Hydrated anions: From clusters to bulk solution with quasi-chemical theory

TL;DR

This paper evaluates the accuracy of quasi-chemical theory in predicting hydration free energies of halide anions, demonstrating excellent agreement with experiments and proposing improvements for modeling inner-shell structures.

Contribution

It introduces an inverse procedure using AIMD data to enhance quasi-chemical theory's predictions for ion hydration energies.

Findings

QCT accurately predicts hydration free energies of halide ions.

Inner-shell complexes show asymmetric hydration structures, especially for heavier halides.

The inverse procedure improves agreement with experimental hydration energies.

Abstract

The interactions of hydrated ions with solution and interface partners are strong on a chemical energy scale. Here, we test the foremost \textit{ab initio theory} for evaluation of hydration free energies of ions, namely, \textit{quasi-chemical theory} (QCT). We focus on halide anions, but also the hydroxide anion, since they have been outstanding challenges for all theories. QCT is built by identification of inner-shell clusters, separate treatment of those clusters, then integration of those results into the broader-scale solution environment. We exploit a close comparison with mass-spectrometric measurements of ion-hydration equilibria. That theory-experiment comparison is excellent with moderate computational effort here. This agreement reinforces both theory and experiment, and provides a numerically accurate inner-shell contribution to QCT. The inner-shell complexes involving heavier halides display strikingly asymmetric hydration clusters. QCT provides a favorable setting for exploitation of the polarizable continuum model (PCM) when the inner-shell material shields the ion from the outer solution environment. For the asymmetrically hydrated, and less effectively shielded, heavier halide ions, we investigate an inverse procedure in which the inner-shell structures are sampled from readily available AIMD calculations on the bulk solutions. This inverse procedure is a remarkable improvement and our final results are in close agreement with a standard tabulation of hydration free energies. Comparison of anion hydration cluster structures with bulk solutions from AIMD simulations emphasize slight differences: the asymmetries of bulk solution inner-shell structures are moderated, but still present; and inner shells fill to slightly higher average coordination numbers in bulk solution than in clusters.