The relevant properties of room temperature ionic liquids (RTILs) for solute interaction and their effect on catalysis are largely determined by the nature of the anion rather than the cation. Therefore, knowledge of the effect of the anion on the properties of RTILs is of great importance for choosing the appropriate RTIL for a particular application. In this context, Ralf Lungwitz and Stefan Spange, from the University of Technology Chemnitz, Germany, are interested in the investigation of the required properties of anions to undergo hydrogen bonding. Within the priority program "Ionic liquids" of the Deutsche Forschungsgemeinschaft (German Research Foundation), the team investigates room temperature ionic liquids either in their pure form or when adsorbed onto inorganic particles.

In this work, Lungwitz and Spange have determined the hydrogen bond accepting (HBA) ability of anions of 1-butyl-3-methylimidazolium salts by ¹H NMR. In particular, they showed that the imidazolium cation served both as a component of RTIL and ¹H NMR probe. Correlations with the solvatochromically determined beta parameter also showed that a general parameter scale for anions could be established. To this effect, 3-(4-amino-3-methylphenyl)-7-phenylbenzo-[1,2-b:4,5-b]-difuran-2,6-dione dye, whose UV/vis band shift is strongly affected by the nature of the anion, was used (Figure 1).

Figure 1. Correlation of the beta parameter determined with 3-(4-amino-3-methylphenyl)-7-phenyl-benzo-[1,2-b:4,5-b]-difuran-2,6-di-one dye in pure RTILs with the 1H NMR chemical shift of the proton in the 2-position of the imidazolium ring measured in CD2Cl2. 

A remarkable consequence of these findings is that the ¹H NMR scale of the 1-butyl-3-methylimidazolium cation may be applied as a tool to determine the HBA parameters of a variety of structurally different anions that cannot be investigated by UV/vis spectroscopy

This conceptual approach will now be used to develop novel structures of RTILs and related compounds.