H/D and D/H exchange rates in alpha-cyclodextrin and alpha-cyclodextrin inclusion compounds - Raman spectroscopic study

The H/D and D/H exchange rates of hydroxy groups in crystalline alpha-cyclodextrin (CD) exposed to H2O and D2O vapour, respectively, have been studied by monitoring the relative integrated Raman intensities of the O-H and O-D stretching bands. The time-dependent Raman intensities suggest a fast diffusion-controlled H2O/D2O equilibrium process in shorter times, followed by a much slower H/D exchange for the longer times. The deuteriation process is found to be ca. 10 times slower than the reverse process, in excellent agreement with the expected isotopic effect in an exchange mechanism whose rate-determining step involves the cleavage of a O-H(-D) bond of a water molecule. The slow H/D exchange rate observed for alpha-CD hexahydrate in comparison with the beta-CD dodecahydrate is explained by its structural rigidity, also evident from the discrete bands observed in the O-H and O-D stretching region. This structural rigidity is no longer present in the inclusion compounds of alpha-CD with an aromatic guest, which exhibit an H/D exchange rate much faster than the pure alpha-CD hexahydrate.