Shielding and Deshielding of Protons

The chemical shift observed for any proton is related to its shielding or dishelming in the molecule. This is because different protons have different electronic environment i.e. the electrons present in the bond close to the proton and the electron present in the neighboring bonds. These electrons indices their own magnetic field which may reinforce the applied magnetic field or oppose it. Now for NMR to occur the proton has to flip its spin form lower energy level to higher energy level i.e. it has to change its spin form to. If  the induced magnetic field reinforces the applied magnetic field, a smaller external magnetic field will be required to flip the spin of the proton. Such a proton is said to be dishelmed and the absorption is said to be downfield. On the other hand, if the induced magnetic field opposes the external magnetic field, stronger magnetic field will be required for the flip. The proton is then said to be shielded and the absorption is said to be updated. The situation may be represented as show in fig.

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