Issue 8, 2000

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

Abstract

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 C50-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 π-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 σ-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.

Supplementary files

Article information

Article type
Paper
Submitted
12 Oct 1999
Accepted
11 Feb 2000
First published
31 Mar 2000

Phys. Chem. Chem. Phys., 2000,2, 1667-1675

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. Keller, R. Kovacs and K.-H. Homann, Phys. Chem. Chem. Phys., 2000, 2, 1667 DOI: 10.1039/A908190I

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