Introduction to Carbon-13 NMR (13C NMR)

Carbon-13 nuclear magnetic resonance, usually written as \(^{13}C\) NMR, is a spectroscopic technique that identifies how many chemically different carbon atoms are present in an organic molecule, and what kind of environment each one sits in. Like Proton NMR, it places a sample in a strong magnetic field and records the radiofrequency signal absorbed by resonating nuclei, but here the nucleus being observed is carbon rather than hydrogen.

Ordinary carbon-12 has an even number of protons and neutrons, giving it no nuclear spin, so it cannot absorb energy in a magnetic field and is invisible to NMR. Carbon-13 has an odd mass number and does possess nuclear spin, which makes it NMR active. The catch is that only about 1 in every 100 carbon atoms in a natural sample is \(^{13}C\), so \(^{13}C\) NMR signals are much weaker than \(^1H\) signals and spectra often need to be run for longer, or with more concentrated samples, to get a clear result.

As with proton spectra, chemical shift is measured in parts per million (ppm) relative to tetramethylsilane (TMS), which is set at \(\delta = 0\). Every other carbon environment is reported as a shift value somewhere along this scale.

The position of a peak along the \(\delta\) scale tells you what the carbon atom is bonded to. Carbons next to electronegative atoms, or involved in double bonds, are deshielded and appear further downfield (higher \(\delta\)). The rough ranges below are the ones most commonly needed at A-level.

Notice how the ranges overlap. A peak at 60 ppm could belong to a carbon bonded to oxygen, while a peak above 190 ppm almost always belongs to a carbon in an aldehyde or ketone \(C=O\) group. Combining the shift data with other evidence, such as a molecular ion from mass spectrometry or a carbonyl absorption from IR spectroscopy, narrows down the possible structure much faster than any single technique alone.

The single most useful rule in \(^{13}C\) NMR is this: the number of peaks in the spectrum equals the number of chemically distinct carbon environments, not the total number of carbon atoms. Carbons that are related by symmetry are equivalent and give only one peak between them.

For example, propan-1-ol, \(CH_3CH_2CH_2OH\), has three carbons and three different environments, so it produces three peaks. Propan-2-ol, \(CH_3CH(OH)CH_3\), also has three carbons, but the two \(CH_3\) groups are equivalent by symmetry, so it produces only two peaks.

Propanal, \(CH_3CH_2CHO\), and propanone, \(CH_3COCH_3\), are structural isomers with the molecular formula \(C_3H_6O\), so a spectrum alone must distinguish them.

• Propanal has three different carbon environments (the \(CH_3\), the \(CH_2\), and the \(CHO\) carbon), so its spectrum shows three peaks. The aldehyde carbon appears close to 200 ppm.
• Propanone has only two carbon environments, because its two \(CH_3\) groups are equivalent by symmetry. Its spectrum shows two peaks, with the ketone carbon also near 205 to 215 ppm.

Counting the peaks and comparing shift values is enough to tell the two isomers apart without needing any other data.

Standard \(^{13}C\) spectra are usually run in proton-decoupled mode, which removes coupling to nearby hydrogen nuclei so that every carbon environment gives a single sharp peak rather than a split multiplet. This makes \(^{13}C\) spectra far simpler to read than Proton NMR spectra, where splitting patterns and integration traces carry extra structural information. The trade-off is that \(^{13}C\) NMR gives you environment count and shift, but not the proton coupling detail that helps pin down neighbouring groups in \(^1H\) NMR.

In practice, chemists combine both techniques: \(^{13}C\) NMR quickly confirms how many distinct carbon environments a molecule has and flags functional groups such as carbonyls, while proton NMR fills in the finer detail of the carbon skeleton.

Related lessons

• Uk A Level Chemistry