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Journal of Chemical Physics / Volume 126 / Issue 20 / ARTICLES / Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

J. Chem. Phys. 126, 204506 (2007); http://dx.doi.org/10.1063/1.2737047 (10 pages)

Free energy calculation of water addition coupled to reduction of aqueous RuO4−

Yoshitaka Tateyama1, Jochen Blumberger2, Takahisa Ohno1, and Michiel Sprik3

1National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
2Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom; Center for Molecular Modeling, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323;
3Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom

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(Received 2 February 2007; accepted 12 April 2007; published online 30 May 2007)

Free energy calculations were carried out for water addition coupled reduction of aqueous ruthenate, RuO4−+H2O+e−→[RuO3(OH)2]2−, using Car-Parrinello molecular dynamics simulations. The full reaction is divided into the reduction of the tetrahedral monoanion, RuO4−+e−→RuO42−, followed by water addition, RuO42−+H2O→[RuO3(OH)2]2−. The free energy of reduction is computed from the fluctuations of the vertical energy gap using the MnO4−+e−→MnO42− reaction as reference. The free energy for water addition is estimated using constrained molecular dynamics methods. While the description of this complex reaction, in principle, involves multiple reaction coordinates, we found that reversible transformation of the reactant into the product can be achieved by control of a single reaction coordinate consisting of a suitable linear combination of atomic distances. The free energy difference of the full reaction is computed to be −0.62 eV relative to the normal hydrogen electrode. This is in good agreement with the experimental value of −0.59 eV, lending further support to the hypothesis that, contrary to the ruthenate monoanion, the dianion is not tetrahedral but forms a trigonal-bipyramidal dihydroxo complex in aqueous solution. We construct an approximate two-dimensional free energy surface using the coupling parameter for reduction and the mechanical constraint for water addition as variables. Analyzing this surface we find that in the most favorable reaction pathway the reduction reaction precedes water addition. The latter takes place via the protonated complex [RuO3(OH)]− and subsequent transport of the created hydroxide ion to the fifth coordination site of Ru.

© 2007 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. COMPUTATIONAL METHODS
    1. Energy gap method for redox reactions
    2. Constrained molecular dynamics
    3. Reaction free energy for bond change coupled to electron transfer
    4. Method specification
  3. RESULTS AND DISCUSSION
    1. Equilibrium simulations
    2. Reduction free energies
    3. Reaction free energy for water addition
    4. Free energies of reduction coupled to water addition
    5. Reaction pathways
  4. CONCLUSION

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KEYWORDS and PACS

Keywords

free energy, water, association, reduction (chemical), ruthenium compounds, negative ions, molecular dynamics method, manganese compounds, charge exchange, thermochemistry, ion-molecule reactions

PACS

  • 82.30.Fi

    Ion-molecule, ion-ion, and charge-transfer reactions

  • 82.30.Nr

    Association, addition, insertion, cluster formation

  • 82.20.Wt

    Computational modeling; simulation

  • 82.60.Lf

    Thermodynamics of solutions

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1063/1.2737047

PUBLICATION DATA

ISSN

0021-9606 (print)  
1089-7690 (online)

Publisher

$abstract.society.value is a Member of CrossRef American Institute of Physics

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