Solvent dynamic influence, electrode dynamics

Phelps et al. [180] have studied the influence of seven solvents on the rate constants for three sesquibicyclic hydrazine-radical cation redox systems. The authors have found that the electrode kinetics of these systems depends on the overdamped solvent dynamics, though the activation barrier due to reactant vibrational rearrangements is substantial. 

The relationship between the rate constants kei for an electrode reaction and fee for the corresponding self-exchange electron transfer reaction is not obvious because kgi can be strongly influenced by the nature and history of the electrode surface and by solvent dynamic effects if present. Electrode properties, however, are not expected to be sensitive to pressures in the 0-200 MPa range. Moreover, the signature of solvent dynamical effects is a dependence of reaction rate on solvent viscosity, but the viscosity of water is effectively independent of such pressures at near-ambient temperatures. Consequently, for typical aqueous electrode reactions, Ai/, = O.SAV, regardless of any involvement of solvent dynamics, and so AVg can be predicted from transition state theory (TST) according to Eqs (5.5)-... 

In the last two decades, studies on the kinetics of electron transfer (ET) processes have made considerable progress in many chemical and biological fields. Of special interest to us is that the dynamical properties of solvents have remarkable influences on the ET processes that occur either heterogeneously at the electrode or homogeneously in the solution. The theoretical and experimental details of the dynamical solvent effects on ET processes have been reviewed in the literature [6], The following is an outline of the important role of dynamical solvent properties in ET processes. 

Though much research on the influence of the solvent on the rate of electrode reactions has been done in recent years the problem is still far from a profound understanding. The basic question is the role of the dynamic and energetic terms in the control of the kinetics of simple electron-transfer electrode reactions. To answer this question it is essential to have reliable kinetic data for analysis. Unfortunately some kinetic data are too low and should be redetermined, preferably using submicroelectrodes. 

One major complication that distinguishes electrocatalytic reactions from catalytic reactions at metal-gas or metal-vacuum interfaces is the influence of the solvent. Modeling the role of the solvent in electrode reactions essentially started with the pioneering work of Marcus [68]. Originally these theories were formulated to describe relatively simple electron-transfer reactions, but more recently also ion-transfer reactions and bond-breaking reactions have been incorporated [69-71]. Moreover, extensive molecular dynamics simulations have been carried out to obtain a more molecular picture of the role of the solvent in charge-transfer processes, either in solution or at metal-solution interfaces. 

The steady state and dynamic photocurrent responses of nanostructured electrodes are clearly strongly influenced by their structure and by the interaction between the solid and solvent phases, but our present understanding of charge carrier transport... 

A high-speed channel electrode was used to measure the rates of electron transfer (kP) for 9,10-diphenylanthracene (DPA) in a variety of solvents. The oxidation of DPA is an outer-sphere one-electron process. It was foimd that the measured values of k were influenced by the reorientation dynamics of the solvent. 

First-principles quantum chemical methods have allowed elucidation of reaction mechanisms for a variety of heterogeneous catalytic reactions. As discussed above, incorporating the nature of the electrochemical double layer into quantum models is limited by the challenges associated with following the structure and dynamics of the electrolyte over the electrode. Further, to capture electro-catalytic reaction mechanisms accurately using DFT methods, the chemical potential of electrons and ionic species that participate in elementary steps must be evaluated. Several DFT modeling approaches have been developed to include the influence of solvent and/or electrochemical potential on surface reactions and to take into account the chemical potential of ionic species. 

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