Bonding and Interactions in UO 2 2+ for Ground and Core Excited States: Extracting Chemistry from Molecular Orbital Calculations.

Theoretical analyses of actinyls are necessary in order to understand and correctly interpret the chemical and physical properties of these molecules. Here, wave functions of Uranyl, UO ₂ ²⁺ , are considered for the ground state and for the core excited states where an electron is promoted from the U 3d _5/2 shell into a low-lying unoccupied orbital that is U 5f antibonding with the ligand, O, orbitals. A focus is on the application of novel theoretical methods to the analysis of these wave functions so that measurements, especially with X-ray absorption, can be related to the UO ₂ ²⁺ chemical bonding. The bond covalency is examined with these theoretical methods. The study includes how the covalent character is different for the ground and excited configurations and how this character changes as the U-O distance is changed. Furthermore, analyses are mode of how many-body effects may modify excitation energies and X-ray adsorption intensities. This includes determining the extent to which a single configuration provides a satisfactory model for the UO ₂ ²⁺ wave functions. Two distinct types of many-body effects are considered. One involves the angular momentum coupling of the open shell electrons in the excited states to yield correct multiplets. The second adds excitations from shells that are bonding into the antibonding open shell space. These excitations are essential to properly describe the X-ray adsorption. While these many-body effects must be taken into account, their importance and their role can be explained and understood using orbitals and orbital occupations.