Prediction of the Aqueous Redox Properties of Functionalized Quinones Using a New QM/MM Variational Formulation

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Physical Chemistry

Background:

• Accurately describing solvent effects in quantum mechanical (QM) calculations is crucial for understanding chemical systems.
• Mixed quantum-classical (QM/MDFT) approaches offer a promising route to model these complex interactions.
• Quinones are of interest as electrolytes for aqueous redox-flow batteries, necessitating accurate computational studies.

Purpose of the Study:

• To develop and apply a geometry optimization procedure within the QM/MDFT framework for mixed quantum-classical systems.
• To compute the two-electron redox potentials of benzoquinone/hydroquinone couples in aqueous solution.
• To validate the QM/MDFT approach by comparing predictions with QM continuum solvent models and experimental data.

Main Methods:

• Introduced a new variational formulation for the grand potential of mixed quantum-classical systems.
• Employed a mean-field approximation to express the grand potential as a variational problem dependent on the nuclear density matrix.
• Reduced computation to a sequence of density optimizations: first solvent and electronic densities, then solute geometry.

Main Results:

• Successfully implemented a geometry optimization procedure within the QM/MDFT framework.
• Computed redox potentials for benzoquinone/hydroquinone couples in aqueous solution.
• Achieved good agreement between QM/MDFT predictions, QM continuum model results, and experimental data.

Conclusions:

• The developed QM/MDFT geometry optimization method is effective for studying complex molecules in solution.
• This approach provides accurate redox potential predictions for potential battery electrolytes.
• The QM/MDFT framework advances the computational modeling of mixed quantum-classical systems.