Theoretical Evaluation of Metal–Ligand Bonding in Neptunium Compounds in Relation to ²³⁷Np Mössbauer Spectroscopy

Abstract

The 237 Np Mossbauer isomer shift and quadrupole splitting (QS) are powerful probes for the metal−ligand bonding of neptunium, a 5f-element of vital importance in the nuclear fuel cycle. A large set of Np compounds with different oxidation states (III) to (VII) is studied to investigate, by first-principles calculations, isomer shifts and the QS trends in relation to the Np oxidation state. Natural Bond Orbital analysis reveals that in addition to donation bonding to the 5f shell, participation of the 6d and 7s neptunium shells in covalent (donation) bonding substantially impacts the isomer shifts. The isomer shift cannot be interpreted solely by the 5f shell electron count. The isomer shift for Np(II) compounds is estimated to be in the range of 31− 34 mm/s, less positive than for Np(III) compounds. For the QS, density functional calculations fail to reproduce the quadrupole splitting for some Np(VI) ionic solids. A multiconfigurational wave function approach reproduces the observed QS trends. The calculations give a semiquantitative interpretation of the trends for Np oxidation states (V) to (VII). The contrasting QS for standard and "reverse" neptunyl(VI), at the opposite extremes of the observed QS scale, arises predominantly from the different crystal environments.

Actinide inverse trans influence versus cooperative pushing from below and multi-center bonding

“…For the orbital entanglement measurements, we found it necessary to use relatively large active spaces [e.g., MPS(24 e , 24 o )]. Previous MPS calculations of neptunium organometallic compounds demonstrated that small CASSCF spaces lead to overlocalization of the 5f shell and an underestimation of the ligand donation (dative bonding) 49 , 50 . Although (12 e , 12 o ) 51 and (8 e , 8 o ) 5 active spaces have been used previously to investigate the electronic structure of CUO, these active spaces appear to be insufficient to bring about a full picture of the bonding.…”

Calculation of electric field gradients with the exact two-component (X2C) quasi-relativistic method and its local approximations

“…11 To date, the EFG and NQCC values of a large number of nuclides have been theoretically studied in the literature, mainly focusing on Fe. 12–33 For systems containing heavy atoms, such as Bi, 6,34–39 U, 40,41 and Np, 42 relativistic effects become crucial and have been taken into account through various approximate or (nearly) exact methodologies. In the early 1970s, Desclaux and Bessis presented the first four-component Dirac calculation of EFGs.…”

“…58 Several comparisons have shown that the precision achieved by X2C in the EFG calculations is very close to that achieved in the four-component calculations. 36,55 Regarding the inclusion of electron correlation, most EFG calculations have been performed using density functional theory (DFT) methods, although several notable exceptions using advanced ab initio methods, such as coupled cluster methods [34][35][36][37][38][39][44][45][46]48,49 or multi-configurational methods 42 can also be found in the literature. It is noteworthy that DFT methods cannot accurately describe the electronic structure and properties of strongly correlated systems.…”