Models of slow rotational mobility of paramagnetic probes in glassy media

Abstract

Experimental ESR spectra of the 2,2,6,6-tetramethyl-4-oxopiperidinoxyl (TEMPON) radical probe in the glycerol, polystyrene, and polyvinylbutyral matrices measured in the temperature range 77–373 K were quantitatively compared with the ESR spectra calculated using the known theoretical models of rotational mobility. It was shown that simulation of ESR spectra by the nonlinear least-squares method is an efficient procedure for discriminating between theoretical models. The temperature ranges were determined in which it is possible to achieve quantitative agreement between experimental and theoretical spectra as well as the ranges in which theoretical models are insufficient to quantitatively describe the experimental results. It was established that the widely used model of Brownian diffusion in isotropic medium is inadequate to describe the ESR spectra in the case of slow motions of small probe molecules. It was found that specific interactions (formation of weak complexes) between the probe molecules and the molecules of the medium results in strongly anisotropic molecular rotational motions.