Authors: Angiras Menon, Jacob W. Martin, Gustavo Leon, Dingyu Hou, Laura Pascazio, Xiaoqing You, and Markus Kraft*

• Spin density and Clar analyses are applied to quantify the degree of localization of various PAH radicals
• Density Functional Theory and variational transition state theory calculations are used to compute the process rate constants and for the formation of a localized π-radical on five-member ring containing PAHs.
• Kinetic simulations in a 0-dimensional reactor showed that the localized π-radical product competes with σ-radical products at temperatures up to even 1500 K, suggesting they may be important to consider for combustion mechanisms.
• Kinetic Monte Carlo simulations suggest that multiple such localized π-radical sites can exist on a single PAH, suggesting potential for chemical polymerization mechanisms.

The impact of localized π-radicals on soot formation is explored by considering their electronic structure and computing their relative concentrations in flame conditions. Electronic structure calculations reveal that the presence of localized π-radicals on rim-based pentagonal rings is due to aromaticity. We further calculated a complete mechanism for the formation and elimination of the site from hydrogen additions and abstractions. A batch reactor with flame concentrations of H• and H₂ was used to determine the time-dependent concentration of localized π-radicals. Low temperatures ( < 1000 K) favored the fully saturated rim-based pentagonal ring. Soot nucleation temperatures (1000–1500 K) give way to unsaturated rim-based pentagons being favored. Localized π-radicals on rim-based pentagonal rings are found to be in higher concentration than the aryl-type σ-radical on the rim-based pentagon (mole fractions of ) in below < 1500 K, consistent with recent experimental observations. Higher temperatures favor the σ-radical and the concentration of the localized π-radical on rim-based pentagons becomes negligible. A kinetic Monte Carlo treatment of multiple sites indicates that multiple localized π-radicals are possible on a single molecule. These results reveal the importance of localized π-radicals on rim-based pentagonal rings for PAH chemistry leading to formation of soot nanoparticles in flames involving aromatic rim-linked hydrocarbons (ARLH).

• This paper draws from preprint 251: Reactive localized π-radicals on rim-based pentagonal rings: properties and concentration in flames
• Access the article at the publisher: DOI: 10.1016/j.proci.2020.07.042

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