Line pairs, atomic spectroscopy

High resolution 29si NMR spectroscopy can provide considerable insights into the structure and distribution of silicate and aluminosilicate anions present in solutions and gels from which zeolites are synthesized. The narrowness of individual lines and the sensitivity of the chemical shift to details of the local chemical environment make it possible in many instances to identify exact chemical structures. Studies using Si NMR have shown that the distribution of anionic structures is sensitive to pH and the nature of the cations in solution. Alkali metal cation NMR has demonstrated the formation of cation-anion pairs the formation of which is postulated to affect the dynamics of silicate and aluminosilicate formation and the equilibium distribution of these species. A NMR has proven useful in identifying the connectivity of A1 to Si, but because of quadrupolar broadening cannot be used to define the precise environment of A1 atoms. 

Because of the dependence on the efg, high-resolution spectroscopy is possible for fairly weakly quadrupolar nuclei if the electronic environment is sufficiently symmetric. Examples are H, Li, Be, N, or Cs. Indeed, quadrupolar relaxation for aqueous Li" is so inefficient that relaxation times of 1000 s have been observed. Asymmetry of the efg is marked for atoms carrying lone pair electrons with s character. Thus N lines are sharp for NH/ but quite broad ca. 200 Hz) for pyridine. The lines for monocovalent bromine or iodine are usually broadened out of existence, but can be observed for aqueous 1 and lO, for example, because of the high local symmetry (Chapter 17). 

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