Isotropic Nuclear Resonance Shifts

It is shown that isotropic (or average) nuclear resonance shifts for a nucleus in a paramagnetic molecule in solution, and in a polycrystalline solid, can be used to distinguish between Fermi contact and ``pseudocontact'' contributions to isotropic nuclear‐hyperfine interactions. The pseudocontact interaction is that isotropic hyperfine coupling which arises from the combined effects of (electron‐spin)‐(nuclear‐spin) coupling, (electron‐orbit)‐(nuclear‐spin) coupling, and electron spin‐orbit interaction. When the magnetic hyperfine interaction between the electronic moment and nuclear spin is approximated by a point dipolar interaction, and the isotropic hyperfine interaction is exclusively pseudocontact, then the isotropic nuclear shift in a polycrystalline solid exceeds the solution shift by the factor ${\text{3(g}}_{\parallel }{\mathrm{+g}}_{\perp }{\mathrm{)/(g}}_{\parallel }{\text{+2g}}_{\perp })$ where g_∥ and g_⊥ are the spectroscopic splitting factors parallel and perpendicular to the molecular symmetry axis. Isotropic shifts due to the Fermi contact interaction are the same for both solid state and solution cases.