B30H8, B39H92−, B42H10, B48H10, and B72H12: polycyclic aromatic snub hydroboron clusters analogous to polycyclic aromatic hydrocarbons
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Calculations performed at the ab initio level using the recently reported planar concentric π-aromatic B18H62+(1) [Chen Q et al. (2011) Phys Chem Chem Phys 13:20620] as a building block suggest the possible existence of a new class of B3nHm polycyclic aromatic hydroboron (PAHB) clusters—B30H8(2), B39H92−(3), B42H10(4/5), B48H10(6), and B72H12(7)—which appear to be the inorganic analogs of the corresponding CnHm polycyclic aromatic hydrocarbon (PAHC) molecules naphthalene C10H8, phenalenyl anion C13H9−, phenanthrene/anthracene C14H10, pyrene C16H10, and coronene C24H12, respectively, in a universal atomic ratio of B:C = 3:1. Detailed canonical molecular orbital (CMO), adaptive natural density partitioning (AdNDP), and electron localization function (ELF) analyses indicate that, as they are hydrogenated fragments of a boron snub sheet [Zope RR, Baruah T (2010) Chem Phys Lett 501:193], these PAHB clusters are aromatic in nature, and exhibit the formation of islands of both σ- and π-aromaticity. The predicted ionization potentials of PAHB neutrals and electron detachment energies of small PAHB monoanions should permit them to be characterized experimentally in the future. The results obtained in this work expand the domain of planar boron-based clusters to a region well beyond B20, and experimental syntheses of these snub B3nHm clusters through partial hydrogenation of the corresponding bare B3n may open up a new area of boron chemistry parallel to that of PAHCs in carbon chemistry.