Structural and Electronic Properties of the Tetraatomic B₂Be₂ Cluster

The ab initio molecular orbital study of the 10 valence electron diberyllium diboride (B₂Be₂) potential energy hypersurface yields a singlet tetrahedrallike form 1s as the global minimum at the HF and MP2/6-31G^⋆ levels of theory. At MP4/6-31G^⋆ the energy difference of Is with any other isomer is at least 30 kcal/mol. The topological electron density analysis, based on the theory of atoms in molecules, shows 1s to have its Be atoms polar π-complexed to the B-B bond with an angle of 80.1° between the π planes. The inversion barrier (1s ⇌ 2s ⇌ 1s) amounts to 11.7 kcal/mol at MP4/6-31G^⋆ + ZPE and 14.6 kcal/mol at MP2/6-31 1G(df) + ZPE. The D_2h structures 2s and 3s are related by means of “bond-stretch” isomerism. The electron density analysis shows that the electron density is concentrated in the center of structure 2s whereas 3s shows a depletion of electron density in the center as compared to the periphery of this rhombic structure. The trapezoidal 5s and the Be-substituted B₂Be ring 4s are discussed for their structural and electronic properties. All B₂Be₂ structures show short B-B bonds with the exception of 3s. The effects of electron correlation and the binding energies are discussed for the global minimum.