Investigations On Geometrical Structures And Stabilities Of Boron-rich Hydroboron Clusters

Recently, the research of the boron-rich hydride clusters and all-boron graphene have been attracted significant attention. However, the number of the boron-rich hydrides is limited and the bonding patterns of all boron a-sheet are still unclear. In this paper, using the density functional theory and wave function theory, we investigated a series of perfect planar or quasi-planar boron-rich hydride clusters, including their geometrical structures, electronic structures, the bonding nature and the thermodynamic stabilities. In addition, The bonding nature of all-boron a-sheet was analyzed using the AdNDP approach. The main contents and conclusions of this paper are as follows:1.π-Aromatic B16H6:A Neutral Boron Hydride Analogue of NaphthaleneSystematic ab inito calculations are carried out in this work to investigate the geometrical and electronic structures of B16H6neutral and its anion B16H6-. The quasi-planar C2v B16H6(1A1) with10delocalized p-electrons proves to be the neutral boron hydride analogue of naphthalene (D2h C10H8). This π-aromatic neutral may be obtained from the experimentally known p-antiaromatic C2h B16(Sergeeva et al., J. Am. Chem. Soc.130:7244,2008) upon hydrogenation at the six corner positions and is expected to be undistinguishable from a perfect planar D2h B16H6in experiments. Detailed adaptive natural density partitioning (AdNDP) analyses clearly reveal the bonding pattern of B16H6and the calculated nucleus independent chemical shifts (NICS) strongly support its global π-aromaticity. C2v B16H6-(2B2) anion with one extra electron appears to have a similar quasi-planar structure with even a less severe out-of-plane distortion. Ultraviolet (UV) absorption spectrum of B16H6and photoelectron spectroscopy (PES) spectrum of B16H6-are simulated to facilitate their spectroscopic characterizations.2. Perfectly Planar Concentric π-Aromatic B18H3-, B18H4, B18H5+, and B18H62+:Inorganic Analogues of [10] AnnuleneBased upon extensive density functional theory and wave function theory investigations, we predict the existence of the perfectly planar concentric π-aromatic D3h B18H3-, D2h B18H4, C2v B18H5+, and D6h B18H62+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 B18clusters, tentatively referred to as borannulenes in this work, prove to possess [10]annulene character with10delocalized π-electrons. Detailed adaptive natural density partitioning (AdNDP) analyses unravel the bonding patterns of the π plus a doubly aromatic D3h B18H3-and C2v B18H5+and the π aromatic and a antiaromatic D2h B18H4, and D6h B18H62+. Borannulenes prove to possess negative nucleus-independent chemical shifts (NICSv) 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-(which is essentially the same as D3h B18H3-) with a high first vertical detachment energy of3.71eV and the perfectly planar D2h B18H4neutral with a huge first excitation energy of1.89eV are predicted to be the most possible borannulenes to be targeted in future experiments.3. The Nature of Bonding Patterns in AH Boron Graphene a-SheetBoron, the neighbor of carbon in the Periodic Table, could be the next element to form the2D-materials similar to graphene. From the theoretical calculations, the most stable planar all-boron graphene is the so-called a-sheet. Detailed adaptive natural density partitioning (AdNDP) analysis reveals the different bonding patterns of the a-sheet. On every filled hexagon borons, there are three3c-2e a-bonds (bordering upon the holes), three4c-2e σ-bonds (at the junction of two filled hexagons) and one7c-2e π-bond. Besides, the hexagon holes serve as scavengers of extra electrons from the filled hexagons; which leads to a6c-2e π-bond over the hexagon hole. Negative NICSv values suggest that every filled hexagon and every hexagon holes in the a-sheet are local π-aromatic. Unlike the graphene which contains2c-2e C-C σ-bonds, the a-sheet has no localized2c-2e B-B σ-interactions. Novel delocalized π/σ-bond and special6c-2e π-bond served as scavengers enrich our understanding of chemical bonding in general. It is expected that the presented bonding picture could be an advance toward rational design of future all-boron nanomaterials.