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(+)•DB18C6•(H2O)n (n = 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)n with n = 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)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(+)•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)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.