Alkali Metal Ion Recognition by 18-Crown-6 in Aqueous Solutions: Evidence from Local Structures

The underlying recognition mechanisms of alkali metalions by crownethers in aqueous solutions are yet to be fully understood at themolecular level. We report direct experimental and theoretical evidencefor the structure and recognition sequence of alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) by 18-crown-6 in aqueous solutions by wide-angle X-ray scatteringcombined with an empirical potential structure refinement modelingand ab initio molecular dynamics simulation. Li+, Na+, and K+ are located in the negativepotential cavity of 18-crown-6, with Li+ and Na+ deviating from the centroid of 18-crown-6 by 0.95 and 0.35 angstrom,respectively. Rb+ and Cs+ lie outside the 18-crown-6ring and deviate from the centroid of 18-crown-6 by 0.05 and 1.35 angstrom, respectively. The formation of the 18-crown-6/alkali metalion complexes is dominated by electrostatic attraction between thecations and the oxygen atoms (Oc) of 18-crown-6. Li+, Na+, K+, and Rb+ form the H2O center dot center dot center dot 18-crown-6/cation center dot center dot center dot H2O "sandwich" hydrates, while water molecules only hydratewith Cs+ of the 18-crown-6/Cs+ complex on thesame side of Cs+. Based on the local structure, the recognitionsequence of 18-crown-6 for alkali metal ions in an aqueous solutionfollows K+ > Rb+ >Na+ >Li+, which is completely different from that (Li+ >Na+ > K+ > Rb+ > Cs+) in thegas phase, confirming that the solvation medium seriously affectsthe cation recognition of crown ethers. This work provides atomicinsights into understanding the host-guest recognition andsolvation behavior of crown ether/cation complexes.