Effect of Ion Pairing on the Mechanism and Rate of Electron Transfer. Electrochemical Aspects

Abstract

Ion pairing may have a strong influence on the kinetics and mechanisms of electrochemical reactions as reflected by the location of the corresponding half-wave (or peak) potential. Upon increasing the extent of ion pairing (by increasing the binding constant and/ or the concentration of associating ion), the following changes are expected, taking a reductive formation of the ion pair as example. For moderate ion pairing and fast electron transfer, a positive shift of the reversible half-wave (or peak) potential by 59.6 mV (at 25 °C) per 10-fold increase of the associating ion concentration is predicted. Next, fast and strong ion pairing prompts the forward electron transfer to become rate determining. On the oxidation side, a predissociation mechanism, involving a positive shift of the wave, prevails as long as the extent of ion pairing is not too large. Upon increasing the extent of ion pairing, the height of the predissociation wave rapidly drops to zero. Direct electron transfer to the ion pair, associated with the expulsion of the associating ion, then takes place according to a mechanism in which the breaking of the ion-pair bond is successive to or concerted with electron transfer. In the latter case, the applicability of the dissociative electron-transfer theory previously developed for reductive cleavages is discussed based on appropriate quantum chemical computations.

Electrochemical study of catechol–boric acid complexes

“…This behavior is consistent with what was previously observed for electrochemical oxidation of catechol in the presence of Al 3+ [11]. Obtaining an α value lower than 0.5 (about 0.3) and small values for heterogeneous rate constants (about 2 × 10 −5 ) shows that this process can be a dissociative electron transfer [15,16].…”

Ultrafast Voltammetry for Probing Interfacial Electron Transfer in Molecular Wires

“…Whenever charge compensation is feasible at a sufficient rate, [31] it should then solely be limited by the activation barrier, that is, the reorganization energy l. If no internal molecular coordinate change occurs, as is the case for ferrocene, the rate should then be limited by the solvent longitudinal relaxation. Using ILIT experiments, Feldberg and coworkers evaluated the activation energies and the prefactors for alkane and OPV SAMs bearing a single ferrocene unit.…”

“…When both redox centers are surrounded by the electrolyte solution, enough counterions are available in the vicinity of the redox center that the ionpairing reaction should influence solely the thermodynamics (i.e., the apparent standard potential) but not the rate of the reaction. [31] Contrarily, if the intermediate ferrocene is buried in a hydrophobic medium, one expects a strong impact on the rate of electron transfer, which may only be triggered by ion penetration into the film. In the absence of a counterion and in an apolar environment, the intermediate Fc + /Fc couple necessarily has a higher E 0 value than the terminal one.…”

“…[ 31] We next turn to the dramatic change in the electron-transfer rates when changing the environment of the redox center from hydrophilic to hydrophobic by varying the length of the diluent thiol. In a previous paper concerning single ferrocenyl compounds and alkyl chains (compare to 2 in Figure 5), Creager and Rowe observed a decrease in the rate when the electroactive moiety was purposely buried inside a hydrophobic environment.…”

Uso de eletrodos de cobre e eletrodos modificados como sensores eletroquímicos

“…Atualmente há um grande número de técnicas eletroquímicas úteis para o estudo da cinética de transferência de carga e mecanismos de reação [78][79][80][81][82][83], fenômenos de corrosão [84][85][86], efeito da formação do par iônico [87][88] …”