Computational insight towards the binding affinity and participation of aliphatic unsaturated sidearms of aza-18-crown-6 extractants for Sr encapsulation in different solvent medium.

Context: Macrocyclic extractants capture cations in solution phase; therefore, their structures and energetics in different solvents are worth investigating. In this computational research work, the optimized geometry of three aza-18-crown-6 extractants (1-3) has been obtained in suitable solvent mediums to work out the influence of the solvation on their binding affinity with Sr. The designed macrocyclic extractants are remarkable as the N-substituted side chain bears the rarely examined simple aliphatic unsaturated double and triple bond. All significant structural perturbations of the macro ring wherein Sr binds are examined. Prediction of the auxiliary effect of the simple aliphatic unsaturated side arm on the binding of Sr through solvent competition or cation-pi interaction is undertaken. The binding affinity of the complex formed in the solvent and gas phases is calculated. Results obtained in this study favor the utilization of solvents of low dielectric constant (CHCl and DCM) for effective binding of Sr ions by the extractants.

Method: All DFT calculations were performed using the Gaussian 09 program. The optimized geometries and their structural features were visualized by GaussView. Density functional calculation involving B3LYP functional and LANL2DZ basis set was employed to obtain optimized geometry and energy of the extractants and their Sr complex. The solvation effects were considered by employing the calculations with the polarized continuum model (PCM). The computational method's reliability was assured by comparing the optimized structure with that of X-ray reported structure of a similar type of complex.