On the highest oxidation states of the actinoids in AnO 4 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.