Oxidation State Dependence of the Actinide M_4/5-Edge XANES of Actinyl (An = U, Np, Pu) Systems

Restricted Active Space simulations including spin-orbit coupling are used to predict the An M_4/5-edge XANES spectra of uranyl, neptunyl and plutonyl ions in the +5 and +6 oxidation states. The simulations reproduce the characteristic red-shift in XANES upon lowering the actinide oxidation state. This shift arises from relative changes in the ground- and core-excited state (CES) energies between An(VI) and An(V), which can be interpreted in terms of electron affinities. QTAIM analysis links the electron affinity of An(VI) states to actinide electron localization and to the more familiar concept of effective nuclear charge. The lower excitation energy in An(V) species stem from a comparatively high effective nuclear charge in An(VI) CESs and lower charge in An(V) ground states, producing enhanced electron affinity in the former and reduced core-electron binding in the latter. These combined effects yield more accessible CESs in An(V), manifesting as a red-shift. This study also examines the proposed correlation between ground-state covalency and the separation between 5f-δ/ϕ and 5f-σ* peaks. Although experimental trends suggest decreasing covalency across the series, QTAIM metrics do not support this. Instead, reduced covalency correlates with smaller peak separations only when the oxidation state is lowered within a given actinyl system.