Comparison of the Blue-Shifted C−D Stretching Vibrations for DMSO-d6 in Imidazolium-Based Room Temperature Ionic Liquids and in Water

The blue-shifted C−D stretching vibrations (νCD) of deuteriated dimethyl sulfoxide (DMSO-d6) in mixtures of DMSO-d6/ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) were carefully investigated and compared with those in DMSO-d6/water. Results from FTIR studies indicate that the direct interaction of C−D···F between DMSO-d6 and the anion [BF4] plays a minor role in the blue-shift of νCD in the mixtures of DMSO-d6/IL. They also show that the indirect influence of the hydrogen bond formed by the nearby functional group SO with C2−H of the cation significantly contributes to the blue-shift of νCD, as can be inferred from the significant differences of the blue-shift in [Bmim][BF4] and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([Bm2im][BF4]). In contrast, in the case of DMSO-d6/water mixtures, direct hydration of C−H···O between the methyl groups of DMSO-d6 with the oxygen atom of water is mainly responsible for the blue shift of νCD. Furthermore, theoretical calculations revealed that charge migrations as well as rehybridization have a dominant effect on the blue-shift in both mixtures.