Hideaki Shirota

Heavy Atom Substitution Effects in Non-Aromatic Ionic Liquids: Ultrafast Dynamics and Physical Properties

In this study, we have investigated the heavy atom substitution effects on the ultrafast dynamics in nonaromatic cation-based ionic liquids, as well as the static physical properties such as shear viscosity, surface tension, glass transition temperature, and melting point. Phosphonium-based ionic liquids show lower shear viscosities and lower glass transition temperatures than their corresponding ammonium-based ionic liquids. We have also examined the substitution of a (2-ethoxyethoxy)ethyl group for an octyl group in ammonium and phosphonium cations and found that the (2-ethoxyethoxy)ethyl group reduces the shear viscosity and increases the surface tension. From the results of the ultrafast dynamics, including intra- and interionic vibrations and reorientational relaxation in the ammonium- and phosphonium-based ionic liquids measured by means of femtosecond optically heterodyne-detected Raman-induced Kerr spectroscopy, we have found that the first moment of low-frequency Kerr spectrum, omitting the contributions of clear intraionic vibrational modes, correlates to the square root of surface tension divided by density. This fact indicates that heavy atom substitution in ionic liquids provides a weaker interionic interaction arising from the larger ionic volume. On the other hand, the ether group in the cations gives the stronger interionic interaction but with a more flexible and/or less segregated nature in the ILs than the alkyl group.

Ultrafast dynamics of pyrrolidinium cation ionic liquids

We have investigated the ultrafast molecular dynamics of five pyrrolidinium cation room temperature ionic liquids using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The ionic liquids studied are N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P14+/NTf2-), N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bromide P1EOE+, and N-ethoxyethyl-N-methylpyrrolidinium dicyanoamide P1EOE+/DCA-). For comparing dynamics among the five ionic liquids, we categorize the ionic liquids into two groups. One group of liquids comprises the three pyrrolidinium cations P14+, P1EOM+, and P1EOE+ paired with the NTf2- anion. The other group of liquids consists of the P1EOE+ cation paired with each of the three anions NTf2-, Br-, and DCA-. The overdamped relaxation for time scales longer than 2 ps has been fit by a triexponential function for each of the five pyrrolidinium ionic liquids. The fast ( approximately 2 ps) and intermediate (approximately 20 ps) relaxation time constants vary little among these five ionic liquids. However, the slow relaxation time constant correlates with the viscosity. Thus, the Kerr spectra in the range from 0 to 750 cm(-1) are quite similar for the group of three pyrrolidinium ionic liquids paired with the NTf2- anion. The intermolecular vibrational line shapes between 0 and 150 cm(-1) are fit to a multimode Brownian oscillator model; adequate fits required at least three modes to be included in the line shape.

How Does the Ionic Liquid Organizational Landscape Change when Nonpolar Cationic Alkyl Groups Are Replaced by Polar Isoelectronic Diethers

X-ray scattering experiments and molecular dynamics simulations have been performed to investigate the structure of four room temperature ionic liquids (ILs) comprising the bis(trifluoromethylsulfonyl)amide (NTf(2)(-)) anion paired with the triethyloctylammonium (N(2228)(+)) and triethyloctylphosphonium (P(2228)(+)) cations and their isoelectronic diether analogs, the (2-ethoxyethoxy)ethyltriethylammonium (N(222(2O2O2))(+)) and (2-ethoxyethoxy)ethyltriethylphosphonium (P(222(2O2O2))(+)) cations. Agreement between simulations and experiments is good and permits a clear interpretation of the important topological differences between these systems. The first sharp diffraction peak (or prepeak) in the structure function S(q) that is present in the case of the liquids containing the alkyl-substituted cations is absent in the case of the diether substituted analogs. Using different theoretical partitioning schemes for the X-ray structure function, we show that the prepeak present in the alkyl-substituted ILs arises from polarity alternations between charged groups and nonpolar alkyl tails. In the case of the diether substituted ILs, we find considerable curling of tails. Anions can be found with high probability in two different environments: close to the cationic nitrogen (phosphorus) and also close to the two ether groups. For the two diether systems, anions are found in locations from which they are excluded in the alkyl-substituted systems. This removes the longer range (polar/nonpolar) pattern of alternation that gives rise to the prepeak in alkyl-substituted systems.

