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Principle of Proton NMR spectroscopy

The principal of NMR Spectroscopy states that the absorption of radiation in the radio frequency region occurs only by those atoms whose nuclear spin, I is non zero and the organic molecules containing these atoms are placed in external magnetic field. 

In the presence of external magnetic field the number of possible orientations for nuclear spin states for NMR active nucleus are given by 2I + 1.

Taking proton as NMR active nucleus with its nuclear spin, I = 1/2 

The number of possible orientations for proton are as follows = (2 x 1/2) + 1 = 2

 The difference between these two different spin states for proton is represented by ∆E which is given as follows:

                                   ∆E = hν = hγH0/2π

                                   ν = γH0/2π

∆E = energy difference between two different spin States

γ = gyromagnetic ratio (γ = 26750 for proton NMR)

gyromagnetic ratio: It is the ratio of magnetic moment to angular momentum


Then the molecules containing nuclei with non-zero value of nuclear spin, I are placed in external magnetic field, the NMR active nuclei not only spins about its axis but also shows the precessional motion whose frequency is also given by:

                                                                       ν = γH0/2π

Where,

ν = precessional frequency

Thus, when the energy difference between two different spin states becomes equal to both the precessional frequency of the spinning nucleus as well as to the frequency of electromagnetic radiations required to induce the transition from one spin state to another the absorption of radio frequency occurs and NMR signal is observed. That's why it is known as nuclear magnetic resonance.

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