Abstract
The chemical and physical properties of imidazolium-based ionic liquids are determined by the interactions between the counter-ions. The C(2) position plays an important role in these interactions, as it represents the predominant site for interionic hydrogen bonding. This study shows that the directional hydrogen bonds between highly symmetrical anions (iodide, tetrafluoroborate, hexafluorophosphate) and the C(2)-H group of the 1-methyl-3-propylimidazolium cation determine the molecular and macroscopic behavior in terms of the thermal properties. Upon replacing the C(2) proton by a methyl group, the anion repositions itself at the C(4)/(5) moiety, where it forms a new hydrogen bond, apparently with only one of the two CH groups. In addition, the larger the anion is in diameter, the more likely it will establish further interactions with other parts of the cation, such as the propyl chain.
Highlights
Ionic liquids (ILs) are typically composed of bulky organic cations and inorganic or organic anions
The present study aims to shed further light on the role of the C(2) position in imidazolium ionic liquids regarding the interionic interactions with highly symmetrical anions
In order to study the effects of methylation at the macroscopic level, the thermal behavior was analyzed using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements
Summary
Ionic liquids (ILs) are typically composed of bulky organic cations and inorganic or organic anions. They represent fascinating materials in the liquid as well as the solid state, with numerous potential applications in electrochemistry and chemical engineering [1,2]. Interionic interactions between cations and anions are known to govern the molecular and macroscopic properties of. The C(2) position plays a crucial role in this respect, because the charge distribution in the aromatic ring leads to enhanced polarization and a slightly acidic nature of the C(2)-H bond. The C(2) proton is the favorable hydrogen-bond donor. A comprehensive overview has recently been given by Pascoal et al [3]
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