<unk>  is the application of nuclear magnetic resonance with respect to<unk> . It is analogous to proton<unk>  and allows the identification of<unk>  atoms in an organic molecule just as proton<unk>  identifies hydrogen atoms. As such<unk> <unk>  is an important tool in structure elucidation in organic chemistry.
However,<unk> <unk>  has a number of complications that are not encountered in proton<unk> .<unk> <unk>  is much less sensitive than 1H<unk>  since the major isotope of<unk> , the 12C isotope, has a spin quantum number of zero and so is not magnetically active. Only the much less common<unk> C isotope, present naturally at 1.1% abundance, is magnetically active with a spin quantum number of 1/2 (like 1H).  Therefore, only the few<unk> -13 nuclei present resonate in the magnetic field, though this can be overcome by isotopic enrichment of eg. protein samples.  In addition, the gyromagnetic ratio (6.728284 107 rad T-1 s-1) is only 1/4 that of 1H, further reducing the sensitivity.  The overall
receptivity of<unk> C is about 4 orders of magnitude worse than 1H .
Another potential complication results from the presence of large one bond J-coupling constants between<unk>  and hydrogen (typically from 100 to 250 Hz).  In order to suppress these couplings, which would otherwise complicate the spectra and further reduce sensitivity,<unk> <unk>  spectra are proton decoupled to remove the signal splitting. Couplings between<unk> s can be ignored due to the low natural abundance of<unk> C.  Hence in contrast to typical proton<unk>  spectra which show multiplets for each proton position,<unk> <unk>  spectra show a single peak for each chemically non-equivalent<unk>  atom.
In further contrast to 1H<unk> , the intensities of the signals are not normally proportional to the number of equivalent<unk> C atoms and are instead strongly dependent on the number of surrounding spins (typically 1H). Spectra can be made more quantitative if necessary by allowing sufficient time for the nuclei to relax between repeat scans.
High field magnets with internal bores capable of accepting larger sample tubes (typically 10 mm in diameter for<unk> C<unk>  versus 5 mm for 1H<unk> , the use of relaxation reagents , for example Cr(acac)3 (chromium (III) acetylacetonate, CAS number 21679-31-2), and appropriate pulse sequences have reduced the time needed to acquire quantitative spectra and have made quantitative<unk>  a commonly used technique in many industrial labs. Applications range from quantification of drug purity to determination of the composition of high molecular weight synthetic polymers.<unk> C chemical shifts follow the same principles as those of 1H, although the typical range of chemical shifts is much larger than for 1H (by a factor of about 20).
Carbon-13 NMR
Nuclear magnetic resonance spectroscopy
Fourier transform spectroscopy
Earth's field NMR
Near infrared spectroscopy
In vivo magnetic resonance spectroscopy
Infrared spectroscopy
Spectroscopy