Yasuhiro Umebayashi

Liquid Structure of Room-Temperature Ionic Liquid, 1-Ethyl-3-methylimidazolium Bis-(trifluoromethanesulfonyl) Imide

The liquid structure of 1-ethyl-3-methylimidazolium bis-(trifluoromethanesulfonyl) imide (EMI(+)TFSI(-)) has been studied by means of large-angle X-ray scattering (LAXS), (1)H, (13)C, and (19)F NMR, and molecular dynamics (MD) simulations. LAXS measurements show that the ionic liquid is highly structured with intermolecular interactions at around 6, 9, and 15 A. The intermolecular interactions at around 6, 9, and 15 A are ascribed, on the basis of the MD simulation, to the nearest neighbor EMI(+)...TFSI(-) interaction, the EMI(+)...EMI(+) and TFSI(-)...TFSI(-) interactions, and the second neighbor EMI+...TFSI(-) interaction, respectively. The ionic liquid involves two conformers, C(1) (cis) and C(2) (trans), for TFSI(-), and two conformers, planar cis and nonplanar staggered, for EMI(+), and thus the system involves four types of the EMI(+)...TFSI(-) interactions in the liquid state by taking into account the conformers. However, the EMI(+)...TFSI(-) interaction is not largely different for all combinations of the conformers. The same applies alsoto the EMI(+)...EMI(+) and TFSI(-)...TFSI(-) interactions. It is suggested from the 13C NMR that the imidazolium C(2) proton of EMI(+) strongly interacts with the O atom of the -SO(2)(CF(3)) group of TFSI(-). The interaction is not ascribed to hydrogen-bonding, according to the MD simulation. It is shown that the liquid structure is significantly different from the layered crystal structure that involves only the nonplanar staggered EMI(+) and C(1) TFSI(-) conformers.

Dependence of the conformational isomerism in 1-n-butyl-3-methylimidazolium ionic liquids on the nature of the halide anion.

The conformational isomerism of the 1-n-butyl-3-methylimidazolium cation, [C(4)mim](+), in halide-based ionic liquids--[C(4)mim]Cl, [C(4)mim]Br, and [C(4)mim]I--was explored by Raman spectroscopy. The [C(4)mim](+) cation exhibits trans-gauche conformational isomerism with respect to the N1-C7-C8-C9 dihedral angle of its butyl chain. The thermodynamics of trans-gauche conversion were analyzed through the successful evaluation of the corresponding Gibbs free energy, Δ(iso)G°, enthalpy, Δ(iso)H°, and entropy, Δ(iso)S°, of conformational isomerization. The values of Δ(iso)G° obtained are small (a few units of kJ/mol) and show a slight negative variation with the decrease of the size of the halide anion. On the other hand, Δ(iso)H° and Δ(iso)S° values are positive for [C(4)mim]I and decrease with the anion size to yield negative values for [C(4)mim]Cl and [C(4)mim]Br. This suggests that the negative electrostatic field around the halide anions stabilizes the gauche isomer from an enthalpic point of view. In order to study the structure and ion-ion interactions in this type of ionic liquids, high-energy X-ray diffraction experiments were performed for [C(4)mim]Cl at different temperatures and for supercooled [C(4)mim][Br] at ambient temperature. Molecular dynamics (MD) simulations for these systems were also carried out at several temperatures. Δ(iso)G° and Δ(iso)H° values derived from the simulations qualitatively agree with the experimental ones. Experimental X-ray structure factors are also well reproduced by the simulations. The MD results also allowed the calculation of different spatial distribution functions (SDFs) for the three ionic liquids. Although all SDFs exhibit similar trends, [C(4)mim]I shows a reduced anion density facing the C(2)-H atoms of the cation and enhanced anion densities above and below the imidazolium ring plane. This indicates that anions localized near the C(2)-H atoms of the cation can stabilize their gauche conformer, an effect that is stronger with smaller anions. This conclusion is also supported by ab initio calculations at the CCSD(T) level for isolated ion pairs.

