Rate of Electron Exchange between Cuprous and Cupric Ions in Concentrated HCl by Electron Spin Resonance

Abstract

Contrary to previous reports, the linewidth of the electron spin resonance spectrum of cupric ion as measured in gauss between points of maximum slope is a sensitive function of ionic strength, chloride ion concentration and cuprous ion concentration. Assuming that electron spin relaxation rates do not change greatly on the addition of cuprous ion, it is possible to derive an electron exchange rate for the reaction: Cu^++Cu^2+↔Cu^2++Cu^+ of k=1.53×109 l/mol/sec. This value is in good agreement with a previous determination of the same rate constant by McConnell and Weaver using the 63Cu nuclear magnetic resonance (0.5×108). The results for the linewidth changes can be rationalized in all cases by assuming that bridge chloride ions between two cupric ions or one cupric and one cuprous ion change drastically in concentration as such factors as ionic strength or chloride ion concentration changes. The g value of the electron (free) decreases as chloride ion is added, indicating delocalization of the unpaired electron onto chloride ligands.