Rafael R. Pappalardo

Solving the hydration structure of the heaviest actinide aqua ion known: the californium(III) case.

The solution chemistry of actinide ions has been a fundamental question since the beginning of the nuclear technologies, given that the solvent stabilizes the high oxidation states of actinides. The development of procedures to avoid the migration of actinides from the already accumulated nuclear waste into natural water systems is a field of great activity. One of the primary properties of actinide ions in solution is their solvation, as it is intimately joined to complexation, precipitation, and resolution processes. The rareness and hazardousness of the heavier actinide elements, which steeply increase with the atomic number, has prevented a complete examination of the trends along the series, beyond the middle of the series. The curium cation Cm has often been considered as the heaviest actinide species characterized, and it has attracted much attention from both experimental and theoretical views in recent years. Systematic studies of the aqueous trivalent lanthanides have revealed a contraction of the metal–oxygen distance and a decrease of the total first coordination number along the series. Recent investigations using extended X-ray absorption fine structure (EXAFS) techniques have examined if this contraction takes place in a monotone or an irregular way along the series. The data available for the actinide series up to Cm indicates a similar contraction, 5, 7] although a conclusive answer cannot be given owing to the uncertainty of the structural data, particularly concerning the hydration number, and the scarce information on the second half of the series. Beyond the middle of the series, there is only one study reported for berkelium (Bk) and a preliminary EXAFS study for californium(III) carried out by one of us. Owing to the position of Cf in the actinide series, an accurate enough determination of the coordination number and Cf O distance could certainly shed light on the question of the actinide contraction. This objective gives the study a more fundamental than applied character, owing to the extreme rareness of this element. The most similar available crystallographic data of Cf with Cf O bonds is that of single crystals of Cf(IO3)3, which present a significantly distorted tricapped trigonal prism with a wide range of Cf O distances (2.353–2.921 ). This limited information does not meet the required level of accuracy for answering the question on the basis of a conventional EXAFS data analysis. Herein we present an alternative way to study this extreme case, by coupling new highly refined EXAFS data obtained in an actinide-dedicated beamline in the European Synchrotron Radiation Facility (ESRF, Grenoble), with the first Monte Carlo (MC) simulations of Cf in water. Specifically developed Cf OH2 intermolecular potentials based on ab initio quantum mechanical (QM) potential energy surfaces and the polarizable and flexible MCDHO water model have been used. Figure 1 shows the experimental and fitted k-weighted EXAFS spectra of a Cf aqueous solution using two model structures, the square antiprism configuration (SA; see Figure 2a), which represents an octacoordination of water

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.

ON THE HALIDE HYDRATION STUDY: DEVELOPMENT OF FIRST-PRINCIPLES HALIDE ION-WATER INTERACTION POTENTIAL BASED ON A POLARIZABLE MODEL

The development of first-principles halide-water interaction potentials for fluoride and iodide anions is presented. The model adopted is the mobile charge densities in harmonic oscillator that allows for a flexible and polarizable character of the interacting particles. The set of points of the quantum mechanical potential energy surfaces are calculated up to the MP2 level. The nonadditive many-body contributions were included explicitly at the three-body terms. Structural and energetic properties of the [X(H2O)n]− clusters (n=1–6) are studied with the new interaction potentials developed. Halide aqueous solutions are also studied by means of Monte Carlo simulations. The agreement between experimental and our predicted estimations shows the good behavior of the proposed potentials. The developed potentials are able to properly describe both the microsolvation of clusters in gas phase and their hydration in aqueous solutions. The different nature of the interactions among F−, Br−, I− and water appears in ...

Development of first-principles interaction model potentials. An application to the study of the bromide hydration

This work presents the development of first-principles bromide ion–water interaction potentials using the mobile charge density in harmonic oscillators-type model. This model allows for a flexible and polarizable character of the interacting molecules and has already been parametrized for water–water interactions. The prospected potential energy surfaces of the bromide ion-water system were computed quantum-mechanically at Hartree–Fock and Moller–Plesset second-order perturbation levels. In addition to the ion–solvent molecule pair, structures formed by the anion and two or three water molecules were considered in order to include many body effects. Minimizations of hydrated bromide clusters in gas phase [Br(H2O)n]− (n=1–6,10,15,20) and Monte Carlo computations of bromide aqueous solutions were performed to test the new potentials. Both structural and thermodynamic properties have been studied in detail and compared to the available experimental and theoretical values. From these comparisons, it was concl...

A molecular dynamics study of the Cr3+ hydration based on a fully flexible hydrated ion model

A theoretical study of the Cr3+ hydration in aqueous solutions has been carried out by means of molecular dynamics (MD) simulations. Ion–water intermolecular interaction potentials are based on first principles using the idea of the previously developed hydrated ion–water interaction potential: The bare ion, Mn+, is replaced by its corresponding hydrate, [M(H2O)6]n+, and the water molecules interact with the hydrate by means of an ab initio [M(H2O)6]n+–H2O interaction potential. A new ab initio interaction potential has been developed to describe the Mn+–(H2O)first-shell interaction based on an examination of the hexahydrate potential-energy surface section that distorts the position of one of the cluster water molecules, the remaining five fixed at their equilibrium position. These two complementary interaction potentials, which describe ion–water interactions have been combined with the TIP4P model for water molecules. Structural and dynamical results derived from the analysis of 1 ns of simulation for ...

