We are familiar with the idea that electrons possess an intrinsic quantum property known as spin (we met this when discussing how electrons pair up in the orbitals of atoms).
The best way to visualise spin is to imagine that an electron is a sphere spinning on its axis, either in a clockwise direction (in which case the spin magnetic quantum number, ms = +½) or anticlockwise (ms = -½).
It turns out that neutrons and protons in the nucleus of an atom also possess this intrinsic quantum property of spin. Now, if protons and neutrons possess spin then so must atomic nuclei, since they are composed of protons and neutrons.
Certain atomic nuclei are magnetically active because they have an unpaired proton or neutron – in fancy quantum speak, we say that these atomic nuclei have a nuclear magnetic spin quantum number of mI = +½ or mI = -½ which simply means that just like electrons, the nuclei of these particular atoms spin in either a clockwise or anticlockwise direction, and there is no difference in energy between the nuclei.
BUT …. a nucleus with unpaired protons or neutrons generates a small magnetic field when it spins. Placing these atomic nuclei in an external magnetic field, causes the magnetic field of the spinning nuclei to align either with the external magnetic field or against it. The nuclei with mI = +½ now have a slightly lower energy than the nuclei with mI = -½.
If we then irradiate our sample with radiowaves (with a frequency of energy that matches ΔE, the difference between the two energy levels, ΔE = ℎ𝛎), energy is absorbed and nuclei are promoted from the lower energy spin state to the higher one.
This energy is given out when the nuclei return to the ground state and can be detected by a radio frequency receiver (the energy is absorbed by the solvent).
Taking it further …
We can add an additional layer of complexity to this picture to give a more accurate explanation:
Now, it turns out that not all 1H nuclei or 13C nuclei in a molecule experience exactly the same magnetic field so they don’t all absorb the exact same frequency of radiation / energy to come into resonance i.e. the magnetic field experienced by a nucleus is not exactly equal to the external magnetic field applied to the sample.
The nucleus is essentially ‘shielded’ by the circulating electron density around the nucleus and the extent of this shielding depends on the position of the nucleus in the molecule.
E.g. a 1H nucleus bonded to an oxygen atom (-OH) in a molecule will experience less shielding (the oxygen atom is electron withdrawing and pulls electron density away from the 1H). ΔE is slightly larger and so the frequency of the radio energy absorbed by the nucleus to promote it to the higher energy spin state will be higher.
E.g. a 13C nucleus bonded to a methyl group will experience more shielding (methyl groups are electron donating). ΔE is slightly smaller and so the frequency of the radio energy absorbed by the nucleus to promote it to the higher energy spin state will be lower.
An NMR spectrum shows a number of signals that tell us how many different environments there are for either 1H nuclei for 13C nuclei in a molecule and we can use this information to piece together the structure.
E.g. for ethanol, C2H5OH
The absolute value of ΔE for different nuclei is not important. Each signal / peak is in reference to a standard (tetramethylsilane, TMS) and the difference between them is the chemical shift, 𝛅. TMS is an inert, non-toxic solvent which is easily removed from the sample due its volatility.
The 13C NMR spectrum shows us that there are two distinct carbon atom environments in ethanol. We can look up the chemical shift for the various environments for 13C nuclei and 1H nuclei on a data sheet.
The 1H NMR spectrum shows three distinct proton environments in a ratio of 1:2:3. This information is deduced from the integration curve (purple line on the spectrum below – the relative height of each peak, or technically the area under each peak, is proportional to the number of nuclei in each environment) although at A level the numbers of protons in each environment is written on the peaks.
You can find more help on interpreting 13C NMR spectra here, along with some practice questions, and help on interpreting 1H NMR spectra (including getting your head around spin-spin coupling) here, also with some practice questions 😎.