Chemical shifts probe electron distribution

The NMR chemical shift of I29xe adsorbed on molecular sieves reflects all the interactions between the electron cloud of the xenon atoms and their environment in the intracrystalline void volume [1]. This nucleus therefore proved to be an ideal probe for investigating various zeolitic properties such as pore dimensions [2, 3], location of the countercations [4, 5], distribution of adsorbed or occluded phases [6-8] and framework polarisability [8, 9]. 

As already outlined in Chapter 3, the variation in 33S chemical shift with molecular structure has been quite extensively studied in sulphinyl, -SO-, sulphonyl, -S02-, and sulphonic, — SO3, moieties. It has been found that the 33S chemical shift is much more sensitive to structural variation than the chemical shift of other nuclei, such as 13C, 15N or 170. For this reason, 33S NMR is a powerful method for structural elucidation and in many cases a unique probe for assessing the properties of the electronic distribution around the sulphur atom. 

In the last decade, xenon has proven to be an efficient sorbate for probing the pore structure and the internal surface of adsorbents by NMR spectroscopy [28-30]. The advantage of xenon in comparison with other adsorbates is brought about by the large chemical shifts of Xe NMR as a consequence of the large electron shell and by the fact that xenon as a noble gas leaves the adsorbent structure essentially unaffected. In particular, in zeolite research Xe NMR has been successfully applied to probe pore and channel dimensions [31], cation distributions [32], cation sites [33], and cation mobilities [34,35] as well as matter depositions and lattice defects [36-38]. 

Not only the isotropic chemical shift but also the anisotropy of the chemical shielding (CSA) tensor of the nucleus is another unique NMR parameter that could be generally measured. Such a tensor is a probe to the electron density distribution around the nucleus and provides structural information that is very difficult to obtain by other means. 

Solution-state ID NMR spectroscopy is a technique being used to probe the chemical environments of nuclei in molecules, and is recorded on a frequency axis or the chemical shift represented as ID information of the molecules. The chemical shift of a nucleus is determined by the electron density distribution around it and the interaction with its neighboring nuclei. A vast structural information about the probed molecule could be deduced from the chemical shifts of its nuclei. However, such 1D information overlaps and the undoubted molecular structure are hard to be determined for compHcated molecules. 

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