Magnetic hyperfine field principles

F.6.4.1. Introduction. If the surface-related phenomena are negligible, the electronic spins in the particle will fluctuate in unison (see Section D.3.1), and the orientation of the hyperfine field will fluctuate like the orientation of the magnetic moment of the particle m. The Mossbauer nucleus senses the relaxation of m via the hyperfine interaction and thus the Mossbauer spectrum can, in principle, be influenced by all the trajectories of m. 

The transitions in the X-band ESR spectra of triplet species occur in two regions. The so-called Anis = 1 region represents transitions between energetically adjacent pairs of the three triplet sublevels. These are characterized by two so-called zero-field splitting parameters, D and E. The parameter D is inversely proportional to the cube of the average separation of the electron spins, and E is related to the molecular symmetry. The number of lines depends on the molecular symmetry. If all three magnetic axes of the molecular carrier of the spectrum are distinct, the spectrum in the Anis = 1 region will show six major resonances, plus any hyperfine lines that may be visible. If two of the principle axes are equivalent by symmetry, only four lines will be observed. In the latter case, the parameter E has the value of... 

This chapter recalls the principles of the hyperfine coupling between electrons and nuclei in terms of energy and deals with its consequences on chemical shift. The equations for contact and pseudocontact shifts are derived and illustrated in a pictorial way. Their physical/chemical backgrounds are discussed as well as their limits of validity. The mechanisms of spin delocalization are illustrated. The perspectives when high field magnets are used are highlighted. 

Most radicals contain one or more atoms with magnetic nuclei, so that the unpaired electron interacts not only with the applied field but also with internal fields. This results in the splitting of the resonance absorption into two or more components. The principles of hyperfine splitting have been described (Symons, 1963). Interaction with a nucleus of spin I splits the resonance into (2/4-1) components e.g. the e.s.r. spectrum of 3... 

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