Electrons in paramgnetic centers are not “free” but can “feel” the presence of a) a small orbital magnetic moment summing to the spin contribution, and b) the presence of coupled paramagnetic nuclei. These have profound effects on the EPR spectrum and provide relevant information.
a) Orbital contribution
The contribution of the orbital moment is taken into account by considering g as a tensorial quantity: the Zeeman interaction thus depends on the direction of application of the magnetic field with respect to the characteristic direction of the paramagnetic system and so does the EPR spectrum.
b) Interactions with paramagnetic nuclei
Paramagnetic nuclei act as perturbation on the unpaired electron(s) because of the presence of two types of interactions: a contact (isotropic) interaction, and a dipolar (anisotropic) interaction,
The simpler case is that of a fast tumbling radical (S=1/2) in a fluid solution, for which the tumbling motion of the molecule averages to zero the dipolar interaction; in this case the hf tensor reduces to a constant, aiso. For small values of aiso the EPR line due to magnetic electron spin transition is split in two or more lines according to the nuclear angular momentum quantum number, I.