Thermodynamic and kinetic characteristics of intermediates of electrode reactions: A comparative investigation of a number of alkylaryl and alkyl halide radicals by the laser photoemission methods

Abstract

A comparative investigation of thermodynamic and kinetic properties of a number of alkylaryl intermediates (benzyl and benzhydryl radicals) and alkyl halide intermediates (chloromethyl, dichloromethyl, and trifluoromethyl radicals) is performed by methods of laser photoemission. Techniques, aimed at the determination of thermodynamic and kinetic properties of intermediates (standard potentials E 0 of redox pairs R⋅/R-, standard adsorption free energies -ΔG a(R) 0 , values of rate constants W 0 at an equilibrium potential, as well as lifetimes (times of death in the bulk) τR of radicals R⋅ and τX of products of their reduction R-) from a comparison of Tafel plots for quasi-reversible reduction of intermediates with calculated ones and standard potentials E 0—from Tafel plots for irreversible electroreduction of intermediates, are presented. The transition from irreversible to quasi-reversible reduction in aprotic solvents at E∼E 0 is observed only in the case of benzyl, benzhydryl, and trifluoromethyl radicals, for which this particular collection of thermodynamic and kinetic properties is obtained, and is not observed for the chloromethyl and dichloromethyl radicals. In this case redox characteristics of intermediates (E 0, W 0) are estimated from absolute values of rates of their electroreduction. Possible reasons for the differences in the probability of a reversible electron transfer are discussed for the systems studied.