Hydrogen bonding in ionic liquids

Abstract

Ionic liquids (IL) and hydrogen bonding (H-bonding) are two diverse fields for which there is a developing recognition of significant overlap. Doubly ionic H-bonds occur when a H-bond forms between a cation and anion, and are a key feature of ILs. Doubly ionic H-bonds represent a wide area of H-bonding which has yet to be fully recognised, characterised or explored. H-bonds in ILs (both protic and aprotic) are bifurcated and chelating, and unlike many molecular liquids a significant variety of distinct H-bonds are formed between different types and numbers of donor and acceptor sites within a given IL. Traditional more neutral H-bonds can also be formed in functionalised ILs, adding a further level of complexity. Ab initio computed parameters; association energies, partial charges, density descriptors as encompassed by the QTAIM methodology (ρBCP), qualitative molecular orbital theory and NBO analysis provide established and robust mechanisms for understanding and interpreting traditional neutral and ionic H-bonds. In this review the applicability and extension of these parameters to describe and quantify the doubly ionic H-bond has been explored. Estimating the H-bonding energy is difficult because at a fundamental level the H-bond and ionic interaction are coupled. The NBO and QTAIM methodologies, unlike the total energy, are local descriptors and therefore can be used to directly compare neutral, ionic and doubly ionic H-bonds. The charged nature of the ions influences the ionic characteristics of the H-bond and vice versa, in addition the close association of the ions leads to enhanced orbital overlap and covalent contributions. The charge on the ions raises the energy of the Ylp and lowers the energy of the X-H σ* NBOs resulting in greater charge transfer, strengthening the H-bond. Using this range of parameters and comparing doubly ionic H-bonds to more traditional neutral and ionic H-bonds it is clear that doubly ionic H-bonds cover the full range of weak through to very strong H-bonds.

X-ray spectroscopy for chemistry in the 2-4 keV energy regime at the XMaS beamline: ionic liquids, Rh and Pd catalysts in gas and liquid environments, and Cl contamination in γ-Al2O3

“…In addition, the shapes of the XANES spectra are broadly similar, although there are subtle differences. (Hunt et al, 2015). In spite of these different cations, E NEXAFS is the same within the error of the experiment, demonstrating that the cation has little effect on q i of the [HSO 4 ] À anion.…”

A Hydronium Solvate Ionic Liquid: Facile Synthesis of Air-Stable Ionic Liquid with Strong Brønsted Acidity

“…Besides, neutralization of Brønsted acids and bases produces so-called protic ILs, which are preferred as acid-catalyzed reaction media due to the presence of an acidic proton. [3][4][5][6] Since proton transfer is a common chemical transformation, Brønsted acidic ILs receive research attention especially for acid catalysts, fuel cells, biomass processing, polymerization, CO 2 fixation, etc. 6,[14][15][16][17][18][19] The design of stronger Brønsted acidic ILs and evaluation of the acidity in ILs are important for exploring their possible applications as media for acid-base reactions.…”

“…Furthermore, ILs have received growing interest due to their various uses as media in chemical, biochemical, and/or electrochemical systems. [1][2][3][4][5][6] Common anions of ILs such as BF 4 -, PF 6 -, and AlCl 4 -are produced by neutralization, i.e. the adducts of a Lewis acid (BF 3 , PF 5 , or AlCl 3 ) and a Lewis base (F -or Cl -).…”

Organic co-solvents mediated variation in anion-water hydrogen bonding in [Bmim][BF4] ionic liquid through FTIR spectroscopy

“…The reduction of the extent of hydrogen bonding in presence of the cosolvents may be responsible for decrease in viscosity of ionic liquids upon addition of co-solvents because hydrogen bonding in ionic liquid is the signature of co-operative effect as well structural directionality which indirectly contributes to the viscosity of ionic liquids. 5,6 However, further investigations are required to explore…”

“…3 Moreover, existence of hydrogen bonding network in the midst of cations and anions persuades co-operative effect and structural directionality (entropic effect) in the ionic liquids. 5,6 Though ionic liquids possess most of characteristics propagated in 'green chemistry', still wide spread applicability of ionic liquids in the realm of chemical synthesis at large scale is limited due to its high viscosity which retards the chemical reactions by slowing the dynamics or movement of the reactants. 7 This challenge has been overcome by introducing co-solvents which help to reduce viscosity of the ionic liquids keeping other properties least altered to widen its applicability.…”