Comparing intermediate range order for alkyl- vs. ether-substituted cations in ionic liquids

X-ray scattering data from four pairs of ionic liquids (ILs) are compared. The alkyl-substituted cations show a first sharp diffraction peak between 3 and 4 nm(-1) that is not observed for ILs having cations with ether- or hydroxy-substitutions. These observations indicate a significant difference in the intermediate range order for these liquids.

Aqueous dimethyl sulfoxide solutions: Inter- and intra-molecular dynamics

The inter- and intra-molecular dynamics of aqueous dimethyl sulfoxide (DMSO) solutions have been measured using optical heterodyne-detected Raman-induced Kerr effect spectroscopy. Solutions were studied over the entire range of composition at 294 K. The Kerr transients characterize both the underdamped inter- and intra-molecular vibrational motions, as well as the overdamped, diffusive orientational motions. The longer diffusive relaxation time constant τ2 is assigned to DMSO reorientation, and varies strongly with mole fraction of DMSO. The shorter time constant τ1 is assigned to water reorientation, and the value of 1.0 ps is nearly invariant across the range of solution composition. The solutions deviate substantially from hydrodynamic scaling behavior, since the ratio of DMSO reorientation time constant normalized by shear viscosity τ2/η is not a linear function of mole fraction. The peak frequencies for three of five low frequency intramolecular vibrations decrease with increasing water content. Both...

Glass Transition Dynamics of Room-Temperature Ionic Liquid 1-Methyl-3-trimethylsilylmethylimidazolium Tetrafluoroborate

The conductivity relaxation dynamics of the room-temperature ionic liquid 1-methyl-3-trimethylsilylmethylimidazolium tetrafluoroborate ([Si-MIm][BF(4)]) have been studied by broadband conductivity relaxation measurements at ambient pressure and elevated pressures up to 600 MPa. For the first time, several novel features of the dynamics have been found in a room-temperature ionic liquid. In the electric loss modulus M″(f) spectra, a resolved secondary β-conductivity relaxation appears, and its relaxation time τ(β) shifts on applying pressure in concert with the relaxation time τ(α) of the primary α-conductivity relaxation. The spectral dispersion of the α-conductivity relaxation, as well as the fractional exponent (1 - n) of the Kohlrausch-Williams-Watts function that fits the spectral dispersion, is invariant to various combinations of pressure and temperature that keep τ(α) constant. Moreover, τ(β) is unchanged. Thus the three quantities, τ(α), τ(β), and n, are coinvariant to changes in pressure and temperature. This strong connection to the α-conductivity relaxation shown by the β-conductivity relaxation in [Si-MIm][BF(4)] indicates that it is the analogue of the Johari-Goldstein β-relaxation in nonionically conducting glass-formers. The findings have fundamental implications on theoretical interpretation of the conductivity relaxation processes and glass transition in ionic liquids. It is also the first time such a secondary conductivity relaxation or the primitive conductivity relaxation of the coupling model has been fully resolved and identified in M″(f) in any ionically conducting material that we know of.

Dicationic versus Monocationic Ionic Liquids: Distinctive Ionic Dynamics and Dynamical Heterogeneity

The dynamical properties of a dicationic ionic liquid (IL), 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide ([C(6)(MIm)(2)][NTf(2)](2)), compared to 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide ([C(3)MIm][NTf(2)]), as its monocationic imidazolic counterpart, are studied by molecular dynamics simulations. We investigate relaxation processes of the polarizability anisotropy of the system and collective dynamics of both the ILs with mean-squared displacement (MSD), non-Gaussian parameter, and the intermediate scattering functions. The analyses of librational dynamics show that the difference of the Kerr spectra between the ILs could be mainly ascribed to the distinctive angular momentum of [C(6)(MIm)(2)](2+) and [C(3)MIm](+) and related to the difference of relaxation behavior between [C(6)(MIm)(2)](2+) and [C(3)MIm](+). Also, it is indicated that the librational dynamics of [NTf(2)](-) indicate a common resonance-type sharp peak that corresponds to an intermolecular motion coupled to the vibrational mode intrinsic to [NTf(2)](-). In addition, it is exhibited from the total X-ray structure factors calculated for both of the ILs that the low-k peak at 0.20 Å(-1) appears for [C(6)(MIm)(2)][NTf(2)](2), while we do not see it for [C(3)MIm][NTf(2)]. We find that the contribution of the anion-cation and anion-anion correlations to the low-k peak is more significant than the cation-cation correlation. Therefore, it is suggested for [C(6)(MIm)(2)][NTf(2)](2) that dynamical heterogeneous behavior strongly correlates with structural variations or heterogeneity.

