What is shielding and deshielding in NMR? Can you give me an example?

1 Answer
Oct 1, 2015

See explanation.

Explanation:

The basic principle of NMR is to apply an external magnetic field called B0 and measure the frequency at which the nucleus achieves resonance.

Electrons orbiting around the nucleus generate a small magnetic field that opposes B0. In this case we say that electrons are shielding the nucleus from B0.

Shielding:
The higher the electron density around the nucleus, the higher the opposing magnetic field to B0 from the electrons, the greater the shielding. Because the proton experiences lower external magnetic field, it needs a lower frequency to achieve resonance, and therefore, the chemical shift shifts upfield (lower ppms) .

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Deshielding:
If the electron density around a nucleus decreases, the opposing magnetic field becomes small and therefore, the nucleus feels more the external magnetic field B0, and therefore it is said to be deshielded. Because the proton experiences higher external magnetic field, it needs a higher frequency to achieve resonance, and therefore, the chemical shift shifts downfield (higher ppms) .

How would this affect the H NMR spectrum?

Let us compare the chemical shift of CH4 protons and CH3Cl protons.
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Chlorine atom is an electronegative atom that will pull the electron density toward it ( electron withdrawing ), resulting in a deshielding of the hydrogen nucleus; an edit will fell higher external magnetic field B0 increasing the resonance frequency and therefore, shifting to higher ppms.

Hydrogen nucleus is shielded in the case of CH4 and therefore, the peak appears on the lower ppm side.

Images source: Organic Chemistry-Janice Gorzynski Smith 3rd Ed