Nuclear magnetic resonance (NMR) spectroscopy is used to work out the structure of organic molecules. It works for nuclei with an odd number of nucleons, such as 1H and 13C, which have a property called nuclear spin. 12C has an even number of nucleons, so it has no spin and gives no NMR signal.A spinning nucleus behaves like a tiny magnet. In a strong external magnetic field it can line up with the field (lower energy) or against it (higher energy). Nuclei in the lower energy state can absorb radio frequency radiation of exactly the right energy to flip into the higher energy state. This absorption is called resonance.
The exact frequency absorbed depends on the chemical environment of the nucleus. Electrons around a nucleus shield it from the external magnetic field, so nuclei in different environments (for example, close to an oxygen atom or not) absorb at slightly different frequencies. This is what allows NMR to tell the atoms in a molecule apart.In practice:
• the sample is dissolved in a solvent such as CDCl3, a deuterated solvent in which hydrogen has been replaced by deuterium, so the solvent does not produce peaks in a 1H spectrum
• a small amount of the reference compound TMS is added
• the sample is placed in a strong magnetic field and exposed to radio frequency radiation, and the absorptions are recorded as a spectrumThe positions of the absorptions are given as chemical shifts, which are covered in the next lesson.
The exact frequency absorbed depends on the chemical environment of the nucleus. Electrons around a nucleus shield it from the external magnetic field, so nuclei in different environments (for example, close to an oxygen atom or not) absorb at slightly different frequencies. This is what allows NMR to tell the atoms in a molecule apart.In practice:• the sample is dissolved in a solvent such as CDCl3, a deuterated solvent in which hydrogen has been replaced by deuterium, so the solvent does not produce peaks in a 1H spectrum
• a small amount of the reference compound TMS is added
• the sample is placed in a strong magnetic field and exposed to radio frequency radiation, and the absorptions are recorded as a spectrumThe positions of the absorptions are given as chemical shifts, which are covered in the next lesson.