Carbon-13 NMR - Implementation

Implementation

13C NMR has a number of complications that are not encountered in proton NMR. 13C NMR is much less sensitive to carbon than 1H NMR is to hydrogen since 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. Only the much less common 13C isotope, present naturally at 1.1% natural abundance, is magnetically active with a spin quantum number of 1/2 (like 1H) and therefore detectable by NMR. Therefore, only the few 13C nuclei present resonate in the magnetic field, although this can be overcome by isotopic enrichment of e.g. 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 13C is about 4 orders of magnitude lower than 1H.

Another potential complication results from the presence of large one bond J-coupling constants between carbon and hydrogen (typically from 100 to 250 Hz). In order to suppress these couplings, which would otherwise complicate the spectra and further reduce sensitivity, carbon NMR spectra are proton decoupled to remove the signal splitting. Couplings between carbons can be ignored due to the low natural abundance of 13C. Hence in contrast to typical proton NMR spectra which show multiplets for each proton position, carbon NMR spectra show a single peak for each chemically non-equivalent carbon atom.

In further contrast to 1H NMR, the intensities of the signals are not normally proportional to the number of equivalent 13C 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 13C NMR versus 5 mm for 1H NMR), 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 carbon-13 NMR 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.

13C 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). The chemical shift reference standard for 13C is the carbons in tetramethylsilane (TMS), whose chemical shift is considered to be 0.0 ppm.