Paola D’Angelo

Revised Ionic Radii of Lanthanoid(III) Ions in Aqueous Solution

A new set of ionic radii in aqueous solution has been derived for lanthanoid(III) cations starting from a very accurate experimental determination of the ion-water distances obtained from extended X-ray absorption fine structure (EXAFS) data. At variance with previous results, a very regular trend has been obtained, as expected for this series of elements. A general procedure to compute ionic radii in solution by combining the EXAFS technique and molecular dynamics (MD) structural data has been developed. This method can be applied to other ions allowing one to determine ionic radii in solution with an accuracy comparable to that of the Shannon crystal ionic radii.

An extended x‐ray absorption fine structure study of aqueous solutions by employing molecular dynamics simulations

Bromine–oxygen radial distribution functions [g(r)] have been calculated by means of molecular dynamics simulations for aqueous solutions of rubidium bromide, 2‐bromopropane and bromoethane. X‐ray absorption spectra at the bromine K edge have been recorded for these solutions. The water contribution to the extended x‐ray absorption fine structure spectra has been calculated starting from the gBr,O(r) distribution function. Fits of the x‐ray absorption spectra have been performed directly on the raw experimental data, allowing the reliability of the g(r) distribution functions to be verified. The agreement between theoretical and experimental spectra is satisfactory. A procedure to improve model g(r) functions on the basis of the short‐range structural information provided by extended x‐ray absorption fine structure data is proposed.

Hydration properties of the bromide aqua ion: the interplay of first principle and classical molecular dynamics, and X-ray absorption spectroscopy.

The hydration properties of the bromide aqua ion have been investigated using state of the art density functional theory (DFT) based molecular dynamics with dispersion-corrected atom-centered pseudopotentials for water and classical molecular dynamics simulations. The reliability of the theoretical results has been assessed by comparing the attained structural results with the extended X-ray absorption fine structure (EXAFS) experimental data. The EXAFS technique is mainly sensitive to short distances around the bromine atom, and it is a direct probe of the local solvation structure. The comparison shows that the DFT simulation delivers a good description of the EXAFS experimental signal, while classical simulation performs poorly. The main reason behind this is the neglect of polarization effects in the classical ion-water interaction potentials. By taking advantage of the reliable information on the Br(-) local hydration structure it has been possible to highlight the contribution of hydrogen atoms to the EXAFS spectra of halide aqueous systems.

The interpretation of diffraction patterns of two prototypical protic ionic liquids: a challenging task for classical molecular dynamics simulations.

In this study, we discuss the performance of classical molecular dynamics in predicting the experimental X-ray diffraction patterns of liquid ethylammonium nitrate (one of the simplest protic room-temperature ionic liquid showing amphiphilic behavior) and of its hydroxy derivative (2-ethanolammonium nitrate, 2-HOEAN). Newly recorded energy-dispersive X-ray diffraction structure factors are compared with the corresponding quantities extracted from molecular dynamics simulations. Other useful theoretical and experimental indicators are used as a probe of the local structure of the title ionic liquids. We shall show that the use of a general purpose, two-body terms only, force field, such as OPLS/AA is able to describe most of the structural experimental data. However, we shall also point out that an improved description of some key structural features observed in the X-ray radial distribution function, can be obtained very easily by adding a general three-body potential energy term instead of changing the two-body potential parameters, in order to optimize the agreement with experimental data. This three-body term turns out to be naturally able to describe the complex polarization effects due to hydrogen bonding without requiring a quanto-mechanical treatment or a polarizable force field. In addition the present model turns out to be able to account for the presence of a low-Q peak in the scattering patterns of EAN, which has been commonly interpreted as a manifestation of the amphiphilic nature of this compound.

Structural investigation of copper(II) chloride solutions using x-ray absorption spectroscopy

The local environment of the Cu2+ ion in copper chloride solutions has been investigated by x-ray absorption spectroscopy. Three aqueous solutions of CuCl2 with increasing Cl−/Cu2+ ratio have been examined. An advanced data analysis including multiple-scattering effects produced quantitative information on the chlorocuprate complexes present in solution and provided evidence for an increasing degree of complex formation between Cu2+and Cl− ions with increasing chloride concentration. The presence of Cu–Cl interactions at the axial site has been detected in a 0.1 M CuCl2 solution for the first time. At higher chloride concentrations (Cl−/Cu2+ratios equal 10 and 30) the equatorial positions in the distorted octahedral copper coordination are occupied by 3.2 and 3.0 oxygens and 0.8 and 1.0 chloride ions, respectively, while the axial positions are occupied by 1.2 and 1.0 oxygens and 0.8 and 1.0 chloride ions, respectively. The results are found to be consistent with previous x-ray and neutron diffraction stu...

Integrated experimental and theoretical approach for the structural characterization of Hg2+ aqueous solutions.

