques to explore the radical-molecule reactions of 2-naphthyl radicals with phenylacetylene (also known as ethynylbenzene), which is potentially a ring-growth pathway for 2-naphthyl. The potential-energy profile was investigated at B2PLYP-D3/def2-TZVPP//M06-2X/6-311+G(d,p) level of theory. Temperature- and pressure-dependent (500–2,500 K and 3–7,600 Torr) rate constants were obtained from RRKM/master-equation simulations. Although the 2-naphthyl radical can add to multiple sites on phenylacetylene, the kinetics is overwhelmingly governed by the addition of the terminal acetylene carbon (β-carbon). At 760 Torr, the stabilization of initial activated adduct into 2-styrylnaphthalene radical prevails below 1,300 K, while the formally-direct channel producing 2-phenylethynyl-naphthalene and H becomes favorable above 1,300 K. For subsequent reaction of 2-styrylnaphthalene radical (i.e., the initial adduct), the sequential isomerization reactions via intramolecular hydrogen transfer dominate at low temperature, whereas the C-H β-scission to form 2-phenylethynyl-naphthalene and H becomes prominent at high temperature. The synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry experiment indicates a mass peak m/z 228 from the reaction between 2-naphthyl and phenylacetylene with a photon energy of 9.5 eV. Based on experimental measurements and kinetic analysis, the m/z 228 peak is deduced to be 2-phenylethynyl-naphthalene. Kinetic analysis of three aryl radicals (i.e., phenyl, 1-naphthyl, and 2-naphthyl) with phenylacetylene reveals that 1-naphthyl, which features a 'zigzag' edge site, exhibits higher reactivity compared to phenyl and 2-naphthyl radicals, both of which possess 'free' edge sites (the phenyl/phenylacetylene and 1-naphthyl/phenylacetylene reactions were studied in Combustion and Flame, 2022, 243, 112,014 and Journal of the American Chemical Society, 2021, 143, 20,710, respectively). This trend holds across a wide temperature range, except at the very high-temperature end. These findings significantly deepen our understanding of the formation and growth mechanisms of PAHs in combustion systems, providing critical insights into the chemical behavior of aryl radicals and their contributions to the formation and evolution of PAHs.