Molecular dynamics simulation of proton-coupled electron transfer in solution

A new approach for the molecular dynamics simulation of proton-coupled elec tron transfer reactions in solution is presented. The solute is represented by a four-state valence bond model, and the solvent is described by explic it solvent molecules. The nuclear quantum effects of the transferring hydro gen are incorporated with a procedure based on a series of purely classical molecular dynamics simulations. The resulting mixed electronic/vibrational free energy surfaces depend on two solvent reaction coordinates correspond ing to electron and proton transfer. This approach is shown to be equivalen t to adiabatic mixed quantum/classical molecular dynamics, in which the nuc lear quantum effects are included during the simulation, under well-defined , physically reasonable conditions. The results of the application of this approach to a model system are compared to those from a previous study base d on a dielectric continuum treatment of the solvent. In addition, specific molecular motions of the solvent associated with proton-coupled electron t ransfer are identified, and solvent configurations that couple the proton a nd electron transfer reactions are characterized. This methodology may be i mplemented using commercial molecular dynamics software packages with littl e or no modification to the existing programs.