Water dynamics in channel hydrates investigated using H/D exchange

The dynamics and interactions of water with different channel hydrates were studied. Caffeine 4/5-hydrate, theophylline monohydrate and sodium cromoglycate were used as model compounds. The hydrogen/deuterium (H/D) exchange of the different hydrates following exposure to deuterium oxide vapour was studied using FT-Raman spectroscopy. The aim of the work was to (1) investigate the potential for H/D exchange studies to provide information about channel hydrates and (2) correlate the mobility of the water molecules inside lattice channels with structural parameters of the specific hydrates. The rate of exchange in the three different compounds was shown to vary considerably with caffeine 4/5 hydrate undergoing exchange much more rapidly than either sodium cromoglycate or theophylline monohydrate, with exchange in the latter compound being extremely slow. Based on the known crystal structures, it was possible to rationalise the results and to draw conclusions about the mechanism of exchange for the model compounds. It was found that the mobility of the water molecules in a channel hydrate is very dependent on the dimensions of the hydrate channel. Thus H/D-exchange studies may provide very useful structural and energetic information about channel hydrates.