Shielding and Deshielding Effect

Shielding and Deshielding Effect

When a magnetic field is applied in a molecule, its electrons are caused to circulate about the nearby nuclei and thus generates an induced magnetic field. If induced field opposes the applied field at a proton, the proton is said to be shielded and if it reinforces the applied field, the proton is said to be deshielded.
Greater the election density around a proton, greater will be the induced field (local diamagnetic effect) which opposes the applied field and hence greater applied field can cause proton transition and thus absorption is shifted upfield i.e. δ value decreases.

The presence of electronegative atoms or groups lower the electron density around the proton (deshielding) and thus absorption is shifted downfield i.e. δ value increases and 𝜏 value decreases.

Greater the deshilding, larger will be δ value and lower will be 𝜏 value.

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