Microhydration Effects on the Encapsulation of Potassium Ion by Dibenzo-18-Crown‑6

Abstract

We have measured electronic and conformer-specific vibrational spectra of hydrated dibenzo-18-crown-6 (DB18C6) complexes with potassium ion, K⁺•DB18C6•(H₂O)_<i>n</i> (<i>n</i> = 1–5), in a cold, 22-pole ion trap. We also present for comparison spectra of Rb⁺•DB18C6•(H₂O)₃ and Cs⁺•DB18C6•(H₂O)₃ complexes. We determine the number and the structure of conformers by analyzing the spectra with the aid of quantum chemical calculations. The K⁺•DB18C6•(H₂O)₁ complex has only one conformer under the conditions of our experiment. For K⁺•DB18C6•(H₂O)_<i>n</i> with <i>n</i> = 2 and 3, there are at least two conformers even under the cold conditions, whereas Rb⁺•DB18C6•(H₂O)₃ and Cs⁺•DB18C6•(H₂O)₃ each exhibit only one isomer. The difference can be explained by the optimum matching in size between the K⁺ ion and the crown cavity; because the K⁺ ion can be deeply encapsulated by DB18C6 and the interaction between the K⁺ ion and the H₂O molecules becomes weak, different kinds of hydration geometries can occur for the K⁺•DB18C6 complex, giving multiple conformations in the experiment. For K⁺•DB18C6•(H₂O)_<i>n</i> (<i>n</i> = 4 and 5) complexes, only a single isomer is found. This is attributed to a cooperative effect of the H₂O molecules on the hydration of K⁺•DB18C6; the H₂O molecules form a ring, which is bound on top of the K⁺•DB18C6 complex. According to the stable structure determined in this study, the K⁺ ion in the K⁺•DB18C6•(H₂O)_<i>n</i> complexes tends to be pulled largely out from the crown cavity by the H₂O molecules with increasing <i>n</i>. Multiple conformations observed for the K⁺ complexes will have an advantage for the effective capture of the K⁺ ion over the other alkali metal ions by DB18C6 because of entropic effects on the formation of hydrated complexes