Combined Experimental and Theoretical Approach to the Study of Structure and Dynamics of the Most Inert Aqua Ion [Ir(H2O)6]3+ in Aqueous Solution

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-theoretical approach 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 absorption near-edge structure (XANES) spectra of three concentrations of [Ir(H2O)6]3+ in perchloric acid media were measured. 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. The Ir-O radial distribution function shows two well-defined peaks at 2.04 +/- 0.01 and 4.05 +/- 0.05 A corresponding to the first and second hydration shell, respectively; the fundamental frequencies for the aqua ion in water are well reproduced by the MD simulation, and its dynamic properties are similar to the experimental values corresponding to other hexahydrated trivalent ions. Particular attention has been devoted to the experimental determination of the second hydration shell. It has been found that contrarily to what expected on the basis of the inertness of the Ir3+ aquaion, the detection of the second hydration shell by EXAFS for this cation is more difficult than for others less inert aqua ions such as Cr3+ or Rh3+. But when combined with MD simulations it is possible to confirm the coordination distance for this shell at 4.1 +/- 0.1 A. In addition, the computation of LI, LII and LIII XANES spectra were carried out using the structural information obtained from MD. These computations allowed the assignment of special features of the spectra to the second hydration shell on a quantitative basis. Therefore, interestingly XANES spectra have given a stronger support to the second hydration shell than EXAFS. The fit of the LIII-edge EXAFS gives an accurate description of the first hydration shell structure in aqueous solution. The value for Ir-O first shell is 2.04 +/- 0.01 A. The statistical information available with the MD results has allowed the analysis of the standard deviation associated with the computation of the XANES spectrum. It is shown that the standard deviation increases with the number of hydration shells and this increase is nonuniform along the average spectrum.