Dynamics of beta-cyclodextrin hydrate: solid state 2H NMR studies.

Abstract

The 2H NMR spectra and spin-lattice relaxation times of powder samples of C-deuterated beta-cyclodextrin (CD) hydrate (beta-CD-2,3,6,6-d28.11H2O) 1 and O-deuterated beta-CD hydrate (beta-CDd21.11H2 O-d22) 2 were measured in the temperature range from 160 to 295 K at 46 MHz. Information about the degree and time scales of dynamic disorder processes was obtained by comparing the powder pattents with computer simulations and analyzing the multi-exponential recovery curves. The data on 1 provide evidence for small-angle reorientations of the C-D groups with a correlation times tau c approximately 10(-7)s at 295 K and suggest that the beta-CD macrocycles do not move as a rigid entity. The NMR responses of 2 at 295 K show two components and are consistent with the hydroxyl O-D groups undergoing 2-fold reorientational jumps with tau c approximately 10(-8) s and with the water molecules exchanging between all sites and a significant fraction performing reorientational jumps with tau c approximately 10(-8)s. Below 235 K, at least three components are evident two were associated with slow reorientations (tau c approximately 10(-4)s at 60 K) of hydroxyl and water O-D groups, and the third with a small number of water molecules for which rapid reorientations (tau c < 10(-7)s) persist at 160 K. These dynamic features are considered in terms of the known structure and the findings of other techniques. These results suggest that the reorientation jumps of the hydroxyls and water molecules are lower than previously thought.