The structure of bis(trifluoromethanesulfonyl) imide (TFSI-) in the liquid state has been studied by means of Raman spectroscopy and DFT calculations. Raman spectra of 1-ethyl-3-methylimidazolium (EMI+) TFSI- show relatively strong bands arising from TFSI- at about 398 and 407 cm(-1). Interestingly, the 407 cm(-1) band, relative to the 398 cm(-1) one, is appreciably intensified with raising temperature, suggesting that an equilibrium is established between TFSI- conformers in the liquid state. According to DFT calculations followed by normal frequency analyses, two conformers of C2 and C1 symmetry, respectively, constitute global and local minima, with an energy difference 2.2-3.3 kJ mol(-1). The wagging omega-SO2 vibration appears at 396 and 430 cm(-1) for the C1 conformer and at 387 and 402 cm(-1) for the C2 one. Observed Raman spectra over the range 380-440 cm(-1) were deconvoluted to extract intrinsic bands of TFSI- conformers, and the enthalpy of conformational change from C2 to C1 was evaluated. The enthalpy value is in good agreement with that obtained by theoretical calculations. We thus conclude that a conformational equilibrium is established between the C1 and C2 conformers of TFSI- in the liquid EMI+TFSI-, and the C2 conformer is more favorable than the C1 one.

The structure of acetonitrile−water mixtures has been investigated by X-ray diffraction with an imaging plate detector and IR spectroscopy over a wide range of acetonitrile mole fractions (0.0 ≤ X AN ≤ 1.0). Reichardt E T N and Sone-Fukuda D II,I values were also measured for the mixtures. It has been found from the X-ray data that in pure acetonitrile an acetonitrile molecule interacts with two nearest neighbors by antiparallel dipole−dipole interaction together with a small shift of the two molecular centers and that two acetonitrile molecules in the second-neighbor shell interact with a central molecule through parallel dipole−dipole interaction. Thus, acetonitrile molecules are alternately aligned to form a zigzag cluster. On addition of water into pure acetonitrile, water molecules interact with acetonitrile molecules through a dipole−dipole interaction in an antiparallel orientation. The IR spectra of O−D and C⋮N stretching vibrations, observed for mixtures of acetonitrile AN and water containing 20% D2O, suggested that hydrogen bonds are also formed between acetonitrile and water molecules in the mixtures at X AN ≤ 0.8. The average numbers of the first- and second-neighbor acetonitrile molecules gradually increase with increasing water content with an almost constant first-neighbor distance and slightly decreased second-neighbor ones. Thus, acetonitrile molecules are assembled to form three-dimensionally expanded clusters; the acetonitrile clusters are surrounded by water molecules through both hydrogen bonding and dipole−dipole interaction. The X-ray radial distribution functions and IR spectra suggest that the hydrogen bond network of water is enhanced in the mixtures at X AN < 0.6. The concentration dependence of E T N and D II,I values determined reflects well the above-mentioned behavior of water molecules in the mixtures. These findings suggest that both water and acetonitrile clusters coexist in the mixtures in the range of 0.2 ≤ X AN < 0.6, i.e., “microheterogeneity” occurs in the acetonitrile−water mixtures.

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.

Anion conformation of a low-viscosity room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl) imide (EMI+FSI-) has been studied by Raman spectra and theoretical DFT calculations. Three strong Raman bands were found at 293, 328, and 360 cm(-1), which are ascribed to the FSI- ion. These Raman bands show significant temperature dependence, implying that two FSI- conformers coexist in equilibrium. This is supported by theoretical calculations that the FSI- ion is present as either C2 (trans) or C1 (cis) conformer; the former gives the global minimum, and the latter has a higher SCF energy of about 4 kJ mol(-1). Full geometry optimizations followed by normal frequency analyses show that the observed bands at 293, 328, and 360 cm(-1) are ascribed to the C2 conformer. The corresponding vibrations at 305, 320, and 353 cm(-1) were extracted according to deconvolution of the observed Raman bands in the range280-400 cm(-1 )and are ascribed to the C1 conformer. The enthalpy DeltaH degrees of conformational change from C2 to C1 was experimentally evaluated to be ca. 4.5 kJ mol(-1), which is in good agreement with the predicted value by theoretical calculations. The bis(trifluoromethanesulfonyl) imide anion (TFSI-) shows a conformational equilibrium between C1 and C2 analogues (DeltaH degrees = 3.5 kJ mol(-1)). However, the profile of the potential energy surface of the conformational change for FSI- (the F-S-N-S dihedral angle) is significantly different from that for TFSI- (the C-S-N-S dihedral angle).

