EPR Interactions - Hyperfine Couplings

Hyperfine couplings play a central role in electron paramagnetic resonance. They arise from the magnetic interaction of unpaired electrons with nuclear spins and provide one of the most important sources of information from EPR spectra. In this article, the hyperfine interaction and its implication for the shape of EPR spectra are discussed. Starting from a derivation of the general hyperfine spin Hamiltonian, three regimes for EPR spectra are distinguished based on the relative size of the electron Zeeman interaction with respect to the nuclear Zeeman and the hyperfine interactions. Representative examples are provided for EPR spectra of diluted paramagnetic centers in liquids and in the frozen solution state. In the latter case, the combined effects of g and hyperfine anisotropy can be disentangled in EPR spectra recorded at different microwave frequencies. Semiempirical rules permit to rationalize the observed size of isotropic hyperfine couplings and provide a simple tool for assignments of EPR spectra. Finally, large hyperfine couplings can give rise to subtle effects in EPR spectra at conventional microwave frequencies and fields (X-band, 0.34 T). Particularly, hyperfine anisotropy can affect transition probabilities, a fact which is at the basis for several advanced EPR experiments.