Site specific rotational mobility of anhydrous glucose near the glass transition as studied by 2D echo decay C-13 NMR

Site specific C-13 labeling of anhydrous glucose is used to study the time scale and geometry of reorientational motion of the exocyclic CH2OH group i n relation to the main glucose ring. By comparison of 2D echo decay NMR exp eriments with Monte Carlo simulations a bimodal distribution of jump angles , a 75% fraction of 1-2 degrees jumps and a 25% of 7-8 degrees jumps, is fo und to describe the geometry of the reorientational processes of the main r ing. For the CH2OH group the average jump angle of the larger jump process is somewhat larger. The jump rates for both the CH2OH group and the ring ar e similar. The apparent activation energy determined for the rotational mot ion of the CH2OH group and the ring is 480 +/- 40 kJ/mol, which is very sim ilar to an earlier determination using viscometry. It is concluded that the glucose ring and the exocyclic CH2OH-group mobility are strongly correlate d and that the rotational freedom of the CH2OH group should not be used to explain the faster p-relaxation process also found for glucose.