Considerations for NMR in SCFs

In the previous section we have outlined how SCFs are excellent solvents for extraction and chromatography, how they provide an environmentally acceptable alternative to conventional organic solvents, and how they can be superior media for many reaction types. These factors alone would be reason enough to study these fluids by NMR spectroscopy, the premier technique for studying structure in solution, but there are also other factors which make these media attractive to the NMR spectroscopist. Perhaps most pertinent in the context of inorganic chemistry is the low viscosity of SCFs, which leads to narrowing of resonances for quadrupolar nuclei [6,7]. Traditionally NMR spectroscopy has been restricted to a handful of spin-1/2 nuclei, while quadrupolar nuclei have been relatively imfashionable because of the low resolution attainable. However these nuclei make up three-quarters of the NMR active nuclei and include most metals, so increased resolution for quadrupolar nuclei would make the whole of the periodic table amenable to study by NMR spectroscopy. 

In the following sections we will illustrate the use of these new fluids in NMR spectroscopy with examples taken from the literature and also from the results of our own research. Table 2 lists the NMR properties of the nuclei to be discussed in the remainder of this review. 

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