On the physical origin of the cation–anion intermediate bond in ionic liquids Part I. Placing a (weak) hydrogen bond between two charges

Abstract

The intermediate bond forces in ionic liquids are investigated from static quantum chemical calculations at various methods and two basis sets. The experimentally observed red-shift of the donor-proton bond stretching frequency due to a bond elongation is confirmed by all methods. Comparing Hartree-Fock to second-order Møller-Plesset perturbation theory, the Hartree-Fock method gives in many cases an erroneous description of the geometries. Furthermore, the Hartree-Fock interaction energies can deviate up to 60 kJ mol(-1) from Møller-Plesset perturbation theory indicating the importance of dispersion interaction. While the usual trends of decreasing stability or interaction energies with increasing ion sizes are found, the geometries involving hydrogen atoms do not change this order of total interaction energies. Therefore, the hydrogen bond is not the most important interaction for ion pairs with regard to the total interaction energy. On the other hand, the different established analysis methods give rise to hydrogen bonding in several ion pairs. Charge analysis reveals the hydrogen-bonding character of the ion pair and shows, depending on the type of ions combined and further on the type of conformers considered, that a hydrogen bond can be present. The possibility of hydrogen bonding is also shown by an analysis of the frontier orbitals. Calculating potential energy surfaces and observing from this the change in the donor proton bond indicates that regular hydrogen bonds are possible in ion pairs of ionic liquids. Thereby, the maximum of bond elongation exceeds the one of a usual hydrogen bond by far. The more salt-like hydrogen-bonded ion pair [NH(4)][BF(4)] exhibits a steeper maximum than the more ionic liquid like ion pair [EtNH(3)][BF(4)]. The fact that imidazolium-based ionic liquids as [Emim][Cl] can display two faces, hydrogen bonding and purely ionic bonding, points to a disturbing rather than stabilizing role of hydrogen bonding on the interaction of the counterions in imidazolium-based ionic liquids. While geometry and charge analysis provides attributes of weak (blue-shifted) hydrogen bonds, large bond elongations accompanied by red-shifts are obtained for the ion pairs investigated. This can be understood by the simple fact that these imidazolium-based ionic liquid ion pairs constitute weak hydrogen bonds placed between two delocalized charges.

Effects of C(2) Methylation on Thermal Behavior and Interionic Interactions in Imidazolium-Based Ionic Liquids with Highly Symmetric Anions

“…This is particularly true for the molecular anions, [BF 4 − ] and [PF 6 − ]. Consequently, the overall molecular system gains symmetry when the C (2) number of other dialkylimidazolium ILs, e.g., NTf 2 and other halogenide anions, and in computational studies [4,5,7,28,29,38]. This suggests that the above findings represent a rather universal phenomenon.…”

“…In the real liquid, a distribution of several cation/anion configurations will be present simultaneously [43], and the sketch must be seen as one example of what the data suggest exists in larger amounts. in computational studies [4,5,7,28,29,38]. This suggests that the above findings represent a rather universal phenomenon.…”

“…In summary, the IR and Raman spectra of the C(2) protonated ILs show that the anion interacts with the cation via formation of a strong hydrogen bond at C(2)-H. Upon methylation, the anion moves to the C(4) and C(5) moiety, where it forms a new hydrogen bond with one of the two protons. The spectra do not suggest that other interaction mechanisms, such as those via the π-electron system of the aromatic ring (often referred to as the on-top position [37][38][39]), play a major role. Moreover, Endo et al [26] performed an experimental and computational study on methylated and nonmethylated butylimidazolium ILs with spherical anions.…”

Theoretical reconstruction and elementwise analysis of photoelectron spectra for imidazolium-based ionic liquids

“…For the details of a dispersion correction for ionic liquids, see ref. 50 51 have identified in their quantum-chemical studies using ion pairs that the connection between cations and anions is predominantly established via the most acidic hydrogen bond to this ringcarbon and either the nitrogen or the oxygen of the [Tf 2 N] À anion. Within the large number of our ion pairs we have ascertained that the single point energy varies about DE = 171 meV.…”

Ionic Liquids Treated within the Grand Canonical Adaptive Resolution Molecular Dynamics Technique

“…The all atom forcefield parameters to model imidazolium ionic liquids that has long alkyl chains attached to the cation ([BMIM]) and relatively hydrophobic anions (Cl − , BF − 4 ) were taken from the Dommert et al [42] and Kirchner, B. et al [43]. The model considers reduced/scaled charges of +0.8 e and −0.8 e instead of the full formal charges +1.0 e and −1.0 e. Within the CG region, ionic liquids are modelled as charged/neutral spherical atoms.…”