Kinetic study of the growth of PAHs from biphenyl with the assistance of phenylacetylene

Biphenyl is a crucial precursor to polycyclic aromatic hydrocarbons (PAHs), and phenylacetylene is an abundant product in aromatic hydrocarbons combustion. By exploring the reaction kinetics of phenylacetylene with biphenyl radicals, we further explore the novel hydrogen-abstraction phenylacetylene-addition (HAPaA) mechanism which is recently proposed to account for alternative mass growth pathways of PAHs. A combination of M06–2X/6–311+G(d,p) and PWPB95-D3/def2-QZVPP calculations were performed to construct the potential energy surfaces, and the rate coefficients were determined via solution of transition state theory based master equations. We demonstrate the capability of biphenyl species to grow with the assistance of phenylacetylene by unraveling the ring growth process of biphenyl radicals, establishing the main evolution routes of key intermediates, and quantifying the competition relationship between various channels. Energetic analysis and kinetic calculations demonstrate that the initial orientations of the reacting moieties do have a remarkable impact on the detailed kinetics of the entrance channels. However, the two adducts formed from initial additions achieve a rapid equilibrium because of the largest rate constants of the interconversion reactions between them, which counteracts the orientation effect on the overall kinetics. Further reaction pathways and corresponding products are related to the aryl radical position. Specifically, the aromatics of 4-phenylphenanthrene or phenanthrene, formed through cyclization reactions followed by hydrogen or phenyl eliminations, are preferred for the “armchair” type 2-biphenyl radical. In contrast, products featuring a triple bond generated through CH β-scission reactions are favored for the “free” type 3-biphenyl and 4-biphenyl radicals. The present research can serve as a good basis for further experimental and modeling studies of PAHs and soot formation.