Knowledge of intermolecular forces is a requisite for understanding material properties. These forces determine whether matter sticks together, gases condense to liquids, and liquids freeze to solids. The study of these forces in ionic liquids is particularly interesting. [1][2][3][4] Although the structure and properties of these fluid materials are determined to a large extent by the Coulomb forces, hydrogen bonding and dispersion forces can play a crucial role. The strong anion-cation interaction in these Coulomb fluids is reflected in their extremely low vapor pressures and high enthalpies of vaporization. [5][6][7][8][9][10][11][12] These properties, among others, make ionic liquids (ILs) attractive for science and technology.However, measuring intermolecular interactions in ionic liquids is still a challenge. In principle these interactions can be studied by experimental techniques that cover the frequency range of these interaction energies. Meanwhile there are numerous spectroscopic techniques available covering the frequency range of interest between 1 and 300 cm À1 , corresponding to 0.03 and 9 THz. The palette of spectroscopic methods includes optical heterodyne-detected Ramaninduced Kerr effect (RIKE), far-infrared (FIR), Raman, and THz spectroscopy as well as low-energy neutron scattering. [13][14][15][16][17][18][19][20][21][22][23] Although it has been shown that this spectral region may be extremely useful for studying intermolecular forces, we concede that the measured spectra are quite complicated and difficult to dissect. In particular the unequivocal assignment of the vibrational bands to intermolecular interactions is a challenge. Low-frequency absorption can arise from internal molecular vibrations, librational modes, and torsional modes such as alkyl group rotation. Usually, DFT and MD methods are required for the interpretation of the measured spectra. [24][25][26][27][28][29][30][31] In principle, these methods are suitable to indicate low-frequency intramolecular vibrational modes.However, a serious problem still remains. It is not clear to what extent DFT methods or force fields typically used in classical molecular dynamics (MD) simulations are able to describe intermolecular interactions accurately. [32] In principle ab initio molecular dynamics (AIMD) simulation is the method of choice for analyzing this frequency range, as shown by Heyden et al. for the case of liquid water. [33] However, for the relatively viscous ionic liquids the small system sizes and short simulation runs provide insufficient statistics and result in noisy and unspecific spectra in the far-infrared region. [34] But even if this frequency range is analyzed properly and the vibrational modes giving the intermolecular interaction between anion and cation can be assigned correctly, another problem persists. It is unclear to what extent frequency positions and frequency shifts of intermolecular vibrational bands can be fully referred to changing force constants indicating stronger or weaker interaction between the a...

The properties of ionic liquids are determined by the energy-balance between Coulomb-interaction, hydrogen-bonding, and dispersion forces. Out of a set of protic ionic liquids (PILs), including trialkylammonium cations and methylsulfonate and triflate anions we could detect the transfer from hydrogen-bonding to dispersion-dominated interaction between cation and anion in the PIL [(C6 H13 )3 NH][CF3 SO3 ]. The characteristic vibrational features for both ion-pair species can be detected and assigned in the far-infrared spectra. Our approach gives direct access to the relative strength of hydrogen-bonding and dispersion forces in a Coulomb-dominated system. Dispersion-corrected density functional theory (DFT) calculations support the experimental findings. The dispersion forces could be quantified to contribute about 2.3 kJ mol(-1) per additional methylene group in the alkyl chains of the ammonium cation.

Polarity controls the equilibrium constants and free energies of contact ion pairs (CIPs) and solvent-separated ion pairs (SIPs) in mixtures of protic ionic liquids and molecular solvents. The subtle balance between the ionic species was studied by far-infrared difference spectra and related DFT-calculated properties for solvents of low and high polarity and for different solvent concentrations.

High resolution mass spectrometry was utilized to study the highly complex product mixtures resulting from electrochemical breakdown of lignin. As most of the chemical structures of the degradation products were unknown, enhanced mass defect filtering techniques were implemented to simplify the characterization of the mixtures. It was shown that the implemented ionization techniques had a major impact on the range of detectable breakdown products, with atmospheric pressure photoionization in negative ionization mode providing the widest coverage in our experiments. Different modified Kendrick mass plots were used as a basis for mass defect filtering, where Kendrick mass defect and the mass defect of the lignin-specific guaiacol (C7H7O2) monomeric unit were utilized, readily allowing class assignments independent of the oligomeric state of the product. The enhanced mass defect filtering strategy therefore provided rapid characterization of the sample composition. In addition, the structural similarities between the compounds within a degradation sequence were determined by comparison to a tentatively identified product of this compound series. In general, our analyses revealed that primarily breakdown products with low oxygen content were formed under electrochemical conditions using protic ionic liquids as solvent for lignin.

The cation-anion and cation-solvent interactions in solutions of the protic ionic liquid (PIL) [Et3NH][I] dissolved in solvents of different polarities are studied by means of far infrared vibrational (FIR) spectroscopy and density functional theory (DFT) calculations. The dissociation of contact ion pairs (CIPs) and the resulting formation of solvent-separated ion pairs (SIPs) can be observed and analyzed as a function of solvent concentration, solvent polarity, and temperature. In apolar environments, the CIPs dominate for all solvent concentrations and temperatures. At high concentrations of polar solvents, SIPs are favored over CIPs. For these PIL/solvent mixtures, CIPs are reformed by increasing the temperature due to the reduced polarity of the solvent. Overall, this approach provides equilibrium constants, free energies, enthalpies, and entropies for ion-pair formation in trialkylammonium-containing PILs. These results have important implications for the understanding of solvation chemistry and the reactivity of ionic liquids.