PNNL

The Science                                 

Understanding the electronic structure of actinide elements, such as uranium and plutonium, can help advance the design of next-generation nuclear materials. However, the radioactivity of these elements makes them challenging to study. Researchers synthesized five different hybrid materials containing the [PuCl₆]^2— anion and probed the electronic structure therein. They found that while the Pu–Cl bonds were predominantly ionic, they featured important covalent contributions that were correlated with the participation of the 5f shell in bonding.

The Impact

The results from this study provide context for understanding and predicting the chemical and physical behavior of transuranic elements—an important step toward advancing the design of next-generation nuclear materials and managing the existing inventory of associated waste. By elucidating the role of the 5f shell in Pu–Cl bonds, this research contributes to the collective goal of resolving the f-electron challenge—the overarching goal of the Heavy Element Chemistry program.

Summary

Researchers synthesized five different hybrid materials, (4XPyH)₂[PuCl₆], where X= H, Cl, Br, I, and (4IPyH)₄[PuCl₆]·2Cl, from an acidic, chloride-rich aqueous media. They then probed the electronic structure of the [PuCl₆]^2— anion by using the quantum theory of atoms in molecules and natural localized molecular orbital analysis. The researchers delineated the underlying bond mechanism and hybrid atomic orbital contributions within the Pu-ligand bonds. Moreover, two related compounds, containing the [PuO₂Cl₄]²⁻and [PuCl₃(H₂O)₅] molecular units, published in past contributions by the authors, were subjected to the same level of analysis and trends in bonding were established.

The results revealed the Pu-Cl bonds were predominantly ionic, yet featured important covalent contributions  that increased as bond polarity decreased from [PuCl₃(H₂O)₅] < [PuO₂Cl₄]²⁻ < [PuCl₆]²⁻. Similarly, the Pu-based hybrid atomic orbitals in the Pu–Cl bonds exhibited decreased s and d orbital character, while the f orbital contribution increased. However, the Cl-based hybrid atomic orbitals did not vary significantly. This indicates that the 5f shell contributes to the covalent character of the Pu–Cl bond. This level of analysis provided valuable insight into the role of oxidation state, coordination geometry, and metal-ion charge in plutonium–ligand bond analysis.

Computational resources were provided by the High-Performance Computing Cluster operated by Research Technology Services at George Washington University, the National Laboratory for Scientific Computing (LNCC/MCTI, Brazil, SDumont supercomputer), and the Center for Computational Research (CCR) at the University at Buffalo.

Contact

Robert Gian Surbella III
Chemist, Pacific Northwest National Laboratory
robert.surbella@pnnl.gov

Christopher L. Cahill
Professor of International Affairs, The George Washington University
cahill@gwu.edu  

Funding

The primary source of support for this research is the Department of Energy Office of Science, Basic Energy Sciences program, Division of Chemical Sciences, Geosciences, and Biosciences, Heavy Element Chemistry program at PNNL and GWU. The researchers also acknowledge support from the National Technical Nuclear Forensic Center and the São Paulo Research Foundation (FAPESP).