Nuclear magnetic resonance

Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the magnetic property of an atom's nucleus. NMR studies a magnetic nucleus, like that of a hydrogen atom, by aligning it with an external magnetic field and perturbing this alignment using an electromagnetic field. The response to the field (the perturbing), is what is exploited in NMR spectroscopy and magnetic resonance imaging.

Uses of NMR

The most famous use of NMR is in Magnetic resonance imaging for medical diagnosis, however NMR is also widely used in chemical studies. These studies are possible because nuclei are surrounded by orbiting electrons, which are also spinning charged particles and so will partially shield the nuclei. The amount of shielding depends on the exact local environment. For example, a hydrogen bonded to an oxygen will be shielded differently than a hydrogen bonded to a carbon atom. In addition, two hydrogen nuclei can interact via a process known as spin-spin coupling if they are on the same molecule, which will split the lines of the spectra in a recognisable way. By studying the peaks of NMR spectra, skilled chemists can determine the structure of many compounds. It can be a very selective technique, distinguishing among many atoms within a molecule or collection of molecules of the same type, but which differ only in terms of their local chemical environment.

Related Topics:
Magnetic resonance imaging - Oxygen - Spin-spin coupling

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By studying T2* information, a chemist may determine the identity of a compound by comparing the observed nuclear precession frequencies to known frequencies. Further structural data can be elucidated by observing spin-spin coupling, a process by which the precession frequency of a nucleus can be influenced by the magnetization transfer from nearby nuclei.

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T2 information can give information about dynamics and molecular motion.

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Because the NMR timescale is rather slow (compared to other spectroscopic methods), changing the temperature of an T2* experiment can also give information about fast reactions, such as the Cope rearrangement or about structural dynamics, such as ring-flipping in cyclohexane.

Related Topics:
Cope rearrangement - Cyclohexane

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A relatively recent example of NMR being used in the determination of a structure is that of buckminsterfullerene. This now famous form of carbon has 60 carbon atoms forming a sphere. The carbon atoms are all in identical environments and so should see the same internal H field. Unfortunately, buckminsterfullerene contains no hydrogen and so 13C NMR has to be used (a more difficult form of NMR to do). However in 1985 the spectrum was obtained by R. Curl and R. Smalley of Rice University and sure enough it did contain just the one single spike, confirming the unusual structure of C60.

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NMR is extremely useful for analyzing samples non-destructively. Radio waves and static magnetic fields easily penetrate many types of matter (in practice, anything that is not inherently ferromagnetic). For example, if one wanted to decisively know whether or not a bottle of wine was 'off', NMR could be used to analyze the wine without ever opening the bottle. This also makes NMR a good choice for analyzing dangerous samples.

Related Topics:
Ferromagnetic - Wine

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