NMR Study of the Structure of some Imidazolium-based room temperature ionic liquids

The importance of room temperature ionic liquids (RTILs) as dissolving media for green chemistry is constanlty growing due to their unique physical and chemical properties. Indeed, they show high dissolving power, chemical stability and virtually no vapor pressure. It is known that RTILs can absorb significant amount of water from air and that some properties (e.g., solubility, polarity, viscosity and conductivity) may consequently vary as a function of absorbed water. This, in turn, affects the rates of chemical reactions and efficiencies of various processes carried out in RTILs. The strict relationsh ip between ionic liquid structure and their behaviour as reaction media stimulated studies with a broad repertoire of techniq ues (FT-IR, Near- IR, fluorescence, viscosimetry, conductivity and pulsed-grad ient spin-echo NMR diffusion coefficient measurements. Unfortunately, these techniques cannot provid e direct information on molecular level of structure of RTILs and their interactions with water. In this communication we report on some results of an NMR study via intermolecular nuclear Overhauser enhancements (NOEs) on model compounds 1-n-butyl-3-methylimidazolium tetrafluoborate ([Bmim+ [BF4]-) and 1-n-butyl-3-methyl imidazolium hexafluorophosphate ([Bmim+[PF6-). The comparison of NOEs patterns in the pure liquids and in the samples containing water showed that the presence of water changes the structure of the pure ionic l iquid by introducing water-cat ion interaction s. Fu rther detai ls on the type and site of water­ imidazoli um ion interactions are obtained by the quantitation of water-cation NOEs. Heteronuclear experiments such as 1 H {19F} NOE difference spectra pointed out the presence of ion-pairs with C(sp2)-H··F type hydrogen bonds.