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

Abstract

The 1 H 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 1 H 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 1 H 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 1 H 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 1 H 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.

Ionic Liquid (EmimAc)-Water Mixture Confined in Nanoporous Glass Matrices Studied With High-Resolution Neutron Spectroscopy

“…These changes to the hydrogen bonding network may also cause an increase in the viscosity at very low water content while at higher water content, the viscosity decreases. These findings are supported by Lengvinaitė et al [12], who investigated the structure of the IL with varying amounts of water via NMR. It was observed that the water forms hydrogen-bonded aggregates with the anions and that these aggregates surround the imidazolium ring cations.…”

“…The fundamental importance of the PEM to the operation of fuel cells has prompted a number of studies into the properties of IL/ water mixtures and the behaviour ILs confined in porous materials. These systems have been investigated using a broad range of experimental and computational techniques, including quasielastic neutron scattering (QENS) [7][8][9][10][11], nuclear magnetic resonance (NMR) [7,10,12], and molecular dynamics simulations (MD) [13].…”

Insights into cation–anion hydrogen bonding in mesogenic ionic liquids: an NMR study

“…In addition to experimental observations, the impressive results on HB interactions in ILs and ILCs have been obtained by molecular dynamics (MD) simulations. 8,[11][12][13]16,[27][28][29][30] The MD analysis of the non-mesogenic ionic liquid C 4 mimCl indicated that Cl is strongly localized in front of the acidic hydrogens in the imidazolium group. 30 Recent modelling of this material and its mixture with water has predicted a strong hydrogen bonding interaction between the C2-H2 moiety of imidazolium and the chloride anions.…”

“…30 Recent modelling of this material and its mixture with water has predicted a strong hydrogen bonding interaction between the C2-H2 moiety of imidazolium and the chloride anions. 29 Chlorides were also found to coordinate imidazolium in the vicinity of the C4-H4 and C5-H5 bonds. In the modelling of hydrated ILs, it has been shown that water molecules acted as hydrogen-bonded links between chloride anions.…”

“…In the modelling of hydrated ILs, it has been shown that water molecules acted as hydrogen-bonded links between chloride anions. 29 Recently, a monohydrated ionic smectic phase of C 14 mimClÁH 2 O has been studied. 16 It has been shown that water partly replaces the chloride anions in hydrogen bonding with the acidic protons of the imidazolium ring.…”