The first step toward a predictive insight into a dissolved ion’s behavior is a detailed knowledge of its solution speciation, notably all energy-relevant correlations with both counterions and solvent molecules. Underpinning the theoretical as well as experimental hurdles to achieving this goal are the often small energy differences between competing species that ultimately impact a solute’s stability, reactivity, and solubility. From a theoretical standpoint, the cost and time required to describe the environment about a central ion imparts restrictions on the number of ions to include and their explicit interactions. Often choices must be made in the absence of detailed information that can be used to judge the relative importance of the possible contributions. For example, the competition for inner- and outer-sphere coordination environments is often dependent on very small energy differences and equilibria involving a number of species. Particularly challenging are higher-valent ions where electrostatic forces can extend past the central ion’s outer sphere environment. We will discuss our recent approach to providing speciation metrics with the accuracy required to inform such computational efforts. Using high-energy X-ray scattering (HEXS) as a probe of metal ion correlations in solution, and employing a novel approach to data analyses, we can obtain difference pair-distribution functions (dPDF) that can provide accurate inner- and outer-sphere coordination numbers in addition to information about correlations extending well into the solvent. We will discuss selected examples that highlight some of our unexpected findings and some new information that may be helpful to theorists wishing to accurately model and predict the solution chemistry of solute ions in aqueous solutions.

New insights into metal-ion correlations in solution

253rd National Meeting (2017)

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