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

Incipient soot early in the flame was studied by high-resolutionatomic force microscopy and scanning tunneling microscopy to resolvethe atomic structure and orbital densities of single soot moleculesprepared on bilayer NaCl on Cu(111). We resolved extended catacondensedand pentagonal-ring linked (pentalinked) species indicating how smallaromatics cross-link and cyclodehydrogenate to form moderately sizedaromatics. In addition, we resolved embedded pentagonal and heptagonalrings in flame aromatics. These nonhexagonal rings suggest simultaneousgrowth through aromatic cross-linking/cyclodehydrogenation and hydrogenabstraction acetylene addition. Moreover, we observed three classesof open-shell & pi;-radical species. First, radicals with an unpaired & pi;-electron delocalized along the molecule's perimeter.Second, molecules with partially localized & pi;-electrons at zigzagedges of a & pi;-radical. Third, molecules with strong localizationof a & pi;-electron at pentagonal- and methylene-type sites. Thethird class consists of & pi;-radicals localized enough to enablethermally stable bonds, as well as multiradical species such as diradicalsin the open-shell triplet state. These & pi;-diradicals can rapidlycluster through barrierless chain reactions enhanced by van der Waalsinteractions. These results improve our understanding of soot formationand the products formed by combustion and could provide insights forcleaner combustion and the production of hydrogen without CO2 emissions.