Monocyclic aromatic compounds BnRgn(n−2)+ of boron and rare gases

Abstract

The monocyclic compounds (BRg)₃⁺(D_3h), (BRg)₄²⁺(D_4h), (BRg)₅³⁺(D_5h) and (BRg)₆⁴⁺(D_6h) formed between boron and rare gases Rg (He–Rn) are theoretically predicted to be stable structures and have π-aromaticity with a delocalized nc-2e π-system. For heavier rare gases Ar–Rn, the B–Rg bond energy is quite high and ranges from 15 to 96 kcal mol⁻¹, increasing with the ring size and the atomic number of rare gases; the B–Rg bond length is close to the sum of covalent radii of B and Rg atoms; NBO and AIM analyses show that the B–Rg bonds for Ar–Rn have a typical covalent character. The B–Rg bond is stabilized mainly by σ-donation from the valence p orbital of Rg to the vacant valence orbital of the boron ring. We searched for a large number of isomers for the systems of Ar and found that the titled monocyclic compounds (BAr)₃⁺(D_3h), (BAr)₄²⁺(D_4h) and (BAr)₅³⁺(D_5h) should be global energy minima. For (BAr)₆⁴⁺ the global energy minimum is an octahedral caged structure, but the titled monocyclic compound is the secondary stable local energy minimum. The energy and thermodynamic stability of the ring B_nRg_n⁽ⁿ⁻²⁾⁺ cations indicate that these rare gas compounds may be viable species in experiments.