On the highest oxidation states of the actinoids in AnO₄ molecules (An = Ac - Cm): A DMRG-CASSCF study

Actinoid tetroxide molecules AnO(4) (An = Ac - Cm) are investigated with the ab initio density matrix renormalization group (DMRG) approach. Natural orbital shapes are used to read out the oxidation state (OS) of the f-elements, and the atomic orbital energies and radii are used to explain the trends. The highest OSs reveal a "volcano"-type variation: For An = Ac - Np, the OSs are equal to the number of available valence electrons, that is, Ac-III, Th-IV, Pa-V, U-VI, and Np-VII. Starting with plutonium as the turning point, the highest OSs in the most stable AnO(4) isomers then decrease as Pu-V, Am-V, and Cm-III, indicating that the 5f-electrons are hard to be fully oxidized off from Pu onward. The variations are related to the actinoid contraction and to the 5f-covalency characteristics. Combined with previous work on OSs, we review their general trends throughout the periodic table, providing fundamental understanding of OS-relevant phenomena.