Secondary magnetic fields:

At this point we can see how a proton is detected through nmr spectroscopy, but if that was all there was to it we would just only see one signal for each proton in a molecule. This would tell us nothing concerning to the structure apart from the fact that protons are present. Fortunately, not all protons need similar energy for resonance. This is since there are secondary magnetic fields in the molecule that affect the magnetic field experienced by each proton. Secondary magnetic fields are generated by the electrons in the molecule and are much smaller in magnitude as compared to the applied magnetic field - in the order of 0-10 parts per million (ppm). Though, they are adequately large enough to result in different signals for different protons. Here this means that there should be one signal for every different (or non-equivalent) proton in the structure. Hence, it is helpful to identify the number of non-equivalent protons in a molecule to identify the number of signals that should be present in the spectrum.

Note: the protons in a methyl group are equal and do not give separate signals since they are in the same molecular environments.

This is also right for the protons in a CH₂ group. (Though, there are two situations in which the two protons on a CH₂ group become non equivalent, that is when they are constrained within a ring system and when they are next to an asymmetric center.) The size and direction of secondary magnetic fields depends upon electron density, diamagnetic circulation and spin-spin coupling.