The concentration of carbon-13 in a compound is very small. Therefore, the chances of having more than one carbon-13 nucleus in the same molecule are vanishingly small; indeed, the chances of two adjacent carbon atoms being carbon-13 are next to nothing. For this reason, homonuclear (carbon-carbon) coupling is rarely observed.
However, protons attached to carbon-13 atoms will couple with the carbon nucleus to split the resonance peaks observed for these carbon atoms. Figure 17 shows how a carbon-13 nucleus will be split by attached hydrogen atoms.
Figure 17. Proton-coupled 13C splitting patterns.
Spectra which show the spin-spin splitting, or coupling, between carbon-13 and the protons directly attached to it are called proton-coupled spectra or nondecoupled spectra.
Proton-coupled spectra for large organic molecules are often difficult to interpret, because the multiplets from different carbons will overlap. This is due to the fact that the coupling constants for 13C-H coupling are frequently larger than the chemical shift differences of the carbons in the spectrum. As we will find, proton-coupled carbon-13 spectra are rarely used for chemical identification.
The figure below shows the proton-coupled 13C NMR spectrum for ethyl phenylacetate. Although the alkyl carbon peaks are widely separated in this example, in more complicated structures the alkyl portion of the spectrum can be very difficult to interpret.
Figure 18. Proton-coupled 13C spectra for ethyl phenylacetate.
Typical coupling constants for 13C-H one-bond couplings are between J = 100 to 250 Hz.
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