Large molecules, ions, radicals and small soot particles in fuel-rich hydrocarbon flames. Part IV. Large polycyclic aromatic hydrocarbons and their radicals in a fuel-rich benzene-oxygen flame

A low-pressure benzene-oxygen flame (C/O = 0.80, p = 2.66 kPa) has been analyzed for large PAH molecules and radicals using molecular beam sampling combined with REMPI ionization and time-of-flight mass spectrometry. In the mass range studied (18-70 carbon atoms per molecule) PAH occur with every number of C atoms. The variance in the number of H is a general phenomenon. Growth of PAH does not follow a narrow path in a C-H diagram but a broad band in which H-rich PAH play an important role. The nearly exponential decrease in maximum flame PAH concentration with increasing number of C up to about coronene (C24H12) is followed by an increase up to about C-50-PAH and a subsequent smooth decrease to still larger species. Large PAH with an even number of C are mainly closed-shell molecules whereas those with an odd number of C are mainly present as pi-radicals. However, there is no more difference in the concentration and reactivity of large PAH molecules and radicals. The distributions in concentration as a function of the number of C for constant numbers of H (C-distribution functions) follow the same rules for molecules and radicals. The different H content of molecules and radicals can be correlated with the structure of their carbon skeleton. Accordingly, PAH are mainly present with a peri-condensed aromatic ring system that has 4C bays when they are H-rich. There is no indication that large PAH need be activated for further growth by H abstraction forming sigma-radicals as postulated by the so-called HACA (hydrogen abstraction --> acetylene addition) mechanism. It is concluded that neither the radical nor the non-radical character is important for the reactivity of large PAH but that their structure is. With increasing size of the aromatic system, unimolecular rearrangements and partial decomposition play a role in forming reactive structures.