Isotopomers (isotopic isomers) are isomers having the same number of each isotopic atom but differing in their positions. For example, CH3CHDCH3 and CH3CH2CH2D are a pair of constitutional isotopomers. An isotopomer should not be confused with an isotopologue.
The peaks seen in a 13C NMR spectra are a result of different Isotopomers. For example consider ethanol (CH3CH2OH). Ethanol will have 2 simple isotopomers one in which the 13C atom is on the methyl group i.e. 13CH3CH2OH and one isotopomer in which the 13C atom is on the methylene i.e. CH313CH2OH.
This gives rise to 2 peaks in the 13C NMR spectrum.
Note the 12C atoms do not show up in the 13C NMR.
Also given that 13C is only present in 1% abundance, there is a very small chance of also seeing the isotopologue in the 13C NMR, wherein both carbon atoms are 13C atoms (i.e. 13CH313CH2OH). These atoms would couple, giving satellites off the main peaks in a normal 13C-NMR spectrum and forming the basis for INADEQUATE (13C–13C correlation) experiments.
Isotopmers have been used to determine reaction pathways e.g. A m-Benzyne to o-Benzyne Conversion Through a 1,2-Shift of a Phenyl Group. Blake, M. E.; Bartlett, K. L.; Jones, M. Jr. J. Am. Chem. Soc. 2003, 125, 6485