Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Carbon-13 NMR

Carbon-13 NMR (13C NMR 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 and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms. As such 13C NMR is an important tool in chemical structure elucidation in organic chemistry. 13C NMR detects only the Carbon-13 isotope of carbon, whose natural abundance is only 1.1%, because the main carbon isotope, Carbon-12, is not detectable by NMR since it has zero spin.

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.

Vanillin C-13 NMR spectrum

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.

Typical chemical shifts in 13C-NMR

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

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).

The peaks seen in a 13C NMR
The peaks seen in a 13C NMR spectrum are a result of different Isotopomers. For example consider ethanol (CH3CH2OH). Ethanol will have 2 simple isotopomers one in which the 13C atom is on the methyl group i.e. 13CH3CH2OH and one isotopomer in which the 13C atom is on the methylene i.e. CH313CH2OH.

This gives rise to 2 peaks in the 13C NMR spectrum.

Note the 12C atoms do not show up in the 13C NMR.

Also given that 13C is only present in 1% abundance, there is a very small chance of also seeing the isotopologue in the 13C NMR, wherein both carbon atoms are 13C atoms (i.e. 13CH313CH2OH). These atoms would couple, giving satellites off the main peaks in a normal 13C-NMR spectrum and forming the basis for INADEQUATE (13C–13C correlation) experiments.