Comparison of the selectivity of [M(12-Crown-4)]+ (M = Li+, Na+, K+) complexes for halide anions and some neutral molecules; a computational study

A theoretical study on the selectivity of a series of [M(12C4)](+) (M = Li+, Na+, K+, 12C4 = 12-crown- 4) complexes for F-, Cl- and Br- anions and a number of neutral molecules (CH3CN, CH3OH, NH3, H2O, py, and 12C4) is reported. At first, it was shown that in the gas phase among all studied halide anions and neutral molecules, halides have much more bonding interaction with all [M(12C4)](+) cations. Calculated interaction energies of above anions and [M(12C4)](+) cations decrease from F- to Br-. Also the interaction energy of halide anions with [M(12C4)](+) complexes, decreases from [Li(12C4)](+) to [K(12C4)](+). The electron decomposition analysis showed that the bond between [M(12C4)](+) complexes and both the neutral and anion guests is mainly electrostatic in nature. Then the selectivity of [M(12C4)](+) complexes for studied anions and neutral molecules are compared in methanol, acetone, acetonitrile, and nitromethane solutions. It was shown that both the desolvation process of reactants and the strength of host-guest interactions have significant effect on the selectivities. Thus the selectivity of [Li(12C4)](+) cation for NH3 and H2O neutral molecules in solution, in contrast to the gas phase, is higher than that for bromide anion. The results of calculations showed that all [M(12C4)](+) complexes, specially [Li(12C4)]+, have high selectivity for F- over other halide anions and neutral molecules.