Kinetic electron spin resonance spectroscopy. Part II. Termination rates for ketyl and semiquinone radicals in alcoholic solvents

Abstract

An intermittent illumination method using e.s.r. detection has been employed to measure rate constants for the termination reactions of free radicals generated during the photoreduction of pyruvic acid, 2-oxobutyric acid, biacetyl, 1,4-benzoquinone, and 1,4-naphthoquinone in organic solvents. For these ketyl and semiquinone radicals, second-order termination rate constants are in the range 10⁸–10⁹ l mol^–1 s^–1 and show a dependence on solvent viscosity in accord with theories of diffusion-controlled reactions which include rotational diffusion within the solvent cage as a rate-influencing factor. Activation energies were measured for the combination of MeĊ(OH)CO₂H and MeĊ(OH)COMe radicals in alcoholic solvents. Observed values of 10–20 kJ mol^–1 are in general less than the temperature coefficients for viscous flow in these solvents, again in agreement with this model.