Mechanism and kinetics of hydrated electron diffusion

TL;DR

This study uses molecular dynamics simulations to elucidate the mechanism and kinetics of hydrated electron diffusion, revealing a Brownian motion driven by water librations and quantifying the exchange process rate and activation energy.

Contribution

It provides a detailed molecular-level understanding of hydrated electron diffusion, including the exchange mechanism and kinetic parameters, supported by simulation data.

Findings

Electron exhibits Brownian diffusion with a higher diffusion coefficient than water.

Diffusive steps involve exchange of the electron between water molecules, a second-order process.

Computed exchange rate constant and activation energy align with experimental data.

Abstract

Molecular dynamics simulations are used to study the mechanism and kinetics of hydrated electron diffusion. The electron center of mass is found to exhibit Brownian-type behavior with a diffusion coefficient considerably 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 constitutes the principal mode of motion. The diffusive mechanism can be understood within the traditional framework of transfer diffusion processes, where the diffusive step is akin to the exchange of an extramolecular electron between neighboring 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 behavior over the temperature range of 298-400 K. We compute an activation energy of 8.9 kJ/mol. Through analysis of Arrhenius plots 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.