Exploring the Aromaticity Differences of Isoelectronic Species of Cyclo[18]carbon (C 18 ), B 6 C 6 N 6 , and B 9 N 9 : The Role of Carbon Atoms as Connecting Bridges.

The cyclo[18]carbon (C 18 ) has piqued widespread interest in recent years for its geometrical aesthetic and unique electronic structure. Inspired by it, theoretical investigations of its isoelectronic B 9 N 9 have been published occasionally; however, few studies considered their other companion B 6 C 6 N 6 . In this work, we study the geometric structure, charge distribution, bonding characteristic, aromaticity, and electron delocalization of B 6 C 6 N 6 and B 9 N 9 for the first time and compare the relevant results with those of C 18 . 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 B 6 C 6 N 6 has definitely a double aromatic character similar to C 18 and the aromaticities of the two are very close, while B 9 N 9 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 B 9 N 9 to B 6 C 6 N 6 . This work elucidates that within the framework of the isoelectronicity of C 18 , different methods of atomic doping can achieve molecules with completely different properties.