Collaborative Research: Polycyclic Aromatic Hydrocarbon Growth Mechanisms in Combustion involving Cyclopentadiene and Indene

本文对含有环戊二烯（CPD）和茚的燃烧体系中多环芳烃（PAH）生长过程进行了基本的分子分析。这些化合物存在于燃烧废水中，因为它们在含有五元环的潜在致突变多环芳烃的形成以及富勒烯和烟灰的形成中扮演中间体的角色而引起人们的兴趣。环戊二烯和茚在燃烧过程中对多环芳烃的生长很重要，因为它们在多个位点形成共振稳定的自由基。对这些化合物在精心设计的分子体系中发生的反应进行研究，可以更好地了解燃烧中多环芳烃生长的潜在化学性质，以及控制有毒空气污染物和烟灰形成的条件。先前的研究表明，环戊二烯基和茚二基自由基加入到含有外部CPD片段的母体分子（CPD和茚二基）的pi键上。随后的反应通过以下两种途径之一形成邻熔多环芳烃：一种是通过类似于环戊二烯自由基结合形成萘的途径，将两个五元环扩展成只有六元环的多环芳烃；另一种涉及降冰片烯基型桥接中间体的形成，随后环打开和C1物种的损失，形成保留一个CPD片段的多环芳烃。第二种途径是先前未被提出的多环芳烃形成途径。佐治亚理工学院的实验将CPD和独立化学的工作扩展到这些化合物与其他三种物质（苯乙烯、苊和菲）的热反应。环戊二烯基和独立基自由基加入到其他类型的pi？研究化学键。苯乙烯是含有乙烯基取代基的最简单的芳香分子；由于大量的乙炔，这些物质在燃烧系统中很常见。苊是含外五元环的全共轭多环芳烃的代表；在燃烧系统中发现的其他例子包括苯乙烯和乙炔。菲是一种多环芳烃，含有π ？比其他芳香键共轭性差的键；另一种具有这种特征的燃烧产生的多环芳烃是芘。因此，所提出的研究将大大扩展对燃烧系统中碳部分的增长的理解，包括环戊二烯基和茚基自由基的加入以及随后的多环芳烃融合。对CPD/ indee体系进行了半经验分子模拟，以研究不同的多环芳烃形成途径。实验观察到的多环芳烃产物通道之间的分配与计算得到的划分在定性上一致。量子力学理论已被应用于涉及苊的碳生长过程的研究。本研究包括在犹他州对CPD/ indee系统进行从头算建模，以改进计算方法并验证半经验建模结果。对实验研究的新环戊二烯和茚二烯加成途径进行了计算研究。本研究将利用实验和计算方法的紧密协调，改进燃烧系统中多环芳烃和烟灰形成的化学机制。该项目结合了两个机构的研究人员的实验和计算专业知识。这种合作将需要密切的互动，并将扩大每个小组的研究能力。

This is a fundamental molecular analysis of polycyclic aromatic hydrocarbon (PAH) growth processes in combustion systems involving cyclopentadiene (CPD) and indene, which contains the CPD moiety. These compounds, which are present in combustion effluents, are of interest because of their roles as intermediates in the formation of potentially mutagenic PAH which contain five-membered rings, as well as in the formation of fullerenes and soot. Cyclopentadiene and indene are important in growth of PAH from combustion processes because they form resonance-stabilized radicals that are reactive at multiple sites. Study of reactions involving these compounds in well-designed molecular systems leads to better understanding of the underlying chemistry of PAH growth from combustion and of conditions that control the formation of toxic air pollutants and soot. Previous work indicates that cyclopentadienyl and indenyl radicals add to the pi bonds of the parent molecules (CPD and indene) that contain external CPD moieties. Subsequent reactions lead to the formation of ortho-fused PAH via one of two pathways: one involving expansion of both five-membered rings to form PAH with only six-membered rings by a route similar to that for cyclopentadienyl radical combination to form naphthalene; the other involving formation of a norbornenyl-type bridged intermediate followed by ring opening and loss of a C1 species to form PAH that retain one CPD moiety. This second pathway is a route of PAH formation not previously proposed. Experiments at Georgia Tech extend work on CPD and indene chemistry to thermal reactions of these compounds with three other species: styrene, acenaphthylene, and phenanthrene. Formation of PAH and growth by addition of cyclopentadienyl and indenyl radicals to other types of pi?bonds is studied. Styrene is the simplest aromatic molecule that contains a vinyl substituent; these species are common in combustion systems due to the abundance of acetylene. Acenaphthylene is representative of fully conjugated PAH containing external five-membered rings; other examples found in combustion systems include acephenanthrylene and aceanthrylene. Phenanthrene is a PAH that contains a pi?bond that is less conjugated than other aromatic pi?bonds; another combustion-generated PAH with this feature is pyrene. Thus, the proposed study will significantly expand the understanding of the growth of carbon moieties in combustion systems from addition of cyclopentadienyl and indenyl radicals and subsequent PAH fusion. Semi-empirical molecular modeling has been performed on the CPD/indene system to study alternative PAH formation pathways. Qualitative agreement was obtained between experimentally observed and computed partitioning between PAH product channels. Quantum mechanical theory has been applied to the study of carbon growth processes involving acenaphthylene. This study includes ab initio modeling at Utah of the CPD/indene system to refine computational methods and verify semi-empirical modeling results. A computational study of the new cyclopentadienyl and indenyl addition pathways being investigated experimentally is conducted.Broader impactThis research will improve chemical mechanisms of PAH and soot formation in combustion systems, using a tight coordination of experimental and computational approaches. The project combines experimental and computational expertise of investigators at two institutions. This collaboration will require a close interaction and will broaden the research capabilities of each group.