Magnetic resonance in paramagnetic systems

From NMR Wiki

The magnetic moment of an electron is around 660 times stronger than that of a proton, so the interaction will have a much longer-range influence, which explains the shorter relaxation observed in EPR than in NMR.

The presence of an unpaired electron could have diverse influences on the system :

Paramagnetic relaxation enhancement : A relaxation phenomon, that occurs with the isotope part of the electron magnetic moment

Pseudocontact chemical shift : Change in the chemical shift due to anisotropy g factor ( the factor that tells at which frequency the electron will resonate for a given field )

It's important to highlight that the g factor could vary by a very large range in comparison to the chemical shift on one element. This explains why it's possible that metals induce relaxation much farther away than nitroxy ions.

Enhancing relaxation through paramagnetic molecules is a very important property that allows the effect of contrast agents in magnetic resonance imaging.