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

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