An Introduction to Electron Spin Resonance and its Application to the Study of Free Radical Metabolites

Abstract

Electron spin resonance (ESR) is one of a class of magnetic resonance experiments. Nuclear magnetic resonance (NMR) is a better known member of this class, but these preliminary comments will apply equally to electron spin resonance and nuclear magnetic resonance. From the names alone it is clear that electrons and nuclei, which are fundamental particles, are involved, and that the magnetic interactions of these particles are of predominant importance. The principal ideas behind these magnetic resonance spectroscopies depend on two characteristics of these fundamental particles. The first is that they are charged, and the second is that they have angular momentum or spin. Angular momentum is the motion of a body about a center, like the rotation of the earth. Angular momentum is a vector quantity which has both magnitude and direction. Another characteristic of fundamental particles is that quantum theory governs their energy, angular momentum and other physical properties. The main result of quantum mechanics is that the physical properties are discrete and not continuous as in classical mechanics. Quantum theory demands that the component of an electron’s angular momentum in any given direction, call it Jz, be quantized, $${J_z} = m\tfrac{h}{{2\pi }}(erg{\kern 1pt} \sec )$$ (1) where J, is the component of angular momentum in the direction z, where h is Planck’s constant and m is the angular momentum quantum number. For an electron, m is limited to the discrete values ±1/2. You can think of this as (clockwise and counter clockwise) rotation or spin.