Linearity and Chemical Bond of UO 2 2+ Revisited: A Comparison Study with UN 2 and UE 2 2+ (E = S, Se, and Te) Based on Relativistic Calculations.

The stability and electronic structure of UO are compared with those of UN and UE (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 UO and UN. In addition to the conventional mechanism based on valence orbitals, we proposed another mechanism wherein the small energy difference between U's 6p and O's σ(2s) orbitals destabilizes the bent structure of UO. The validity of the analysis based on the DFT method was evaluated using the coupled-cluster method. The slightly bent structures of UO and UN are feasible from the viewpoint of energetic stability: the destabilized energy at 160° is 0.144 and 0.059 eV for UO and UN, 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 UE, core-valence orbitals mainly affect the stability of these molecules, like UO and UN. In UE, the 6p hole is fairly modest, and the 6p hole in UO is a unique feature in uranium-chalcogen systems.