Electron spin resonance spectroscopy theoretical aspects

Harriman, J.E. Theoretical Foundations of Electron Spin Resonance. New York Academic Press 1978. Symons, M. C. R. Chemical and Biochemical Aspects of Electron Spin Resonance Spectroscopy. New York van Nostrand-Reinhold 1978.-... 

EPR spectroscopy is a technique that affords a means for the detection and quantification of paramagnetism, i.e. the presence of unpaired electrons. It is applicable to solids, liquids or even gases. A compound or molecular fragment containing an odd number of electrons is paramagnetic. In this article we give examples to show the power and breadth of the technique. The reader is advised, however, to read the Encyclopedia article on the theoretical aspects of EPR to appreciate fully the strength of the technique and also the excellent series Specialist Periodical Reports on Electron Spin Resonance, published by the Royal Society of Chemistry, which has now reached volume 16. 

This article will discuss the concept and outline the theory of electron paramagnetic resonance (EPR), sometimes known as electron spin resonance (ESR), spectroscopy. Paramagnetism arises as a consequence of unpaired electrons present within an atom or molecule. It can be said that EPR is the most direct and sensitive technique to investigate paramagnetic materials. It will not be possible within this article to fully expound on all the theoretical aspects of EPR and so the reader is encouraged to refer to the excellent texts listed in the Further reading section. 

To resolve hf and nuclear quadrupole interactions which are not accessible in the EPR spectra, George Feher introduced in 1956 a double resonance technique, in which the spin system is simultaneously irradiated by a microwave (MW) and a radio frequency (rf) field3. This electron nuclear double resonance (ENDOR) spectroscopy has widely been applied in physics, chemistry and biology during the last 25 years. Several monographs2,4 and review articles7 11 dealing with experimental and theoretical aspects of ENDOR have been published. 

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