Hydrated electron diffusion: the importance of hydrogen-bond dynamics

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Chemical Physics

Background:

• The behavior of hydrated electrons is crucial for understanding electron transfer processes in aqueous solutions.
• Experimental studies have observed a transition from Arrhenius to non-Arrhenius diffusion behavior for hydrated electrons.

Purpose of the Study:

• To investigate the diffusion mechanism of hydrated electrons using mixed quantum-classical molecular dynamics simulations.
• To elucidate the role of water molecule dynamics and hydrogen bond network fluctuations in hydrated electron diffusion.

Main Methods:

• Mixed quantum-classical molecular dynamics simulations were performed.
• Simulations covered a temperature range of 283-400 K.
• Equilibrium constants for H-bonded and non-H-bonded configurations were computed.

Main Results:

• The simulations reproduced the experimentally observed Arrhenius to non-Arrhenius transition at approximately 320 K.
• Above the transition temperature, the activation energy for diffusion was determined to be 7.5 ± 0.3 kJ mol⁻¹.
• Hydrated electron diffusion is driven by fluctuations in the hydrogen bond network of surrounding water molecules.

Conclusions:

• The activation energy for diffusion corresponds to the energy change associated with breaking hydrogen bonds.
• Above 320 K, librational dynamics of water molecules dominate hydrogen bond breaking.
• Below 320 K, both librational and translational dynamics of water molecules influence hydrated electron diffusion.