Dipolar and spin exchange effects in the ESR spectra of nitroxide radicals in solution: Part I. Pure water solution

Abstract

The electron spin—electron spin interactions in aqueous solutions of a negatively charged nitroxide radical (2,2,5,5-tetramethylpyrrolidine-1-oxyl-3-carboxylate, Tempydo −) have been analyzed as a function of concentration and of temperature in order to suggest possible uses of this kind of probe in colloidal and biological systems. The dipole—dipole broadening was effective starting from a radical concentration > 2.5 × 10 −3 M, whereas the Heisenberg spin exchange began to be effective above 7.5 × 10 −3 M. Both spin—spin interactions followed the temperature and viscosity dependence predicted by the current theories. The Smoluchowsky theory for diffusion-controlled collision between nitroxide radicals was quite well fulfilled in water.