Nuclear magnetic resonance studies on the rotational and translational motions of ionic liquids composed of 1-ethyl-3-methylimidazolium cation and bis(trifluoromethanesulfonyl)amide and bis(fluorosulfonyl)amide anions and their binary systems including lithium salts Studies on the translational and rotational motions of ionic liquids composed of-methyl-propyl-pyrrolidinium cation and bis(trifluoromethanesulfonyl)amide and bis(fluorosulfonyl)amide anions and their binary systems including lithium salts Communication: Anomalous temperature dependence of the intermediate range order in phosphonium ionic Experimental evidences for molecular origin of low-Q peak in neutron/x-ray scattering of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquids

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The state of water in room-temperature ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI(+)BF(4)(-)), has been investigated by measurements of absorption and desorption isotherms, attenuated total reflectance infrared (ATR-IR) spectroscopy, and (2)H NMR relaxation method. The absorption enthalpies of water for the ionic liquid were estimated from the absorption isotherms. The enthalpies in the water mole fraction range of x(w) approximately 0.3. In addition, the activation energies for the rotational motion of a water molecule estimated from the (2)H NMR relaxation rates have indicated that the motion of water molecules in EMI(+)BF(4)(-)-D(2)O solutions gradually becomes freer with increasing water content from x(w) = 0.10 to 0.30, but is retarded again at x(w) = 0.33. Therefore, all the present findings have suggested that the state of water molecules in EMI(+)BF(4)(-) significantly changes at x(w) approximately 0.3. On the other hand, to directly observe the effect of water on structure of EMI(+)BF(4)(-), LAXS experiments have been made on EMI(+)BF(4)(-)-water solutions. It has been suggested that the interactions between the C(2) atom within the imidazolium ring of EMI(+) and BF(4)(-) are strengthened with increasing water content, while those at the C(4) and C(5) atoms weaken. Thus, the present LAXS experiments have clarified the beginning of formation of ion pair in EMI(+)BF(4)(-) by adding water at the molecular level.

Thermal properties, structure, and dynamics of supercooled water in porous silica of two different pore sizes (30 and 100 Å) have been investigated over a temperature range from 298 down to 193 K by differential scanning calorimetry (DSC), neutron diffraction, neutron quasi-elastic scattering, and proton NMR relaxation methods. Cooling curves by DSC showed that water in the 30 Å pores freezes at around 237 K, whereas water in the 100 Å pores does at 252 K. Neutron diffraction data for water in the 30 Å pores revealed that with lowering temperatures below 237 K hydrogen bond networks are gradually strengthened, the structure correlation being extended to 10 Å at 193 K. It has also been found that crystalline ice is not formed in the 30 Å pores in the temperature range investigated, whereas cubic ice (I c) crystallizes in the 100 Å pores at 238 K. The self-diffusion coefficients of water protons in both pores determined from the quasi-elastic neutron scattering measurements showed that the translational motion of water molecules is slower by a factor of two in the 30 Å pores than in the 100 Å pores, the motion of water molecules in the 100 Å pores being comparable with that of bulk water. The self-diffusion coefficients of water in both pores at different temperatures showed that the translational motion of water molecules is gradually restricted with decreasing temperature. The spin-lattice relaxation time (T 1) and the spin-spin relaxation time (T 2) data obtained by the proton NMR relaxation experiments suggested that the motions of water molecules in the 100 Å pores are faster by a factor of 2−3 than those of water molecules in the 30 Å pores. The peak area, the half-width at half maximum, the relaxation rates (T 1 -1 and T 2 -1) of water molecules at the various temperatures all showed an inflection point at 238 and 253 K for the 30 and 100 Å pores, respectively, suggesting the freezing of water in the pores.