In the spectrum you see above, each peak represents a different kind of carbon atom: each one absorbs energy (or resonates—hence the term ‘nuclear magnetic resonance’) at a different frequency. But why should carbon atoms be ‘different’? We have told you two factors that affect the energy difference (and therefore the frequency)—the magnetic field strength and what sort of nucleus is being studied.

So you might expect all 13C nuclei to resonate at one particular frequency and all protons (1H) to resonate at one (different) frequency. But they don’t. The variation in frequency for different carbon atoms must mean that the energy jump from ‘nucleus-aligned-with’ to ‘nucleus-aligned-against’ the applied magnetic field must be different for each type of carbon atom. The reason is that the 13C nuclei in question experience a magnetic field that is not quite the same as the magnetic field that we apply. Each nucleus is sur rounded by electrons, and in a magnetic field these will set up a tiny electric current. This current will set up its own magnetic field (rather like the magnetic field set up by the electrons of an electric current moving through a coil of wire or solenoid), which will oppose the magnetic field that we apply. The electrons are said to shield the nucleus from the external magnetic field. If the electron distribution varies from ¹³C atom to ¹³C atom, so does the local magnetic field experienced by its nucleus, and so does the corresponding resonating frequency.

As an example, consider ethanol (right). The red carbon attached to the OH group will have a smaller share of the electrons around it compared to the green carbon since the oxygen atom is more electronegative and pulls electrons towards it, away from the red carbon atom. The magnetic field that the red carbon nucleus feels will therefore be slightly greater than that felt by the green carbon, which has a greater share of the electrons, since the red carbon is less shielded from the applied external magnetic field—in other words it is deshielded. Since the carbon attached to the oxygen feels a stronger magnetic fi eld (it is more ‘exposed’ to the fi eld as it has lost some of its electronic shielding) there will be a greater energy difference between the two alignments of its nucleus. The greater the energy difference, the higher the resonant frequency (energy is proportional to frequency).

So, for ethanol we would expect the red carbon with the OH group attached to resonate at a higher frequency than the green carbon, and indeed this is exactly what the ¹³C NMR spectrum shows. ¹³C NMR spectrum of ethanol