Role of -Radical Localization on Thermally Stable Cross-Links Between Polycyclic Aromatic Hydrocarbons

The thermal stability of cross-linksbetween polycyclic aromatichydrocarbons (PAHs) is critical for understanding the formation ofsoot pollutants, graphite, and carbon blacks. Recently, a varietyof different & pi;-radicals have been directly imaged and suggestedto enable thermally stable bonding; however, a systematic study ofreactivity has been lacking. In this work, we use density functionaltheory to study the reactivity of PAH & pi;-radicals. The Mullikenspin densities are initially used to categorize the different classesof localization, and the bond energy is computed to determine thedegree of localization required for thermal stability. The resultsshowed that the bond energies of PAHs are strongly correlated withthe calculated spin densities, but bond energies do not exist withthe bond lengths due to significant rearrangement and steric effectsduring bond formation. A threshold for & pi;-radical localizationis suggested that will be stable in combustion and pyrolysis environmentsof & rho;M & alpha; & GE; 0.5. Finally, the formation of multicenter bonds betweenlocalized and delocalized & pi;-radicals was investigated usingthe nudge elastic band (NEB) scan, and it was found that only delocalized & pi;-radicals provided local energy minima. These results showthat the localization of & pi;-radicals is critical for the formationof thermally stable single-center bonds between aromatic radicals.