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.