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 B_0 and measure the frequency at which the nucleus achieves resonance.

Electrons orbiting around the nucleus generate a small magnetic field that opposes B_0. In this case we say that electrons are shielding the nucleus from B_0.

Shielding:
The higher the electron density around the nucleus, the higher the opposing magnetic field to B_0 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 B_0, 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 CH_4 protons and CH_3Cl 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 B_0 increasing the resonance frequency and therefore, shifting to higher ppms.

Hydrogen nucleus is shielded in the case of CH_4 and therefore, the peak appears on the lower ppm side.

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