Figure 9.35
Splitting diagram for an AMX system.3,5-octanedionato)europium(III) (Eu(dpm) 3 and Eu(fod) 3 and Eu(fod) 3 respectively). When added to
solutions of molecules containing a heteroatom with an unshared electron pair, e.g. aldehydes, ketones,
alcohols, esters, ethers, amines, etc., further coordination by these molecules to the europium complex
occurs. Resulting changes in electron densities around protons in the coordinating molecules lead to
downfield or sometimes upfield shifts of their resonances. The shift is greatest for the group nearest to
the coordinating heteroatom and becomes progressively less with distance from it. The spectrum is
therefore spread out, enabling the originally overlapping signals to be observed clearly. An example of
this technique is shown in Figure 9.36 for a sample of n-hexanol. Although the shift reagent. Eu(dpm) 3 ,
causes a degree of line broadening, the expected multiplicity of each group can easily be discerned.
Exchangeable Protons
Protons attached to oxygen, nitrogen or sulphur (acids, alcohols, phenols, amines, etc.) are labile in
solution and are essentially decoupled from neighbouring protons if the rate of exchange is fast. The
resulting spectrum is simplified because the exchanging proton senses only an averaged environment
and appears as a sharp singlet. At intermediate and slow rates of exchange, the absorption may be broad
or resolved into the component peaks of a multiplet. Traces of acid or alkali in the sample catalyse the
exchange process while treatment with D 2 O removes the resonance altogether. The latter is often used
as a diagnostic test for an exchangeable proton.