Microhydration of Dibenzo-18-Crown-6 Complexes with K+, Rb+, and Cs+ Investigated by Cold UV and IR Spectroscopy in the Gas Phase

In this Article, we examine the hydration structure of dibenzo-18-crown-6 (DB18C6) complexes with K+, Rb+, and Cs+ ion in the gas phase. We measure well-resolved UV photodissociation (UVPD) spectra of K+·DB18C6·(H2O)n, Rb+·DB18C6·(H2O)n, and Cs+·DB18C6·(H2O)n (n = 1–8) complexes in a cold, 22-pole ion trap. We also measure IR-UV double-resonance spectra of the Rb+·DB18C6·(H2O)1–5 and the Cs+·DB18C6·(H2O)3 complexes. The structure of the hydrated complexes is determined or tentatively proposed on the basis of the UV and IR spectra with the aid of quantum chemical calculations. Bare complexes (K+·DB18C6, Rb+·DB18C6, and Cs+·DB18C6) have a similar boat-type conformation, but the distance between the metal ions and the DB18C6 cavity increases with increasing ion size from K+ to Cs+. Although the structural difference of the bare complexes is small, it highly affects the manner in which each is hydrated. For the hydrated K+·DB18C6 complexes, water molecules bind on both sides (top and bottom) of the boat-type K+·DB18C6 conformer, while hydration occurs only on top of the Rb+·DB18C6 and Cs+·DB18C6 complexes. On the basis of our analysis of the hydration manner of the gas-phase complexes, we propose that, for Rb+·DB18C6 and Cs+·DB18C6 complexes in aqueous solution, water molecules will preferentially bind on top of the boat conformers because of the displaced position of the metal ions relative to DB18C6. In contrast, the K+·DB18C6 complex can accept H2O molecules on both sides of the boat conformation. We also propose that the characteristic solvation manner of the K+·DB18C6 complex will contribute entropically to its high stability and thus to preferential capture of K+ ion by DB18C6 in solution.