A molecular density functional theory approach to electron transfer reactions

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A molecular density functional theory approach to electron transfer reactions , Chem. Sci.2019, Advance Article 

 

Electron transfer (ET) reactions play a central role in a wide range of chemical systems, including energy storage and harvesting in electrochemical devices or biological processes such as aerobic respiration and photosynthesis. This ubiquity can explain the considerable amount of experimental, theoretical and simulation studies that have been devoted to this class of reactions. The widely accepted theory of ET reaction in solution was proposed by Marcus. It is based on the description of the solvent by a dielectric continuum. The macroscopic fluctuations of the solvent are represented by an out-of-equilibrium polarization field, and the free energy is a functional depending quadratically on this polarization. It eventually provides a simple two-state picture, where the free energy of each state depends quadratically on a reaction coordinate. This famous two-parabola picture has been used with great success to interpret experimental results and to make predictions. However, Marcus theory does not take into account the molecular nature of the solvent which can break the linear assumption of solvent response. In such cases, we must resort to molecular simulation.

 

 

Marcus theory plays a crucial role in the study of ET reactions. This explains why its validity has been investigated extensively using molecular dynamics simulation. However, MD remains computationally very demanding, and has so far been essentially limited to simple systems. Molecular density functional theory has been proposed as an alternative to study solvation because it is computationally much faster, while retaining a molecular description of the solvent. In the present paper, we develop tools to use MDFT to study electron transfer reactions in water using MDFT. We have first derived how to compute the relevant reaction coordinate: the average vertical energy gap. We have also shown how to compute the free energy curves and the reorganization free energies

 

N'hésitez pas à consulter le communiqué de presse associé à cet article : Quand la théorie de Marcus se confronte à la DFT

 

 

Résumé: 

Chem. Sci.2019, Advance Article 

 

Beyond the dielectric continuum description initiated by Marcus theory, the standard theoretical approach to study electron transfer (ET) reactions in solution or at interfaces is to use classical force field or ab initio molecular dynamics simulations. We present here an alternative method based on liquid-state theory, namely molecular density functional theory, which is numerically much more efficient than simulations while still retaining the molecular nature of the solvent. We begin by reformulating molecular ET theory in a density functional language and show how to compute the various observables characterizing ET reactions from an ensemble of density functional minimizations. In particular, we define within that formulation the relevant order parameter of the reaction, the so-called vertical energy gap, and determine the Marcus free energy curves of both reactant and product states along that coordinate. Important thermodynamic quantities such as the reaction free energy and the reorganization free energies follow. We assess the validity of the method by studying the model Cl0 / Cl+ and Cl0 / Cl- ET reactions in bulk water for which molecular dynamics results are available. The anionic case is found to violate the standard Marcus theory. Finally, we take advantage of the computational efficiency of the method to study the influence of a solid–solvent interface on the ET, by investigating the evolution of the reorganization free energy of the Cl0 / Cl+ reaction when the atom approaches an atomistically resolved wall.

 

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Physico-chimie Théorique
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A molecular density functional theory approach to electron transfer reactions 

 

Guillaume Jeanmairet, Benjamin Rotenberg, Maximilien Levesque, Daniel Borgis  and Mathieu Salanne

 

Chem. Sci.2019, Advance Article 

 

DOI: 10.1039/C8SC04512G