By using a combination of spectroscopic and theoretical methods, this work provides a systematic study of intermolecular interactions in ionic liquids (ILs) with a focus on hydrogen bonding. It can be shown that far infrared spectroscopy serves as a direct probe for the hydrogen bond strength between anion and cation. The stretching frequencies of H bonds are directly linked to inter-molecular binding energies.
Hydrogen bonds fluidize ionic liquids, which is in contrast to molecular liquids. This finding can be explained by introducing symmetry defects into the liquid’s charge network. This effect is of fundamental importance for the synthesis of novel low viscosity, low melting ILs on the one hand and for the development of force fields in molecular dynamics simulations on the other hand.
The correlation of intermolecular vibrational frequencies with measured enthalpies of vaporization for imidazolium-based ionic liquids allows estimating this property, which is difficult to measure, for new ILs yet to be characterized. In protic ILs, hydrogen bond networks can be identified, which are reminiscent of water.
The H bond contribution to the overall interaction energy can be determined by means of quantum chemical calculations and NBO analysis. Variation of the hydrogen bond strength enables “tuning” of the physico-chemical properties and serves as a step towards ILs as “designer solvents”.
Correlating IR and NMR properties allows a simple determination of experimentally not available quadrupole coupling constants for these liquids. As a consequence, molecular rotational correlation times can be calculated from measured NMR relaxation rates, representing single cation dynamics in ILs. This single particle dynamics is of paramount importance for bulk properties such as the viscosity and the applicability of hydrodynamic theories to ionic liquids.