Abstract
The 1H NMR spectra of 10–5 mole fraction solutions of 1-decyl-3-methyl-imidazolium chloride ionic liquid in water, acetonitrile, and dichloromethane have been measured. The chemical shift of the proton at position 2 in the imidazolium ring of 1-decyl-3-methyl-imidazolium (H2) is rather different for all three samples, reflecting the shifting equilibrium between the contact pairs and free fully solvated ions. Classical molecular dynamics simulations of the 1-decyl-3-methyl-imidazolium chloride contact ion pair as well as of free ions in water, acetonitrile, and dichloromethane have been conducted, and the quantum mechanics/molecular mechanics methods have been applied to predict NMR chemical shifts for the H2 proton. The chemical shift of the H2 proton was found to be primarily modulated by hydrogen bonding with the chloride anion, while the influence of the solvents—though differing in polarity and capabilities for hydrogen bonding—is less important. By comparing experimental and computational results, we deduce that complete disruption of the ionic liquid into free ions takes place in an aqueous solution. Around 23% of contact ion pairs were found to persist in acetonitrile. Ion-pair breaking into free ions was predicted not to occur in dichloromethane.
Highlights
IntroductionBecause Ionic liquids (ILs) are typically composed of asymmetric organic cations and organic or inorganic anions, many of them remain liquid at or near room temperature.[1−4] ILs are generally regarded as being environmentally friendly solvents because they are nonvolatile, thermally stable, and recyclable
Ionic liquids (ILs) are salts with a melting point below 100 °C
In the QM/MM scheme utilizing nonpolarizable potentials, the inclusion of some of the solvent molecules to the quantum mechanically treated region of the model offers an improved description of electrostatic interactions between the solute and the solvent
Summary
Because ILs are typically composed of asymmetric organic cations and organic or inorganic anions, many of them remain liquid at or near room temperature.[1−4] ILs are generally regarded as being environmentally friendly solvents because they are nonvolatile, thermally stable, and recyclable. Many of their chemical and physical properties, such as acidity, electrical conductivity, ability to solvate solutes of different polarities, or miscibility with water as well as with organic solvents, can in principle be tailored for specific needs due to the seemingly unlimited flexibility in choosing the cationic and anionic species. Mixtures of IL and acetonitrile were seen to possess an improved electrochemical window as compared to those of pure components,[31] and they have been used as electrolytes in electrochemical production of graphene[32] or for the electrocatalytic enhancement of the reduction of CO2 to CO.[33]
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