Reaction pathways for the formation of five-membered rings onto polyaromatic hydrocarbon framework

Abstract

Experimental and theoretical evidences accumulated over the years have highlighted the role of polycyclic aromatic hydrocarbons as molecular precursors to soot particles. However, many of their physical and chemical characteristics are still under debate, as well as the mechanisms that drive their transition from gaseous species to solid carbonaceous particles. The formation of five-membered rings can be described with three types of reactive sites present on hydrocarbons: (1) a “free-edge” reactive site and a C3 gas phase species (C2 + C3), (2) a “zig-zag” site and a C2 gas phase species (C3 + C2), and (3) an “armchair” site and a C1 gas phase species (C4 + C1). In this work, we focus our attention on the last two categories and use ab initio G3-type electronic structure calculations to explore systematically possible reaction pathways leading to the formation of five-membered rings. Specifically, our study reports on reaction pathways leading to the formation of acenaphthene and acenaphthylene starting from the zig-zag type site on naphthalene and to the formation of five-membered ring in the form of fluorene and 4H-cyclopenta[def]phenanthrene starting from the “armchair” site on biphenyl or phenanthrene. The relative importance of the new reaction pathways has been investigated in a 0-D Closed Homogeneous Batch Reactor, where the conditions are taken from experimental data. Furthermore, the new reaction pathways, together with temperature dependent rate constants, provide a more complete description of the formation of embedded five-membered rings, such as methylene-bridged PAHs identified in catechol pyrolysis, that can play an important role not only in the gas-phase chemistry of aromatics but also as pathways to five-membered rings on armchair sites and cross-linking for soot models.