Linearity and Chemical Bond of UO₂²⁺ Revisited: A Comparison Study with UN₂ and UE₂²⁺ (E = S, Se, and Te) Based on Relativistic Calculations

Abstract

The stability and electronic structure of UO₂²⁺ are compared with those of UN₂ and UE₂²⁺ (E = S, Se, and Te) based on four- and two-component relativistic Hamiltonians. We observed that the Hartree–Fock method overestimates the stability of the linear structures of UO₂²⁺ and UN₂. In addition to the conventional mechanism based on valence orbitals, we proposed another mechanism wherein the small energy difference between U’s 6p₃/₂ and O’s σ(2s) orbitals destabilizes the bent structure of UO₂²⁺. The validity of the analysis based on the DFT method was evaluated using the coupled-cluster method. The slightly bent structures of UO₂²⁺ and UN₂ are feasible from the viewpoint of energetic stability: the destabilized energy at 160° is 0.144 and 0.059 eV for UO₂²⁺ and UN₂, respectively. The U–X bond (X = N, O) is rigid in the slightly bent structure, and it corresponds to the conservation of the feature of the chemical bond. For UE₂²⁺, core–valence orbitals mainly affect the stability of these molecules, like UO₂²⁺ and UN₂. In UE₂²⁺, the 6p hole is fairly modest, and the 6p hole in UO₂²⁺ is a unique feature in uranium-chalcogen systems.