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Using high-pressure infrared methods, we have investigated close interactions of charge-enhanced C-H-O type in ionic liquid/dimethyl sulfoxide (DMSO) mixtures. The solvation and association of the 1-butyl-3-methylimidazolium tetrafluoroborate (BMI+BF4-) and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BMM+BF4-) in DMSO-d6 were examined by analysis of C-H spectral features. Based on our concentration-dependent results, the imidazolium C-H groups are more sensitive sites for C-H-O than the alkyl C-H groups and the dominant imidazolium C-H species in dilute ionic liquid/DMSO-d6 should be assigned to the isolated (or dissociated) structures. As the dilute mixtures were compressed by high pressures, the loss in intensity of the bands attributed to the isolated structures was observed. In other words, high pressure can be used to perturb the association-dissociation equilibrium in the polar region. This result is remarkably different from what is revealed for the imidazolium C-H in the BMM+BF4-/D2O mixtures. DFT-calculations are in agreement with our experimental results indicating that C4-H-O and C5-H-O interactions seem to play non-negligible roles for BMM+BF4-/DMSO mixtures. [ABSTRACT FROM AUTHOR]

The effects of alkyl side-chain length on the interactions between imidazolium ionic liquids (ILs) and β-cyclodextrin (β-CD) were studied at ambient pressure and up to 2.5GPa. No noticeable changes in the C-H band frequencies of 1-methyl-3-propylimidazolium hexafluorophosphate ([MPIM][PF6]) were observed upon β-CD addition under ambient pressure. Nevertheless, β-CD addition inhibited the formation of the [MPIM][PF6] aggregated structure under pressures of ≤0.7 GPa. As the pressure was raised to 1.1–2.5 GPa, [MPIM][PF6] aggregated and formed self-associated configurations. This pressure-induced dissociation was not detected in the 1-butyl-3-methylimidazolium hexafluorophosphate and β-CD mixture ([BMIM][PF6]-β-CD) due to the long side chain in the [BMIM] cation. This indicates that the alkyl side-chain length of the imidazolium ILs plays a non-negligible role in controlling the order and strength of IL-β-CD interactions. It is well known that the validity of infrared spectroscopy for detecting the inclusion complexation is questionable in the literature. However, this study demonstrates that the combination of high pressure and IR spectroscopy may provide a suitable way to monitor the IL-β-CD interactions. [ABSTRACT FROM AUTHOR]