High resolution electron spin resonance of organic free radicals

The amount of information on the structure of a free radical and on the extension of the orbital of the unpaired electron obtainable with electron-spin resonance (ESR) depends on the resolution of the spectrum. Since it is in principle always possible to fullfill the technical requirements for optimum resolution, the linewidth depends on the physical phenomena in the sample itself, i.e. the relaxation.Some aspects of high resolution ESR are demonstrated with examples including the line broadening effect of spin-orbit coupling and of the molecular oxygen of the air. The importance of symmetry for reducing the maximum number of hyperfine structure components is stressed. Finally, the present limit of high resolution is demonstrated with the example of the 1,3-bisdiphenylene-allyl which exhibits about 500 hyperfine structure components with a spacing of 60 milligauss and a minimum linewidth of 17 milligauss