General quantum-mechanical study on the hydrolysis equilibria for a tetravalent aquaion: the extreme case of the Po(IV) in water.

A systematic study of the different hydrolyzed species derived from the hydrated Po(IV) in water, [Po(H(2)O)(n)(OH)(m)]((4-m)) for 1 m 4, and 4 m + n 9, has been carried out by means of quantum mechanical computations. The effects of outer solvation shells have been included using a polarizable continuum dielectric model. For a fixed number of hydroxyl groups, the preferred hydration number for the Po(IV) can be determined in terms of Gibbs energy. It is shown that the hydration number (n) systematically decreases with the increase in the number of hydroxyl groups (m) in such a way the total coordination number (n + m) becomes smaller, being 9 in the aquocomplex and 4 in the neutral hydroxo-complex. Free energies for the hydrolysis processes involving Po(IV) complexes and a different number of hydroxyl groups have been computed, revealing the strong tendency toward hydrolysis exhibited by these complexes. The predominant species of Po(IV) in aqueous solutions are ruled by a dynamical equilibrium involving aggregates containing in the first coordination shell OH(-) groups and water molecules. Although there is not experimental information to check the theoretical predictions, theoretical computations in solution seem to suggest that the most likely clusters are [Po(H(2)O)(5)(OH)(2)](2+) and [Po(H(2)O)(4)(OH)(2)](2+). The geometry of the different clusters is ruled by the trend of hydroxyl groups to be mutually orthogonal and to promote a strong perturbation of the water molecule in trans-position by lengthening the Po-H(2)O distances and tilting the corresponding bond angle. A general thermodynamic cycle is defined to compute the Gibbs free energy associated to the formation of the different hydrolyzed forms in solution. From it, the estimates of pK(a) values associated to the different protolytic equilibria are provided and discussed. Comparison of the relative values of pK(a) along a hydrolysis series with the experimental values for other tetravalent cations supports its consistency.

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.

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.

Optimized end station and operating protocols for reflection extended x-ray absorption fine structure (ReflEXAFS) investigations of surface structure at the European Synchrotron Radiation Facility beamline BM29.

The development of the capability to engineer the surface properties of materials to match specific requirements demands high quality surface characterization techniques. The ideal tool should provide chemically specific structural characterization as well as surface sensitivity and depth profiling. Ideally the characterization method should also be applicable to systems both with and without long range order. X-ray absorption spectroscopy fine structure, when using the standard transmission detection system, provides all this information with the significant exception of surface sensitivity. In contrast, by detecting the reflected instead of the transmitted beam, it encompasses all these requirements because when the incident beam impinges onto a sample surface at glancing angles, in conditions close to the total reflection, only the outermost regions of the system under study are sampled. Such a technique provides information about the local structure as a function of depth as well as thin layer structure in the case of layered samples. Although it is potentially the ideal tool to study surface modified materials, experimental difficulties have hampered its widespread use in the fields of surface and materials sciences. As a solution to the experimental challenges, we provide a detailed description of an appropriate experimental station, the sample requirements, the measuring protocols, and software routines needed to optimize the collection of the data. To illustrate the capabilities of the technique the results obtained for a model multilayer sample are presented and analyzed under the total external reflection approximation.

The Structure of the Hydrated Gallium(III), Indium(III), and Chromium(III) Ions in Aqueous Solution. A Large Angle X-ray Scattering and EXAFS Study.

The structure of the hydrated gallium(III), indium(III), and chromium(III) ions has been determined in aqueous perchlorate and nitrate solutions by means of the large-angle X-ray scattering (LAXS) and extended X-ray absorption fine structure (EXAFS) techniques. The EXAFS studies have been performed over a wide concentration range, 0.005-1.0 mol.dm(-)(3) (2.6 mol.dm(-)(3) for chromium(III)), while the LAXS studies are restricted to concentrated solutions, ca. 1.5 mol.dm(-)(3). All three metal ions were found to coordinate six water molecules, each of which are hydrogen bonded to two water molecules in a second hydration sphere. The metal-oxygen bond distance in the first hydration sphere of the gallium(III), indium(III), and chromium(III) ions was determined by LAXS and EXAFS methods to be 1.959(6), 2.131(7), and 1.966(8) Å. The LAXS data gave mean second sphere M.O distances of 4.05(1), 4.13(1), and 4.08(2) Å for the gallium(III), indium(III), and chromium(III) ions, respectively. The perchlorate ion was found to be hydrogen bonded to 4.5(7) water molecules with the O.O distance 3.05(2) Å and Cl.O 3.68(3) Å. Analyses of the Ga, In, and Cr K-edge EXAFS data of the aqueous perchlorate and nitrate solutions showed no influence on the first shell M-O distance by a change of concentration or anion. The minor contribution from the second sphere M.O distance is obscured by multiple scattering within the tightly bonded first shell. EXAFS data for the alum salts CsM(SO(4))(2).12H(2)O, M = Ga or In, showed the M-O bond length of the hexahydrated gallium(III) and indium(III) ions to be 1.957(2) and 2.122(2) Å, respectively.

