ABSTRACT
In an external magnetic field, the nucleus of an atom may exist in various spin states; it can have different energies. When this nucleus is irradiated with the electromagnetic waves of an appropriate frequency, it absorbs the energy and jumps from the ground state to the excited state, producing a nuclear magnetic resonance (NMR) signal. In NMR spectroscopy, nuclei with nonzero magnetic moments are studied. In the absence of an external magnetic field, the nuclear magnetic moment is randomly directed. If two nuclei are separated by more than three bonds, the corresponding long-range coupling is typically so weak that it is only observed in conjugated systems. Some provision must be made for nuclei to return back to the ground state again to ensure a steady NMR signal, which is referred to relaxation. Two types of relaxation exist: spin-lattice relaxation and spin-spin relaxation.
