Hydrogen bonds are very important in chemistry and biology. [1][2][3] The properties of liquids and solutions consisting purely of neutral molecules are characteristically determined by the strength and number of hydrogen bonds. When water freezes to form ice, each water molecule forms four strong hydrogen bonds to its neighbors in tetrahedral fashion giving a periodical H-bond network.[4] In nonpolar solvents peptides retain their helical secondary structure up to very high temperature as a result of intramolecular H bonds.[5] Nucleic acids when neutralized in aqueous electrolyte solutions build the famous double-helical structure on the basis of strong two-and threefold hydrogen bonds between base pairs. [6] What all these important structures have in common is that they are stabilized by hydrogen bonds; they usually become more rigid and less flexible with increasing strength and number of H bonds.In this study we show that the opposite behavior can be found for ionic liquids (ILs), which are composed solely of ions rather than neutral molecules. ILs constitute a remarkably promising class of technologically useful and fundamentally interesting materials. [7][8][9][10][11][12] Herein we show that strong and directional H bonds formed between cations and anions destroy the charge symmetry and thus can fluidize ionic liquids. H bonds introduce "defects" into the Coulomb network of ILs and increase the dynamics of the cations and anions, resulting in decreased melting points and reduced viscosities. Thus the properties of ILs can be altered by adjusting the ratio between Coulomb forces and van der Waals interactions represented by H bonds. This possibility is demonstrated by FTIR measurements of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C 2 mim]-[NTf 2 ] (1) and 1-ethyl-2,3-dimethylimidazolium bis(trifluoro methylsulfonyl)imide [C 2 C 1 mim][NTf 2 ] (2), wherein characteristic H-bond contributions can be switched off by methylation at C(2).Recently, we presented low-frequency vibrational spectra of imidazolium-based ionic liquids in the range between 30 and 300 cm À1 obtained by far-infrared spectroscopy.[13] We could show that the absorptions at wavenumbers above 150 cm À1 can be assigned to intramolecular bending and wagging modes of cations and anions in the ionic liquid. The contributions below 150 cm À1 were assigned to the intermolecular interactions between cations and anions that describe the bending and stretching vibrational modes of hydrogen bonds. 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 region. Further proof of the intermolecular interactions came from a nearly linear relation between the average binding energies of calculated IL aggregates and the measured wavenumbers for maxima of the low-frequency vibrational bands for a series of ionic liquids containing the same imidazolium cation but different anions. Although this assignment is supported by recent THz...

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.

Understanding cohesion energies and studying intermolecular forces are real challenges. Cohesion energies determine whether matter sticks together, gases condense to liquids, or liquids freeze to solids. Knowledge of intermolecular forces is in particular interesting for the class of ionic liquids. Although ionic liquids consist purely of ions they show a broad liquid range, and some of them have melting points well below 08C. [1][2][3] On the other hand, ionic liquids show extremely low vapor pressures and high enthalpies of vaporization, which make them attractive as "green" solvents that could replace traditional industrial solvents. [4,5] Thus in some cases ionic liquids show typical liquid behavior, whereas in other cases they display more molten salt like behavior. However, the understanding of intermolecular forces is crucial for the development of special and tuneable properties of ionic liquids.In principle, the interaction energies between cations and anions of an ionic liquid can be calculated by ab initio and DFT calculations. This has been done for a large number of ionic liquids comprising various cations and anions. Typical interaction energies lie between 300 and 400 kJ mol À1 . [6][7][8][9][10][11][12] However, these are calculated interaction energies for selected ion-pairs, and do not give the average interaction energies in the bulk ionic liquids. So far there is no direct evidence for the cation-anion interaction in ionic liquids. In principle, these interactions can be studied by experimental methods, such as optical heterodyne-detected Ramaninduced Kerr-effect spectroscopy, [13][14][15][16][17] THz spectroscopy, [18][19][20] and low-energy neutron scattering, [21] which cover the frequency range of these interaction energies. FTIR and Raman studies on ionic liquids have focused on the mid infrared range and on investigations of the intramolecular stretching and bending modes. [22][23][24][25][26][27][28][29][30][31][32][33][34][35] The very little Raman work known does not discuss the low frequency range (0-300 cm À1 ) in terms of intermolecular forces. [36] To the best of our knowledge we present here the first FTIR measurements of imidazolium-based ionic liquids [C 2 Figure 1. Overall it can be seen that the spectra show significant differences. Because we kept the imidazolium cation (C 2 mim + ) constant, the differences can only arise from weak intramolecular vibrations of various anions and/or specific cation-anion interactions. Beside wavenumbers also the vibrational intensities vary significantly with the anions used.Strong support for the interpretation of the low-frequency vibrational bands is provided by ab initio calculations of ionic liquid aggregates ([C 2 mim][A]) x , where x is the number of ion-pairs contributing to the overall cluster, and A À represents the chosen anion. It is assumed that the largest clusters

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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.