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 ¹H, ²⁹Si, and ¹³C NMR. In oven-dried hectorite clay, ¹H 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 ¹H peak is observed. The assignment is further confirmed by the efficient cross-polarization observed in two-dimensional (2D) ¹H-²⁹Si HETCOR spectra, and by ²⁹Si-detected REDOR experiments with ¹H-dephasing in the ²⁹Si dipolar field, which yield a ¹H-²⁹Si distance of 2.9±0.4 Å. In these ¹H-²⁹Si experiments, the sensitivity of the ²⁹Si signal is enhanced at least fivefold by refocusing the decay resulting from the inhomogeneous broadening of the single ²⁹Si peak, stretching the ²⁹Si signal out over 80 ms. The small ¹H 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 ²⁹Si detection open excellent perspectives for NMR studies of composites of clays with organic molecules. Two-dimensional ¹H-²⁹Si and ¹H-¹H chemical-shift correlation experiments enable efficient detection of the ¹H spectrum of organic segments near the clay surface. Combined with ¹H spin diffusion, the organic segments at up to several nanometers from the clay surfaces can be probed. A 2D ¹H-¹³C correlation experiment yields the ¹³C spectrum of the organic species near the clay surfaces. A mobility gradient of intercalated poly(ethylene oxide), PEO, segments is proven in ¹H-²⁹Si WISE experiments with spin diffusion.