B27-: Appearance of the smallest planar boron cluster containing a hexagonal vacancy

Photoelectron spectroscopy and ab initio calculations have been carried out to probe the structures and chemical bonding of the B-27(-) Cluster. Comparison between the experimental spectrum and the theoretical results reveals a two-dimensional (2D) global minimum with a triangular lattice containing a tetragonal defect (I) and two low-lying 2D isomers (II and III), each with a hexagonal vacancy. All three 2D isomers have 16 peripheral boron atoms and 11 inner boron atoms. Isomer I is shown to be mainly responsible for the observed photoelectron spectrum with isomers II and III as minor contributors. Chemical bonding analyses of these three isomers show that they all feature 16 localized peripheral B-B sigma-bonds. Additionally, isomer I possesses 16 delocalized sigma bonds and nine delocalized pi bonds, while isomers II and III each contain 17 delocalized s bonds and eight delocalized pi bonds. It is found that the hexagonal vacancy is associated generally with an increase of delocalized sigma bonds at the expense of delocalized pi bonds in 2D boron clusters. The hexagonal vacancy, characteristic of borophenes, is found to be a general structural feature for mid-sized boron clusters. The current study shows that B-27(-) is the first boron cluster, where a hexagonal vacancy appears among the low-lying isomers accessible experimentally. (C) 2015 AIP Publishing LLC.