Ion-pair formation in the ionic liquid 1-ethyl-3-methylimidazolium bis(triflyl)imide as a function of temperature and concentration

The structures and ion-pair formation in the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are studied by a combination of FTIR measurements and DFT calculations. We could clearly distinguish imidazolium cations that are completely H-bonded to anions from those that are single H-bonded in ion pairs. Ion-pair formation already occurs in the neat IL and rises with temperature. Ion-pair formation is strongly promoted by dilution of the IL in chloroform. In these weakly polar environments ion pairs H-bonded via C(2)H are strongly favored over those H-bonded via C(4,5)H. This finding is in agreement with DFT (gas phase) calculations, which show a preference for ion pairs H-bonded via C(2)H as a result of the acidic C(2)H bond.