The interaction between the spin magnetic moment of an unpaired electron and the nuclear spin magnetic moments resulting in the splitting of the α (spin up) and β (spin down) energy levels in an external magnetic field and, thus, in the multiplet pattern of the ESR spectra of radical-like species and transition metal compounds. Two main contributions to the hyperfine @C01025@ are usually considered, @F02339@ contact and dipolar interactions. The contact interaction is @I03353@ and related to the unpaired @S05864@ at the @N04256@, \(|\psi _{\unicode[Times]{x3C3}}|^{2}\). The dipolar interaction is anisotropic, and related to \(r^{-3}\), where \(r\) is the distance between the atom holding the unpaired electron and the @N04256@ with non-zero spin.