Free radical reaction rates by electron paramagnetic resonance. Termination rates of substituted benzophenone ketyl radicals in alkanes

Abstract

PART I: A general method for the measurement of absolute rate constants for the termination of photochemically produced organic free radicals in solution has been developed. Radicals are generated in the cavity of an EPR spectrometer by high intensity UT irradiation of a suitable sample; an EPR spectrum is obtained from the steady-state concentration of intermediate radicals. The (second-order) decay of the EPR signal with time is followed after irradiation is abruptly stopped by mechanical means. Many such decay curves are averaged by computer. From the averaged decay curve plus a measurement of the steady-state radical concentration (by electronic double integration), absolute rate constants for radical termination are obtained. Rate constants as fast as diffusion-controlled may be determined by this technique. PART II: Using the flash photolysis technique, the self-termination rate constants of benzophenone ketyl, 4, 4'-dichlorobenzophenone ketyl, and 4, 4'-dimethoxybenzophenone ketyl radicals were measured in a series of eight alkanes. These rate constants were all at least about 0.5 times, but clearly less than, the corresponding theoretical diffusion-controlled rate constants. Rate variations with solvent were satisfactorily accounted for by the theory of the effect of diffusion coefficient on an observed bimolecular rate constant. There was no significant rate variation with ketyl radical. Thus, the widely different self-termination rate constants of the above ketyl radicals that have been measured in other solvents are due to substituent-dependent solvent effects. In benzene, these substituent effects may be due to charge-transfer complexing between radical and solvent.