Combining EXAFS and Computer Simulations to Refine the Structural Description of Actinyls in Water.

EXAFS spectroscopy is one of the most used techniques to solve the structure of actinoid solutions. In this work a systematic analysis of the EXAFS spectra of four actinyl cations, [UO2]2+, [NpO2]2+, [NpO2]+ and [PuO2]2+ has been carried out by comparing experimental results with theoretical spectra. These were obtained by averaging individual contributions from snapshots taken from classical Molecular Dynamics simulations which employed a recently developed [AnO2]2+/+ -H2O force field based on the hydrated ion model using a quantum-mechanical (B3LYP) potential energy surface. Analysis of the complex EXAFS signal shows that both An-Oyl and An-OW single scattering paths as well as multiple scattering ones involving [AnO2]+/2+ molecular cation and first-shell water molecules are mixed up all together to produce a very complex signal. Simulated EXAFS from the B3LYP force field are in reasonable agreement for some of the cases studied, although the k= 6-8 Å-1 region is hard to be reproduced theoretically. Except uranyl, all studied actinyls are open-shell electron configurations, therefore it has been investigated how simulated EXAFS spectra are affected by minute changes of An-O bond distances produced by the inclusion of static and dynamic electron correlation in the quantum mechanical calculations. A [NpO2]+-H2O force field based on a NEVPT2 potential energy surface has been developed. The small structural changes incorporated by the electron correlation on the actinyl aqua ion geometry, typically smaller than 0.07 Å, leads to improve the simulated spectrum with respect to that obtained from the B3LYP force field. For the other open-shell actinyls, [NpO2]2+ and [PuO2]2+, a simplified strategy has been adopted to improve the simulated EXAFS spectrum. It is computed taking as reference structure the NEVPT2 optimized geometry and including the DW factors of their corresponding MD simulations employing the B3LYP force field. A better agreement between the experimental and the simulated EXAFS spectra is found, confirming the a priori guess that the inclusion of dynamic and static correlation refine the structural description of the open-shell actinyl aqua ions.

A general study of actinyl hydration by molecular dynamics simulations using ab initio force fields.

A set of new ab initio force fields for aqueous [AnO2]2+/+ (An = Np(vi,v), Pu(vi), Am(vi)) has been developed using the Hydrated Ion (HI) model methodology previously used for [UO2]2+. Except for the non-electrostatic contribution of the HI-bulk water interaction, the interaction potentials are individually parameterized. Translational diffusion coefficients, hydration enthalpies, and vibrational normal mode frequencies were calculated from the MD simulations. Physico-chemical properties satisfactorily agree with experiments validating the robustness of the force field strategy. The solvation dynamics and structure for all hexavalent actinoids are extremely similar and resemble our previous analysis of the uranyl cation. This supports the idea of using the uranyl cation as a reference for the study of other minor actinyls. The comparison between the NpO2 2+ and NpO2 + hydration only provides significant differences in first and second shell distances and second-shell mean residence times. We propose a single general view of the [AnO2]2+/+ hydration structure: aqueous actinyls are amphiphilic anisotropic solutes which are equatorially conventional spherically symmetric cations capped at the poles by clathrate-like water structures.

Hydration Structure of the Elusive Ac(III) Aqua Ion: Interpretation of X-ray Absorption Spectroscopy (XAS) Spectra on the Basis of Molecular Dynamics (MD) Simulations.

Knowledge of actinoid solution chemistry has been enriched with the recent synthesis and characterization of the elusive Ac(III) aqua ion, the first one of the series, for which extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra has been recorded. Structural analysis combined with Born-Oppenheimer molecular dynamics simulations lead to suggest a 2.63-2.69 Å range for the Ac-O distance, and a coordination number between 9 and 11. A hydration number as high as 11 would imply the appearance of a sharp coordination number contraction at the beginning of the series. In this work, we present a specific Ac(III)-H2O first-principles-based intermolecular potential, which has been developed following the exchangeable Hydrated Ion model. This potential has been used in classical molecular dynamics (MD) simulations of Ac(III) in water. Results show a well-defined Ac(III) ennea-hydrated aqua ion with a mean Ac-O distance of 2.66 ± 0.02 Å, surrounded by a compact second hydration shell formed by ∼20 H2O centered at 4.9 ± 0.1 Å. The results obtained for the first element of the actinoid series confirm the regular contraction of their aqua ions along the series. Simulated EXAFS and XANES spectra have been computed from the structural information provided by the MD simulation. The agreement with the experimental spectra is satisfactory, validating the results from the computer simulation. An observed hump in the experimental XANES spectrum is interpreted and ascribed to the second hydration shell, being an evidence of the consistency of the Ac(III) hydration shells.

