Structural Studies of the Ionic Liquid 1-Ethyl-3-methylimidazolium Tetrafluoroborate in Dichloromethane Using a Combined DFT-NMR Spectroscopic Approach

DFT methods in combination with NMR spectroscopy are used to investigate possible variants of the mol. structure of the ion pairs of the ionic liq. (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF4], in dichloromethane. According to the computations of the chem. shifts, exptl. NMR spectra can be rationalized by an equil. between ca. 70-80% of structures with the anion positioned near to the C2 atom of the imidazolium ring and ca. 20-30% of structures with the anion close to the C5 and/or C4 atoms. The content of the latter structures, according to the computed Gibbs free energies, does not exceed 10%. Both the computations and the exptl. NMR data suggest that the ratio of the two above-mentioned types of structures of the imidazolium-based ILs is influenced by the concn./polarity of their dichloromethane solns.