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

University Of Utah, Salt Lake City UT

Investigators

Abstract

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 impact This 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.