Hou S S, Beyer F L, Schmidt-Rohr K
Ames Laboratory, Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA.
Solid State Nucl Magn Reson. 2002 Sep-Nov;22(2-3):110-27. doi: 10.1006/snmr.2002.0075.
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 A. 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-3Si WISE experiments with spin diffusion.
通过1H、29Si和13C核磁共振技术，研究了顺磁性离子含量低的蒙脱石粘土的核磁共振（NMR）特性，以及用于高灵敏度检测硅酸盐表面附近有机物质的NMR实验。在烘干的锂皂石粘土中，1H NMR在0.35 ppm处显示出一个尖锐信号，该信号随旋转速度显著变窄。它被归属于硅酸盐层的“内层”OH质子。在氟代锂皂石中，OH基团被氟取代，未观察到这样的1H峰。二维（2D）1H-29Si HETCOR谱中观察到的有效交叉极化，以及在29Si偶极场中进行1H去相的29Si检测REDOR实验，进一步证实了该归属，实验得出1H-29Si距离为2.9±0.4埃。在这些1H-29Si实验中，通过重新聚焦单个29Si峰不均匀展宽导致的衰减，将29Si信号的灵敏度提高了至少五倍，使29Si信号在80毫秒内展开。在超过4 kHz的旋转频率下，该信号较小的1H线宽归因于粘土中较大的质子-质子距离。OH基团的高场各向同性化学位移是由于它们位于粘土结构的六边形“空腔”中，难以与氢键伙伴接触。分辨良好、易于选择的OH质子信号和高灵敏度的29Si检测为粘土与有机分子复合材料的NMR研究开辟了极好的前景。二维1H-29Si和1H-1H化学位移相关实验能够有效检测粘土表面附近有机链段的1H谱。结合1H自旋扩散，可以探测到距离粘土表面高达几纳米的有机链段。二维1H-13C相关实验得出粘土表面附近有机物种的13C谱。在具有自旋扩散的1H-3Si WISE实验中，证明了插层聚环氧乙烷（PEO）链段的迁移率梯度。
