Reactions in nmr tubes

In many cases it is necessary to monitor closely the progress of a small scale reaction by methods other than tic one very useful alternative is nuclear magnetic resonance (nmr). With larger scale reactions this is simply done by removing an aliquot of the reaction mixture and recording its nmr spectrum. Obviously this approach cannot be applied to reactions involving relatively small quantities of material. The answer is to carry out the reaction in an nmr tube. Both 5mm and lOmm diameter nmr tubes can conveniently be used as reaction vessels (Fig. 12.5) in the same way as test-tubes were employed in Section 12.2, but without magnetic stirrer bars. 

The reaction can then be monitored by recording the spectrum of the reaction mixture directly. There are however several important points to note when using nmr tubes as reaction vessels  

As a general rule you should use the reaction solvent that is normally employed for the type of reaction being carried out, however its deuterated equivalent must be used if H nmr spectra are to be observed. Agitation of ie reaction mixture is probably best achieved using sonication in an ltrasonic bath (Chapter 9), although periodic shaking will often suffice. 

If you require to carry out the reaction at elevated temperatures then it is jsually advizable to use a sealed nmr tube. Thick-walled nmr tubes are commercially available if the reaction mixture is required to withstand increased reaction pressures typical of those obtained in sealed tube experiments. 

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If the stirring speed is zero, the system is no longer a stirrer tank. A very common case is an NMR tube. This is mentioned because some researchers monitor homogeneous cataljdic reactions in NMR tubes, and unexpected or irrepro-ducible results are frequently obtained (vide infra). 

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