The interplay of the 3d9 and 3d10L electronic configurations in the copper K-edge XANES spectra of Cu(II) compounds.

A theoretical analysis of the X-ray absorption near-edge structure spectra at the Cu K-edge in several divalent copper [Cu(II)] compounds showing a distorted nearest-neighborhood around copper is presented. The experimental spectra of CuO and KCuF(3) have been compared with computations performed in the framework of the multiple-scattering theory. The results show that ab initio single-channel multiple-scattering calculations are not able to reproduce the experimental spectra. On the contrary, the experimental spectra can be accounted for by using two excitation channels and the sudden limit of the multichannel multiple-scattering theory. The comparison between experimental data and computations indicates that both 3d(9) and 3d(10)L electronic configurations are needed to account for the absorption process in these systems, suggesting that this is the general case for the K-edge XANES of divalent copper compounds.

The hydration of Cu2+: Can the Jahn-Teller effect be detected in liquid solution?

The long elusive structure of Cu(II) hydrate in aqueous solutions, classically described as a Jahn-Teller distorted octahedron and recently proposed to be a fivefold coordination structure [Pasquarello et al., Science 291, 856 (2001)], has been probed with x-ray-absorption spectroscopy by performing a combined theoretical and experimental analysis. Two absorption channels were needed to obtain a proper reproduction of the x-ray-absorption near-edge structure (XANES) region spectrum, as already observed in other Cu(II) complexes [Chaboy et al., Phys. Rev. B 71, 134208 (2005)]. The extended x-ray-absorption fine-structure (EXAFS) spectrum was analyzed as well within this approach. Quite good reproductions of both XANES and EXAFS spectra were attained for several distorted and undistorted structures previously proposed. Nevertheless, there is not a clearly preferred structure among those including four-, five-, and sixfold coordinated Cu(II) ions. Taking into account our results, as well as many more from several other authors using different techniques, the picture of a distorted octahedron for the Cu(II) hexahydrate in aqueous solution, paradigm of the Jahn-Teller effect, is no longer supported. In solution a dynamical view where the different structures exchange among themselves is the picture that better suits the results presented here.

Exploring the capabilities of X-ray absorption spectroscopy for determining the structure of electrolyte solutions: computed spectra for Cr(3+) or Rh(3+) in water based on molecular dynamics.

Extended X-ray absorption fine structure (EXAFS) spectra of Cr(3+) and Rh(3+) in aqueous solution are analyzed and compared with computed spectra derived from structural results obtained by molecular dynamics (MD) simulation. This procedure quantifies the reliability of the EXAFS structural determination when applied to ions in solution. It provides guidelines for interpreting experimental spectra of octahedrally coordinated metal cations in aqueous solution. A set of relationships among Debye-Waller factors is proposed on the basis of MD results to reduce the number of independent fit parameters. The determination of the second hydration shell is examined. Calculated XANES spectra compare well with experimental ones. Indeed, the splitting observed on the main peak of the Rh K-edge was anticipated by the calculations. Simulated spectra from MD structures of increasing cluster size show a relationship between the second hydration shell and features of the XANES region at energies just above the edge. The combination of quantum and statistical calculations with the XANES spectrum is found to be very fruitful to get insight into the quantitative estimation of structural properties of electrolyte solutions.

Nature of metal binding sites in Cu(II) complexes with histidine and related N-coordinating ligands, as studied by EXAFS.

Knowledge of the complexes formed by N-coordinating ligands and Cu(II) ions is of relevance in understanding the interactions of this ion with biomolecules. Within this framework, we investigated Cu(II) complexation with mono- and polydentate ligands, such as ammonia, ethylenediamine (en), and phthalocyanine (Pc). The obtained Cu-N coordination distances were 2.02 A for [Cu(NH(3))(4)](2+), 2.01 A for [Cu(en)(2)](2+), and 1.95 A for CuPc. The shorter bond distance found for CuPc is attributed to the macrocyclic effect. In addition to the structure of the first shell, information on higher coordination shells of the chelate ligands could be extracted by EXAFS, thus allowing discrimination among the different coordination modes. This was possible due to the geometry of the complexes, where the absorbing Cu atoms are coplanar with the four N atoms forming the first coordination shell of the complex. For this reason multiple scattering contributions become relevant, thus allowing determination of higher shells. This knowledge has been used to gain information about the structure of the 1:2 complexes formed by Cu(II) ions with the amino acids histidine and glycine, both showing a high affinity for Cu(II) ions. The in-solution structure of these complexes, particularly that with histidine, is not clear yet, probably due to the various possible coordination modes. In this case the square-planar arrangements glycine-histamine and histamine-histamine as well as tetrahedral coordination modes have been considered. The obtained first-shell Cu-N coordination distance for this complex is 1.99 A. The results of the higher shells EXAFS analysis point to the fact that the predominant coordination mode is the so-called histamine-histamine one in which both histidine molecules coordinate Cu(II) cations through N atoms from the amino group and from the imidazole ring.