Splitting of 13C Signals

NMR Spectroscopy Explained Simplified Theory, Applications and Examples for Organic Chemistry and Structural Biology, by Neil E Jacobsen Copyright 2007 John Wiley Sons, Inc. 

In addition, there are isotopomers with two 13C isotopes in one molecule  

Each of these gives rise to an AB pattern due to 13C-13C splitting ( Jcc for the first two and 2JCC for the third species). These additional 13C signals appear as weak satellite peaks (0.55% of the main peak) around the main peaks from the first three species, and because signal-to-noise ratios are typically much less than 200 for 13C spectra, these signals will be buried in the noise. Finally, there is one isotopomer with three 13C nuclei  

This species is invisible to 13C NMR and does not contribute at all to the 13C spectrum. 

The advantage of 13C s low natural abundance can be seen clearly in this example each carbon resonance in the spectrum represents a pure isotopomer with 13C only at that position and 12C at all other positions within the molecule. Any species with two or more 13 C atoms in the molecule is present at a concentration of at most two orders of magnitude lower than the one-13 C isotopomers, so we will never see any contribution from these species in our 13 C spectrum. 

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