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 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 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 D_3h B₁₈H₃⁻(6) and C_2v B₁₈H₅⁺(10) and the π aromatic and σ antiaromatic D_2h B₁₈H₄(8) and D_6h B₁₈H₆²⁺(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 B₁₈H₃⁻(15) (which is essentially the same as D_3hB₁₈H₃⁻) with a high first vertical detachment energy of 3.71 eV and the perfectly planar D_2hB₁₈H₄ 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.