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Using density functional theory (DFT) calculations and the available crystallographic data we have investigated the binding of hydrated Cs+ ions to the pumpkin-shaped cucurbituril macrocycles, CB[n] with n = 5–7. The calculations indicate that besides the interactions between caesium ions and the carbonyl-laced portals, also the hydrogen bonds established between the coordinated water molecules and the macrocycle do contribute to the overall binding affinity. Although the other alkali metal ions compete for binding with caesium, the partial dehydration of the caesium aqua ions is likely favoured by the relatively small interaction energy associated with the water–Cs+ bond. The inclusion inside the macrocycle's cavity of either one water molecule or one chloride anion enhances the binding of Cs+ due to the additional ion–dipole or ion–ion interactions, respectively, established within the complexes. An advantage in using cucurbituril macrocycles for the sequestration of caesium ions from an aqueous solution is the possibility of binding two hydrated metal ions by the carbonyl-laced portals thereby forming 1:2 complexes.
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