A double-QM/MM method for investigating donor-acceptor electron-transfer reactions in solution

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Biophysics

Background:

• Electron transfer (ET) is fundamental in condensed matter, impacting electrochemistry and biochemistry.
• Investigating outer-sphere ET requires methods that can handle donor-acceptor interactions over varying distances.
• Conventional QM/MM methods struggle with intermediate distances crucial for long-range ET.

Purpose of the Study:

• To develop and validate a double-quantum mechanical/molecular mechanical (d-QM/MM) method for outer-sphere electron transfer (ET) studies.
• To enable detailed investigation of ET processes involving multiple quantum mechanical regions.
• To accurately model redox potentials, reorganization energies, and electronic coupling for donor-acceptor systems.

Main Methods:

• Developed a novel d-QM/MM approach with multiple QM regions.
• Applied the method to Fe(2+/3+) self-exchange and Fe(3+) + Ru(2+) → Fe(2+) + Ru(3+) reactions in aqueous solution.
• Utilized density functional theory (DFT) hybrid functionals for calculations.

Main Results:

• The d-QM/MM method demonstrated reasonable accuracy for redox potential, reorganization free energy, and electronic coupling.
• Clear depiction of donor-acceptor distance dependencies for key ET parameters.
• Successful exploration of the intermediate donor-acceptor distance range.

Conclusions:

• The d-QM/MM method provides a robust framework for studying outer-sphere ET, especially at intermediate distances.
• This method overcomes limitations of conventional QM/MM for long-range ET phenomena.
• Applicable to diverse systems in electrochemistry and biochemistry involving transition metals.