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

We have measured electronic and conformer-specific vibrational spectra of hydrated dibenzo-18-crown-6 (DB18C6) complexes with potassium ion, K+center dot DB18C6 center dot(H2O)(n) (n = 1-5), in a cold, 22-pole ion trap. We also present for comparison spectra of Rb+center dot DB18C6 center dot(H2O)(3) and Cs+center dot DB18C6 center dot(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+center dot DB18C6 center dot(H2O)(1) complex has only one conformer under the conditions of our experiment. For K+center dot DB18C6 center dot(H2O)(n) with n = 2 and 3, there are at least two conformers even under the cold conditions, whereas Rb+center dot DB18C6 center dot(H2O)(3) and Cs+center dot DB18C6 center dot(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+center dot DB18C6 complex, giving multiple conformations in the experiment. For K+center dot DB18C6 center dot(H2O)(n) (n = 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+center dot DB18C6; the H2O molecules form a ring, which is bound on top of the K+center dot DB18C6 complex. According to the stable structure determined in this study, the K+ ion in the K+center dot DB18C6 center dot(H2O)(n) complexes tends to be pulled largely out from the crown cavity by the H2O molecules with increasing n. 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.