Preferential interaction of charged alkali metal ions (guest) within a narrow cavity of cyclic crown ethers (neutral host): A quantum chemical investigation

The binding interaction of alkali metal ions (charged guest) within the narrow cavity of crown ethers (neutral host) of different cavity size has been studied using quantum chemical density functional theory. Different conformational structures, binding energies and various thermodynamic parameters of free crown ethers and their metal ion complexes have been determined with the B3LYP functional using a large split valence 6-311++G(d,p) basis set. Geometry optimization was performed using guess structures obtained from semi-empirical PM3 optimized structures without imposing any symmetry restriction. The calculated values of binding enthalpy increase with increase in cavity size, i.e., with increase in donor O atoms and are found to be in good agreement with gas phase experimental results. We have demonstrated the effect of micro-solvation on the binding interaction between the alkali metal ions (Li+ and Na+) and the macrocyclic crown ethers by considering micro-solvated metal ions up to six water molecules directly attached to the metal ion. A metal ion exchange reaction involving the replacement of lithium ion in metal ion-crown ether complexes with sodium ion contained within a metal ion-water cluster serves as the basis for modeling binding preferences in solution. An attempt has been made to study the effect of micro-solvation on the binding interaction of metal ions with crown ethers by considering two water molecules attached to metal ion-crown ether complexes. The calculated OH stretching frequency of H2O molecule in micro-solvated metal ion-crown complexes is less blue-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.