Interfacial electron transfer: Effects of ion solvation and solvent dynamics

In this project we want to develop methods for a quantitative prediction of the electron transfer kinetics in electrolyte solutions. In particular, we want to study the effects of the composition of the electrolyte solution on the kinetics and understand the underlying mechanisms. In order to achieve this goal, we must address a number of different issues: Electrolyte structure and dynamics in bulk and interfacial regions. Electron transfer rate dependence on solution microstructure. Structural and dynamical peculiarities of mixed - solvent electrolytes. Charge transfer between electrode and solution. The systems we want to study are aqueous solutions of metal ions and complexed metal ions. By adding other molecules like hydrocarbons and glycols we change the structure of these solutions and determine their electrochemical properties. The combination of molecular dynamics simulations and quantum chemical approaches with stochastic theory makes it possible to obtain results which can be readily used for direct and multifarious comparison with experimental data such as polarization curves, symmetry coefficients, exchange currents and adsorption isotherm parameters. This is a joint collaboration project between two partners in Russia and one in Austria. The Austrian team leader will be responsible for "structure elucidation" by modeling. Molecular dynamics simulations of different kind will be performed and analysed in terms of static and thermodynamic properties. The Russian partner at Moscow State University will be responsible for supporting experiments and the Team leader at Kazan Technological University will be responsible for the electrochemical theories that bridge the gap between simulations and experimental results.

Normally one would think that a chemical reaction depends mostly on the partners reacting. For many reactions, however, the solvent in which it takes place is at least equally important, not only regarding its structure but also because of its dynamics, as has become more and more clear in recent years. Solvent effects come in many flavours and modelling them is a challenging but necessary task.One of these flavours investigated by us is the solvent reorientation dynamics. One might simplistically think that a slow solvent slows down a reaction because it remains in a position unfavourable for the transition state and the products. In contrast, a fast solvent solvates all states optimally without delay. While such dynamics complicate the picture, they could potentially be useful for tuning reactions by modifying the solvent. We investigated an exemplary reaction and also studied solvent properties in detail. This reaction was an electron transfer in ethylene glycol / water mixtures for which experimental so?called dielectric spectra are available. We constructed a five?dimensional free?energy surface describing the electron transfer. The solvent reorganization energies were derived from molecular dynamics simulations. The importance of solvent dynamics is clearly revealed by the large difference of the reorganization energies for reduction and oxidation. Brownian dynamics was used to estimate electron?transfer rates which are in good agreement with experimentsAnother effect investigated by was the structure of liquid helium on surfaces of charged clusters and fullerenes. Despite of the low interaction energies we found pronounced structuring and shell formation. In a similar study with hydrogen molecules ordered layers of H2 were found as well. Water, in contrast, behaved differently, due to its tendency to form water clusters. Ann these studies involved extensive quantum chemical calculations on various model systems and molecular dynamics simulations.The stabilization of gold?cyanide, a linear anion with an interesting charge distribution, by its solvation shell of nitromethane molecules was investigated by quantum chemical methods as well as by molecular dynamics simulations. It was found that minimal?energy cluster structures survive up to room temperature.Another process, the effect of microsolvation on the decomposition rate of sulfides was investigated quantum chemically. It was possible to explain the unexpected behaviour of the reaction rate with a new mechanism that might also be of more general importance.