Atom Substitution Effects of [XF6]- in Ionic Liquids. 1. Experimental Study

We have investigated the interionic vibrational dynamics of 1-butyl-3-methylimidazolium cation ([BMIm]+) based ionic liquids with the anions of [PF6]-, [AsF6]-, and [SbF6]- as well as the static physical properties, such as shear viscosity and liquid density. Shear viscosity for the ionic liquids becomes lower with the heavier atom anion: [BMIm][PF6]>[BMIm][AsF6]>[BMIm][SbF6]. This tendency for heavy atom substitution for the anion results from weaker interionic interaction caused by larger anion volume. Femtosecond optically heterodyne-detected Raman-induced Kerr effect spectroscopy has been used to observe the interionic vibrational dynamics of ionic liquids. The interionic vibration in the frequency region of less than 50 cm(-1) clearly shows the heavy atom substitution effect; that is, the heavy atom substitution of [XF6]- critically affects the interaction-induced motion. The forthcoming paper will further provide the molecular-level insights of the heavy atom substitution effect of the [XF6]- anion on the interionic dynamics and interaction for the three ionic liquids by a molecular dynamics simulation approach.

Comparison of interionic/intermolecular vibrational dynamics between ionic liquids and concentrated electrolyte solutions.

In this study, we have compared the interionic/intermolecular vibrational dynamics of ionic liquids (ILs) and concentrated electrolyte solutions measured by femtosecond optically heterodyne-detected Raman-induced Kerr effect spectroscopy. A typical anion in ILs, bis(trifluoromethanesulfonyl)amide ([NTf(2)](-)), has been chosen as the anion for the sample ILs and concentrated electrolyte solutions. ILs used in this study are 1-butyl-3-methylimidazolium, 1-butylpyridinium, N-butyl-N,N,N-triethylammonium, and 1-butyl-1-methylpyrrolidinium with [NTf(2)](-). Li[NTf(2)] solutions (approximately 3.3 M) of water, methanol, propylene carbonate, and poly(ethylene glycol) have been selected as control samples. Kerr transients of the ILs and electrolyte solutions show intra- and interionic/intermolecular vibrational dynamics followed by slow picosecond overdamped relaxation. Fourier transform Kerr spectra have shown a difference in the relative intensities of intraionic vibrational bands of [NTf(2)](-) (280-350 cm(-1)) between the ILs and electrolyte solutions. The origin of the difference is attributed to the change in the conformational equilibrium between cisoid and transoid forms of [NTf(2)](-), which is caused by a favorable stabilization of dipolar cisoid form due to Li(+) and dipolar solvent molecules in the electrolyte solutions. Low-frequency Kerr spectra (0-200 cm(-1)) exhibit unique features with the variation of cation and solvent species. The aromatic ILs have a prominent high-frequency librational motion at about 100 cm(-1) in contrast to the case for the nonaromatic ones. The common structure of the spectra observed at about 20 cm(-1) likely comes from an interionic motion of [NTf(2)](-). The nonaromatic ILs allow a fair comparison with the electrolyte solutions of propylene carbonate and poly(ethylene glycol) because of the structural similarities. The comparison based on the first moment of the interionic/intermolecular vibrational spectrum suggests the stronger interionic/intermolecular interaction in the concentrated electrolyte solutions than the ILs.

Solvation in highly nonideal solutions: A study of aqueous 1-propanol using the coumarin 153 probe

We have investigated the anomalous behavior of aqueous 1-propanol binary solutions using a typical fluorescence probe molecule, coumarin 153. We present data on the fluorescence lifetimes, fluorescence anisotropies, and solvent reorganization dynamics, as well as the steady-state absorption and emission spectra of coumarin 153 in the binary solutions. The rotational diffusion and solvation time constants depend strongly on the content of 1-propanol, especially at low 1-propanol mole fractions. Spectroscopic results presented here are consistent with prior light scattering [G. H. Grosmann and K. H. Ebert, Ber. Bunsenges. Phys. Chem. 85, 1026 (1981)], small angle x-ray scattering [H. Hayashi, K. Nishikawa, and T. Iijima, J. Phys. Chem. 94, 8334 (1990)], and dielectric relaxation [S. Mashimo, T. Umehara, and H. Redlin, J. Chem. Phys. 95, 6257 (1991)] data. The anomalous dynamics features likely arise from the effect of the preferential solvation due to the 1-propanol clustering.