Kinetic electron spin resonance spectroscopy of complex reaction systems

Abstract

Kinetic electron spin resonance spectroscopy with intermittent radical generation has been used to obtain the rate constants of various simultaneous reactions in systems containing 2,6-di-t-butylphenoxyl or 2,6-di-t-butyl-4-methylphenoxyl radicals, the latter terminating reversibly. The radicals were generated in various solvents by photolysis of di-t-butyl peroxide in presence of the parent phenols. A data-analysis procedure is presented for reaction systems also involving species not detectable by e.s.r. The results show that self-termination rate constants of the phenoxyl radicals are up to two orders of magnitude lower than expected for diffusion control. This is usually attributed to steric hindrance due to the bulky t-butyl groups, but the activation energies found for most solvents contradict this interpretation. An explanation is presented which involves an intermediate in the termination reaction and an extra stabilization of the free radicals by dipole–induced-dipole interactions with the solvents, the latter being characterized by their molar polarization.