Incomplete Averaging of Anisotropies

Careful examination of the isotropic ESR spectra of organic radicals may reveal variations in line widths from one hyperfine component to another. Such effects 

Much of the width arises from incomplete averaging of anisotropies in the g-and hyperfine matrices (Chapter 3). For radicals with axial symmetry the parameters of eqn (2.8) depend on Ag = - g , AA, = AiM - A and tr, 

Since these terms are proportional to tr, they increase with decreasing temperature.1 There are several line-width contributions, included in oc0, which do not depend on m,-. These include magnetic field inhomogeneity and the spin rotation interaction, the latter increasing with 1/tr and thus with increasing temperature. These and other line-width effects have been studied in some detail and are discussed elsewhere.13 

If the g- and hyperfine anisotropies are known from analysis of a solid-state spectrum, the line-width parameters (1, and yt can be used to compute the rotational correlation time, tr, a useful measure of freedom of motion. Line widths in ESR spectra of nitroxide spin labels, for example, have been used to probe the motional freedom of biological macromolecules.14 Since tr is related to the molecular hydrodynamic volume, Ft, and the solution viscosity, r, by a relationship introduced by Debye 15 

ESR line widths can provide a measure of the effective size of a paramagnetic molecule or ion - useful information when there is a suspicion that a radical has polymerized. 

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