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

Carbon-13 NMR ($^{13}$C 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 $^{13}$C NMR is an important tool in chemical structure elucidation in organic chemistry. $^{13}$C 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

$^{13}$C NMR has a number of complications that are not encountered in proton NMR. $^{13}$C NMR is much less sensitive to carbon than $^1$H NMR is to hydrogen since the major isotope of carbon, the $^{12}$C 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 $^{13}$C isotope, present naturally at 1.1% natural abundance, is magnetically active with a spin quantum number of 1/2 (like $^1$H) and therefore detectable by NMR. Therefore, only the few $^{13}$C 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 10$^7$ rad T$^{−1}$ s$^{−1}$) is only 1/4 that of $^1$H, further reducing the sensitivity. The overall receptivity of $^{13}$C is about 4 orders of magnitude lower than $^1$H.
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 $^{13}$C. 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 $^1$H NMR, the intensities of the signals are not normally proportional to the number of equivalent $^{13}$C atoms and are instead strongly dependent on the number of surrounding spins (typically $^1$H). 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 $^{13}$C NMR versus 5 mm for $^1$H 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.
$^{13}$C chemical shifts follow the same principles as those of $^1$H, although the typical range of chemical shifts is much larger than for $^1$H (by a factor of about 20). The chemical shift reference standard for $^{13}$C is the carbons in Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/tetramethylsilane (TMS), whose chemical shift is considered to be 0.0 ppm.

## 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, CH$_2$ and CH$_3$ groups by variation of the selection angle parameter (the tip angle of the final $^1$H 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 CH$_3$ in a phase opposite to CH$_2$
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 $^1$H to $^{13}$C has the secondary advantage of increasing the sensitivity over the normal $^{13}$C spectrum (which has a modest enhancement from the NOE (Nuclear Overhauser Effect) due to the $^1$H decoupling).

### The peaks seen in a 13C NMR

The peaks seen in a $^{13}$C NMR spectrum are a result of different Isotopomers. For example consider ethanol (CH$_3$CH$_2$OH). Ethanol will have 2 simple isotopomers one in which the $^{13}$C atom is on the methyl group i.e. $^{13}$CH$_3$CH$_2$OH and one isotopomer in which the $^{13}$C atom is on the methylene i.e. CH$_3$$^{13}$CH$_2$OH.
This gives rise to 2 peaks in the $^{13}$C NMR spectrum.
Note the $^{12}$C atoms do not show up in the $^{13}$C NMR.
Also given that $^{13}$C is only present in 1% abundance, there is a very small chance of also seeing the isotopologue in the $^{13}$C NMR, wherein both carbon atoms are $^{13}$C atoms (i.e. $^{13}$CH$_3$$^{13}$CH$_2$OH). These atoms would couple, giving satellites off the main peaks in a normal $^{13}$C-NMR spectrum and forming the basis for INADEQUATE ($^{13}$C–$^{13}$C correlation) experiments.

## References

* ↑ R. M. Silverstein, G. C. Bassler and T. C. Morrill (1991). Spectrometric Identification of Organic Compounds. Wiley.
* ↑ Caytan, Elsa; Remaud, Gerald S.; Tenailleau, Eve; Akoka, Serge (2007), "Precise and accurate quantitative 13C NMR with reduced experimental time", Talanta, 71 (3): 1016–1021, doi:10.1016/j.talanta.2006.05.075{{citation}}: CS1 maint: multiple names: authors list (link)