Effects of Dipolar and Exchange Interactions on the EPR Spectrum. Biradicals and Polynuclear Complexes

Abstract

The nature of the exchange interaction is illustrated by a simple example, which is treated in detail, and the phenomenological nature of the Heisenberg Hamiltonian is emphasized. First-order perturbation theory is then used to calculate the splitting of the resonance lines of two paramagnetic centers coupled by weak exchange and dipolar interactions. The magnitude of the splitting is compared with that obtained from numerically calculated spectra. The cases of polynuclear transition ion complexes and biradicals, in which strong exchange interactions exist, are treated by using equivalent operators. They are illustrated by several spectra displayed by typical pairs. The intensity of spectra given by coupled paramagnetic centres and their relaxation properties are discussed.