Unbuckling the 18-Crown-6 Ether Belt Around Metal Ions: Forging the Connection to the Condensed Phase

Crown ethers are central to supramolecular chemistry, recognizing and binding specific ions in solution. The most well-known, 18-Crown-6 (18C6), preferentially captures K⁺ in an aqueous solution, while gas phase binding of 18C6 with alkali metal ions decreases linearly with an increasing ionic radius. Why the high affinity for Li⁺ and Na⁺ in the gas phase is dramatically reduced with hydration remains an open question in understanding the K⁺ selectivity in the aqueous phase. A combined spectroscopic and computational study of M⁺18C6(H₂O)_<i>n</i>=0-3 (M = Li, Na, and K) in the CH stretch region has revealed how stepwise hydration unbuckles the crown ether belt from Li⁺ and Na⁺, substantially changing the backbone structure of 18C6. In contrast, the structure of the K⁺18C6 complex is unbuckled and is unaffected by hydration. Combined with new measurements of the OH stretch, a direct connection is provided between the stepwise hydration of M⁺18C6 and the selectivity for K⁺ in an aqueous solution. It demonstrates and validates at the molecular level the application of gas-phase measurements to condensed-phase studies.