Trapping Aryl Radicals with Acetylene: Evidence for C2-Accretion as a Mechanism for Polycyclic Aromatic Hydrocarbon Growth1

Abstract

The formation of polycyclic aromatic hydrocarbons (PAH), fullerenes, soot, and other carbonaceous materials during the combustion or pyrolysis of low-molecular weight hydrocarbons requires, at a minimum, that small molecules and/or reactive intermediates somehow become joined to make larger ones. Most likely, more than one type of intermolecular C-C bond-forming reaction plays a role. The accretion of C{sub 2}-units has long been considered a probable pathway for the stepwise growth of PAH in flames, but evidence also points to the operation of bimolecular processes in which both partners can be relatively large. The experiments reported here address the former paradigm and provide clear support for a specific C{sub 2}-accretion pathway in which the key C-C bond-forming step involves the simple trapping of aryl radicals by acetylene (C{sub 2}H{sub 2}), both of which are abundant species in flames.