Activation energy for a model ferrous-ferric half reaction from transition path sampling

Area of Science:

• Computational chemistry
• Physical chemistry

Background:

• Accurate calculation of activation parameters is crucial for understanding chemical reactions.
• Molecular simulation techniques offer powerful tools for studying reaction dynamics and energetics.

Purpose of the Study:

• To compare different molecular simulation techniques for determining activation parameters of the aqueous ferrous ion oxidation half-reaction.
• To evaluate the accuracy and efficiency of umbrella integration, Marcus theory-based thermodynamic integration, and transition path sampling.

Main Methods:

• Umbrella integration and Marcus theory-based thermodynamic integration were used, relying on the diabatic gap as the reaction coordinate.
• Transition path sampling was employed, which does not require prior knowledge of the reaction coordinate.
• Activation entropy and energy were derived from the temperature dependence of activation free energy for the first two methods.

Main Results:

• Benchmark activation energies from transition path sampling showed good agreement with standard methods.
• The activation energy for the model system was found to be significantly smaller (approximately half) than the activation free energy predicted by the Marcus model.
• This discrepancy suggests a substantial entropic contribution to the reaction energetics.

Conclusions:

• Transition path sampling provides a reliable and direct method for calculating activation energies without needing a predefined reaction coordinate.
• The model ferrous ion oxidation reaction exhibits a significant entropic contribution, leading to a lower activation energy than predicted by models assuming linear response.
• This study highlights the importance of considering entropic effects and comparing different simulation methodologies for accurate reaction parameter determination.