Hydrated Electron Diffusion: The Importance of Hydrogen-Bond Dynamics

Mixed quantum-classical molecular dynamics simulations have been used to investigate hydrated electron diffusion within the temperature range 283-400 K. The Arrhenius to non-Arrhenius behavior observed in experiments is reproduced with a transition temperature, T(t), estimated at 320 K. Above this temperature, the activation energy for diffusion is 7.5 +/- 0.3 kJ mol(-1). By computing equilibrium constants for interconversions between H-bonded and non-H-bonded configurations, we show that hydrated electron diffusion is driven by fluctuations in the H-bond network of hydrating water molecules. The computed activation energy is in fact the energy change associated with H-bond breakings. Above T(t), librational dynamics appear to dominate H-bond breakings in hydrating water molecules. Below T(t), in the non-Arrhenius region, hydrated electron diffusion is driven by both the librational and translational dynamics of the hydrating water molecules. A kinetic analysis is presented to complement the above findings.