Shielding and Deshielding- Chemical Shift

The changes in absorption frequency due to shielding is called the chemical. Electrons, like nuclei, have an inherent magnetic field(both have spin=1/2). But the strength of the magnetic field of electron is several thousands of times stronger than a nuclei. As the electrons charge is negative, the polarity of the magnetic field generated by an electron is opposite that of the nucleus i.e., in an external magnetic field, the lowest energy nuclei "align with" the external field where as the lowest energy electrons "align against" the external magnetic field.

In a molecule, the nucleus is always surrounded by an electron cloud. as a result the nucleus will experience an effective magnetic field (Heff).

Note: Heff   is a combination of the external applied magnetic field and the magnetic field generated by the electron cloud surrounding the nucleus.

 With in a molecule there are factors which can increase or decrease the electron density surrounding a nucleus. Factors which reduce the electron density are said to deshield the nucleus(Heff  at the nucleus increases. Similarly, factors which increase the electron density are said to shield the nucleus since Heff will decrease.

H_eff       =          H_o   +   σ