Some Requisite Theoretical Background in ESR Spectroscopy

The first experimental observation of electron spin resonance was made on an iron compound by Zavoisky in Russia in 1944. A year later Purcell and Bloch and their collaborators observed independently nuclear magnetic resonance in condensed matter. While the celebrated Purcell and Bloch discoveries have sparked an explosive wave of research on nuclear magnetic resonance throughout the world, little activity on electron spin resonance followed immediately after the first success. In fact, even Zavoisky did not continue working in the field of ESR, and the development of the technique was almost exclusively left to the physicists in the decade after the war. Indeed, the potential diversity of applications of ESR to chemistry could hardly have been foreseen in the earlier days. 

The basic theory of the phenomenon of electron spin resonance was developing even before the first 

Magnetic Properties of Radicals, Molecules, and Excited Molecules 

The photochemist s first concern is whether or not a particular transient in a photochemical reaction can be detected by ESR under certain experimental conditions. 

This requires the knowledge of the magnetic properties of the species in question. Atoms having odd numbers of electrons are paramagnetic and we will not consider them here. 

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