In order to retain the information about the attached hydrogens that a proton-coupled spectrum provides, a compromise technique called off-resonance proton-decoupling is used.
In this technique, the sample is swept by a radio frequency generator which is either slightly upfield or downfield of normal proton resonances (i.e., off resonance). When off-resonance decoupling is used, the apparent coupling constant is greatly reduced, and peak overlap is minimized.
In point of fact, off-resonance proton-decoupled spectra often show only singlets for each carbon atom, but the multiplicity of the peak (the number of protons directly attached) is reported as a letter designation above the peak (s (singlet), d (doublet), t (triplet), or q (quartet)).
The figure below shows the off-resonance proton-decoupled 13C NMR spectrum for ethyl phenylacetate. Each carbon atom gives a single signal with the letter indicating the number of attached hydrogens.
Figure 20. Off-resonance proton-coupled 13C spectra for ethyl phenylacetate.
Although off-resonance proton-decoupled spectra contain a great deal of information, the technique has been supplanted by the DEPT (Distortionless Enhancement by Polarization Transfer) experiment.
Return to: NMR Spectroscopy Page