Analysis of resolved ESR spectra of neutral nitroxide radicals in ethanol and pyridine: the dynamic behavior in fast motion conditions

Abstract

The ESR spectra of ethanol and pyridine solutions of the spin probes 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol) and 3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-oxyl (Tempyo) were analyzed to obtain information on the dynamic behavior of the radicals in solution. In order to evaluate the true line widths, the dependence of the proton and carbon-13 hyperfine coupling constants on temperature and on solvent polarity were analyzed as a function of a/sub N/. Tempol and Tempyo showed an opposite dependence of a/sub H/ on a/sub N/. Furthermore the structure of Tempol was affected more than that of Tempyo by variations of solvent polarity. From the line widths the correlation times for motion were evaluated. A weak hydrogen bond was formed by Tempol and Tempyo with ethanol. However, only Tempol in ethanol gave rise to an anisotropic rotational reorientational motion, with the fastest rotation about the x axis. All samples, except for Tempol in pyridine, showed a deviation from the Debye spectral densities. The hydrodynamic rotational radii were about 3 times lower than the effective geometric spherical radii. However, Tempyo exerted slightly greater torques than Tempol during its rotational motion. At the highest temperatures (T > 298 K), the dynamic behavior was as expected on the basis of the free volumemore » model, whereas at lower temperatures an increase in the H-bonding strength was found between radical and ethanol molecules.« less