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 molecular structure of the ion pairs of the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF<sub>4</sub>], in dichloromethane. According to the computations of the chemical shifts, experimental NMR spectra can be rationalized by an equilibrium between <i>ca.</i> 70−80% of structures with the anion positioned near to the C2 atom of the imidazolium ring and <i>ca.</i> 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 experimental NMR data suggest that the ratio of the two above-mentioned types of structures of the imidazolium-based ILs is influenced by the concentration/polarity of their dichloromethane solutions.