The response of water to a change of charge of a solvated ion is, to a good approximation, linear for the type of iron-like ions frequently used as a model system in classical force field studies of electron transfer. Free energies for such systems can be directly calculated from average vertical energy gaps. Exploiting this feature, we have computed the free energy and the reorganization energy of the M2+/M3+ and M1+/M2+ oxidations in a series of model systems all containing a single Mn+ ion and an increasing number of simple point charge water molecules. Long-range interactions are taken into account by Ewald summation methods. Our calculations confirm the observation made by Hummer, Pratt, and Garcia (J. Phys. Chem. 1996, 100, 1206) that the finite size correction to the estimate of solvation energy (and hence oxidation free energy) in such a setup is effectively proportional to the inverse third power (1/L3) of the length L of the periodic cell. The finite size correction to the reorganization energy is found to scale with 1/L. These simulation results are analyzed using a periodic generalization of the Born cavity model for solvation, yielding three different estimates of the cavity radius, namely, from the infinite system size extrapolation of oxidation free energy and reorganization energy, and from the slope of the linear dependence of oxidation free energy on 1/L3. The cavity radius for the reorganization energy is found to be significantly larger compared to the radius for the oxidation (solvation) free energy. The radius controlling the 1/L3 dependence of oxidation free energy is found to be comparable to the radius for reorganization. The implication of these results for density functional theory-based ab initio molecular dynamics calculation of redox potentials is discussed.

The geometrical structure of the tetraaquo and tetrachloro Pt(II) complexes in aqueous solutions has been studied by means of X-ray absorption spectroscopies (EXAFS and XANES), combined with quantummechanical computations. The latter were carried out to supply independent information about the arrangement of water molecules around the complexes. To this aim thewere optimized and the XANES spectra computed using this theoretical structural information were compared with the experimental spectra. From this comparison it was deduced that the hydration shell of the tetraaquo complex was responsible for a small feature of the XANES spectrum above the white line. Pt-Cl distance in [PtCl 4 ] 2units, both in the crystalline compound, K 2 PtCl 4 , and in aqueous solution, was found to be 2.30 Å. Pt-O distance in [Pt(H 2 O) 4 ] 2+ species was 2.02 Å. No evidence of stable axial water molecules was found in the aquo complex case. Quantum-mechanical optimization of [PtCl 4 ] 2-•(H 2 O) 2 aggregate indicated that water molecules adopt axial orientation with a Pt-O distance of 3.3 Å.

In this paper a theoretical study of the bromide solvation in three different polar solvents is presented:  water, methanol, and acetonitrile. DFT (B3LYP) calculations on the structure and energetics of [Br(Solv) n ]- clusters, for n = 1−9 and Solv = H2O, CH3OH, and CH3CN, have been carried out. Different structures where the anion is placed inside or on the surface of the cluster have been explored. The relative importance of solvent−solvent vs ion−solvent interactions determines the geometrical distribution of the microsolvation. Aggregates of solvent molecules within the bromide clusters are more defined in the case of water. Methanolated bromide clusters show a defined trend to place some solvent molecule at the second solvation shell. The bigger acetonitrile complexes (n > 5) are the more representative cases of interior complexes where the solvent molecules surround quite symmetrically the bromide anion whereas, in water and methanol, the microsolvation is more compromised between bromide−solvent and solvent−solvent interactions, then favoring arrangements with the ion on the surface of the cluster, particularly for n < 5. To rationalize the key components of the microsolvation, ion−solvent and solvent−solvent interaction energies have been decomposed in terms of two-body, three-body, and four-body contributions. Three-body terms are important for methanol and acetonitrile clusters due to the bromide−solvent contribution, whereas for aqueous clusters a significant cancellation between bromide−water and water−water interactions largely reduces the total three-body component.

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