Para-xylyl linked bis-imidazolium ionic liquids: A study of the conformers of the cation and of the anion-cation hydrogen bonding

In this work, two new para-xylyl linked bis-imidazolium Ionic Liquids (DILs), namely [p-C6H4(CH2ImMe)2][Cl-]2 and [p-C6H4(CH2ImMe)2 +][BF4 -]2, are prepared from imidazolium salts using a simple procedure in two steps. A computational study of the conformers of [p-C6H4(CH2ImMe)2], performed by means of DFT calculations with the 6-31G** basis set and the B3LYP theory, evidences that nine possible geometries of the cation are stable points of the potential energy surface and, therefore, could be experimentally found in real compounds. The infrared and Raman spectra of the nine conformers are calculated and compared to the experimental data. The occurrence of the trans configuration of the imidazolium rings in both compounds is strongly supported by this comparison. Finally, the cation-anion interactions were investigated in both DILs by means of NMR and Raman measurements, with a particular attention to the occurrence of hydrogen bonding. Both techniques indicate that hydrogen bonding is stronger in the sample containing Cl as anion. Moreover, the preferred site for hydrogen bonding is the C2 proton of the imidazolium ring; however, also the C(4) and C(5) protons of imidazolium, the methylene group and the phenyl ring participate to the hydrogen bonding