| Abstract
| - Quantitative determination of the hydration structure of hexaaquairidium(III), [Ir(H2O)6]3+, in aqueous solution,the most inert aqua ion known, has been achieved for the first time by a combined experimental−theoreticalapproach employing X-ray absorption spectroscopy and molecular dynamics (MD) simulations. The Ir LIII-edge extended X-ray absorption fine structure (EXAFS) spectrum and LI-, LII-, and LIII-edge X-ray absorptionnear-edge structure (XANES) spectra of three concentrations of [Ir(H2O)6]3+ in perchloric acid media weremeasured. To carry out classical MD simulations of the aqua ion in water, a new set of first-principles Ir−H2O intermolecular potentials, based on the hydrated ion concept, has been developed. Structural, dynamics,and energetic properties have been obtained from the analysis of the statistical trajectories generated. TheIr−O radial distribution function shows two well-defined peaks at 2.04 ± 0.01 and 4.05 ± 0.05 Å correspondingto the first and second hydration shell, respectively; the fundamental frequencies for the aqua ion in water arewell reproduced by the MD simulation, and its dynamic properties are similar to the experimental valuescorresponding to other hexahydrated trivalent ions. Particular attention has been devoted to the experimentaldetermination of the second hydration shell. It has been found that contrarily to what expected on the basisof the inertness of the Ir3+ aquaion, the detection of the second hydration shell by EXAFS for this cation ismore difficult than for others less inert aqua ions such as Cr3+ or Rh3+. But when combined with MDsimulations it is possible to confirm the coordination distance for this shell at 4.1 ± 0.1 Å. In addition, thecomputation of LI, LII and LIII XANES spectra were carried out using the structural information obtainedfrom MD. These computations allowed the assignment of special features of the spectra to the second hydrationshell on a quantitative basis. Therefore, interestingly XANES spectra have given a stronger support to thesecond hydration shell than EXAFS. The fit of the LIII-edge EXAFS gives an accurate description of the firsthydration shell structure in aqueous solution. The value for Ir−O first shell is 2.04 ± 0.01 Å. The statisticalinformation available with the MD results has allowed the analysis of the standard deviation associated withthe computation of the XANES spectrum. It is shown that the standard deviation increases with the numberof hydration shells and this increase is nonuniform along the average spectrum.
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