Ionic liquids (ILs) have many favorable physicochemical properties such as low melting points, wide liquid ranges, negligible vapor pressures, high thermal stability, excellent solubility, and tunable hydrophobicity. As a structure-directing force, Hydrogen Bonds (H-bonds) are responsible for ionic pairing, stacking and self-assembling. In particular, H-bonds is considered to be critical in the structure and interaction modes of ILs.

In 2007, Prof. Seddon investigated the interactions of ions in the solid state for a series of representative [C_nC₁im][PF₆] salts and found that there are specific directional H-bonds to [PF₆]^- anions or the formation of interstitial voids. Recently, Prof. Ludwig et al. focused their attention on the characterization of H-bonds in ILs by low-frequency spectroscopy combined with DFT calculations and molecular dynamics (MD) simulations. They found that the H-bonds in imidazolium-based ILs perturb the Coulomb forces to deviate from charge symmetry, finally resulting in decreased viscosities and enhanced fluidization of the ILs.

The H-bonds have important effect on the properties of ionic liquid, leading to a decrease in their melting points and viscosities and thus fluidizing them (Figure 1). Up to now, reports on H-bonds in ILs have emerged in large numbers, however, a review on the uniqueness of H-bonds and their roles in reactions has not been presented. In this work, the structural features of H-bonds in common ILs and PILs are discussed based on the results of experiments and calculations, then H-bonds are correlated with the properties of ILs and finally the roles of H-bonds in typical reactions are reviewed.