Carbon-13 NMR

Overview


Carbon-13 NMR is the application of nuclear magnetic resonance with respect to carbon. It is analogous to proton NMR and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms. As such carbon NMR is an important tool in structure elucidation in organic chemistry.

However, carbon NMR has a number of complications that are not encountered in proton NMR. Carbon NMR is much less sensitive than 1H NMR since the major isotope of carbon, the 12C isotope, has a spin quantum number of zero and so is not magnetically active. Only the much less common 13C isotope, present naturally at 1.1% abundance, is magnetically active with a spin quantum number of 1/2 (like 1H). Therefore, only the few carbon-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 13C is about 4 orders of magnitude worse 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).

DEPT spectra
DEPT stands for Distortionless Enhancement by Polarization Transfer. It is a very useful method for determining the presence of primary, secondary and tertiary carbon atoms. The DEPT experiment differentiates between CH, CH2 and CH3 groups by variation of the selection angle parameter (the tip angle of the final 1H pulse):
 * 45° angle gives all carbons with attached protons (regardless of number) in phase
 * 90° angle gives only CH groups, the others being suppressed
 * 135° angle gives all CH and CH3 in a phase opposite to CH2

Signals from quaternary carbons and other carbons with no attached protons are always absent (due to the lack of attached protons).

The polarization transfer from 1H to 13C has the secondary advantage of increasing the sensitivity over the normal 13C spectrum (which has a modest enhancement from the NOE (Nuclear Overhauser Effect) due to the 1H decoupling).