Mechanism and kinetics of hydrated electron diffusion

Molecular dynamics simulations are used to study the mechanism and kinetics of hydrated electron diffusion. The electron centre of mass is found to exhibit Brownian-like behaviour with a diffusion coefficient con-siderably greater than that of the solvent. As previously postulated by both experimental and theoretical works the instantaneous response of the electron to the librational motions of surrounding water molecules consti-tutes the principal mode of motion. The diffusive mechanism can be un-derstood within the traditional framework of transfer diffusion processes, where the diffusive step is akin to the exchange of an extra-molecular electron between neighbouring water molecules. This is a second-order process with a computed rate constant of 5.0 ps−1 at 298 K. In agreement with experiment the electron diffusion exhibits Arrhenius behaviour over the temperature range 298 K–400 K. We compute an activation energy of 8.9 kJ mol−1. Through analysis of Arrhenius plo ts and the application of a simple random walk model it is demonstrated that the computed rate constant for exchange of an excess electron is indeed the phenomenological rate constant associated with the diffusive process. 1 a