Molecular Structure of Solvates and Coordination Complexes in Solution as Determined with EXAFS and XANES

Muñoz‐Páez, Adela; Marcos, Enrique Sánchez

doi:10.1016/b978-0-08-097774-4.00905-0

Cited by 9 publications

(4 citation statements)

References 111 publications

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“…Molecular dynamics simulations are a useful tool to help in the interpretation and analysis of complex EXAFS spectra [ 15 , 21 , 54 ]. Conversely, comparison of experimental spectra with simulated ones generated from an ensemble of MD configurations is a useful and sensitive tool to validate force fields [ 18 , 21 ].…”

Section: Methodsmentioning

confidence: 99%

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

et al. 2020

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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.

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Section: Methodsmentioning

confidence: 99%

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

et al. 2020

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“…The difficulty in obtaining accurate results on metal ion solvation structures in disordered liquid systems is well-known; therefore, we used both the EXAFS and the XANES (X-ray absorption near-edge structure) techniques to obtain a unified picture of the hydration structure. XAS spectroscopy is known to be a very powerful tool for the study of the first shell structure surrounding a metal because of its high sensitivity to the closest environment of the photoabsorber. , The LAXS technique provides vital information about long and diffuse distances not accessible by XAS. In particular, the low-energy part of a XAS spectrum provides unique information about the three-dimensional arrangement of the scattering atoms, and to the best of our knowledge, this is the first time that XANES is employed in the study of Ag + coordination in water.…”

Section: Introductionmentioning

confidence: 99%

Elusive Coordination of the Ag⁺ Ion in Aqueous Solution: Evidence for a Linear Structure

et al. 2020

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X-ray absorption spectroscopy (XAS) has been employed to study the coordination of the Ag + ion in aqueous solution. The conjunction of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) data analysis provided results suggesting the preference for a first shell linear coordination with a mean Ag−O bond distance of 2.34(2) Å, different from the first generally accepted tetrahedral model with a longer mean Ag−O bond distance. Ab initio molecular dynamics simulations with the Car−Parrinello approach (CPMD) were also performed and were able to describe the coordination of the hydrated Ag + ion in aqueous solution in very good agreement with the experimental data. The high sensitivity for the closest environment of the photoabsorber of the EXAFS and XANES techniques, together with the long-range information provided by CPMD and large-angle X-ray scattering (LAXS), allowed us to reconstruct the three-dimensional model of the coordination geometry around the Ag + ion in aqueous solution. The obtained results from experiments and theoretical simulations provided a complex picture with a certain amount of water molecules with high configurational disorder at distances comprised between the first and second hydration spheres. This evidence may have caused the proliferation of the coordination numbers that have been proposed so far for Ag + in water. Altogether these data show how the description of the hydration of the Ag + ion in aqueous solution can be complex, differently from other metal species where hydration structures can be described by clusters with well-defined geometries. This diffuse hydration shell causes the Ag−O bond distance in the linear [Ag(H 2 O) 2 ] + ion to be ca. 0.2 Å longer than in isolated ions in solid state.

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“…From an experimental side, X-ray absorption spectroscopies (EXAFS and XANES) are appropriate techniques to help in the structural elucidation of the species in solution . However, this complex behavior makes difficult the collection of XAS spectra of the [Bi(H 2 O) m ] 3+ aqua ion, an important reference for the follow up of the chemical changes induced by the medium.…”

Section: Introductionmentioning

confidence: 99%

Effect of Basicity on the Hydrolysis of the Bi(III) Aqua Ion in Solution: An Ab Initio Molecular Dynamics Study

Ayala

Martı́nez²,

Pappalardo³

et al. 2018

J. Phys. Chem. A

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Hydrolysis of the Bi(III) aqua ion under a range of solution conditions has been studied by means of ab initio molecular dynamics simulations. While the Bi(III) aqua ion is stable in pure water, there is an increasing degree of hydrolysis with the number of hydroxide anions in the medium. This is accompanied by a monotonic decrease of the total coordination number to an asymptotic value of ∼6, reached under extreme basicity conditions. Comparison of the simulated Bi(III) hydrolyzed species with the experimental species distribution at different degrees of basicity suggests that, at the PBE/DFT level of theory here employed, liquid water shows an overly acidic character. Predictions of theoretical EXAFS and XANES spectra were generated from the AIMD trajectories for different Bi hydrolyzed species, [Bi(HO)(HO)], m = 0-3 and n = 7-2. Comparison with available experimental spectra is presented. Spectral features joined to the degree of hydrolysis and hydration are analyzed.

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Molecular Structure of Solvates and Coordination Complexes in Solution as Determined with EXAFS and XANES

Cited by 9 publications

References 111 publications

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

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

Elusive Coordination of the Ag⁺ Ion in Aqueous Solution: Evidence for a Linear Structure

Effect of Basicity on the Hydrolysis of the Bi(III) Aqua Ion in Solution: An Ab Initio Molecular Dynamics Study

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Molecular Structure of Solvates and Coordination Complexes in Solution as Determined with EXAFS and XANES

Cited by 9 publications

References 111 publications

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

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

Elusive Coordination of the Ag+ Ion in Aqueous Solution: Evidence for a Linear Structure

Effect of Basicity on the Hydrolysis of the Bi(III) Aqua Ion in Solution: An Ab Initio Molecular Dynamics Study

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Elusive Coordination of the Ag⁺ Ion in Aqueous Solution: Evidence for a Linear Structure