Free Energies of Hydrated Halide Anions: High Through-Put Computations on Clusters to Treat Rough Energy-Landscapes.

Diego T Gomez,David M Rogers,Susan B Rempe,Lawrence R Pratt

doi:10.3390/molecules26113087

Abstract

With a longer-term goal of addressing the comparative behavior of the aqueous halides F, Cl, Br, and I on the basis of quasi-chemical theory (QCT), here we study structures and free energies of hydration clusters for those anions. We confirm that energetically optimal clusters, with X = Cl, Br, and I, exhibit surface hydration structures. Computed free energies, based on optimized surface hydration structures utilizing a harmonic approximation, typically (but not always) disagree with experimental free energies. To remedy the harmonic approximation, we utilize single-point electronic structure calculations on cluster geometries sampled from an AIMD (ab initio molecular dynamics) simulation stream. This rough-landscape procedure is broadly satisfactory and suggests unfavorable ligand crowding as the physical effect addressed. Nevertheless, this procedure can break down when , with the characteristic discrepancy resulting from a relaxed definition of clustering in the identification of clusters, including ramified structures natural in physical cluster theories. With ramified structures, the central equation for the present rough-landscape approach can acquire some inconsistency. Extension of these physical cluster theories in the direction of QCT should remedy that issue, and should be the next step in this research direction.

Highlights

As a step toward enhancing our understanding of ion-specific effects, this paper studies the structures and free energies of hydration clusters of the anions F−, Cl−, Br−, and I− in the dilute gas-phase, with the longer term goal of addressing the comparative behavior of this series of ions in liquid water on the basis of quasi-chemical theory (QCT) [12,13,14,15]
Because our analyses here will be limited to gas phase systems, we restrict our work to physical cluster theories [16,17,18,19,20,21,22], progenitors of molecular QCT [23,24]
We evaluated structures and formation free energies of Wn X clusters in gas phase

Summary

Introduction

Molecular-level mechanisms of ion-specific effects is a topic of current research [7,8,9]. Important aspects of those mechanisms in aqueous solution involve the local ion hydration properties, and exchange of hydrating water molecules for molecular ligating groups [3,5,10,11,12]. Because our analyses here will be limited to gas phase systems, we restrict our work to physical cluster theories [16,17,18,19,20,21,22], progenitors of molecular QCT [23,24]

Objectives

Methods

Results

Discussion

Conclusion