Alexander Wulf

Hydrogen bonding in protic ionic liquids: reminiscent of water.

Similarities and differences: Far-infrared spectra of protic ionic liquids could be assigned to intermolecular bending and stretching modes of hydrogen bonds. The characteristics of the low-frequency spectra resemble those of water. Both liquids form three-dimensional network structures, but only water is capable of building tetrahedral configurations. EAN: ethylammonium nitrate, PAN: propylammonium nitrate, DMAN: dimethylammonium nitrate.

Spectroscopic Evidence for an Enhanced Anion–Cation Interaction from Hydrogen Bonding in Pure Imidazolium Ionic Liquids

Ionic liquids (ILs) have many valuable applications in chemistry and technology. They are understood as liquids consisting entirely of ions and having melting points below 100 8C. The interesting properties of ionic liquids are governed by the type and strength of interaction between its constituents. It is assumed that hydrogen bonding plays an important role for the properties and reaction dynamics of these Coulomb systems. The presence of hydrogen bonding in the structure of 1-alkyl-3-methylimidazolium salt was first reported by Seddon et al. in 1986. Since then, evidence for hydrogen bonding has been obtained from X-ray diffraction and mid-infrared and NMR spectroscopy. Local and directional interactions, such as hydrogen bonds, in imidazoliumbased ILs are indicated by shorter C H···anion distances, redshifted C H frequencies, and downfield-shifted C H proton chemical shifts. Indications of hydrogen bonding is also provided by theoretical studies. Recently however, some authors have strongly challenged the presence of hydrogen bonds in ionic liquids, and claimed that hydrogen bonding need not be invoked for explaining IL properties. For this reason, we initiated a program of direct spectroscopic observation of hydrogen bonds by successively increasing hydrogen bond abilities in a set of well-chosen imidazolium-based ionic liquids. Studying and understanding these interactions is a real challenge, and in particular for ILs. For imidazolium-based ILs, hydrogen bonding in infrared spectroscopy has been primarily concerned with the shift (Dns) of the C H stretching frequency in the mid-infrared region. However, it is more pertinent to observe the stretching (ns) and bending (nb) frequencies of hydrogen bonds themselves in far-infrared (FIR) spectra. These modes are shown and assigned in Scheme 1. The force constants obtained from these frequencies provide information about the hydrogen-bonding potential function as well as being a measure of the bond strength. Unambiguous assignment of the hydrogen-bond frequencies has provided a major difficulty in the FIR investigations. In particular, the low-frequency spectrum is surprisingly rich in information. Even for light molecules, low-energy intramolecular vibrations, such as torsions and certain skeletal motions, fall in the FIR region. Recently, we presented the low-frequency vibrational spectra of imidazolium-based ionic liquids in the range 30–300 cm 1 obtained by FIR spectroscopy. We could show that the wavenumbers above 150 cm 1 can be assigned to intramolecular bending and wagging modes of cations and anions in the ionic liquid. The contributions below 150 cm 1 were assigned to the bending and stretching vibrational modes of the intermolecular anion– cation interactions. This assignment was supported by DFT calculations, which gave wavenumbers for the bending and stretching modes of ion pairs and ion-pair aggregates in this frequency range. We also suggested that the frequencies and intensities of the FIR vibrational bands may contribute to the development of forcefields in molecular dynamics simulations. However, important issues could not be clarified definitively. To what extend does the intermolecular vibrational band stem from localized short-ranged H-bonds and/or from non-localized long-ranged Coulomb forces? This question is addressed herein by choosing the same anion for all the ILs and successively increasing the H-bond abilities of the depicted cations. The second unclear point concerns the origin of the frequency shifts for the intermolecular vibrational bands. Following the solution of the equation for simple harmonic oscillators, w = (k/m), those shifts can occur either from the changing force constant and/or the reduced masses. This problem is addressed herein by choosing cations that give comparable or even the same reduced masses in combination with the same anion. If that is the case, the frequency shifts can be attributed to changing force constants and thus the changing strength of cation–anion interaction only. Scheme 1. The stretching (ns) and bending (nb) frequencies of a hydrogen bond shown for the C2 H···A interaction in a 1,3-dimethylimidazolium cation.

The potential role of hydrogen bonding in aprotic and protic ionic liquids

Cohesion energies determine the phase behavior of materials. The understanding of interaction energies is in particular interesting for ionic liquids. Here we show experimentally that, in accord with theoretical work, the intermolecular cation-anion interactions in ionic liquids can be detected by far FTIR spectroscopy. The measured vibrational bands of aprotic and protic ionic liquids in the low-frequency range can be referred to the interaction strength between cations and anions in various combinations. It can be shown by DFT B3LYP calculations that these interactions are described by characteristic ratios between Coulomb forces and hydrogen bonds. These ratios can be tuned towards increasing hydrogen bond contributions which is reflected in important macroscopic properties of ionic liquids such as enthalpies of vaporization and viscosities. This opens a new path for tuning the desired properties of this new class of material.

Imidazolium Salt Ion Pairs in Solution

The formation, stabilisation and reactivity of contact ion pairs of non-protic imidazolium ionic liquids (ILs) in solution are conceptualized in light of selected experimental evidence as well theoretical calculations reported mainly in the last ten years. Electric conductivity, NMR, ESI-MS and IR data as well as theoretical calculations support not only the formation of contact ion pairs in solution, but also the presence of larger ionic and neutral aggregates even when dissolved in solvents with relatively high dielectric constants, such as acetonitrile and DMSO. The presence of larger imidazolium supramolecular aggregates is favoured at higher salt concentrations in solvents of low dielectric constant for ILs that contain shorter N-alkyl side chains associated with anions of low coordination ability. The stability and reactivity of neutral contact species are also dependent on the nature of the anion, imidazolium substituents, and are more abundant in ILs containing strong coordinating anions, in particular those that can form charge transfer complexes with the imidazolium cation. Finally, some ILs display reactivities as contact ion pairs rather than solvent-separated ions.

Water vibrational bands as a polarity indicator in ionic liquids

With a combination of FTIR and ab initio calculations we studied the structure of water molecules confined in ionic liquids (ILs). Most of the single water molecules are H-bonded in double donor (DD) configurations either to one anion or to two anions in a 1 : 2 complex. We show that the vibrational stretching modes of such water molecules can be used as sensitive probes for the polarity of the ILs, expressed through dielectric constants. We apply our method to imidazolium salts of the third and fourth generation. We discuss the anion- and cation-dependence and compare our results with other polarity parameters.

The Effects of Temperature and H/D Isotopic Dilution on the Transmission and Attenuated Total Reflection FTIR Spectra of Water

Abstract Using a set of IR spectroscopy methods the OH- and OD-stretch region of H2O, D2O and HDO were investigated. With a combination of attenuated total reflection infrared (ATR-IR) and transmission infrared Fourier Transform spectroscopy, temperature dependent experiments were carried out. HDO experiments were done with varying isotopic concentrations. We focused on contributions at higher wavenumbers within the OH/OD-stretch region to discuss coupling effects. We also compared results from ATR and transmission spectroscopy in order to check the comparability of these techniques. In addition we tried to investigate how the overtone of the bending vibration may be included into the analysis of the OH-stretch region of pure water.