B26−: The smallest planar boron cluster with a hexagonal vacancy and a complicated potential landscape.

Anionic boron clusters have been systematically investigated both experimentally and theoretically up to 30 atoms and have all been proved to be planar or quasi-planar (2D) in their global minima. However, the B 26 − cluster has remained elusive in this size range up to now, because of its complicated potential landscape. Here we present a joint photoelectron spectroscopy (PES) and first-principles study on the structures and bonding of this seemingly enigmatic cluster. Extensive global minimum searches, followed by high-level calculations and Gibbs free energy corrections, reveal that at least three 2D isomers, I ( C 1 , 2 A), II ( C 1 , 2 A), and III ( C 1 , 2 A), could contribute to the observed PE spectrum for the B 26 − cluster. Isomer I , which has the lowest free energy at finite temperatures, is found to dominate the experimental spectrum and represents the smallest 2D boron cluster with a hexagonal vacancy. Distinct spectral features are observed for isomer III , which has a pentagonal hole and is found to contribute to the measured PE spectrum as a minor species. Isomer II with a close-packed triangular 2D structure, which is the global minimum at 0 K, may also contribute to the observed spectrum as a minor species. Chemical bonding analyses show that the principal isomer I can be viewed as an all-boron analog of the polycyclic aromatic hydrocarbon C 17 H 11 + in terms of the π bonds. [ABSTRACT FROM AUTHOR]