Effects of Tetrafluoroborate and Bis(trifluoromethylsulfonyl)amide Anions on the Microscopic Structures of 1-Methyl-3-octylimidazolium-Based Ionic Liquids and Benzene Mixtures: A Multiple Approach by ATR-IR, NMR, and Femtosecond Raman-Induced Kerr Effect Spectroscopy.

The microscopic aspects of the two series of mixtures of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF4])-benzene and 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)amide ([MOIm][NTf2])-benzene were investigated by several spectroscopic techniques such as attenuated total reflectance IR (ATR-IR), NMR, and fs-Raman-induced Kerr effect spectroscopy (fs-RIKES). All three different spectroscopic results indicate that the anions more strongly interact with the cations in the [MOIm][BF4]-benzene mixtures than in the [MOIm][NTf2]-benzene mixtures. This also explains the different miscibility features between the two mixture systems well. The xC6H6 dependences of the chemical shifts and the C-H out-of-plane bending mode of benzene are similar: the changes are large in the high benzene concentration (xC6H6 > ∼ 0.6) compared to the low benzene concentration. In contrast, the linear xC6H6 dependences of the first moments of the low-frequency spectra less than 200 cm(-1) were observed in both the [MOIm][BF4]-benzene and [MOIm][NTf2]-benzene systems. The difference in the xC6H6 dependent features between the chemical shifts and intramolecular vibrational mode and the intermolecular/interionic vibrational bands might come from the different probing space scales. The traces of the parallel aromatic ring structure and the T-shape structure were found in the ATR-IR and NMR experiments, but fs-RIKES did not observe a clear trace of the local structure. This might imply that the interactions between the imidazolium and benzene rings are not strong enough to librate the imidazolium and benzene rings together. The bulk properties, such as miscibility, density, viscosity, and surface tension, of the two ionic liquid-benzene mixture series were also compared to the microscopic aspects.