The hydration structure of the heavy-alkalines Rb+ and Cs+ through molecular dynamics and X-ray absorption spectroscopy: surface clusters and eccentricity.

Physicochemical properties of the two heaviest stable alkaline cations, Rb+ and Cs+, in water have been examined from classical molecular dynamics (MD) simulations. Alkaline cation-water intermolecular potentials have been built from ab initio interaction energies of [M(H2O)n]+ clusters. Unlike in the case of other monatomic metal cations, the sampling needed the inclusion of surface clusters to properly describe the interactions. The first coordination shell is found at an average M-O distance of 2.87 Å and 3.12 Å for Rb+ and Cs+, respectively, with coordination numbers of 8 and 10. Structural, dynamical and energetic properties are discussed on the basis of the delicate compromise among the ion-water and water-water interactions which contribute almost on the same foot to the definition of the solvent structure around the ions. A significant asymmetry is detected in the Rb+ and Cs+ first hydration shell. Reorientational times of first-shell water molecules for Cs+ support a clear structure-breaking nature for this cation, whereas the Rb+ values do not differ from pure water behavior. Experimental EXAFS and XANES spectra have been compared to simulated ones, obtained by means of application of the FEFF code to a set of statistically significant structures taken from the MD simulations. Due to the presence of multi-excitations in the absorption spectra, theoretical-experimental agreement for the EXAFS spectra is reached when the multi-excitations are removed from the experimental spectra.

Axial structure of the Pd(II) aqua ion in solution.

Solution chemistry of Pd(II) and Pt(II) complexes is relevant to many fields of chemistry given the widespread applications of their compounds in homogeneous and heterogeneous catalysis, intermediate reaction synthesis, and antitumoral drugs. The well-defined square-planar arrangement of their complexes contrasts with the rather diffuse axial environment in solution. A theoretical proposal for a characteristic hydration shell in this axial region, called the meso-shell, stimulated further experimental and theoretical studies which have led to different pictures. The present work characterizes the structure of the axial region of the Pd(II) aqua ion in solution using a combination of neutron and X-ray diffraction and extended X-ray absorption fine structure (EXAFS) spectroscopy, with empirical potential structure refinement (EPSR). The results confirm the existence of the axial region and structurally characterize the water molecules within it. An important finding not previously reported is that the counterion, in this case the perchlorate anion, competes with water molecules for the meso-shell occupancy. The important role played by the axial region in many ligand substitution reactions is therefore intimately connected with the presence of the counterion and not just hydration water. This must call the attention of the experimental community to the important role that the counterion of the precursor salt must play in the synthesis.

A theoretical study of the hydrogen bond donor capability and co-operative effects in the hydrogen bond complexes of the diaza-aromatic betacarbolines.

In this work, we present a quantum mechanical investigation on the hydrogen bond interactions of N(9)-methyl-9H-pyrido[3,4-b]indole, MBC, and N(2)-methyl-9H-pyrido[3,4-b]indole, BCA, with different hydrogen bond donors. Thus, it has been analysed the influence that the hydrogen bond donor strength and the co-operative effect of the increasing number of donor molecules have on the shape of the potential energy surfaces versus the N···H distances, r(N­H). To rationalize the nature of the interactions, the Bader theory has been applied and the characteristics of the bond critical points analysed. The results show that two different hydrogen bond complexes can be formed depending on the donor capabilities or the number of donor molecules included in the calculations. The topological parameters from the Bader theory are used to justify the statement that the analysed interactions can be classified as weak or partially covalent hydrogen bond interactions, respectively. As experimentally observed, weak hydrogen bond donors form weak hydrogen bond complexes, called HBC. Upon the increase of the donor strength the N···H proton is shifted nearest to the nitrogen atom giving rise to the observation of a stronger hydrogen bond complex, the proton transfer complex, PTC. The most outstanding result of these studies is the fact that the formation of the PTC can also be managed just by changing the number of donor molecules, that is, by a co-operative effect of the hydrogen bonds.

Coupling CP-MD simulations and X-ray absorption spectroscopy: exploring the structure of oxaliplatin in aqueous solution.

