NMR Investigation on Ion Selectivity of 18-Crown-6 Ether

The ion selectivity of 18-crown-6 (18C6) ether has been investigated by DFT method. The following conclusions are drawn with the analyses about the geometrical structure, the binding energy, the natural bond orbital charge population, the infrared spectra, and the spin-spin coupling constant. First of all, the ion selectivity of 18C6 attributes primarily to the valence state, the periodic number, and the volume of the cation. Secondly, for the cation encapsulated complex of the same main group, the variation of distance between two para-oxygen atoms in the complex and in 18C6 (ΔR_OO) is proportionate to the binding energy of cation and 18C6. Then, the cation, bearing most of the electropositive charges, is encapsulated in the center of six electronegative oxygen atoms. Fourthly, the value of spin-spin coupling constant of cation and peripheral oxygen atom (¹J(M, O)) mainly dominated by the Fermi-contact term. The swing of cation along the para-position oxygen atoms is typical and can be taken as a characteristic for ion recognition.