Ekaterina I. Izgorodina

Electrochemistry at Negative Potentials in Bis(trifluoromethanesulfonyl)amide Ionic Liquids

The bis(trifluoromethanesulfonyl)amide (TFSA) anion is widely studied as an ionic liquid (IL) forming anion which imparts many useful properties, notably electrochemical stability. Here we present electrochemical and spectroscopic evidence indicating that reductive decomposition of the bis(trifluoromethanesulfonyl)amide (TFSA) anion begins at ~ −2.0 V vs. Fc+/Fc, well before the reported cathodic limit for many of these ILs. These processes are shown to be dependent upon the electrode substrate and are influenced by the water content of the IL. Supporting ab initio calculations are presented which suggest a possible mechanism for the anion decomposition. The products appear to passivate the electrode surface and the implications of this behaviour are discussed.

Ion-Pair Binding Energies of Ionic Liquids: Can DFT Compete with Ab Initio-Based Methods?

In this work, we examine the performance of a range of local and hybrid DFT functionals (including BLYP, PBE, PW91, B3P86, B3LYP, and TPSS), new generation DFT functionals (including KMLYP, BMK, M05, and M05-2X), and DFT functionals with the explicit empirical correction for dispersion interactions (including BLYP-D, PBE-D, and B3P86-D) in calculating ion-pair binding energies of pyrrolidinium-based ionic liquids, [C(n)mpyr][X] (n = Me, Et, n-Pr and n-Bu and X = Cl, BF(4), PF(6), CH(3)SO(3) (mesylate), CH(3)PhSO(3) (tosylate), N(CN)(2), and NTf(2)). Calculated IPBEs were compared to the results of the selected benchmark method, MP2/6-311+G(3df,2p). Modified MP2 methods such as SCS-MP2 and SOS-MP2 were also considered in the study. Errors of the DFT-based and ab initio-based methods in calculations of IPBEs of ionic liquids, trends in relative IPBEs, and basis-set superposition errors are discussed in depth. DFT functionals that can potentially be used to study binding energies of ionic liquids are identified.

Protic ionic liquids based on the dimeric and oligomeric anions: [(AcO)xHx−1]−

We describe a fluidity and conductivity study as a function of composition in N-methylpyrrolidine-acetic acid mixtures. The simple 1 : 1 acid-base mixture appears to form an ionic liquid, but its degree of ionicity is quite low and such liquids are better thought of as poorly dissociated mixtures of acid and base. The composition consisting of 3 moles acetic acid and 1 mole N-methylpyrrolidine is shown to form the highest ionicity mixture in this binary due to the presence of oligomeric anionic species [(AcO)(x)H(x-1)](-) stabilised by hydrogen bonds. These oligomeric species, being weaker bases than the acetate anion, shift the proton transfer equilibrium towards formation of ionic species, thus generating a higher degree of ionicity than is present at the 1 : 1 composition. A Walden plot analysis, thermogravimetric behaviour and proton NMR data, as well as ab initio calculations of the oligomeric species, all support this conclusion.

Nature of Hydrogen Bonding in Charged Hydrogen-Bonded Complexes and Imidazolium-Based Ionic Liquids

The nature of hydrogen bonding was compared in neutral complexes and negatively charged complexes consisting of either the HF molecule or the halide anion (fluoride and chloride) and the C-H bond in the methane molecule with a varying degree of fluorination (such as CH(4), CH(2)F(2), and CHF(3)). Both linear (C(3v) symmetry) and nonlinear (C(2v) symmetry) hydrogen-bonded complexes were studied. Symmetry-adapted perturbation theory was used to decompose interaction energies into fundamental components such as Coulomb, repulsion, induction and dispersion to analyze the interplay among these forces in stabilizing hydrogen bonding. In the linear charged complexes, both Coulomb attraction and induction significantly contributed to the stabilization of hydrogen bonding. In the nonlinear charged complexes, mainly Coulomb attraction contributed to the HB complex stabilization, with the inductive forces playing a less important role. Contrary to the neutral complexes, dispersion forces played only a marginal role in the charged complexes. Interplay between the fundamental forces was also investigated in the ion pairs of the imidazolium-based ionic liquid, [C(2)mim]Cl, that were categorized as either (1) typical ion-ion interaction, with the anion interacting from above or below the imidazolium plane; or (2) hydrogen-bonding interaction, with the anion interacting with the C2-H bond of the imidazolium cation. Both types of interactions were found to induce similar charge transfers, and the analysis of the energetic components revealed only a slight difference in the ion pairs studied: (1) both interactions were electrostatically driven, between 86% and 88% of the overall attractive energy, with the electrostatic component being slightly lower in the hydrogen-bonded ion pairs by ~8 kJ mol(-1); and (2) dispersion forces were found to be stronger in the typical ion-ion interactions by ~15 kJ mol(-1) and could be possible only due to the fact that the anion was able to move closer to the cation with no steric hindrance. From the experimental point of view, a vibrational red shift is expected in the hydrogen-bonded complexes of imidazolium-based ionic liquids, whereas the ion-ion interactions are more likely to produce a slight blue shift.

