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 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.

Typical chemical shifts in ¹³C-NMR

DEPT spectra

DEPT spectra of propyl benzoate

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).

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.