Comparison of the Blue-Shifted C−D Stretching Vibrations for DMSO-<i>d</i><sub>6</sub> in Imidazolium-Based Room Temperature Ionic Liquids and in Water

The blue-shifted C−D stretching vibrations (ν<sub>CD</sub>) of deuteriated dimethyl sulfoxide (DMSO-<i>d</i><sub>6</sub>) in mixtures of DMSO-<i>d</i><sub>6</sub>/ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF<sub>4</sub>]) were carefully investigated and compared with those in DMSO-<i>d</i><sub>6</sub>/water. Results from FTIR studies indicate that the direct interaction of C−D···F between DMSO-<i>d</i><sub>6</sub> and the anion [BF<sub>4</sub><sup>−</sup>] plays a minor role in the blue-shift of ν<sub>CD</sub> in the mixtures of DMSO-<i>d</i><sub>6</sub>/IL. They also show that the indirect influence of the hydrogen bond formed by the nearby functional group SO with C<sub>2</sub>−H of the cation significantly contributes to the blue-shift of ν<sub>CD</sub>, as can be inferred from the significant differences of the blue-shift in [Bmim][BF<sub>4</sub>] and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([Bm<sub>2</sub>im][BF<sub>4</sub>]). In contrast, in the case of DMSO-<i>d</i><sub>6</sub>/water mixtures, direct hydration of C−H···O between the methyl groups of DMSO-<i>d</i><sub>6</sub> with the oxygen atom of water is mainly responsible for the blue shift of ν<sub>CD</sub>. Furthermore, theoretical calculations revealed that charge migrations as well as rehybridization have a dominant effect on the blue-shift in both mixtures.