Simplification of H spectra

The chemist can adopt a variety of procedures to simplify complex spectra to make them more amenable to first-order analysis or analysis by inspection. A simple example is deuterium exchange which was described in the previous section. Acidic protons attached to carbon may also be exchanged under basic conditions. 

Considerable simplification of a complex spectrum may be achieved by running the spectrum at higher magnetic fields (up to 300 MHz), which causes linear expansion of the spectrum but leaves the relative chemical shifts unchanged. The instrumentation required, however, is expensive and may not be available. 

The most commonly used shift reagents are tris-chelates of lanthanide ions with the /1-diketones, 2,2,6,6-tetramethylheptane-3,5-dione (dipivaloylmethane, (1)) and l,l,l,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dione (2). Typical reagents are tris-(dipivaloylmethanato) europium and tris-1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dionato europium, the names of which are normally abbreviated to Eu(dpm)3 and Eu(fod)3. 

A wide range of lanthanide shift reagents is now commercially available including some derived from diketones in which all the protons have been replaced by deuterium, thus preventing any interference in the p.m.r. spectrum. 

The two major applications of lanthanide shift reagents are firstly the simplification of the spectrum, and secondly the confirmation of the assignment of signals by relating the extent of the shift to the concentration of the shift reagent. 

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Although placed towards the end, this section deals with the double resonance experiments that are probably applied most routinely. None of the principles involved is peculiar to a particular nuclear species but it is convenient to consider the possibilities of double resonance by dealing with specific nuclei and situations commonly met by practising chemists. Two important areas are the simplification of H spectra and the assignment of resonances in spectra. 

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