DFT modeling on the suitable crown ether architecture for complexation with Cs+ and Sr2+ metal ions

Crown ether architectures were explored for the inclusion of Cs+ and Sr2+ ions within nano-cavity of macrocyclic crown ethers using density functional theory (DFT) modeling. The modeling was undertaken to gain insight into the mechanism of the complexation of Cs+ and Sr2+ ion with this ligand experimentally. The selectivity of Cs+ and Sr2+ ions for a particular size of crown ether has been explained based on the fitting and binding interaction of the guest ions in the narrow cavity of crown ethers. Although, Di-Benzo-18-Crown-6 (DB18C6) and Di-Benzo-21-Crown-7 (DB21C7) provide suitable host architecture for Sr2+ and Cs+ ions respectively as the ion size match with the cavity of the host, but consideration of binding interaction along with the cavity matching both DB18C6 and DB21C7 prefers Sr2+ ion. The calculated values of binding enthalpy of Cs metal ion with the crown ethers were found to be in good agreement with the experimental results. The gas phase binding enthalpy for Sr2+ ion with crown ether was higher than Cs metal ion. The ion exchange reaction between Sr and Cs always favors the selection of Sr metal ion both in the gas and in micro-solvated systems. The gas phase selectivity remains unchanged in micro-solvated phase. We have demonstrated the effect of micro-solvation on the binding interaction between the metal ions (Cs+ and Sr2+) and the macrocyclic crown ethers by considering micro-solvated metal ions up to eight water molecules directly attached to the metal ion and also by considering two water molecules attached to metal-ion-crown ether complexes. A metal ion exchange reaction involving the replacement of strontium ion in metal ion-crown ether complexes with cesium ion contained within a metal ion-water cluster serves as the basis for modeling binding preferences in solution. The calculated O-H stretching frequency of H2O molecule in micro-solvated metal ion-crown complexes is more red-shifted in comparison to hydrated metal ions. The calculated IR spectra can be compared with an experimental spectrum to determine the presence of micro-solvated metal ion-crown ether complexes in extractant phase.