Resolving Heterogeneous Dynamics of Excess Protons in Aqueous Solution with Rate Theory

Area of Science:

• Chemical kinetics
• Physical chemistry
• Computational chemistry

Background:

• Rate theories offer physical insights into chemical dynamics.
• Proton transfer in aqueous solutions is fundamental to many chemical processes.
• Understanding ion-pair dynamics in concentrated acid solutions is crucial.

Purpose of the Study:

• To apply transition state theory and Marcus theory to study proton transfer rates in HCl solutions.
• To investigate the pathways and lifetimes of proton transfer using ab initio molecular dynamics.
• To quantify concentration-dependent effects on proton transfer and ion-pairing.

Main Methods:

• Long ab initio molecular dynamics simulations.
• Application of transition state theory and Marcus theory.
• Isolation and quantification of specific reaction rates (proton transfer, ion-pair dissociation, solvent exchange).

Main Results:

• Good agreement between transition state theory and Marcus theory for proton transfer rates.
• Predicted concentration dependence for proton transfer and ion-pairing in HCl solutions.
• Estimated Zundel species lifetimes: 0.8 ps (2 M HCl) and 1.3 ps (8 M HCl).

Conclusions:

• Combined simulation and theory accurately quantify concentration effects in acid solutions.
• Identical molecular data sets provide a self-consistent theoretical picture of aqueous proton transfer.
• Revealed important correlations between hydronium cations and counterions in acid solutions.