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E.M. Adkins, J.A. Giaccai, J.H. Miller, Computed electronic structure of polynuclear aromatic hydrocarbon agglomerates, Proceedings of the Combustion Institute. 36 (2017) 957–964. doi:10.1016/j.proci.2016.06.186.

A technique for linking the physical composition of polynuclear aromatic hydrocarbon (PAH) stacks and clusters to their electronic properties is reported. Kohn–Sham HOMO–LUMO gaps are reported for a series of monomers, stacks, and clusters of six, high-symmetry PAHs (pyrene, coronene, ovalene, circumpyrene, circumcoronene, and circumovalene) generated by DFT calculations with the 6-31G* basis set and a B3LYP exchange correlation functional in NWChem. A previously published, atom-pair minimization algorithm was used to optimize the geometries of the PAH stacks and clusters. HOMO–LUMO gaps decrease with an increase in monomer size; homogeneous stacks and clusters indicate substantial lowering of the HOMO–LUMO gap because of agglomeration effects with the formation of dimers and formation of clusters (two or more stacks) being the most dominant contributions. Heteromolecular particulates had HOMO–LUMO gaps that were strongly influenced by the larger components in the system. The HOMO–LUMO gaps of homogeneous clusters approached a maximum agglomeration effect because of the localization of electronic interactions among adjacent stacks. Previously published, experimentally determined optical band gaps (OBG) from Tauc/Davis–Mott analysis of extinction spectra in various laminar, non-premixed flames had an average OBG of 2.1 eV. Based on the computations presented here, this work suggests that clusters with this OBG are comprised of modest molecular size PAH, about the size of ovalene.

Detection of Trace Hydrocarbons in Flames Using Direct Sampling Mass Spectrometry Coupled with Multilinear Regression Analysis

Maria A. Puccio and J. Houston Miller
Analytical Chemistry 2010 82 (12), 5160-5168

DOI: 10.1021/ac1003823

 

Concentration contours (top, from left to right) of acetylene, butadiene, 1-buten-3-yne, and butadiyne and (bottom, from left to right) of benzene, toluene, phenylacetylene, and naphthalene.

A technique for the determination of species’ concentrations from the molecular growth regions of flames is presented. Samples are obtained by microprobe extraction from a nitrogen-diluted, methane/air, nonpremixed laminar flame supported on a coannular burner. Quantification of measurements is accomplished by doping the flame’s fuel flow with argon at a level to match that in the laboratory’s air. A library of 70 eV fragmentation patterns for several flame species is used in conjunction with a simplex algorithm to analyze mass spectra obtained at each flame location. Each fragmentation pattern is normalized for its integrated intensity and its ionization cross-section relative to argon. This technique provided sub-part-per-million sensitivity of a large range of major and minor carbon-containing species ranging in size from C2 to C12 hydrocarbons. This flame can be acoustically forced to oscillate at a frequency emulating natural flame flickering behavior. Time-resolved measurements are obtained using a modified quartz microprobe synchronized to open and close with the flame oscillations. The near real-time sampling and analysis time and the relatively high sensitivity make this technique preferable to other extraction-based flame measurements.

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