Cross relaxation of free radicals in partially ordered solids

Magnetic energy transfer or cross relaxation between paramagnetic centers in solids depends strongly on the separations of the paramagnetic species involved. The cross-relaxation rate and kinetic form are calculated for three different radial distribution functions. The results show that spin probe-spin label cross-relaxation studies can be quite sensitive to the local structure around free radicals. Cross relaxation appears to be a simple probe of the structure of many systems of biological or chemical interest for which diffraction methods are difficult or impossible. Potential applications are measurements of distances in proteins and protein complexes, micelles and biological membranes, radiation-damaged solids, surfaces, and interfaces between condensed phases.