Charge Density Influence on Enthalpy of Formation of Uranyl Peroxide Cage Cluster Salts

More than 60 unique uranyl peroxide cage clusters have been reported that contain as many as 124 uranyl ions and that have overall diameters extending to 4 nm. They self-assemble in water under ambient conditions, are models for understanding structure-size-property relations as well as testing computational models for actinides, and have potential applications in nuclear fuel cycles. High-temperature drop solution calorimetry has been used to derive the enthalpies of formation of the salts of seven topologically diverse uranyl peroxide cage clusters containing from 22 to 28 uranyl ions that are bridged by various combinations of peroxide, pyrophosphate, and phosphite. The enthalpies of formation of these seven salts, as well as three salts of other uranyl peroxide clusters reported earlier, are dominated by the interactions of the alkali countercations with the clusters. There is an approximately linear relationship between the enthalpies of formation of the cluster salts and the charge density of the corresponding uranyl peroxide cluster, wherein salts containing clusters with higher charge densities have more negative enthalpies of formation.