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

The underlying recognition mechanisms of alkali metal ions by crown ethers in aqueous solutions are yet to be fully understood at the molecular level. We report direct experimental and theoretical evidence for 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 scattering combined with an empirical potential structure refinement modeling and ab initio molecular dynamics simulation. Li⁺, Na⁺, and K⁺ are located in the negative potential cavity of 18-crown-6, with Li⁺ and Na⁺ deviating from the centroid of 18-crown-6 by 0.95 and 0.35 Å, respectively. Rb⁺ and Cs⁺ lie outside the 18-crown-6 ring and deviate from the centroid of 18-crown-6 by 0.05 and 1.35 Å, respectively. The formation of the 18-crown-6/alkali metal ion complexes is dominated by electrostatic attraction between the cations and the oxygen atoms (Oc) of 18-crown-6. Li⁺, Na⁺, K⁺, and Rb⁺ form the H₂O···18-crown-6/cation···H₂O "sandwich" hydrates, while water molecules only hydrate with Cs⁺ of the 18-crown-6/Cs⁺ complex on the same side of Cs⁺. Based on the local structure, the recognition sequence of 18-crown-6 for alkali metal ions in an aqueous solution follows K⁺ > Rb⁺ >Na⁺ >Li⁺, which is completely different from that (Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺) in the gas phase, confirming that the solvation medium seriously affects the cation recognition of crown ethers. This work provides atomic insights into understanding the host-guest recognition and solvation behavior of crown ether/cation complexes.