Determination of two- and three-body correlation functions in ionic solutions by means of MD and EXAFS investigations

Abstract

The solvation structure of Sr 2+ ions in acetonitrile has been studied by x-ray absorption spectroscopy (XAS) and molecular dynamics (MD) simulations. The extended x-ray absorption fine structure (EXAFS) above the Sr K-edge has been interpreted in the framework of the multiple scattering (MS) formalism and, for the first time, clear evidence of MS contributions has been found for noncomplexing ions in solution. Molecular dynamics has been used to generate the partial pair 9(r) and the three-body 9(rl, r2, 0) distribution functions from which a model x(k) has been constructed. An excellent agreement has been found between the theoretical and experimental data. This result demonstrates the ability of the XAS technique in probing three-body correlation functions in solutions.

Molecular Simulation Analysis and X-ray Absorption Measurement of Ca2+, K+ and Cl- Ions in Solution

“…Due to the advance of modern EXAFS algorithms and the availability of more accurate ion and solvent potential models, the number of studies that combine MD with EXAFS has increased considerably. − Our work is distinguished from early contributions in that we combine MD, ab initio quantum chemical calculations and EXAFS measurement to address the ion solvation in aqueous solution. We have recently developed a method to predict EXAFS spectra: an ensemble of molecular configurations is generated from an intermolecular potential and is subsequently used in electron scattering analysis.…”

“…With the help of the present theory, the number and type of surrounding atoms, their average distance to the central absorbing atom, and their distribution about the absorbing center can be ascertained. A more complete understanding of the structural information contained in the EXAFS spectra has been derived in many instances from MD simulations of the aqueous system. − In standard EXAFS analysis, the fine structure factor is defined by where μ( E ) is the absorption coefficient as a function of the X-ray energy E = E 0 + ℏ 2 k 2 /2 m e , μ 0 ( E ) is the background absorption coefficient, and Δμ 0 ( E 0 ) is the jump in the absorption background at the absorption edge, E 0 . EXAFS spectra provide predictions of the number of water molecules in the first solvation shell, which is the average distance of these scatterers from the central atom, and a measure of the static disorder of this first solvation shell, as well as the vibrational disorder.…”

X-ray Absorption Spectroscopy Study of the In-Solution Structure of Ni2+, Co2+, and Ag+ Solvates in Acetonitrile Including Multiple Scattering Contributions

“…All these structural parameters are compatible with the obtained values within the crystal structure of [Ag(CH 3 CN) 4 ]ClO 4 . A useful approach to identify the reason for the observed shortening is to obtain the pair and three body distributions functions of the investigated system from molecular dynamics calculations, as already done in the study of the solvation structure of Sr 2+ cations in acetonitrile solutions, what allowed the accurate determination of the M 2+ −N−C angle, which was equal to 163° …”