The great majority of carbon-13 spectra are obtained as proton-decoupled spectra. The decoupling destroys all interaction between carbon-13 nuclei and attached protons.
The decoupling process is accomplished by irradiating the sample with a broad spectrum of high intensity radio frequency radiation between 40 and 70 MHz while scanning at normal intensities at frequencies about 15 MHz. All protons become saturated and can no longer couple with carbon-13 nuclei.
The figure below shows the proton-decoupled 13C NMR spectrum for ethyl phenylacetate. Each carbon atom gives a single signal.
Figure 19. Proton-coupled 13C spectra for ethyl phenylacetate.
Although proton-decoupled spectra are much easier to interpret, a great deal of information is lost (namely, the number of hydrogen atoms attached to each carbon atom). A more advanced technique, off-resonance decoupling can restore this information while still presenting an easily interpretable spectrum.
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