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

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.

多年积累的实验和理论证据凸显了多环芳烃作为 soot（炭黑、烟灰）颗粒分子前体的作用。然而，它们的许多物理和化学特性以及促使它们从气态物质转变为固态含碳颗粒的机制仍存在争议。五元环的形成可以用烃类上存在的三种反应位点来描述：（1）一个“自由边缘”反应位点和一种C3气相物质（C2 + C3）；（2）一个“之字形”位点和一种C2气相物质（C3 + C2）；（3）一个“扶手椅”位点和一种C1气相物质（C4 + C1）。在这项工作中，我们将注意力集中在后两类上，并使用从头算G3型电子结构计算来系统地探索导致五元环形成的可能反应途径。具体而言，我们的研究报告了从萘的“之字形”位点开始形成苊和苊烯以及从联苯或菲的“扶手椅”位点开始形成芴和4H - 环戊[def]菲形式的五元环的反应途径。在一个0 - D封闭均相间歇式反应器中研究了新反应途径的相对重要性，其中条件取自实验数据。此外，新的反应途径以及与温度相关的速率常数，为嵌入式五元环的形成提供了更完整的描述，例如在邻苯二酚热解中发现的亚甲基桥连的多环芳烃，它们不仅在芳烃的气相化学中，而且作为扶手椅位点上形成五元环以及 soot模型交联的途径都可能发挥重要作用。