Dipolar and spin exchange effects in the ESR spectra of nitroxide radicals in solution: Part II. Water solutions adsorbed on porous silica gels

Abstract

The analysis of the ESR lineshape of a negative nitroxide radical (2,2,5,5-tetramethylpyrrolidine-1-oxyl-3-carboxylate, Tempydo −) introduced as aqueous solutions into silica gels with different pore diameters indicated that the probe was prevalently localized in the intermediate and central regions of the pores. As a function of concentration inside the pores, line broadening effects that depend on the water mobility and therefore on the pore diameter were observed. Dipole-dipole broadening prevailed at low temperature and in narrow pores, while Heisenberg spin exchange dominated at high temperatures. Adsorbed water did not behave as predicted by the Smoluchowsky theory for diffusion-controlled collisions, thus proving anomalous properties of the adsorbed water.