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 sp(2) 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 C(60) 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 (C(18)H(6) and C(54)H(18)) 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.