Structural Motifs and Evolution of Boron Nanoclusters

Boron is a chemically versatile element, capable of forming diverse chemical bonds (e.g., single, double, triple, 3-center 2-electron bonds, and more), which determines its chemical behavior as a pure substance and in compounds with other elements. Electron deficiency and tendency to form multicenter bonds give rise to the ubiquitous presence of clusters in structures of boron allotropes and of many boron compounds in bulk and molecular forms. Here we investigate a wide range of neutral boron clusters B_n (n = 2–60) using the first-principles evolutionary algorithm USPEX. We find clear preference for planar structures for n < 10, while there is a competition between planar, cage, bilayer, and tubular structures for n > 10. We identify magic clusters as those having positive second-order differences of the total energy (and additionally analyze their fragmentation energy and HOMO–LUMO gap). Most of the magic clusters have even n, the most notable exception being magnetic cluster B₃₉ with cuboctahedral shape. Investigating the concept of aromaticity of inorganic compounds, we applied such approaches as nuclear independent chemical shift (NICS) and adaptive natural density partitioning (AdNDP) to a number of boron clusters and found that two clusters, B₁₀ and B₁₃, are aromatic (the former being magic).