Nuclear Magnetic Resonance

Nuclear magnetic resonance (NMR) spectroscopy is an analytical technique that gives information about the position of ¹³C or ¹H atoms in a molecule. It is one of the most powerful tools chemists have for working out the structure of an organic compound.Some nuclei, including ¹H and ¹³C, have a property called nuclear spin, which makes them behave like tiny magnets. Nuclei with an even number of protons and an even number of neutrons, such as ¹²C, have no overall spin and give no NMR signal.In an NMR spectrometer the sample is placed in a very strong magnetic field. The nuclei line up either with the field (lower energy) or against it (higher energy). The sample is then exposed to radio-frequency radiation. Nuclei in the lower energy state absorb radiation of the right frequency and flip to the higher energy state. This absorption is called resonance, and it is recorded as a peak.The exact frequency absorbed depends on the environment of each nucleus. Electrons around a nucleus shield it from the external field, so nuclei in different parts of a molecule, next to different atoms or groups, absorb at slightly different frequencies. The spectrum therefore shows a separate peak for each different environment.Two types are used at A level:
• ¹³C NMR, which shows the different carbon environments
• ¹H NMR (proton NMR), which shows the different hydrogen environments and how many H atoms are in each.NMR does not destroy the sample, so it can be recovered afterwards.