H-2 NMR spectra for D2O adsorbed on oriented cellulose fibers

2H NMR spectra for D2O adsorbed on cellulose fibers which are macroscopically oriented demonstrate that the axis of cellulose fibers can be regarded as the director of the average molecular motion of the adsorbed D2O. The spectra for samples prepared from fiber sheets depend on the orientation of the sheets with respect to the magnetic field as well as the fiber orientation in the sheets. When the sheets are oriented perpendicular to the magnetic field, all the fibers in the sheets are perpendicular to the field irrespective of the fiber orientations in the sheets, and a partially overlapped quadrupole splitting is observed. When the sheets are oriented parallel to the magnetic field, the observed spectra deviate significantly from the expected 2D powder patterns, and the distribution of fiber orientations in the sheets has a significant influence on the observed band shape. An N-site mutual exchange model was used to account for the lateral diffusion of water molecules from one fiber to another. In a comparison of the simulated spectra based on the N-site mutual exchange model with the experimentally recorded spectra, the average lifetime for the adsorbed D2O molecules to reside on cellulose fibrils was estimated to be about 1.5 ms.

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