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

Abstract

Based upon extensive density functional theory and wave function theory investigations, we predict the existence of the perfectly planar concentric π-aromatic D(3h) B(18)H(3)(-)(6), D(2h) B(18)H(4)(8), C(2v) B(18)H(5)(+)(10), and D(6h) B(18)H(6)(2+)(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 π-electrons. Detailed adaptive natural density partitioning (AdNDP) analyses unravel the bonding patterns of the π plus σ doubly aromatic D(3h) B(18)H(3)(-)(6) and C(2v) B(18)H(5)(+)(10) and the π aromatic and σ antiaromatic D(2h) B(18)H(4)(8) and D(6h) B(18)H(6)(2+)(12). Borannulenes prove to possess negative nucleus-independent chemical shifts (NICS(zz)) 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) B(18)H(3)(-)(15) (which is essentially the same as D(3h) B(18)H(3)(-)) with a high first vertical detachment energy of 3.71 eV and the perfectly planar D(2h) B(18)H(4) 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.