Understanding the effect of the C2 proton in promoting low viscosities and high conductivities in imidazolium-based ionic liquids: part I. weakly coordinating anions

1,3-Disubstituted imidazolium ionic liquids have been the subject of numerous theoretical and experimental studies due to their low viscosity-often the very lowest for any given cation/anion family. One of the mysteries in the imidazolium family of salts is the sharp increase in viscosity that is observed on methylating at the C2 position in the ring. In the nonmethylated case, the C2 proton is observed to be distinctly acidic and, where this is undesirable, substitution of the C2 position removes the problem, but produces an unexpected increase in viscosity. Methylation at other positions on the ring does not produce such a significant effect. In this study, two possible structural or energetic sources of the increased viscosity were investigated: (1) ion association, as probed by the Walden rule, and (2) differences in the potential energy surface profiles that favor ionic transport in the non C2-methylated imidazolium ionic liquids. The second hypothesis was investigated using high-level ab initio theory. The higher viscosity of C2-methylated imidazolium ionic liquids is shown to be a result of high potential energy barriers (significantly above the available thermal energy) between the energetically preferred conformations on the potential energy surface, thus restricting movement of ions in the liquid state to only small oscillations and inhibiting the overall ion transport.

Assessment of Kohn–Sham density functional theory and Møller–Plesset perturbation theory for ionic liquids

We present high-level benchmark calculations of interaction energies of 236 ion pair structures of ionic liquids constituting a new IL-2013 set. 33 different approaches using various basis sets are validated against these benchmark data. Overall, traditional functionals like B3LYP, without an explicit dispersion correction, should be avoided when investigating ionic liquids. We can recommend the third version of Grimmes empirical dispersion correction (DFT-D3) and the LC-BOP functional, as well as most functionals of the Minnesota family of the M0X type. Our results highlight the importance of diffuse basis set functions for the accurate prediction of the IL energetics using any DFT functional. The best combination of reasonable accuracy and reasonable cost was found to be the M06-L functional in combination with the 6-31++G** basis set, producing a remarkable mean absolute deviation of only 4.2 kJ mol(-1) and a maximum deviation of -12.5 kJ mol(-1). Second-order Møller-Plesset perturbation theory (MP2) in combination with counterpoise-corrected triple-ζ basis sets can also be recommended for reliable calculations of energetics of ionic liquids.

Aluminium speciation in 1-Butyl-1-Methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide/AlCl3 mixtures

Electrodeposition of aluminium is possible from solutions of AlCl(3) dissolved in the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (C(4)mpyrNTf(2)) ionic liquid. However, electrodeposition is dependant on the AlCl(3) concentration as it only occurs at concentrations >1.6 mol L(-1). At these relatively high AlCl(3) concentrations the C(4)mpyrNTf(2)/AlCl(3) mixtures exhibit biphasic behaviour. Notably, at 1.6 mol L(-1) AlCl(3), aluminium can only be electrodeposited from the upper phase. Conversely, we found that at 3.3 mol L(-1) aluminium electrodeposition can only occur from the lower phase. The complex chemistry of the C(4)mpyrNTf(2)/AlCl(3) system is described and implications of aluminium speciation in several C(4)mpyrNTf(2)/AlCl(3) mixtures, as deduced from Raman and (27)Al NMR spectroscopic data, are discussed. The (27)Al NMR spectra of the C(4)mpyrNTf(2)/AlCl(3) mixtures revealed the presence of both tetrahedrally and octahedrally coordinated aluminium species. Raman spectroscopy revealed that the level of uncoordinated NTf(2)(-) anions decreased with increasing AlCl(3) concentration. Quantum chemical calculations using density functional and ab initio theory were employed to identify plausible aluminium-containing species and to calculate their vibrational frequencies, which in turn assisted the assignment of the observed Raman bands. The data indicate that the electroactive species involved are likely to be either [AlCl(3)(NTf(2))](-) or [AlCl(2)(NTf(2))(2)](-).

Liquids intermediate between “molecular” and “ionic” liquids: Liquid Ion Pairs?

Ionic liquids comprised of tetradecyltrihexyl- and tetrabutyl-phosphonium cations paired with chloride or sulfonyl amide anions exhibit properties that reflect strong ion association, including comparatively low viscosity as well as a degree of volatility, and hence exemplify an interesting intermediate state between true ionic and true molecular liquids.

Towards a Better Understanding of ‘Delocalized Charge’ in Ionic Liquid Anions

One of the main characteristics that are attributed to ionic liquids (especially those with a low melting point) is that the anions comprising the ionic liquids possess a certain degree of charge delocalization as compared to anions in traditional molten salts. Based on the proton affinity equilibrium we proposed a new energetic criterion that can be used as a measure of charge delocalization. The proposed proton affinity comparison quantifies the extent to which ionic liquid anions are delocalized. Thus it should lead to a better understanding towards the design of task-specific ionic liquids. Therefore, this criterion can be applied to newly designed anions to assure that the extent of charge delocalization falls within the same range of values on the proton affinity scale as other commonly used ionic liquid anions.

Quantum Chemical Methods for the Prediction of Energetic, Physical, and Spectroscopic Properties of Ionic Liquids

The accurate prediction of physicochemical properties of condensed systems is a longstanding goal of theoretical (quantum) chemistry. Ionic liquids comprising entirely of ions provide a unique challenge in this respect due to the diverse chemical nature of available ions and the complex interplay of intermolecular interactions among them, thus resulting in the wide variability of physicochemical properties, such as thermodynamic, transport, and spectroscopic properties. It is well understood that intermolecular forces are directly linked to physicochemical properties of condensed systems, and therefore, an understanding of this relationship would greatly aid in the design and synthesis of functionalized materials with tailored properties for an application at hand. This review aims to give an overview of how electronic structure properties obtained from quantum chemical methods such as interaction/binding energy and its fundamental components, dipole moment, polarizability, and orbital energies, can help shed light on the energetic, physical, and spectroscopic properties of semi-Coulomb systems such as ionic liquids. Particular emphasis is given to the prediction of their thermodynamic, transport, spectroscopic, and solubilizing properties.