The Phenomenon of Magnetic Resonance: Theoretical Considerations
The detailed understanding of the effects of molecular diffusion upon electron paramagnetic resonance (EPR) responses, whether they be conventional EPR, saturation-transfer (ST-EPR), or time-domain (TD-EPR), requires mastery of some of the more mathematical topics of chemical physics, including quantum and statistical mechanics of spin systems and molecular transport theories. Much of the methodology of EPR spectroscopy involves simply the qualitative measurement of changes in or analysis of spectra employing highly simplified and approximate theoretical methods. Although such qualitative analysis has its place, there is a danger of misapplying various approximate treatments. Of particular concern is the need to realize how instrumental variables and unresolved magnetic interactions influence measurements and interpretations. In this chapter we discuss the phenomena of magnetic resonance at several levels and in sufficient detail to understand how various approximate treatments relate to the more rigorous theory. Modulation has also been employed to effect a more precise definition of resonance frequencies.