Growth of Polycyclic Aromatic Hydrocarbons by C₂H₂ Mediated by Five-membered Rings: Acenaphthylene Conversion to Phenanthrene

To investigate the role of five-membered rings in the growth of PAHs, the conversion of acenaphthylene to phenanthrene is studied using density functional theory and composite method. Two pathways are considered that initiate with H-abstraction from different sites on acenaphthylene and are followed by acetylene addition on the radical sites. The reactions in both pathways progress with the formation of two fused five-membered rings and H-migration reactions that eventually get converted to a six-membered ring to form either phenanthrene or phenanthryl radical. The reaction kinetics are calculated using RRKM theory, and the new reactions are added to a detailed hydrocarbon mechanism to test their effect on the profiles of phenanthrene and acenaphthylene. The flame simulations indicate that the phenanthrene concentration increases by 58% with the addition of the proposed reactions to the mechanism and helps in matching the experimental phenanthrene profile with the computed one. Thus, the pathways studied in this work can be considered to be important for phenanthrene formation.