A unified superatomic-molecule theory for local aromaticity in π-conjugated systems

Aromaticity is one of the most important concepts in chemistry. However, there is still no unified chemical insight for various systems with conjugated sp² carbon. Herein, we proposed a superatomic-molecule theory to build a generalized electron rule for polycyclic conjugated hydrocarbons, fullerenes and 2D periodic materials. Taking benzenoid units as 2D superatoms, polycyclic conjugated hydrocarbons and C₆₀ can be seen as superatomic molecules consisting of bonded superatoms, resulting in local aromaticity. In superatomic molecules, π electrons are not totally delocalized, but localized in a single superatom forming superatomic lone pairs or shared by two atoms forming a superatomic bond, mimicking rules in classical valence bond theory. Moreover, two 2D superatomic crystals (C₁₈H₆ and C₅₄H₁₈) are predicted to have fairly large band gaps (∼1.8 eV), although the π electrons are conjugated and delocalized. The proposed superatomic-molecule theory provides generalized chemical insights into the nature of local aromaticity, which can be qualitatively evaluated by the chemical intuition given by superatomic Lewis structures.