Quantum versus classical electron transfer energy as reaction coordinate for the aqueous Ru2+/Ru3+ redox reaction |
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Authors: | J Blumberger M Sprik |
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Institution: | (1) Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, United Kingdom |
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Abstract: | Applying density functional theory (DFT)-based molecular dynamics simulation methods we investigate the effect of explicit
treatment of electronic structure on the solvation free energy of aqueous Ru2+ and Ru3+.Our approach is based on the Marcus theory of redox half reactions, focussing on the vertical energy gap for reduction or
oxidation of a single aqua ion. We compare the fluctuations of the quantum and classical energy gap along the same equilibrium
ab initio molecular dynamics trajectory for each oxidation state. The classical gap is evaluated using a standard point charge
model for the charge distribution of the solvent molecules (water). The quantum gap is computed from the full DFT electronic
ground state energies of reduced and oxidized species, thereby accounting for the delocalization of the electron in the donor
orbital and reorganization of the electron cloud after electron transfer (ET). The fluctuations of the quantum ET energy are
well approximated by gaussian statistics giving rise to parabolic free energy profiles. The curvature is found to be independent
of the oxidation state in agreement with the linear response assumption underlying Marcus theory. By contrast, the diabatic
free energy curves evaluated using the classical gap as order parameter, while also quadratic, are asymmetric reflecting the
difference in oxidation state. The response of these two order parameters is further analysed by a comparison of the spectral
density of the fluctuations and the corresponding reorganization free energies. |
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