Electronic Structure of Actinyls: Orbital Properties

Abstract

A detailed analysis is presented for the covalent character of the orbitals in the actinyls: UO2 2+, NpO2 2+, and PuO2 2+. Both the initial, or ground state, GS, configuration and the excited configurations where a 3d electron is excited into the open valence, nominally the 5f shell, are considered. The orbitals are determined as fully relativistic, four component Dirac-Coulomb Hartree–Fock solutions. Several measures, which go beyond the commonly used population analyses, are used to characterize the covalent character of an orbital in order to obtain reliable estimates of the covalency. Although there are differences in the covalent character of the orbitals for the initial and excited configurations of the different actinyls, there is a surprising similarity in the covalent character for all of the states considered. This is true both between the initial and excited configurations as well as between the different actinyls. The analysis emphasizes the 5f covalent character in the closed shell bonding orbitals and the open shell antibonding orbitals since the focus is on characterizing orbitals needed in a many-body treatment of the actinyl wave functions. However, estimates are also made of the participation of the actinide 6d in the covalent bonding.

Actinyl Electronic Structure Probed by XAS: The Role of Many-Body Effects

“…For NpO 2 2+ , there are seven, 2-fold degenerate multiplets for the initial 5f 1 configuration. The ligand and spin–orbit splitting of the open shell, 5f, orbitals is, in energetic order, 5fϕ 5/2 , 5fδ 3/2 , 5fϕ 7/2 , 5fδ 5/2 , 5fπ 1/2 , 5fπ 3/2 , and 5fσ 1/2 . As expected from the ordering and splitting of the NpO 2 2+ orbital energies, the lowest multiplet is Ω = 5/2 u and the first excited multiplet, at Δ E = 0.32 eV is Ω = 3/2 u .…”

“…In order to determine the M 4,5 → 5f XANES, WFs are needed for the GS multiplet and for the excited configuration, 3d → 5f, multiplets. Details of the variationally optimized Dirac–Hartree–Fock orbitals that are used to construct the ab initio CI WFs have been described elsewhere . Here, we review the many-electron WFs for the initial and excited multiplets.…”

“…We stress that different sets of orbitals are used for the CI WFs for the initial and excited configurations. Although the use of different sets of orbitals complicates the calculation of the XANES intensities, , it does take into account the different potentials for the electrons between the initial and excited configurations which significantly influences the multiplet energies. Two parameters define the many-body effects included in the ab initio CI WFs; these are the active orbital space and the active configuration spaces which will be described in more detail below.…”

“…Indeed, the need to include excitations from closed shell bonding to open shell antibonding orbitals is known even for very simple molecular systems, for example, CO and NO, where it improves the description of their properties including the XPS of the 1s orbitals . Thus, it is reasonable to expect that inclusion of such many-body effects will improve the M 4,5 → 5f XANES for the actinyls where the covalent character of the An–O interaction leads to a significant An 5f character in some of the closed shell orbitals . In the OCSA CI, determinants are included where electrons from the closed shell An–O bonding orbitals are promoted into the open shell antibonding, nominally 5f orbitals.…”

“…Most of the actinyl electronic structure was studied in a range of theoretical efforts; see, for example, refs ,− ,− . In a previous paper, we presented a detailed analysis of the covalent bonding where we compared the covalent character of the actinyl orbitals for UO 2 2+ , NpO 2 2+ , and PuO 2 2+ . As well as comparing the covalent character of the different actinyls, for the initial, ground state (GS), configuration, comparison of the covalent character between the GS configuration and excited configurations where a 3d electron was excited into an actinyl 5f orbital was also made.…”