Characterization of a Hexanuclear Plutonium(IV) Nanostructure in an Acetate Solution via Visible-Near Infrared Absorption Spectroscopy, Extended X-ray Absorption Fine Structure Spectroscopy, and Density Functional Theory

A new hexanuclear plutonium cluster has been stabilized in aqueous media with acetate ligands. To probe the formation of such a complex structure, visible-near infrared (vis-NIR) absorption spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory(DFT)werecombined.ThepresenceofPu6O4(OH)4(CH3COO)12species in solution was first detected by vis-NIR and EXAFS spectroscopy. To confirm unambiguouslythis structure, EXAFS spectra were simulated fromab initiocalculations. Debye-Waller factors and structural parameters werederived from DFT calculations. A large number of 5f electronswere treated as valence or core electrons using small- and large-core relativistic effective pseudopotentials. It is possible to reproduce accurately the EXAFS spectrum of the octahedral hexamer cluster at both levels of calculations. Further DFT and EXAFS calculations were performed on clusters of lower or higher nuclearities and of different geometries using the 5f-coreapproximation. The result shows that trimer, tetramer, flat hexamer, and even 16-mer clusters exhibit different EXAFS patterns and confirm the very specific octahedral hexa nuclear EXAFS signature