A combined experimental-theoretical approach applying X-ray absorption spectroscopy and ab initio molecular dynamics (CP-MD) simulations is used to get insight into the structural determination of oxaliplatin, a third-generation anticancer drug of the cisplatin family, in aqueous solution. Experimental Pt L(III)-edge EXAFS and XANES spectra of oxaliplatin in water are compared with theoretical XAS spectra. The latter are obtained as statistically averaged spectra computed for a set of selected snapshots extracted from the MD trajectory of ethyldiamineoxalatoplatinum(II) (EDO-Pt) in liquid water. This compound is a simplified structure of oxaliplatin, where the outer part of the cyclohexane ring contained in the cyclohexanediamine ligand of oxaliplatin has been removed. We show that EDO-Pt is an appropriate model to simulate the spectroscopical properties of oxaliplatin given that the cyclohexane ring does not generate particular features in neither the EXAFS nor the XANES spectra. The computation of average EXAFS spectra using structures from the MD simulation in which atoms are selected according to different cutoff radii around the Pt center allows the assignment of spectral features to particular structural motifs, both in k and R-spaces. The outer oxygen atoms of the oxalate ligand (R(Pt-O(II)) = 3.97 +/- 0.03 A) are responsible for a well-defined hump at around 6.5 A(-1) in the k(2)-weighted EXAFS spectrum. The conventional EXAFS analysis data procedure is reexamined by its application to the simulated average EXAFS spectra. The structural parameters resulting from the fit may then be compared with those obtained from the simulation, providing an estimation of the methodological error associated with the global fitting procedure. A thorough discussion on the synergy between the experimental and theoretical XAS approaches is presented, and evidence for the detection of a slight hydration structure around the Pt complex is shown, leading to the suggestion of a new challenge to experimental XAS measurements.

Po(IV) hydration: a quantum chemical study.

This work presents a theoretical study on the hydration of Po(IV) in solution. Three points have been addressed: (i) the level of calculation needed to properly describe the system under study, (ii) the hydration number of Po(IV), and (iii) the nature of the polonium-water bonding. The condensed medium effects have been included by means of a continuum solvation model, thus different [Po(H(2)O)(n)](4+) hydrates were embedded in a cavity surrounded by a polarizable dielectric medium. Among the quantum-mechanical calculation levels here considered, the MPW1PW91 functional was shown to be the most suitable, allowing a proper description of the Po-H(2)O interactions at affordable cost. The hydration number of Po(IV) was found to be between 8 and 9. This value is ruled by a dynamic equilibrium involving the octa- and ennea-hydrates, although the 7-fold coordination cannot be completely excluded. The hydration free energy of Po(IV) is estimated to be around -1480 kcal/mol. The Po-H(2)O bonding is dominated by strong electrostatic contributions although a small covalent contribution is responsible for the peculiar arrangement adopted by the smaller hydrates (n < or = 5). A natural bond order (NBO) analysis of the hydrate wave functions shows that the covalent bond involves the empty 6p orbitals of the polonium ion and one lone pair on the oxygen atom of the water molecule. A parallel investigation to the hydrate study, where the polonium ion was replaced by a tetravalent point charge plus a repulsion potential, was carried out. These results allowed a detailed examination of the electrostatic and nonelectrostatic contributions to the polonium hydrate formation.

Explaining Asymmetric Solvation of Pt(II) versus Pd(II) in Aqueous Solution Revealed by Ab Initio Molecular Dynamics Simulations.

The solvation behavior of Pt(II) versus Pd(II) has been studied in ambient water using ab initio molecular dynamics. Beyond the well-defined square-planar first solvation shell encompassing four tightly bonded water molecules as predicted by ligand field theory, a second coordination shell containing about 10 H2O is found in the equatorial region. Additional solvation in the axial regions is observed for both metals which is demonstrated to be induced by the condensed phase. For the Pt(II) aqua complex, however, this water molecule is bonded with one of its hydrogen atoms toward the cation, thus establishing a typical anionic solvation pattern, which is traced back to the electronic structure of Pt(2+) versus Pd(2+) cations, in particular to the anisotropic polarizability of their tetrahydrates. Systematic model calculations based on suitable aqua complex fragments embedded in a polarizable continuum solvent support the idea that anionic hydration is facilitated by the liquid. Furthermore, transient protolysis of water molecules in the first shell is observed for both divalent transition metal cations, being more pronounced for Pt(II) versus Pd(II). The relevance of these solvation features is discussed with respect to the different acidity of Pt(2+) versus Pd(2+) aqua ions in water, their different water ligand exchange rates, and force field modeling approaches.

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.

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.