When a proton-decoupled spectrum is obtained, the intensities of the carbon resonances are enhanced. This effect is known as the nuclear Overhauser enhancement (NOE).
The NOE effect is quite general; it occurs whenever two dissimilar atoms are irradiated simultaneously. In the proton- decoupled experiment, the carbon-13 nuclei are observed at the same time that the hydrogen nuclei are irradiated.
Why does this type of experiment cause an enhancement in the observed signals? Well, when the hydrogen nuclei are irradiated, the number of nuclei in the higher energy "spin-opposed" state is increased. The carbon atoms to which these protons are attached adjust their equilibrium distribution to increase the population of carbon atoms in the lower energy "spin-aligned" state due to the interaction of spin dipoles. This larger population of carbon atoms in the lower spin state results in a larger number of nuclei that can be detected, and therefore an increased signal.
Not all carbon-13 signals will be enhanced to the same extent. Generally, the larger the number of attached protons the larger the enhancement will be.
In more advanced experiments, the NOE effect can be used to verify peak assignments. The NOE effect is distance dependent. The closer a hydrogen atom is to a particular carbon atom, the larger the enhancement for that carbon atoms signal. As an example, consider dimethyl formamide. Because of resonance, the two methyl groups attached to nitrogen are not equivalent. Irradiation of the aldehyde hydrogen will cause an enhancement of the syn methyl group and verify the identity of that peak.
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