Carbon-13 nuclear magnetic resonance

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Carbon-13 (C13) nuclear magnetic resonance (most commonly known as carbon-13 NMR spectroscopy or ¹³C 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 (¹
H
NMR) and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms. ¹³C NMR detects only the ¹³
C
isotope. The main carbon isotope, ¹²
C
does not produce an NMR signal. Although ca. 1 mln. times less sensitive than ¹H NMR spectroscopy, ¹³C NMR spectroscopy is widely used for characterizing organic and organometallic compounds, primarily because 1H-decoupled 13C-NMR spectra are more simple, have a greater sensitivity to differences in the chemical structure, and, thus, are better suited for identifying molecules in complex mixtures.^[1] At the same time, such spectra lack quantitative information about the atomic ratios of different types of carbon nuclei, because nuclear Overhauser effect used in 1H-decoupled 13C-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 13C 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 ¹²C isotope, has a spin quantum number of zero and so is not magnetically active and therefore not detectable by NMR. ¹³C, with a spin quantum number of 1/2, is not abundant (1.1%), whereas other popular nuclei are 100% abundant, e.g. ¹H, ¹⁹F, ³¹P.