Hydrogen-bond acidity of ionic liquids: an extended scale

Abstract

One of the main drawbacks comprising an appropriate selection of ionic liquids (ILs) for a target application is related to the lack of an extended and well-established polarity scale for these neoteric fluids. Albeit considerable progress has been made on identifying chemical structures and factors that influence the polarity of ILs, there still exists a high inconsistency in the experimental values reported by different authors. Furthermore, due to the extremely large number of possible ILs that can be synthesized, the experimental characterization of their polarity is a major limitation when envisaging the choice of an IL with a desired polarity. Therefore, it is of crucial relevance to develop correlation schemes and a priori predictive methods able to forecast the polarity of new (or not yet synthesized) fluids. In this context, and aiming at broadening the experimental polarity scale available for ILs, the solvatochromic Kamlet-Taft parameters of a broad range of bis(trifluoromethylsulfonyl)imide-([NTf 2 ] − )-based fluids were determined. The impact of the IL cation structure on the hydrogen-bond donating ability of the fluid was comprehensively addressed. Based on the large amount of novel experimental values obtained, we then evaluated COSMO-RS, COnductor-like Screening MOdel for Real Solvents, as an alternative tool to estimate the hydrogen-bond acidity of ILs. A three-parameter model based on the cation-anion interaction energies was found to adequately describe the experimental hydrogen-bond acidity or hydrogen-bond donating ability of ILs. The proposed three-parameter model is also shown to present a predictive capacity and to provide novel molecular-level insights into the chemical structure characteristics that influence the acidity of a given IL. It is shown that although the equimolar cation-anion hydrogen-bonding energies (E HB ) play the major role, the electrostaticmisfit interactions (E MF ) and van der Waals forces (E vdW ) also contribute, admittedly in a lower extent, towards the hydrogen-bond acidity of ILs. The new extended scale provided for the hydrogen-bond acidity of ILs is of high value for the design of new ILs for task-specific applications. † Electronic supplementary information (ESI) available: Comparison between experimental and literature solvatochromic values; correlation between experimental and predicted hydrogen-bond acidity; equation to calculate the absolute relative deviation. See

Understanding the effect of ionic liquids as adjuvants in the partition of biomolecules in aqueous two-phase systems formed by polymers and weak salting-out agents

“…[P 4444 ]Cl was kindly supplied by Cytec Ind. The chemical structures, predictive hydrogen-bond acidity ( α ) [21] and molar volume ( V m ) of the studied ILs are presented in Table 1.…”

Photoinitiated polymerization in ionic liquids and its application

“…However, polarity is the sum of all possible, nonspecific and specific, interactions between solute ions or molecules and solvent molecules. To separate specific effects of the local electric fields from the hydrogen bonding effects, the classical approach of Kamlet–Taft has been adapted using sets of closely related dyes that were selected to probe particular solvent properties . The polarity was broken down into three solvatochromic parameters, α , β and π *: The α scale of solvent HBD (hydrogen bond donor) acidities describes the ability of the solvent to donate a proton in a solvent‐to‐solute hydrogen bond. The β scale of HBA (hydrogen bond acceptor) basicities provides a measure of the ability of the solvent to accept a proton (donate an electron pair) in a solute‐to‐solvent hydrogen bond. The π * scale is an index of solvent dipolarity/polarizability, which measures the ability of the solvent to stabilize a charge or a dipole by virtue of its dielectric effect …”

“…Kamlet–Taft parameters were determined and gathered in a number of papers . Examples of Kamlet–Taft parameters for some ILs and non‐ionic solvents are given in Table .…”

Ionic liquid‐based organic–inorganic hybrid electrolytes: Impact of in situ obtained and dispersed silica

“…Also, immidazolium cations have been found to possess high acidic character as compared to other IL cations such as pyridinium, pyrrolidinium, and so on. The high acidity is due to the hydrogen attached at the C(2) position in the immidazolium cation . Furthermore, PEO is considered a “hard base” which makes the dissolution easy of salts having acidic nature.…”

“…The high acidity is due to the hydrogen attached at the C(2) position in the immidazolium cation. 35 Furthermore, PEO is considered a "hard base" 36 which makes the dissolution easy of salts having acidic nature. Tetraethylorthosilicate (TEOS) has been chosen as a silica precursor due to its slow and manageable reaction rate and easy purification.…”