Chloride ions as integral parts of hydrogen bonded networks in aqueous salt solutions: the appearance of solvent separated anion pairs

Ildikó Pethes,László Pusztai,Imre Bakó

doi:10.1039/d0cp01806f

Abstract

Hydrogen bonding to chloride ions has been frequently discussed over the past 5 decades. Still, the possible role of such secondary intermolecular bonding interactions in hydrogen bonded networks has not been investigated in any detail. Here we consider computer models of concentrated aqueous LiCl solutions and compute the usual hydrogen bond network characteristics, such as distributions of cluster sizes and of cyclic entities, both for models that take and do not take chloride ions into account. During the analysis of hydrogen bonded rings, a significant amount of 'solvent separated anion pairs' have been detected at high LiCl concentrations. It is demonstrated that taking halide anions into account as organic constituents of the hydrogen bonded network does make the interpretation of structural details significantly more meaningful than when considering water molecules only. Finally, we compare simulated structures generated by 'good' and 'bad' potential sets on the basis of the tools developed here, and show that this novel concept is, indeed, also helpful for distinguishing between reasonable and meaningless structural models.

Highlights

The term ‘hydrogen bonding’ (H-bonding) is traditionally connected to water and ice, where oxygen atoms form bonds with hydrogen atoms of neighboring water molecules by enhancing the electronic density between O and H by the lone electron pairs of the O atoms.[1,2,3] The same mechanism works for many other compounds with hydroxyl (–OH) groups: examples are alcohols, organic acids, sugars, proteins, DNA, etc
All calculations were repeated using the energetic definition of hydrogen bonds (HBs):[34] findings of which are shown in the Electronic supplementary information (ESI).† According to the geometric definition, two water molecules are identified as H-bonded if the intermolecular distance between an oxygen and a hydrogen atom is less than 2.5 Å, and the OÁ Á ÁO–H angle is smaller than 30 degrees
The calculations providing the atomic assemblies (‘particle configurations’) were identical to those reported recently by one of us.[18]. Note that these concentration values are rather high, the highest one representing an ion/water ratio of about 50 : 75. The reason why such systems have been selected is the expectation that the role of the ions in enhancing/disrupting the hydrogen bonded network would be most apparent under such circumstances

Summary

Introduction

The term ‘hydrogen bonding’ (H-bonding) is traditionally connected to water and ice, where oxygen atoms form bonds with hydrogen atoms of neighboring water molecules by enhancing the electronic density between O and (nonbonded) H by the lone electron pairs of the O atoms.[1,2,3] The same mechanism works for many other compounds with hydroxyl (–OH) groups: examples are alcohols, organic acids, sugars, proteins, DNA, etc. Most of the (faintly) related discussions in earlier papers have been about how ions break/disrupt the hydrogen bonding network of water molecules in these solutions

Results

Conclusion