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

Actinoid tetroxide molecules AnO (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, Th, Pa, U, and Np. Starting with plutonium as the turning point, the highest OSs in the most stable AnO isomers then decrease as Pu, Am, and Cm, 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.