Structural Features of the [C4mim][Cl] Ionic Liquid and Its Mixtures with Water: Insight from a 1H NMR Experimental and QM/MD Study.

Dovilė Lengvinaitė,Kęstutis Aidas,Stasė Bielskutė,Vytautas Balevicius,Vytautas Klimavicius,Aatto Laaksonen,Sonata Kvedaraviciute,Francesca Mocci

doi:10.1021/acs.jpcb.1c08215

Abstract

The 1H NMR chemical shift of water exhibits non-monotonic dependence on the composition of an aqueous mixture of 1-butyl-3-methylimidazolium chloride, [C4mim][Cl], ionic liquid (IL). A clear minimum is observed for the 1H NMR chemical shift at a molar fraction of the IL of 0.34. To scrutinize the molecular mechanism behind this phenomenon, extensive classical molecular dynamics simulations of [C4mim][Cl] IL and its mixtures with water were carried out. A combined quantum mechanics/molecular mechanics approach based on the density functional theory was applied to predict the NMR chemical shifts. The proliferation of strongly hydrogen-bonded complexes between chloride anions and water molecules is found to be the reason behind the increasing 1H NMR chemical shift of water when its molar fraction in the mixture is low and decreasing. The model shows that the chemical shift of water molecules that are trapped in the IL matrix without direct hydrogen bonding to the anions is considerably smaller than the 1H NMR chemical shift predicted for the neat water. The structural features of neat IL and its mixtures with water have also been analyzed in relation to their NMR properties. The 1H NMR spectrum of neat [C4mim][Cl] was predicted and found to be in very reasonable agreement with the experimental data. Finally, the experimentally observed strong dependence of the chemical shift of the proton at position 2 in the imidazolium ring on the composition of the mixture was rationalized.

Highlights

To disclose the molecular mechanisms behind the physicochemical properties of ionic liquid (IL)/water mixtures, various experimental and theoretical techniques have been called in order to provide a detailed insight into the molecular structure and dynamics of these heterogeneous systems.[8,13−15] In particular, classical molecular dynamics (MD) simulations have led to the general conclusion that solitary water molecules are dispersed throughout the bulk of the IL when water content is rather low.[14,16−21] Under these circumstances, the isolated water molecules are found to primarily form hydrogen bonds with the anions,[16,17,21] acting as bridges between them.[19,20,22]
[C4mim][Cl] IL, we show in Figure 2a the radial distribution function (RDF) between the H2 hydrogen atom in the C4mim+ cations and the chloride anions
These findings are confirmed by the spatial distribution function (SDF) of chloride anions around the imidazolium ring of the C4mim+ cation shown in this work are scaled by appropriate number densities to Figure 4a

Summary

Introduction

While earlier often regarded as an undesirable contaminant,[1,2] water has over the last decade been increasingly considered as an integral part of ionic liquid (IL) or more broadly salt-based materials, greatly extending their limits of functionalization and applicability.[3,4] The mixtures of IL and water may acquire unique properties which are not necessarily associated with any of the two components.[5−9] For example, hydrated choline dihydrogenphosphate was found to be a superior solvating medium for some proteins as compared to the ordinary aqueous buffer solutions,[5,6] suppressing protein denaturation over significantly extended periods of time and preserving their function.[7]. To disclose the molecular mechanisms behind the physicochemical properties of IL/water mixtures, various experimental and theoretical techniques have been called in order to provide a detailed insight into the molecular structure and dynamics of these heterogeneous systems.[8,13−15] In particular, classical molecular dynamics (MD) simulations have led to the general conclusion that solitary water molecules are dispersed throughout the bulk of the IL when water content is rather low.[14,16−21] Under these circumstances, the isolated water molecules are found to primarily form hydrogen bonds with the anions,[16,17,21] acting as bridges between them.[19,20,22] When the molar fraction of water increases, clusters of water molecules begin to emerge,[17,18,22] and nanostructural organization of the mixture is enhanced.[19,23] Eventually, a continuous water network is formed, which percolates the entire system and surrounds the ionic clusters.[18,20−22]

Methods

Results

Conclusion