The structural and dynamic properties of the solvated Hg2+ ion in aqueous solution have been investigated by a combined experimental-theoretical approach employing x-ray absorption spectroscopy and molecular dynamics (MD) simulations. This method allows one to perform a quantitative analysis of the x-ray absorption near-edge structure (XANES) spectra of ionic solutions using a proper description of the thermal and structural fluctuations. XANES spectra have been computed starting from the MD trajectory, without carrying out any minimization in the structural parameter space. The XANES experimental data are accurately reproduced by a first-shell heptacoordinated cluster only if the second hydration shell is included in the calculations. These results confirm at the same time the existence of a sevenfold first hydration shell for the Hg2+ ion in aqueous solution and the reliability of the potentials used in the MD simulations. The combination of MD and XANES is found to be very helpful to get important new insights into the quantitative estimation of structural properties of disordered systems.

Using a Combined Theoretical and Experimental Approach to Understand the Structure and Dynamics of Imidazolium-Based Ionic Liquids/Water Mixtures. 1. MD Simulations

The structural and dynamic properties of 1-butyl-3-methylimidazolium bromide ([C4mim]Br)/water mixtures with different molar ratios have been investigated using classical molecular dynamics (MD) simulations, and the reliability of the results has been assessed by comparison with extended X-ray absorption fine structure experimental data. The analysis of the MD trajectories has highlighted the presence of a complex network of interactions among cations, anions, and water molecules, even if water molecules have been found to interact preferentially with the Br(-) anion. The existence of solvent-shared ion pairs has been detected in all of the investigated mixtures with one or more water molecules acting as a bridge between the cation and the anion, also when water is present in great excess ([C4mim]Br/water ratio of 1:200). The dynamic behavior of the systems has been characterized starting from the MD trajectories. Water molecules have been found to quicken the dynamics of the IL cations and anions, and acceleration involves all of the investigated motions.

Structural Investigation of Lanthanoid Coordination: a Combined XANES and Molecular Dynamics Study

This is the first systematic study exploring the potentiality of the X-ray absorption near edge structure (XANES) technique as a structural tool for systems containing lanthanoid(III) ions. A quantitative analysis of the XANES spectra at the K- and L(3)-edges has been carried out for three hydrated lanthanoid(III) ions, namely, Yb, Nd, and Gd, in aqueous solution and in the isostructural trifluoromethanesulfonate salts. The structural and dynamic properties of the hydrated lanthanoid(III) ions in aqueous solution have been investigated by a combined experimental-theoretical approach employing X-ray absorption spectroscopy and molecular dynamics (MD) simulations. This method allows one to perform a quantitative analysis of the XANES spectra of ionic solutions using a proper description of the thermal and structural fluctuations. XANES spectra have been computed starting from the MD trajectory, without carrying out any minimization in the structural parameter space. A comparative K- and L(3)-edge XANES data analysis is presented, demonstrating the clear advantages of the L(3)-edge XANES analysis over the K-edge studies for structural investigations of lanthanoid compounds. The second hydration shells provide a detectable contribution to the L(3)-edge spectra while the K-edge data are insensitive to the more distant coordination spheres because of the strong damping and broadening of the signal caused by the extremely large core hole widths. The XANES technique has been found to be a new valuable tool for the structural characterization of metal complexes both in the solid and in the liquid state, especially in the presence of low symmetry.

Effect of the Zn2+ and Hg2+ ions on the structure of liquid water.

The effect of ions on the structure of liquid water is still not completely understood, despite extensive experimental and theoretical studies. A combined XANES and molecular dynamics investigation on diluted Zn(2+) and Hg(2+) aqueous solutions reveals that the influence of a single ion on the bonding pattern of water molecules is strongly dependent on the nature of the ion. While the structure of water is not altered by the presence of the Zn(2+) ion, the Hg(2+) cation has a strong impact on the hydrogen-bond network of water that extends beyond the first coordination shell.

An extended x‐ray absorption fine structure study by employing molecular dynamics simulations: Bromide ion in methanolic solution

X‐ray absorption spectroscopy is widely employed in the structural analysis of disordered systems. In the standard extended x‐ray absorption fine structure (EXAFS) analysis the coordination of the photoabsorber is usually defined by means of Gaussian shells. It is known that this procedure can lead to significant errors in the determination of the coordination parameters for systems which present anharmonic thermal vibrations or interatomic asymmetric pair distribution functions. An efficient method has been recently employed in the study of the hydration shells of bromide and rubidium ions and brominated hydrocarbon molecules in diluted aqueous solutions. According to this method, pair distribution functions [g(r)] obtained from molecular dynamics simulations can be used as relevant models in the calculation of the EXAFS signals. Moreover, asymmetric shells modeled on the g(r) first peaks, have been employed in the EXAFS analysis and the parameters defining the asymmetric peaks have been optimized during...