| Abstract
| - We report 17O and 27Al NMR results for 17O-enriched synthetic and natural layer silicates with significantly improved resolution of oxygen sites including basal and apical oxygens using 17O 3QMAS NMR spectroscopy. These oxygen sites have well-defined ranges of 17O isotropic chemical shifts and quadrupolar coupling constants.
- We report 17O and 27Al NMR results for 17O-enriched synthetic and natural layer silicateswith significantly improved resolution of oxygen sites including basal and apical oxygensusing 17O 3QMAS NMR spectroscopy. These oxygen sites have well-defined ranges of 17Oisotropic chemical shifts (δiso) and quadrupolar coupling constants (Cq). The Cq of basal oxygen([4]Si−O−[4]Si) range from 4.1 to 5.2 MHz for these phases, and δiso varies from 39 to 55ppm. The δiso value of the basal oxygen sites increases with increasing Cq. The Cq and δisovalues of apical oxygens ([4]Si−O−2[6]Al) are about 3.4 MHz and about 64 ppm, respectively,and are similar among the phases examined. The hydroxyl group (2[6]Al−OH) has the largestCq (about 7 MHz) among the oxygen sites, as well as the most shielded chemical shift ofabout 37−44 ppm. Analysis of the 27Al MAS NMR spinning sideband manifolds yields preciseNMR parameters for the [6]Al sites. 27Al MAS and 3QMAS NMR spectroscopies at 14.1 Tyield excellent quantification of the [6]Al/[4]Al ratio of 2 for natural muscovite. The methodsdescribed here, in particular 17O 3QMAS at high H0 field (14.1 T), are effective probes ofcrystallographically distinct sites with similar Cq and δiso value in layer silicates and arethus potentially useful for characterizing the complex and heterogeneous natural layersilicates and other nanoscale earth materials, such as crystalline and amorphous oxyhydoxides and oxides.
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