Coupling a polarizable water model to the hydrated ion–water interaction potential: A test on the Cr3+ hydration

A strategy to build interaction potentials for describing ionic hydration of highly charged monoatomic cations by computer simulations, including the polarizable character of the solvent, is proposed. The method is based on the hydrated ion concept that has been previously tested for the case of Cr3+ aqueous solutions [J. Phys. Chem. 100, 11748 (1996)]. In the present work, the interaction potential of [Cr(H2O6)]3+ with water has been adapted to a water model that accounts for the polarizable character of the solvent by means of a mobile charge harmonic oscillator representation (MCHO model) [J. Chem. Phys. 93, 6448 (1990)]. Monte Carlo simulations of the Cr3+ hexahydrate plus 512 water molecules have been performed to study the energetics and structure of the ionic solution. The results show a significant improvement in the estimate of the hydration enthalpy [ΔHhydr(Cr3+)=−1109.6±70 kcal/mol] that now matches the experimental value within the uncertainty of this magnitude. The use of the polarizable wate...

Aqueous PdII and PtII: anionic hydration revealed by Car-Parrinello simulations.

The aqua ions of Pd and Pt form well-defined square–planar structures in aqueous solutions. Their hydration and related physicochemical properties are relevant in a twofold sense. The first one is for understanding the solvent structure in the non-equatorial regions. In general terms, a highly charged atomic cation interacts with the solvent, thus generating a roughly spherical shell of strongly perturbed solvent molecules surrounding the cation, named first solvation shell. At a distance far enough from the cation, the solvent recovers the bulk behavior. Then, an intermediate region, with a spherical shell shape, can be defined in which the solvent molecules slowly lose their perturbed character as they are further away from the cation and closer to the bulk. This intermediate region is called second solvation shell. This widely used description of the interaction between the cation and the solvent is known as the concentric shell model of Frank and Evans. However, when the electronic properties of the aqua ion impose a planar coordination in the first hydration shell, this Copernican view of Frank and Evans is no longer applicable. Thus, the characterization of the axial regions in Pd and Pt aqua ions is attracting much attention from both experimental and theoretical points of view. The second aspect of interest is associated to the highly relevant antitumoral applications of Pt square–planar complexes, such as cisplatin cis-PtACHTUNGTRENNUNG(NH3)2(Cl)2, which become medically active when some of the first-shell ligands exchange for water molecules. The homologous compounds of Pd are inert in this respect, which is supposedly due to their extremely different exchanging rate constants relative to those of the Pt compounds. This dissimilar behavior may have its origin either in the structural or in the electronic properties of the compounds. A first step towards unraveling this issue consists in studying the tetrahydrates in water solution. This Communication reports Car–Parrinello molecular dynamics (CP–MD) simulations of the Pd and Pt aqua ions in water, thereby providing unusual insight into both dynamical and electronic structure phenomena occurring in the first and second solvation shells. The focus is on two remarkable features: 1) the “anionic solvation” of axial H2O molecules and 2) the transient proton transfer between equatorial firstand second-shell H2O molecules.

AM1 study of a β-carboline set: structural properties and potential reactivity

A set of β-carbolines derived from norharman and its corresponding dehydro and tetrahydro derivatives has been studied by means of the semiempirical AM1 method. Geometrical parameters, protonation affinities and static reactivity indices have been examined. Structural properties and protonation sites are well described by calculations. Orientation of electrophilic attack on different centres is only partially predicted by the frontier indices. The role of the protonated molecules as reactive species is also discussed.

Interplay of computer simulations and x-ray absorption spectra in the study of the bromide hydration structure

X-ray absorption spectra (EXAFS and XANES) were generated from snapshots of a Monte Carlo (MC) simulation of a bromide ion aqueous solution and from model structures. The MC simulation relies on a recently developed and tested polarizable potential based on ab initio potential energy surfaces. A comparison with the experimental K-edge Br spectrum of a 0.3 M YBr3 aqueous solution was performed. XANES spectra are reproduced acceptably only if statistical fluctuations are included, which is performed in this work by using snapshots from computer simulation. As expected, single scattering BrO contributions are dominant in the case of the EXAFS region. Due to this fact, Br− in water is a good model system for studying the influence of the distribution of distances on the determination of structural parameters. Then, a parallel study of the data analysis procedure of the experimental EXAFS spectrum and those theoretically computed from the structures supplied by the MC simulation, was carried out. The shape of ...

Hydration of Cisplatin Studied by an Effective Ab Initio Pair Potential Including Solute-Solvent Polarization.

The hydration of cis-[PtCl2(NH3)2] (cisplatin) has been studied by means of classical molecular dynamics simulations using a new interaction potential obtained by fitting about 4000 ab initio interaction energies calculated at the MP2 level. The functional form included several r(-n) terms (n = 4, 6, 8, 12) to achieve an accurate description of the interactions in the different regions around the cisplatin. Bulk solvent effects on the cisplatin-water molecule interactions have been included by means of a continuum model. Radial Distribution Function (RDF) analysis does not provide a clear enough description of the hydration pattern due to the intricate solvent arrangement around the solute. Angle-solved RDFs and spatial distribution functions have been used to provide more detailed pictures of the local hydration structure around the two ligands, chloride and ammine groups, and the axial region. Based on this information, it is shown a more convenient way to compute the running coordination number for the first hydration shell by simultaneously considering angle-solved RDFs centered on the ligand representative atoms of the complex: ammino N, Cl, and Pt atoms. This way, the hydration number is obtained by integrating over an interlocking-sphere volume built by the spheres centered on the cation and the main atoms of each ligand. Compared to previous works dealing with cisplatin hydration, the global hydration number for the first coordination shell is now higher and involves about 27 water molecules. The importance of the structural sampling, the computational level, as well as the functional form adopted for the interaction potential are thoroughly discussed with respect to the previous proposed intermolecular potential.