Electron spin-lattice relaxation in organic free radicals in solutions

Abstract

The electron spin-lattice relaxation times of several semiquinones have been measured as a function of free radical concentration, temperature, solvent, and viscosity, using the pulsed saturation recovery technique. It is found that the recovery of the signal can be fitted to an exponential in all cases within the limits of experimental error. It is also found that the relaxation time T1 is independent of the hyperfine component for a given free radical. Three different models for the liquid state have been employed to analyze the data and it has been postulated that the data, both in liquids and frozen liquids can be explained on the basis of a hindered motion, in hydrogen bonding solvents. A brief review of the earlier theoretical and experimental studies on electron spin-lattice relaxation of free radicals in solutions is also presented.

Electron spin relaxation rates for semiquinones between 25 and 295K in glass-forming solvents

“…As indicated by the fit lines, the contribution from spin rotation is much larger than from modulation of g and A anisotropy [42–44, 46, 47]. …”

“…Knowledge of semiquinone relaxation rates is needed for relaxation enhancement measurements of interspin distances such as between the iron-sulfur cluster and the flavin adenine dinucleotide (FAD) semiquinone of electron transfer flavoprotein ubiquinone oxidoreductase (ETF-QO) [40, 41]. There are a few reports in the literature concerning the temperature dependence or mechanism of spin lattice relaxation of semiquinones [42–48] at temperatures between about 200 K and room temperature at X-band. The dependence of semiquinone relaxation on microwave frequency was tested only at room temperature [46].…”

“…The dependence of semiquinone relaxation on microwave frequency was tested only at room temperature [46]. Values of the spin lattice relaxation rate, 1/T 1 , obtained by pulse methods as a function of temperature and viscosity [42–44, 46, 47] have been fitted with eq. (10).…”

Frequency (250MHz to 9.2GHz) and viscosity dependence of electron spin relaxation of triarylmethyl radicals at room temperature

“…The relative importance of these contributions depends on resonant frequency and molecular tumbling correlation times [10]. For semiquinones the dominant contributions to spin-lattice relaxation in fluid solution at X-band are spin rotation and a thermally activated process [11][12][13][14].…”

Epr: The Indian Scene

“…Semiquinones were studied in detail to determine the relaxation mechanisms and their dependence on radical-radical and solvent-radical interactions (44). The hindered motion of the semiquinone ions in a solvent cage was postulated for alcohol type solvents; the solvent cage precluded intermolecular relaxation mechanisms leading to a concentration independent T t in alcohols; while in other solvents T!…”