Carbon-13 nuclear magnetic resonance

Carbon-13 (C13) nuclear magnetic resonance (most commonly known as carbon-13 NMR spectroscopy or C NMR spectroscopy or sometimes simply referred to as carbon NMR) is the application of nuclear magnetic resonance (NMR) spectroscopy to carbon. It is analogous to proton NMR (
H NMR) and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms. C NMR detects only the
Cisotope. The main carbon isotope,
C does not produce an NMR signal. Although about 1 million times less sensitive than H NMR spectroscopy, C NMR spectroscopy is widely used for characterizing organic and organometallic compounds, primarily because H-decoupled C-NMR spectra are simpler, have a greater sensitivity to differences in the chemical structure, and thus are better suited for identifying molecules in complex mixtures. At the same time, such spectra lack quantitative information about the atomic ratios of different types of carbon nuclei, because the nuclear Overhauser effect used in H-decoupled C-NMR spectroscopy enhances the signals from carbon atoms with a larger number of hydrogen atoms attached to them more than from carbon atoms with a smaller number of H's, and because full relaxation of C nuclei is usually not attained (for the sake of reducing the experiment time), and the nuclei with shorter relaxation times produce more intense signals.

The major isotope of carbon, the C isotope, has a spin quantum number of zero, so is not magnetically active and therefore not detectable by NMR. C, with a spin quantum number of 1/2, is less abundant (1.1%), whereas other popular nuclei are 100% abundant, e.g. H, F, P.