Magnetic properties and electronic structure of neptunyl(VI) complexes: Wavefunctions, orbitals, and crystal-field models

The electronic structure and magnetic properties of neptunyl(VI), NpO ₂²⁺, and two neptunyl complexes, [NpO₂(NO ₃)₃]^- and [NpO₂Cl₄] ^2-, were studied with a combination of theoretical methods: ab initio relativistic wavefunction methods and density functional theory (DFT), as well as crystal-field (CF) models with parameters extracted from the ab initio calculations. Natural orbitals for electron density and spin magnetization from wavefunctions including spin-orbit coupling were employed to analyze the connection between the electronic structure and magnetic properties, and to link the results from CF models to the ab initio data. Free complex ions and systems embedded in a crystal environment were studied. Of prime interest were the electron paramagnetic resonance g-factors and their relation to the complex geometry, ligand coordination, and nature of the nonbonding 5f orbitals. The g-factors were calculated for the ground and excited states. For [NpO ₂Cl₄]^2-, a strong influence of the environment of the complex on its magnetic behavior was demonstrated. Kohn-Sham DFT with standard functionals can produce reasonable g-factors as long as the calculation converges to a solution resembling the electronic state of interest. However, this is not always straightforward. Making waves: The electronic structure and magnetic properties of neptunyl(VI), NpO₂²⁺, and two neptunyl complexes, [NpO₂(NO₃)₃]^- (see figure) and [NpO₂Cl₄]^2-, were studied with a combination of theoretical methods: ab initio relativistic wavefunction methods and density functional theory, as well as crystal-field models with parameters extracted from the ab initio calculations.