PAH growth assisted by five-membered ring: pyrene formation from acenaphthylene

Abstract

To investigate the role of five-membered rings in the growth of polycyclic aromatic hydrocarbons (PAHs) in flames, the conversion of acenaphthylene to pyrene with substituted phenalene as intermediate is studied through quantum calculations using CBS-QB3 composite method. Two pathways are considered, where one initiates with H-abstraction from the five membered ring on acenaphthylene, while the other initiates with H-addition on it. Both the pathways are then followed by subsequent C2H2 addition reactions on the radical sites. With the first C2H2 addition, a substituted phenalene-type species is formed, which undergoes another C2H2 addition reaction to lead to pyrene formation. The reaction kinetics are calculated using transition state theory, and the new reactions are added to a detailed hydrocarbon mechanism to test their effect on the profiles of pyrene and other PAHs. Among the H-abstraction-C2H2-addition (HACA)-type mechanisms for pyrene formation, the proposed reactions have a minor contribution in pyrene formation as they mainly form 1-ethynyl-acenaphthylene and phenanthrene. Through mechanism analysis, the reactions primarily responsible for pyrene formation are highlighted that require further investigation.