Perfectly planar concentric pi-aromatic B18H3-, B18H4, B18H5+, and B18H62+ with [10]annulene character

Based upon extensive density functional theory and wave function theory investigations, we predict the existence of the perfectly planar concentric pi-aromatic D(3)h B18H3-(6), D-2h B18H4(8), C-2v B18H5+(10), and D-6h B18H62+(12) which are the smallest boron hydride clusters composed of a hybrid of the triangular and hexagonal motifs with a hexagonal hole at the center. These partially hydrogenated B-18 clusters, tentatively referred to as borannulenes in this work, prove to possess [10]annulene character with 10 delocalized pi-electrons. Detailed adaptive natural density partitioning (AdNDP) analyses unravel the bonding patterns of the pi plus sigma doubly aromatic D-3h B18H3- (6) and C-2v B18H5+(10) and the pi aromatic and sigma antiaromatic D-2h B18H4(8) and D(6)h B18H62+(12). Borannulenes prove to possess negative nucleus-independent chemical shifts (NICSzz) comparable with that of [10] annulene and huge negative anisotropies of the magnetic susceptibility (AMS) much bigger than the latter. The slightly non-planar C-s B18H3-(15) (which is essentially the same as D-3h B18H3-) with a high first vertical detachment energy of 3.71 eV and the perfectly planar D(2)h B18H4 neutral with a huge first excitation energy of 1.89 eV are predicted to be the most possible borannulenes to be targeted in future experiments.