Microscopic interactions of an imidazolium-based ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C2mimTFSI), with dimethyl sulfoxide (DMSO), methanol (MeOH), and acetonitrile (AN) have been analyzed by means of Raman, attenuated total reflectance infrared (ATR-IR), (1)H and (13)C NMR spectroscopy techniques. The magnitude of the red-shift of the C(2)-H vibration mode of the imidazolium ring and the deshielding of the C(2)-H hydrogen and carbon atoms, compared with that of the other atoms of the ring or the anion, indicated a strong interaction between the C(2)-H hydrogen atom and the molecular liquids in the following order; DMSO ≫ MeOH > AN. This correlates with the order of the electron donicities of these molecular liquids which allows us to suggest a hydrogen bonding character of these interactions. The behavior of S= O vibration of DMSO as a function of the DMSO molar fraction xDMSO also suggested that DMSO molecules are stoichiometrically hydrogen-bonded with the three hydrogen atoms, C(2,4,5)-H, of the ring. In contrast, the hydrogen bonding between MeOH and the C(4,5)-H atoms is much weaker than that in DMSO. AN hardly forms hydrogen bonds with the C(4,5)-H atoms. Instead, AN molecules may interact with the imidazolium ring through the π-π interaction. The interactions between the imidazolium ring and the molecular liquids lead to the loosening of the TFSI anion from the cation; this correlates with both the blue-shift of the S=O stretching vibration of TFSI and the deshielding of the trifluoromethyl carbon atoms with an increase in the molar fraction of the molecular liquid xML. The latter is weak in the MeOH solutions, and may be explained by the possible hydrogen bonding of the MeOH hydroxyl group as an electron-acceptor with the TFSI anion. Furthermore, the organization of MeOH molecules around the ethyl and methyl groups of the cation is discussed in terms of the chemical shift of the hydrogen and carbon atoms in these groups as a function of xML.

Soft X-ray absorption spectroscopy (XAS) has been performed on the N K-edge of two imidazolium-based ionic liquids (ILs), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C2mim][TFSA]) and 1-ethyl-3-methylimidazolium bromide ([C2mim][Br]), to clarify the electronic structures of the ILs. Soft X-ray emission spectroscopy (XES) has also been applied to the ILs by excitation at various X-ray energies according to the XAS spectra. It was possible to fully associate the XAS peaks with the XES peaks. Additionally, both XAS and XES spectra of the ILs were well reproduced by the theoretical spectra for a single-molecule model on [C2mim](+) and [TFSA](-) using density functional theory. The assignments for the XAS and XES peaks of the ILs were accomplished from both experimental and theoretical approaches. The theoretical XAS and XES spectra of [C2mim](+) and [TFSA](-) did not significantly depend on the conformations of the ions. The reproducibility of the theoretical spectra for the single-molecule model suggested that the interactions between the cations and anions are very weak in the ILs, thus scarcely influencing the electronic structures of the nitrogen atoms.

The thermodynamics of complex formation of Ni with molecular liquids (ML), dimethyl sulfoxide (DMSO), methanol (MeOH), and acetonitrile (AN) in the ionic liquid (IL) of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([Cmim][TFSA]) has been elucidated using ultraviolet (UV)-visible spectroscopy. X-ray structural analyses for single crystals grown from Ni-[Cmim][TFSA]-DMSO and -AN solutions at high ML contents have shown that six DMSO oxygen or AN nitrogen atoms coordinate with Ni to form octahedral structures of [Ni(dmso)](TFSA) and [Ni(an)](TFSA), respectively. This is the same in the case of the Co complex of [Co(dmso)](TFSA). UV-visible spectroscopic experiments have revealed that the TFSA anions that initially combine with Ni in the IL are replaced with ML molecules in the IL-ML systems in three steps with increasing ML content. The electron donicities of the three MLs are larger in the order of DMSO > MeOH > AN. However, the stability of each complex does not simply depend on this order; the stability is higher in the order of [Ni(dmso)] > [Ni(an)] > [Ni(meoh)]. In other words, the stability of the MeOH complexes is lower than that of the AN ones, despite the higher electron donicity of MeOH. The reasons for the order of the complex stabilities have been interpreted on the molecular scale, according to the stepwise enthalpies and entropies determined, together with the strength of the hydrogen bonding between the MLs and the imidazolium ring and the formation of MeOH clusters in [Cmim][TFSA].