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

The thermal stability of cross-links between polycyclic aromatic hydrocarbons (PAHs) is critical for understanding the formation of soot pollutants, graphite, and carbon blacks. Recently, a variety of different π-radicals have been directly imaged and suggested to enable thermally stable bonding; however, a systematic study of reactivity has been lacking. In this work, we use density functional theory to study the reactivity of PAH π-radicals. The Mulliken spin densities are initially used to categorize the different classes of localization, and the bond energy is computed to determine the degree of localization required for thermal stability. The results showed that the bond energies of PAHs are strongly correlated with the calculated spin densities, but bond energies do not exist with the bond lengths due to significant rearrangement and steric effects during bond formation. A threshold for π-radical localization is suggested that will be stable in combustion and pyrolysis environments of ρ_M^α ≥ 0.5. Finally, the formation of multicenter bonds between localized and delocalized π-radicals was investigated using the nudge elastic band (NEB) scan, and it was found that only delocalized π-radicals provided local energy minima. These results show that the localization of π-radicals is critical for the formation of thermally stable single-center bonds between aromatic radicals.