Chemical Shift:
The description of the behaviour of nuclei in presence of magnetic field discussed so far holds good only for bare nuclei. The resonance condition for such nuclei is, v = g N µN B0 / h. It implies that all the nuclei of a given type (for example, of hydrogen atoms) in a sample should absorb the energy corresponding to the above v value. If this was the case, the NMR spectroscopy would have been of no use to the chemists. In real systems, however, we do not deal with bare nuclei. The nuclei are surrounded by electrons and their presence can modify the field experienced by the nuclei by either shielding or deshielding them. This makes the nuclei to come to resonance at different frequencies. Now let us understand this.
The electrons circulating around the nucleus (proton in case of H atom) in a spherical fashion produce an induced magnetic field (Bind) which opposes the applied filed (B0). Therefore, the magnetic field experienced by the proton in a molecule placed in a magnetic field of strength B0 is always less because of shielding or screening of the nucleus by the surrounding electrons. An effective magnetic field experienced through the nucleus (Beff) is given by the following equation:
Beff = B0 - Bind
The induced field instead is proportional to the applied field and is given by the following expression:
Bind = σ B0
The proportionality constant, σ, is known as the shielding constant and is a measure of the extent of shielding of the nucleus by the electrons. Substituting the value of Bind from Eq. 12.10 into Eq. 12.9, we get the following.
Beff = B0 - σ B0
= B0 (1 - σ )
Therefore, in presence of the extra nuclear electronic environment, the resonance condition gets modified as given below.
ν = g N µ N B0 (1 - σ ) / h
In molecular systems containing nuclei other than that of hydrogen and π or conjugated electrons, the field produced by the electrons may augment the applied field. That is the effective field at the nucleus may be more than the applied field. The value of σ will be negative in such a case and we say that the nucleus is deshielded.
When shielding occurs, the Beff is less than B0, hence B0 have to be increased to bring a nucleus to resonance. Instead while deshielding occurs, Beff is more than B0, requiring the field to be reduced to achieve resonance. Thus, the nucleus comes in resonance at lower field. Therefore, because of the shielding (or deshielding) identical nuclei (e.g., H) which have different chemical environment (in other words, different electron density) resonate at different values of the frequencies or applied field. These values being characteristic of the chemical environment can be used to identify various types of environment in which the proton is present. Since the shift in the position of resonance is due to difference in chemical environment, it is called chemical shift.