Quasi-planar aromatic B36 and B36(-) clusters: all-boron analogues of coronene.

Flat boron has recently emerged as a fascinating concept in cluster science. Here we present computational evidence for the quasi-planar all-boron aromatic B36 (C6v, (1)A1) and B36(-) (C2v, (2)A1) clusters, established as the global-minimum structures on the basis of Stochastic Surface Walking (SSW) searches. The energetics for low-lying isomeric structures are evaluated using the validated density-functional method at the PBE0/6-311+G* level. Our global-minimum structures are in line with a recent report (Z. A. Piazza et al., Nat. Commun., 2014, 5, 3113). These structures consist of two-dimensional close-packing boron with a perfect hexagonal hole at the center, which may serve as molecular models for the monolayer boron α sheet. Chemical bonding analysis indicates that B36 and B36(-) are all-boron analogues of coronene (C24H12), featuring concentric dual π aromaticity with an inner π sextet and an outer π sextet. The hydrogenated B36H6 (C6v, (1)A1) model cluster shows similar bonding properties, which possesses concentric triple aromaticity with inner π, outer π, and outer σ sextets.