Basic Principles and Elements of an NMR Experiment

Nuclear spin, characterized by a spin quantum number I, is a quantum mechanical property possessed by some nuclei. This rotation of the nucleus around its spinning axis gives rise to an angular momentum, p, whose value is defined by 

Following quantum mechanical rules, a nucleus with spin I may occupy (21 + 1) different energy levels when placed in a magnetic field. For l3C nuclei, I = 1/2, 

In collections of identical individual nuclei, there is a very slight excess of nuclei in the lower state, on the order of 1 in 105 at 25 °C, such that 

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