The Energy Landscape of Uranyl–Peroxide Species

Abstract

Nanoscale uranyl peroxide clusters containing UO2(2+) groups bonded through peroxide bridges to form polynuclear molecular species (polyoxometalates) exist both in solution and in the solid state. There is an extensive family of clusters containing 28 uranium atoms (U28 clusters), with an encapsulated anion in the center, for example, [UO2(O2)(3-x)(OH)(x)(4-)], [Nb(O2)4(3-)], or [Ta(O2)4(3-)]. The negative charge of these clusters is balanced by alkali ions, both encapsulated, and located exterior to the cluster. The present study reports measurement of enthalpy of formation for two such U28 compounds, one of which is uranyl centered and the other is peroxotantalate centered. The [(Ta(O2)4]-centered U28 capsule is energetically more stable than the [(UO2)(O2)3]-centered capsule. These data, along with our prior studies on other uranyl-peroxide solids, are used to explore the energy landscape and define thermochemical trends in alkali-uranyl-peroxide systems. It was suggested that the energetic role of charge-balancing alkali ions and their electrostatic interactions with the negatively charged uranyl-peroxide species is the dominant factor in defining energetic stability. These experimental data were supported by DFT calculations, which agree that the [(Ta(O2)4]-centered U28 capsule is more stable than the uranyl-centered capsule. Moreover, the relative stability is controlled by the interactions of the encapsulated alkalis with the encapsulated anion. Thus, the role of alkali-anion interactions was shown to be important at all length scales of uranyl-peroxide species: in both comparing clusters to clusters; and clusters to monomers or extended solids.

Uranyl–Peroxide Capsule Self‐Assembly in Slow Motion

“…The Δ H f oxides per molecular unit ( U 1 , U 5 or U 28 ) becomes more negative from K‐U 1 to K‐U 5 to K‐U 28 , but the Δ H f oxides per uranyl unit becomes slightly less negative with increasing size of uranyl species (Figure , Supporting Information Table S13). This is consistent with prior calorimetric studies comparing enthalpy of formation of uranyl monomers and clusters . Moreover, the enthalpy for the pentamer intermediate is along the expected energy landscape between the monomer and the fully formed capsule.…”

“…These data can be obtained free of charge by The Cambridge Crystallographic Data Centre and clusters. [3,12,18] Moreover,t he enthalpy fort he pentamer intermediate is along the expected energy landscape between the monomer and the fully formed capsule.…”

Performance of group additivity methods for predicting the stability of uranyl complexes

“…Actually, the size of such clusters is greatly influenced by the pH, ranging from 20 up to 120 uranium atoms . Further studies have proved that the bending of the U─O 2 ─U dihedral angle encourages curved shapes and thus cage cluster formation . In addition, classical molecular dynamic simulations have been developed so as to address the role of counter cations in the solubility and size of such systems .…”

“…[6] Further studies have proved that the bending of the U─O 2 ─U dihedral angle encourages curved shapes and thus cage cluster formation. [7][8][9] In addition, classical molecular dynamic simulations have been developed so as to address the role of counter cations in the solubility and size of such systems. [10,11] Uranyl complexes can also be found in wheel-shaped polyoxometalates as well as peroxide rings.…”

Group additivity-Pourbaix diagrams advocate thermodynamically stable nanoscale clusters in aqueous environments

“…Theory can provide a complementary means to elucidate some of these questions. For example, quantum mechanical (QM) studies of the structure, electronic and spectroscopic properties, absorption and reaction sites for clusters are well established2425. Recently, molecular dynamics simulations have been used to unravel solute-solvent and ion-pairing interactions as well as the dynamics of specific clusters242627.…”