Ten C8C1Im+ (1‐methyl‐3‐octylimidazolium)‐based ionic liquids with anions Cl−, Br−, I−, NO3−, BF4−, TfO−, PF6−, Tf2N−, Pf2N−, and FAP− (TfO=trifluoromethylsulfonate, ...Tf2N=bis(trifluoromethylsulfonyl)imide, Pf2N=bis(pentafluoroethylsulfonyl)imide, FAP=tris(pentafluoroethyl)trifluorophosphate) and two C8C1C1Im+ (1,2‐dimethyl‐3‐octylimidazolium)‐based ionic liquids with anions Br− and Tf2N− were investigated by using X‐ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While 1H NMR spectroscopy is found to probe very specifically the strongest hydrogen‐bond interaction between the hydrogen attached to the C2 position and the anion, a comparative XPS study provides first direct experimental evidence for cation–anion charge‐transfer phenomena in ionic liquids as a function of the ionic liquid’s anion. These charge‐transfer effects are found to be surprisingly similar for C8C1Im+ and C8C1C1Im+ salts of the same anion, which in combination with theoretical calculations leads to the conclusion that hydrogen bonding and charge transfer occur independently from each other, but are both more pronounced for small and more strongly coordinating anions, and are greatly reduced in the case of large and weakly coordinating anions.
Charges in charged systems: Anion and cation interactions in ionic liquids strongly modify the charges on the ions as proven by XPS, NMR spectroscopy and DFT calculations. The smaller, more basic and, thus, more strongly coordinating the anion, the more pronounced the charge transfer to the cation (see graphic). Hydrogen‐bonding‐type interactions between anion and cation do not significantly influence the amount of charge transferred.
We investigate seven 1-alkyl-1-methylpyrrolidinium-based ionic liquids, C(n)C(1)PyrrX, using X-ray photoelectron spectroscopy (XPS). The electronic environment for each element is analysed and a ...robust fitting model is developed for the C 1s region that applies to each of the ionic liquids studied. This model allows accurate charge correction and the determination of reliable and reproducible binding energies for each ionic liquid studied. The electronic interaction between the cation and anion is investigated for ionic liquids with one and also two anions. i.e., mixtures. Comparisons are made to imidazolium-based ionic liquids; in particular, a detailed comparison is made between C(8)C(1)PyrrX and C(8)C(1)ImX(-), where X(¬) is common to both ionic liquids.
For ionic liquids (ILs), both the large number of possible cation + anion combinations and their ionic nature provide a unique challenge for understanding intermolecular interactions. Cohesive energy ...density, ced, is used to quantify the strength of intermolecular interactions for molecular liquids, and is determined using the enthalpy of vaporization. A critical analysis of the experimental challenges and data to obtain ced for ILs is provided. For ILs there are two methods to judge the strength of intermolecular interactions, due to the presence of multiple constituents in the vapour phase of ILs. Firstly, cedIP, where the ionic vapour constituent is neutral ion pairs, the major constituent of the IL vapour. Secondly, cedC+A, where the ionic vapour constituents are isolated ions. A cedIP dataset is presented for 64 ILs. For the first time an experimental cedC+A, a measure of the strength of the total intermolecular interaction for an IL, is presented. cedC+A is significantly larger for ILs than ced for most molecular liquids, reflecting the need to break all of the relatively strong electrostatic interactions present in ILs. However, the van der Waals interactions contribute significantly to IL volatility due to the very strong electrostatic interaction in the neutral ion pair ionic vapour. An excellent linear correlation is found between cedIP and the inverse of the molecular volume. A good linear correlation is found between IL cedIP and IL Gordon parameter (which are dependent primarily on surface tension). ced values obtained through indirect methods gave similar magnitude values to cedIP. These findings show that cedIP is very important for understanding IL intermolecular interactions, in spite of cedIP not being a measure of the total intermolecular interactions of an IL. In the outlook section, remaining challenges for understanding IL intermolecular interactions are outlined.
Here we report the synthesis and characterisation by X-ray photoelectron spectroscopy (XPS) of eight high purity amino acid-based ionic liquids (AAILs), each containing the ...1-octyl-3-methylimidazolium, C(8)C(1)Im(+), as a standard reference cation. All expected elements were observed and the electronic environments of these elements identified. A fitting model for the carbon 1s region of the AAILs is reported; the C aliphatic component of the cation was used as an internal reference to obtain a series of accurate and reproducible binding energies. Comparisons are made between XP spectra of the eight AAILs and selected non-functionalised ionic liquids. 1-octyl-3-methylimidazolium acetate was also studied as a model of the carboxyl containing amino acid anion. The influence of anionic substituent groups on the measured binding energies of all elements is presented, and communication between anion and cation is investigated. This data is interpreted in terms of hard and soft anions and compared to the Kamlet-Taft hydrogen bond acceptor ability, β, for the ionic liquids. A linear correlation is presented which suggests that the functional side chain, or R group, of the amino acid has little impact upon the electronic environment of the charge-bearing moieties within the anions and cations studied.
We investigate eight 1‐alkylpyridinium‐based ionic liquids of the form CnPyA by using X‐ray photoelectron spectroscopy (XPS). The electronic environment of each element of the ionic liquids is ...analyzed. In particular, a reliable fitting model is developed for the C 1s region that applies to each of the ionic liquids. This model allows the accurate charge correction of binding energies and the determination of reliable and reproducible binding energies for each ionic liquid. Shake‐up/off phenomena are determinedfor both C 1s and N 1s spectra. The electronic interaction between cations and anions is investigated for both simple ionic liquids and an example of an ionic‐liquid mixture; the effect of the anion on the electronic environment of the cation is also explored. Throughout the study, a detailed comparison is made between C8PyA and analogues including 1‐octyl‐1‐methylpyrrolidinium‐ (C8C1PyrrA), and 1‐octyl‐3‐methylimidazolium‐ (C8C1ImA) based samples, where X is common to all ionic liquids.
Pyridinium‐based ionic liquids are studied by X‐ray photoelectron spectroscopy. A fitting model is developed for the C 1s region, which allows the determination of reliable and reproducible binding energies for each ionic liquid. Electronic interactions between the cations and anions are investigated, and the effect of the anion on the electronic environment of the cation is also explored.
PDF
