Unraveling electronic structure and aromaticity differences in cyclo[12]carbon (C12), B4C4N4, and B6N6 isoelectronic ring molecules

Cyclo[12]carbon (C12) is the smallest recently synthesized carbon ring molecule that conforms to Hückel anti-aromaticity. Unraveling the electronic structure and aromaticity differences between C12 and its isoelectronic analogs (B4C4N4, B6N6) is essential for elucidating the impact of C-atom bridging and the physicochemical properties of novel ring systems. Herein, robust first-principle computational methods (including static density functional theory calculations and ab initio molecular dynamics simulations) are employed to investigate the electronic populations, bonding features, and kinetic behavior of different electron types. The molecular aromaticity is also examined by using various analytical indicators, such as anisotropy of induced current density, ZZ component of isochemical shielding surfaces, electron localization function-π, and Fermi holes. It is revealed that C12 and B4C4N4 exhibit pronounced anti-aromatic properties, while B6N6 is non-aromatic. The distinct in-plane and out-of-plane π-orbital features and differences in electronic delocalization capacity are fundamental to their anti-aromatic and non-aromatic nature, contrasting with classical aromatic molecules such as C18. This work provides valuable references for understanding the electronic structures of novel carbon ring molecules and their isoelectronic analogs that lack aromaticity, which can aid in comprehending the physicochemical properties of classic main-group elements and advance the design and synthesis of new ring molecules.