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 M18C6(HO) (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 K18C6 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 M18C6 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.