Theoretical Prediction and Interpretation of ²³⁷Np Mössbauer Isomer Shifts

Abstract

Different theoretical approaches for the calculation of 237 Np Mossbauer isomer shifts are investigated. The traditional contact density route is compared to a previously proposed alternative approach that uses energy derivatives with respect to the nuclear radius. Both approaches yield similar results as long as suitable basis sets augmented with large exponents and relativistic methods are used. Density functional theory (DFT) calculations do not show a strong dependency of the 237 Np isomer shift on the chosen functional. Wavefunction calculations show that dynamic electron correlation can be important when covalent bonding influences the isomer shift. Effects from spin−orbit coupling are small. The isomer shifts of ionic solids and Np(III) organometallic complexes are largely governed by the oxidation state of Np. Isomer shifts of organometallic Np(IV) complexes are strongly affected by donation bonding. Detailed analysis of the wavefunction results with different active spaces demonstrates that correlation among the outer core Np and occupied ligand frontier orbitals contributes significantly to isomer shifts of Np(IV) compounds.

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

“…The crystal embedding of [Np VI O 6 ] −6 in the reverse neptunyl Na 4 NpO 5 effectively quenches the large intrinsic EFG, resulting in a relatively minor QS compared to the EFG intrinsic to the isolated [Np VI O 6 ] −6 cluster. Calculations of actinide Mossbauer isomer shifts and QS remain challenging, but we hope that the present study and its two predecessors 28,29 show that useful information about actinide covalency and 5f bonding can be obtained by a combination of Mossbauer experiments and first-principles theoretical calculations. The final Mossbauer parameter to be explored in this manner is caused by the magnetic electron− nucleus hyperfine interaction.…”

“…237 Np Mössbauer isomer shifts were calculated via the contact density, as described in refs , , and . For a brief overview of relativistic quantum chemistry concepts, see chapter 24 of ref .…”

“…The test set is composed of Np­(VII) to Np­(II) ionic solids and organometallic compounds as listed in Table and Figure . To model the electronic structure of the ionic solids, embedded cluster calculations were carried out using the experimental crystal structures. ,,,− The cluster embedding approach is the same that we utilized and benchmarked previously, , and we refer the reader to these publications regarding the structures of the organometallic compounds and details regarding the cluster embedding for the ionic solids. To estimate the isomer shifts for Np­(II), we used a truncated experimental crystal structure of [K­(crypt)]­[Np II C 5 H 3 (SiCH 3 ) 2 ] .…”

“…The detailed mechanisms, and the roles played by the 5f, 6d, and 7s shells, remain to be uncovered. In two previous studies, , we investigated theoretical approaches, based on density functional and wave function theory (DFT, WFT) for determining 237 Np Mössbauer isomer shifts. The present study is devoted to the application of these methods to investigate the chemical origins of the experimental trends in the isomer shift and the QS.…”

“…e Isomer shifts relative to NpAl 2 reference. f Structures are from our previous study without further modifications 28. Formate is abbreviated as form and glycolate as glyc.…”