High-sensitivity multinuclear NMR spectroscopy of a smectite clay and of clay-intercalated polymer

The nuclear magnetic resonance (NMR) properties of a smectite clay low in paramagnetic ions, and NMR experiments to detect organic material near the silicate surfaces with high sensitivity, have been explored by H-1, Si-29, and C-13 NMR. In oven-dried hectorite clay, H-1 NMR reveals a sharp signal at 0.35 ppm that narrows significantly with spinning speed. It is assigned to the "inner" OH protons of the silicate layers. In fluorohectorite, where the OH groups are replaced by fluorines, no such H-1 peak is observed. The assignment is further confirmed by the efficient cross-polarization observed in two-dimensional (2D) H-1-Si-29 HETCOR spectra, and by Si-29-detected REDOR experiments with H-1-dephasing in the Si-29 dipolar field, which yield a H-1-Si-29 distance of 2.9 +/- 0.4 Angstrom. In these H-1-Si-29 experiments, the sensitivity of the Si-29 signal is enhanced at least fivefold by refocusing the decay resulting from the inhomogencous broadening of the single Si-29 peak, stretching the Si-29 signal out over 80 ms. The small H-1 linewidth of this signal at spinning frequencies exceeding 4 kHz is attributed to the large proton proton distances in the clay. The upfield isotropic chemical shift of the OH groups is explained by their inaccessibility to hydrogen-bonding partners, as a result of their location in hexagonal "cavities" of the clay structure. The well-resolved, easily selectable OH-proton signal and the high-sensitivity Si-29 detection open excellent perspectives for NMR studies of composites of clays with organic molecules. Two-dimensional H-1-Si-29 and H-1-H-1 chemical-shift correlation experiments enable efficient detection of the H-1 spectrum of organic segments near the clay surface. Combined with H-1 spin diffusion, the organic segments at up to several nanometers from the clay surfaces can be probed. A 2D H-1-C-13 correlation experiment yields the C-13 spectrum of the organic species near the clay surfaces. A mobility gradient of intercalated poly(ethylene oxide), PEO, segments is proven in H-1-Si-29 WISE experiments with spin diffusion. (C) 2002 Elsevier Science (USA).