Electron spin resonance (ESR) or Electron paramagnetic resonance (EPR) spectroscopy is an experimental technique for detecting and characterizing chemical systems bearing one or more unpaired electrons. The application of the technique, the background of which is similar to the more diffused nuclear magnetic resonance (NMR) spectroscopy, therefore concerns (i) organic and inorganic free radicals, (ii) trapped radicals in various matrices, including irradiated solids (radiation chemistry), frozen inert matrices and solid surfaces, (iii) transition metal ion compounds in classical inorganic systems or in biological systems, (iv) excited paramagnetic states (triplets) and several other systems of relevant scientific interest. The aim of this article is to provide an overall survey on the basic principles and on the various applications of the technique. This is done by describing, first, the physical basis of the electron resonance phenomenon (microwave absorption when the system is in the line of force of a strong external magnetic field) and, subsequently, the main types of interactions that the unpaired electron undergoes in the chemical system to which it belongs and which determines the features of the experimental spectra. These are essentially (i) the magnetic interaction of the electron with nuclei of nonzero nuclear spin (hyperfine interaction) which determines the multiline structure of the spectrum (hyperfine structure) and (ii) the interaction of the electron spin with the electron orbital angular momentum, occurring through the so‐called spin–orbit coupling. This latter interaction causes the dependence of the resonance on the orientation of the radical in the external magnetic field. Particular emphasis will be given to the different types of spectra observed according to the physical state of the investigated sample (liquid solutions, single solid crystals or microcrystalline powdered solids). In the last case (powders), the essential methodology for understanding the often complex profiles of the experimental spectra is described. Advanced electron resonance techniques, essentially ENDOR (electron–nuclear double resonance) and time‐resolved EPR will also be briefly mentioned.
The final part of the article is devoted to some analytical applications of the technique, including spin trapping (an essential tool for the detection and quantitative evaluation of reactive short‐lived radicals), radiation dosimetry (with applications to medical problems and irradiated foodstuff analysis) and dating of geologically relevant systems.
