Precession Frequency

Some nuclei (eg: 1H, 13C) could exist in more than one spin state, which, in the absence of an external magnetic field, have identical energies.  For example, 1H nucleus could exist in two spin states, which, in the absence of an external magnetic field, have identical energies.



In an external magnetic field, they have different energies.



The frequency corresponding to the difference between the energies of the two spin states, E2-E1, is called the precession frequency.



Thus, the greater the energy difference between the spin states, the higher the precession frequency.  Since the energy difference between the spin states is directly proportional to the external magnetic field (see the graph below), the stronger the external magnetic field, the higher the precession frequency.



see also resonance

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