ABSTRACT
There is a strong theoretical analogy between ESR and NMR. In ESR, the sample (a paramagnetic center PC), is immersed in a strong magnetic field and is submitted to the action of electromagnetic, radiation, the frequency of which is, in principle, changing regularly. A paramagnetic center is a chemical species containing non-paired electrons. The method is based on the fact that electrons possess a magnetic moment, that is to say, a spin angular momentum. The difference with the NMR lies in the fact that in the case of the ESR, it is an electron spin resonance. There is a transition in ESR when the PC, immersed in Field B is submitted to electromagnetic radiation of a particular frequency ν. The PC is characterized by the parameter g which is measured. It can aid in the identification of the PC. There exist hyperfine structures in ESR due to magnetic interactions between the PC and the magnetic dipole moments of the nuclei present in its structure. Some spin labels exist. They permit the study of PC.
