Classical Models of Hydroxide for Proton Hopping Simulations.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Hydronium (H3O+) and hydroxide (OH-) ion diffusion in water occurs via Grotthuss hopping, a proton transfer mechanism.
• Simulating OH- hopping is difficult due to classical force fields producing overcoordinated solvation structures.

Purpose of the Study:

• To develop a classical molecular model for OH- that accurately simulates proton hopping in water.
• To overcome limitations of existing two-particle point-charge models in reproducing solvation properties and coordination numbers.

Main Methods:

• Explored two-particle point-charge models for OH- solvation in TIP3P water.
• Developed a novel three-auxiliary-particle classical model for OH-.
• Parameterized the model in the MOBHY module of CHARMM to match experimental diffusion coefficients.

Main Results:

• Two-particle models required unphysical parameter changes for accurate solvation.
• The new three-auxiliary-particle model achieved a lower, more realistic coordination number for OH-.
• The model successfully reproduced experimental aqueous diffusion coefficients and electrophoretic mobilities.

Conclusions:

• The developed classical OH- model enables accurate simulations of Grotthuss hopping.
• This model overcomes previous challenges in simulating OH- solvation and transport.
• The findings facilitate better understanding and simulation of ion transport in aqueous systems.