A Spontaneous Structural Transition of {U24 Pp12 } Clusters Triggered by Alkali Counterion Replacement in Dilute Solution.

A transition between two isomeric clusters involving the change of the main skeleton structure of a well-defined, rigid molecular cluster [(UO2 )24 (O2 )24 (P2 O7 )12 ]48- , {U24 Pp12 }, is achieved by simply introducing proper alkali cations into its dilute aqueous solution. While the unique structural transition can be triggered by introducing any of the Na+ /K+ /Rb+ /Cs+ alkali ions, the two isomers, Li/Na-{U24 Pp12 } and Na/K-{U24 Pp12 }, as typical macroions, can accurately choose among different alkali counter-cations based on their hydrated sizes, and the ion selectivity process clearly showed endothermic features. The preferred K+ and Rb+ ions have suitable sizes to be incorporated into the proper windows on {U24 Pp12 } nanocapsules, as supported by the transition points in both ITC studies and IR measurements.