The role of hydrogen bonding for the formation of cation dimers in carboxy-functionalized ionic liquids

The properties of ionic liquids (ILs) are governed by a subtle balance of Coulomb interactions, dispersion forces and hydrogen bonding. The latter has a particularly significant influence on the behaviour of those substances and can be divided into two main types: One occuring between a cation and an anion and another between ions of like-charge. In ILs, the hydrogen bonding between cations might not appear to be important. However, structural motifs involving hydrogen-bonded cation clusters were observed in the bulk liquid phase. In this study, we want to analyse the carboxy-functionalized IL N-carboxymethylpyridinium bis(trifluoromethanesulfonyl)azanide, which forms doubly hydrogen-bonded cation dimers already in the solid state. We ask whether this particularly strong hydrogen bond motif between ions of like-charge remains upon melting and how it behaves with increasing temperature.