Assessment of nonadiabaticity in outer-sphere electron-transfer reactions

Abstract

In recent years, Balzani and co-workers have promoted an approach designed to disentangle nuclear and electronic factors in electron-transfer reactions of ground and of electronically excited states.1"4 On the basis of this approach, they have reported values of xn and AG,,* in eq 1 for the self-exchange reactions kn = (ká/k^)KU{kbT/h) exp(--AGXX*/RT) (1) of several redox couples. Definitions in eq 1: ku, second-order rate constant for self-exchange reaction; k¿ and k^, rate constants for formation and for dissociation, respectively, of the outer-sphere complex formed between the two oxidation states; k,, and AG,,*, electronic and nuclear factors, respectively, for internal electron transfer within the outer-sphere complex. The reported1•3 values of ku and AG,,*, when introduced in eq 1, yield values of kn that differ from the measured rate constants by several orders of magnitude.5 Since the authors failed to point out1•3 these large discrepancies,5 they did not offer any explanation. Evidently, this is a highly unsatisfactory situation since a very important and useful result of electron-transfer theory is that the rate constants for cross-reactions can be calculated, in principle, from the measured reduction potentials and self-exchange rate constants of the individual couples, and whenever discrepancies occur and are noted, some insight into the details of the mechanisms of the reactions under consideration are revealed.6•7 It is the purpose of this communication to show that Balzani's approach1-3 cannot be reconciled with the measured self-exchange parameters of the redox couples and to point out several other problems with the approach. Moreover, we present an alternate treatment that makes use of the measured properties of the individual couples and of Marcus' cross relationship modified8 to take into account nonadiabaticity.

Reflections on the outer-sphere mechanism of electron transfer

“…We return to this point below. Other salient points are the conformity to predictions of both 5+/4+ and 3+/2+ charge types, and also of the couple F~( H~o ) ,~+ '~+ , which has been thought to behave anomalously (32,33) and for which an inner-sphere (aqua bridged) self-exchange mechanism has been proposed (32,34), a possibility that the pressure result rules out. Finally, since the theory assumes adiabatic electron transfer, the selfexchange reactions listed in Table 2 are evidently fully adiabatic or close to it; non-adiabatic behavior should be revealed in AV: values that are significantly more negative than predicted (9).…”

Kinetics and mechanism for oxidation of [RuIII(edta)(H2O)]− with peroxydisulfate in aqueous medium

“…On the basis of a pre-equilibrium approach, and by considering peroxydisulfate is stable and is involved in two-electron-transfer reactions [14,19], the above kinetic results for the formation of [(edta)Ru V (¼O)] À in the reaction of [(edta)Ru III (H 2 O)] À with S 2 O 2À 8 can be accounted for in terms of the suggested mechanism outlined in scheme 1.…”

Kinetics and mechanism of the oxidation of glyoxylic acid by peroxodisulfate induced by irradiation with visible light of aqueous solutions containing tris (2,2′-bypyridine)ruthenium(II) ion

“…However, peroxodisulfate is an oxidative quencher 28,29 of Ru(bipy) 3 2+ as the former decomposes on photoreduction into two ions and this also checks the back electron transfer. Ru(bipy) 3 2+ , SO 4• -and SO 4 2are generated on quenching of ⊗Ru(bipy) 3 2+ by peroxodisulfate, and Ru(bipy) 3 3+ and SO 4• -are strong oxidants 30,31 for the oxidation of glyoxylic acid.…”