Exploring the Aromaticity Differences of Isoelectronic Species of Cyclo[18]carbon (C₁₈), B₆C₆N₆, and B₉N₉: The Role of Carbon Atoms as Connecting Bridges

The cyclo[18]carbon (C₁₈) has piqued widespread interest in recent years for its geometrical aesthetic and unique electronic structure. Inspired by it, theoretical investigations of its isoelectronic B₉N₉ have been published occasionally; however, few studies considered their other companion B₆C₆N₆. In this work, we study the geometric structure, charge distribution, bonding characteristic, aromaticity, and electron delocalization of B₆C₆N₆ and B₉N₉ for the first time and compare the relevant results with those of C₁₈. Based on the comprehensive analysis of aromaticity indicators such as AV1245, nucleus-independent chemical shifts, anisotropy of the induced current density, magnetically induced current density, iso-chemical shielding surface, and induced magnetic field (B^ind), we found that B₆C₆N₆ has definitely a double aromatic character similar to C₁₈ and the aromaticities of the two are very close, while B₉N₉ is a nonaromatic species. In response to this novel finding, we delved into its nature from an electron delocalization perspective through a localized orbital locator, electron localization function, Fermi hole, and atomic remote delocalization index analyses. The C atom between B and N as an interconnecting bridge strengthens the electron delocalization of the conjugate path, which is the essence of the significant enhancement of the molecular aromaticity from B₉N₉ to B₆C₆N₆. This work elucidates that within the framework of the isoelectronicity of C₁₈, different methods of atomic doping can achieve molecules with completely different properties.