Potential Energy Landscape of Bis(fluorosulfonyl)amide

The conformational landscape of the bis(fluorosulfonyl)amide, [FSI]-, anion was analyzed using data obtained from Raman spectroscopy, molecular dynamics (MD), and ab initio studies. The plotting of three-dimensional potential energy surfaces and the corresponding MD simulation conformer-population histograms show the existence of two stable isomers, C2 (trans) and C1 (cis) conformers, and confirm the nature of the anion as a flexible molecule capable of interconversion between conformers in the liquid state. In ionic liquids, the two [FSI]- conformers coexist in equilibrium, a result confirmed by the Raman data. The implications of the conformational behavior of the ion [FSI]- are discussed in terms of the solvation properties of the corresponding ionic liquids.

Structural change of ionic association in ionic liquid/water mixtures: A high-pressure infrared spectroscopic study

High-pressure infrared measurements were carried out to observe the microscopic structures of two imidazolium-based ionic liquids, i.e., 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [EMI(+)(CF(3)SO(2))(2)N(-), EMI(+)TFSA(-)] and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide [EMI(+)(FSO(2))(2)N(-), EMI(+)FSA(-)]. The results obtained at ambient pressure indicate that the imidazolium C-H may exist in two different forms, i.e., isolated and network structures. As the sample of pure EMI(+)FSA(-) was compressed, the network configuration is favored with increasing pressure by debiting the isolated form. For EMI(+)TFSA(-)/H(2)O mixtures, the imidazolium C-H peaks split into four bands at high pressures. The new spectral features at approximately 3117 and 3190 cm(-1), being concentration sensitive, can be attributed to the interactions between the imidazolium C-H and water molecules. The alkyl C-H absorption exhibits a new band at approximately 3025 cm(-1) under high pressures. This observation suggests the formation of a certain water structure around the alkyl C-H groups. The O-H stretching absorption reveals two types of O-H species, i.e., free O-H and bonded O-H. For EMI(+)TFSA(-)/H(2)O mixtures, the compression leads to a loss of the free O-H band intensities, and pressure somehow stabilizes the bonded O-H configurations. The results also suggest the non-negligible roles of weak hydrogen bonds in the structure of ionic liquids.

Raman Spectroscopic Study, DFT Calculations and MD Simulations on the Conformational Isomerism of N-Alkyl-N-methylpyrrolidinium Bis-(trifluoromethanesulfonyl) Amide Ionic Liquids

The conformational behaviors of N-alkyl-N-methylpyrrolidinium bis-(trifluoromethanesulfonyl) amide ionic liquids (alkyl; propyl and butyl, [P(1n)][TFSA]; n = 3 and 4) were studied by Raman spectroscopy in the frequency range of 200-1700 cm(-1) at different temperatures. Observed Raman spectra in the frequency range 870-960 cm(-1) for [P(13)][TFSA] and at 860-950 cm(-1) for [P(14)][TFSA] depend on the temperature, indicating that pseudo rotational isomerization of the pyrrolidinium ring exists in the ionic liquids. DFT calculations revealed that the pseudo rotational potential energy surfaces for P(13)(+) and P(14)(+) ions were similar to each other, i.e., the e6 isomer is the global minimum, whereas the three other isomers e1, e4, and e5 are ca. 3 kJ mol(-1) higher in energy. Optimized geometries with no imaginary frequency were successfully obtained for the e6, e1, and e4 isomers. For both cations, the theoretical Raman spectra of the e6 isomers reproduce well the observed data. To explain their observed Raman spectra in a reasonable way, it is necessary to consider one or more species as predicted by DFT calculations, i.e., the e4 isomer of P(13)(+) rather than the e1, or the e1 isomer of P(14)(+) rather than the e4. In addition, the torsion energy potentials of the alkyl chains of the cations were scanned by DFT calculations. It turns out that the alkyl chains of the cations prefer all trans conformations. It should be emphasized that the alkyl chains of the pyrrolidinium cations show remarkably different conformational behaviors comparing with those of the imidazolium. The isomerization enthalpies Delta(iso)H degrees from the e6 to the e4 isomer of P(13)(+) and to e1 of P(14)(+) were reasonably estimated from the temperature dependence of Raman spectra based on our proposed assignments to be 2.9 kJ mol(-1) for P(13)(+) and 4.2 kJ mol(-1) for P(14)(+), respectively. Thus evaluated experimental Delta(iso)H degrees values, which may contain some uncertainties, are in agreement with those predicted by DFT calculations and MD simulations suggesting that pseudo rotational isomerization equilibria are established in the examined N-alkyl-N-methylpyrrolidinium ionic liquids. The conformational behavior of TFSA(-) was also investigated. The Delta(iso)H degrees from the trans (trifluoromethyl groups on opposite sides of the S-N-S plane) to the cis isomer were evaluated to be 4.2 kJ mol(-1) for [P(13)][TFSA] and 3.5 kJ mol(-1) for [P(14)][TFSA], respectively, which are similar to that for the 1-ethyl-3methylimidazolium ionic liquid.

