Polycyclic Aromatic Hydrocarbon Growth Mechanisms in Combustion involving Cyclopentadiene and Indene

Dr. A. Violi, as member of the ICSE, is working on developing a better understanding of the growth processes of polycyclic aromatic hydrocarbons (PAH) containing the cyclopentadiene (CPD) moiety in combustion systems. PAH, which are detected in combustion effluents, are of interest because of their role as intermediates in the formation of potentially mutagenic PAH, which contain five-membered rings, as well as in the formation of fullerenes and soot. Study of reactions involving these compounds in well-designed molecular systems is needed to better understand the underlying chemistry of PAH growth in combustion and conditions that control the formation of toxic air pollutants and soot.

The experimental and computational studies build on previous work of one of the co-investigators, Prof. J. Mulholland at Georgia Tech, who found that cyclopentadienyl (CPDyl) and indenyl radicals add to the non-aromatic π bonds of the parent molecules (CPD and indene) that contain external CPD. Subsequent reactions lead to the formation of ortho-fused PAH via one of two pathways. One involves expansion of both five-membered rings to form PAH with only six-membered rings by a route modified from that proposed for CPDyl radical combination to form naphthalene. The other involves 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 represents a route of PAH formation not previously proposed. Experiments at Georgia Tech are extending work on CPD and indene chemistry to thermal reactions of these compounds with three other species: styrene, acenaphthylene and phenanthrene. PAH formation and growth by addition of CPDyl and indenyl radicals to other types of π bonds is being studied. Styrene is the simplest of aromatic molecules that contain vinyl substituents; 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 π bond that is less conjugated than its other aromatic π bonds; another combustion-generated PAH with this feature is pyrene.

Semi-empirical molecular modeling has been performed on the CPD/indene system by Prof. J. Mulholland to study alternative PAH formation pathways. Qualitative agreement was obtained between experimentally observed and computed partitioning between PAH product channels. At the University of Utah, Dr. A. Violi has applied quantum mechanical theory to the study of carbon growth processes involving acenaphthylene. Here, ab initio modeling of the CPD/indene system is proposed to refine computational methods and verify semi-empirical modeling results. Dr. Violi and Prof. Mullholland are currently working on a computational study of new CPDyl and indenyl addition pathways that are being identified in the laboratory.

This work is sponsored by the National Science Foundation.