Low-symmetry effects in electron paramagnetic resonance

Electron paramagnetic resonance (EPR) is a well-established technique for investigating the properties of metal ions in crystals, chemical complexes and biological molecules. It provides information about electronic structure and the site symmetry or nature of the environment of metal ions. The authors review the historical development of low-symmetry ideas and their theoretical basis as its major uses have been restricted to metal ions in sites of high symmetry. Major experimental results are tabulated and the discussion includes some of the attempts to interpret low-symmetry EPR data using crystal field and molecular orbital models. It is concluded that the role of excited-state orbitals, under the influence of low-symmetry fields, is largely responsible for the different orientations of the principal directions of the various interactions experienced by a paramagnetic ion.