Rapid Water Diffusion in a Cage-Type Crystal Lattice: beta-Cyclodextrin Dodecahydrate

摘要

Crystalline beta-cyclodextrin-dodecahydrate (beta-CD 12H_2O) is an excellent model for research on hydrogen bonding and on water clusters. In the crystal lattice, the beta-CD molecules form molecular cages that host seven water molecules. These are statistically and dynamically distributed over eleven positions, between which they move with jump rates of around 10~(10)-10~(11) s~(-1). There is no direct connection between the cages, so that water diffusion through the crystal lattice should occur only slowly (or not at all) (for further structural information see ret. [7]). Recently, however, we have shown that the water content of betaCD hydrate adjusts itself to humidity changes in the .atmosphere within minutes. This requires exchange of water molecules with the surroundings and water diffusion within the crystals with a rate of about 1 1000 of that in pure water, although the crystal structure is apparently densely packed (Fig, 1, see also legend) This rapid diffusion through the crystal lattice must be associated with structural fluctuations of the beta-CD molecules. which temporarily open suitable diffusion paths. In the following, we investigate these phenomena by using a combination of H_2O/D_2O and H_2 ~(16)O/ H_2~(18)O exchange experiments.