ABSTRACT
Systems containing unpaired electron spins, such as free radicals, biradicals, triplet states, most transition metal and rare-earth ions and some point defects in solids form the playground for electron paramagnetic resonance, EPR, also called electron spin resonance, ESR, or electron magnetic resonance, EMR. The fundamentals of EPR spectroscopy are very similar to the more familiar nuclear magnetic resonance (NMR) technique. Both deal with interactions of electromagnetic radiation with magnetic moments, which in the case of EPR arise from electrons rather than nuclei. With few exceptions, unpaired electrons lead to a non-vanishing spin of a particle that can be used as a spectroscopic probe. In EPR spectroscopy such molecules are studied by observing the magnetic fields at which they come into resonance with monochromatic electromagnetic radiation. Since species with unpaired electron spins are relatively rare compared to the multitude of species with magnetic nuclei, EPR is less widely applicable than NMR or even optical spectroscopy which has clear advantages with its ability to detect diamagnetic as well as paramagnetic states. What appears to be a drawback, however, can turn into an invaluable advantage, for instance, when selectively studying paramagnetic ions or molecules buried in a large protein environment. With its inherent specificity for those reactants, intermediates or products that carry unpaired electron spins, together with its high spectral resolution, EPR has excelled over many other techniques in, for example, unravelling the primary events of photosynthesis. Similarly, many key intermediates in this process have been identified by EPR. By appending a paramagnetic fragment-a so-called ‘spin-label’—to a molecule of biological importance, in effect one has acquired a probe to supply data on the interactions and dynamics of biological molecules. Very many systems of biomedical interest have had their structure and function elucidated by application of modern EPR techniques. Also EPR has allowed chemists to probe into the details of reaction mechanisms by using the technique of spin trapping to identify reactive radical intermediates. As one last example of the many successes of EPR the identification of paramagnetic species in insulators and semiconductors is worth mentioning.
