What's Meant By Shielding And Deshielding?


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Robert Schatten answered
Shielding and deshielding are both processes that are related to chemical shifts. A chemical shift, in nuclear magnetic resonance, describes the dependence of nuclear magnetic energy levels on the electronic environment in a molecule. These chemical shifts are relevant in nuclear magnetic resonance spectroscopy techniques such as carbon-13 nuclear magnetic resonances and proton nuclear magnetic resonance.

Atoms are classified as shielded when they have a higher induced field, i.e. A higher electron density in relation to a lower electron density. This occurs when the electrons around a nucleus circulate a magnetic field and create a secondary induced magnetic field. This field therefore opposes the applied field that is predetermined by Lenz’s law ("An induced current is always in such a direction as to oppose the motion or change causing it"). The chemical milieu of an atom can influence the density of the electrons through the polar effect. Any electron-donating alkyl groups lead to increased shielding, while electron-withdrawing substituents, such as nitro groups, lead to the deshielding of the nucleus.

Bonding electrons can also lead to shielding and deshielding effects; it is not only substituents that cause local induced fields. An example of bonding electrons leading to shielding and deshielding effects are the pi bonds in benzene. A circular current runs through the hyper conjugated system and causes a shielding effect at the molecule’s centre while at its edges there is a deshielding effect.

Shielding and deshielding can also be used to explain any trends in chemical shift. When a higher chemical shift takes place the frequency is higher, the applied effective magnetic field is lower, the nucleus is more deshielded and the signal or shift is downfield or at a low field. In comparison a lower chemical shift is also referred to as a diamagnetic shift. It is upfield and more shielded.

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