Portraits of Soot Molecules Reveal Pathways to Large Aromatics, Five-/Seven-Membered Rings, and Inception through π-Radical Localization

Abstract

Incipient soot early in the flame was studied by high-resolution atomic force microscopy (AFM) and scanning tunneling microscopy (STM) resolving the atomic structure and orbital densities of single soot molecules prepared on bilayer NaCl on Cu(111). We resolved extended catacondensed and pentagonal-ring linked (pentalinked) species indicating how small aromatics crosslink and cyclodehydrogenate to form moderately sized aromatics. In addition, we resolved embedded pentagonal and heptagonal rings for the first time in flame aromatics. These non-hexagonal rings suggest simultaneous growth through aromatic crosslinking/cyclodehydrogenation and hydrogen abstraction acetylene addition (HACA). Moreover, we observed three classes of open-shell π-radical species. Firstly, radicals with an unpaired π-electron delocalized along the molecule’s perimeter. Secondly, molecules with partially localized π-electrons at zig-zag edges of a π-radical. Thirdly, strong localization of a π-electron was found at pentagonal- and methylene-type sites. The third class consists of π-radicals localized enough to enable thermally stable bonds as well as multiradical species. These π-diradicals can rapidly cluster through barrierless chain reactions enhanced by van der Waals interactions. These results improve our understanding of soot formation and the products formed by combustion and could provide insights for cleaner combustion and the production of hydrogen without $ CO_2 $ emissions.