Ab initio molecular dynamics simulation of electron and proton transfer

Activation of the solvent is crucial for electron and proton transfer. At the transition state of the these elementary charge transfer reactions the solvent is not in equilibrium with the geometry of the reactive solutes. Free energy barriers computed assuming equilibrium can therefore be considerably lower compared to estimates which include contributions from solvent fluctuations. This is one of the key observations of the Marcus theory of electron transfer, which was found to apply to proton transfer as well. Marcus derived a number of powerful relations quantifying these observations using a model in which the solvent is described by a dielectric continuum. These results were later verified by atomistic simulations, mostly based on empirical valence bond (EVB) models. A crucial step in this development, pioneered by Warshel, was a proper definition of a microscopic reaction coordinate describing the solvent degrees of freedom. His choice, which proved most successful, was the vertical energy gap between diabatic states. This quantity is natural to EVB models and can be easily computed. The proposed project aims to extend these calculations to density functional ab initio molecular dynamics methods. The idea is to replace the valence bond orbitals defining the diabatic states by suitable constraints on the electron density and position of the exchange proton. While the focus of the project is on method development, the model systems that will be used to develop and validate the method will be realistic electron and proton transfer systems in aqueous solution for which the computed results can be compared to experiment. The motivation behind the project is investigation of hydride and proton coupled electron transfer processes, as exhibited by simple biologically relevant model systems such as nicotine amide, phenols, quinones, thiols and ascorbic acid. The final phase of the project will be devoted to some of these applications. The question that will be investigated is how the kinetics of these systems is controlled by concerted electron and proton transfer (selectivity). The ultimate aim is to go on to compounds that can not be easily treated by EVB models, in particular transition metal coordination complexes, which are models for reactive sites in metallo-proteins.