Boron-nitrogen analogues of cyclo[18]carbon

[Display omitted] •BN analogues of cyclo[18]carbon (C18) are theoretically investigated.•One or more isoelectronic BN units provoke a deformation of the ring.•Some BN-containing rings are highly polar molecules.•The inorganic analogue B9N9 has a larger HOMO–LUMO gap than that of C18. An explorative quantum chemical study of the boron-nitrogen (BN) analogues of cyclo[18]carbon (C18) is performed. The molecular geometries of nineteen BN analogues were optimized and their relative stability, normal modes of vibration, frontier orbitals, and the topology of the electronic charge densities were analyzed. The geometry predicted for the inorganic analogue B9N9 is a nearly-perfect nonagon with nine NBN edges and a HOMO–LUMO gap significantly larger (9.04 vs 5.71 eV) than that of C18. Thus the higher kinetic stability expected for B9N9 makes it an interesting synthetic target.