Cs⁺–π interactions and the design of macrocycles for the capture of environmental radiocesium (Cs-137): DFT, QTAIM, and CSD studies

Using a combination of density functional theory and the quantum theory of atoms in molecules, we investigate the nature of the Cs⁺–π interaction in two types of model complexes. The models considered herein are the charged Cs⁺–arene system and the neutral Cs⁺–arene–I⁻ system. In either model, the electron-withdrawing substituents on the arene moiety do compete with the Cs⁺ ion that interacts equally with the six carbon atoms of the arene. When an iodide ion is present, its electronic charge density is made available to the arene and then transferred to the alkali metal cation, thus enhancing its binding energy. In addition to these models, the complexes of Cs⁺ with the tetraphenylborate anion and calixarenes are investigated starting from the corresponding molecular crystal structures retrieved from the Cambridge Structural Database. These complexes are characterized by the presence of multiple Cs⁺–π interactions which increase the overall binding energy of the alkali metal cation. Our computational results are expected to provide useful guidelines for the design of macrocycles for the selective recognition and efficient extraction of radiocesium (Cs-137) in contaminated fluids.