Planar D2h B26H8, D2h B26H82+, and C2h B26H6: Building Blocks of Stable Boron Sheets with Twin-Hexagonal Holes.

The most stable mono-layer boron sheets were predicted to have both the isolated hexagonal hole and the twin-hexagonal hole. Previous investigations indicate that planar B ₁₈H _n^q ( n = 3–6, q =  n − 4) are the building blocks of boron sheets with isolated hexagonal holes. Extensive DFT investigations performed in this work show that D_2h B ₂₆H ₈, D_2h B ₂₆H ₈²⁺, and C₂ B ₂₆H ₆, may serve as the building blocks of boron sheets with twin-hexagonal holes. These bicyclic clusters possess planar or quasi-planar geometries at B3LYP/6-311+G(d,p) level, with 16, 14, and 14 delocalized π electrons, respectively. Detailed analyses indicate that they are overall aromatic in nature, with the formation of islands of both σ and π aromaticity. They are analogous to D_2h C ₁₆H ₁₄ and D_2h C ₁₆H ₁₄²⁺ in π bonding patterns, respectively, but fundamentally different from the latter in σ-bonding. Remarkably, all of them appear to be energetically the lowest-lying isomers obtained, which are promising targets for future gas phase syntheses. These hydroboron clusters, together with B ₁₈H _n^q clusters, establish the molecular basis for modeling the short-range structures, nucleation, and growth processes of monolayer boron sheets. The results obtained in this work enrich the chemistry of boron hydride clusters and expand the analogy relationship between hydroborons and hydrocarbons. [ABSTRACT FROM AUTHOR]