Solvation and microscopic properties of ionic liquid/acetonitrile mixtures probed by high-pressure infrared spectroscopy

The microscopic features of binary mixtures formed by an ionic liquid (EMI(+)TFSA(-) or EMI(+)FSA(-)) and a molecular liquid (acetonitrile or methanol) have been investigated by high-pressure infrared spectroscopy. On the basis of its responses to changes in pressure and concentration, the imidazolium C-H appears to exist at least in two different forms, i.e., isolated and associated structures. The weak band at approximately 3102 cm(-1) should be assigned to the isolated structure. CD(3)CN can be added to change the structural organization of ionic liquids. The compression of an EMI(+)TFSA(-)/CD(3)CN mixture leads to the increase in the isolated C-H band intensity. Nevertheless, the loss in intensity of the isolated structures was observed for EMI(+)FSA(-)/CD(3)CN mixtures as the pressure was elevated. In other words, the associated configuration is favored with increasing pressure by debiting the isolated form for EMI(+)FSA(-)/CD(3)CN mixtures. The stronger C-H...F interactions in EMI(+)FSA(-) may be one of the reasons for the remarkable differences in the pressure-dependent results of EMI(+)TFSA(-) and EMI(+)FSA(-).

Structure, solvation, and acid-base property in ionic liquids

Ionic liquids (ILs) are expected to have specific properties as solvents for chemical reactions in view of solution chemistry. Among physicochemical properties, liquid structure, acid–base, and electron-pair donating and accepting abilities of solvent play a crucial role in ion-solvation and acid–base, metal-ion complexation, and electrochemical reactions. Various types of ILs have been developed, and among others, the bis(trifluoromethanesulfonyl)amide (TFSA–)-based ILs are extensively used. TFSA– is a flexible molecule to give two stable conformers, cis (C1) and trans (C2), which are present in equilibrium in the liquid state. The conformational equilibrium shifts upon solvation to the metal ion. This is quantitatively studied to obtain thermodynamic parameters of conformational change from C2 to C1 in the bulk and in the solvation sphere of the lithium ion. On the other hand, with ethylammonium nitrate (EAN), a typical protic IL, it is revealed that the ammonium group is hydrogen-bonded with three nitrate ions to form a heterogeneous liquid structure. The solvent acid–base property of EAN and acid dissociation reaction in EAN have been quantitatively revealed, and the results will be discussed in comparison with those in normal molecular solvents.

Phase transition and conductive acceleration of phosphonium-cation-based room-temperature ionic liquid

An unusual ionic conduction phenomenon related to the phase transition of a novel phosphonium-cation-based room-temperature ionic liquid (RTIL) is reported; we found that in the phase change upon cooling, a clear increase in ionic conductivity was seen as the temperature was lowered, which differs from widely known conventional RTILs; clearly, our finding of abnormality of the correlation between temperature change and ionic conduction is the first observation in the electrolyte field.