Based upon extensive density functional theory and wave function theory investigations, we predict the existence of the perfectly planar concentric π-aromatic D3h B18H3−(6), D2h B18H4(8), C2v B18H5+(10), and D6h 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 B18 clusters, tentatively referred to as borannulenes in this work, prove to possess [10]annulene character with 10 delocalized π-electrons. Detailed adaptive natural density partitioning (AdNDP) analyses unravel the bonding patterns of the π plus σ doubly aromatic D3h B18H3−(6) and C2v B18H5+(10) and the π aromatic and σ antiaromatic D2h B18H4(8) and D6h 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 Cs B18H3−(15) (which is essentially the same as D3hB18H3−) with a high first vertical detachment energy of 3.71 eV and the perfectly planar D2hB18H4 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.
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