Tateki Ishida

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.

Thermodynamic analysis of the solvent effect on tautomerization of acetylacetone: An ab initio approach

The keto-enol tautomerism of acetylacetone in solution is studied with the reference interaction site model self-consistent-field (RISM-SCF) method. We choose three solvents, H2O, dimethyl sulfoxide (DMSO), and carbon tetrachloride (CCl4), representing, respectively, protic polar, aprotic polar and nonpolar solvents. The analysis is made taking account of the solute electronic as well as geometrical change of the tautomers due to solvent effect. In addition, the electronic correlation energy of solute molecule and solute vibrational energies are considered. The free energy differences are analyzed by decomposing them into the enthalpy and entropy terms. The theory reproduces the total free energy determined by the experiment fairly well. We also find that, as solvent polarities increase, the keto tautomer shows the drastic geometric change in order to make its dipole moment larger and that the geometric change of the keto tautomer is enthalpically driven in H2O and entropically in DMSO. It is made clear t...

Microscopic Aspects in Dicationic Ionic Liquids through the Low-Frequency Spectra by Femtosecond Raman-Induced Kerr Effect Spectroscopy

The interionic vibrations in imidazolium-based dicationic ionic liquids (ILs) containing the bis(trifluoromethylsulfonyl)amide ([NTf(2)](-)) counteranion were investigated using femtosecond optical-heterodyne-detected Raman-induced Kerr effect spectroscopy. The microscopic nature of the dicationic ILs ([C(n)(MIm)(2)][NTf(2)](2), where n = 6, 10, and 12; MIm = N-methylimidazolium) was compared with that of the corresponding monocationic ILs ([C(n)MIm][NTf(2)], where n = 3, 5, and 6) used as reference samples. Low-frequency Kerr spectra within the frequency range 0-200 cm(-1) of the ILs revealed that (i) the spectral profile of the dicationic ILs as well as that of the corresponding monocationic ILs is bimodal; (ii) the difference in the spectral shapes of the dicationic and monocationic ILs is greater for [C(6)(MIm)(2)][NTf(2)](2) and [C(3)MIm][NTf(2)] than for the ILs with longer alkylene linker/alkyl groups, namely [C(10)(MIm)(2)][NTf(2)](2) and [C(5)MIm][NTf(2)], and [C(12)(MIm)(2)][NTf(2)](2) and [C(6)MIm][NTf(2)]; (iii) the small difference between the dicationic and monocationic ILs is confirmed by the relative intensity of the low-frequency component (ca. 17 cm(-1)) to the high-frequency component (ca. 70 cm(-1)); and (iv) the spectral profiles of the three dicationic ILs are not very different, but the line-shape of the low-frequency Kerr spectrum of 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide ([C(3)MIm][NTf(2)]) is significantly different from those of the other two monocationic ILs whose cations have a longer alkyl group. The distinguished line-shape of the low-frequency Kerr spectrum of [C(3)MIm][NTf(2)] from the other ILs can be accounted for by the homogeneous nature in the microstructure of the IL, but the other ILs indicate microsegregation structures due to the longer nonpolar alkylene linker or alkyl group in the cations.

Atom substitution effects of [XF6]- in ionic liquids. 2. Theoretical study.

Following the preceding spectroscopic study, we further investigate atomic mass effects of [XF6]- in 1-butyl-3-methylimidazolium cation ([BMIm]+) based ionic liquids (ILs) on dynamical natures by a computational approach in this study. We carry out the molecular dynamics simulations for 1-butyl-3-methylimidazolium cation based ILs ([BMIm][PF6], [BMIm][AsF6], and [BMIm][SbF6]) with the development of the force fields of [AsF6]- and [SbF6]- by an ab initio calculation. We have calculated density of state (DOS) and velocity autocorrelation function (VACF) profiles, polarizability time correlation function (TCF) and Kerr spectra, intermediate scattering functions, and dynamical structure factors. The decomposition analysis has been also carried out to understand the ion species and types of motion. From these computational studies, we find that the contribution of the reorientation of cations and anions mainly governs the Kerr spectrum profile in all three ILs, while the contribution of the collision-induced and cross terms, which are related to translational motions including coupling with librational motion, is not large at higher frequencies than 50 cm(-1). It is suggested that, with the atom substitution effects of anion units on interionic interactions, many properties in ILs are controllable. In addition, it is emphasized in this study that atomic mass effects in ILs are accessible through a complementary approach of both experimental and theoretical approaches.

Ultrafast Dynamics in 1-Butyl-3-methylimidazolium-Based Ionic Liquids: A Femtosecond Raman-Induced Kerr Effect Spectroscopic Study

We investigated the ultrafast dynamics in 1-butyl-3-methylimidazolium-based ionic liquids with two series of anions, (1) cyano-group substituted anions (thiocyanate [SCN](-), dicyanamide [N(CN)(2)](-), and tricyanomethide [C(CN)(3)](-)) and (2) trifluoromethylsulfonyl-group substituted anions (trifluoromethanesulfonate [OTf](-), bis(trifluoromethylsulfonyl)amide [NTf(2)](-), and tris(trifluoromethylsulfonyl)methide [CTf(3)](-)). This was done by femtosecond Raman-induced Kerr effect spectroscopy. From the Fourier transform Kerr spectra of the ionic liquids, the low-frequency spectrum of 1-butyl-3-methylimidazolium tricyanomethide shows a low-frequency shift compared to the ILs with the other cyano-group substituted anions due to the planar structures of the cation and the anion. The relative amplitude of the low-frequency band at approximately 20 cm(-1) compared to the entire broad spectrum for the ionic liquids with trifluoromethylsulfonyl-group substituted anions becomes larger with the order [OTf](-) < [NTf(2)](-) ≈ [CTf(3)](-). This vibrational band can be attributed to the librational motion of anions and/or the coupling of the translational and reorientational motions.

Solvation dynamics of benzonitrile excited state in polar solvents: A time-dependent reference interaction site model self-consistent field approach

The solvation dynamics of benzonitrile (C6H5CN) after the 21A1←1 1A1 vertical transition in water (H2O), methanol (CH3OH), and acetonitrile (CH3CN) solvents is studied with the reference interaction site model self-consistent field (RISM-SCF) method. The evolution of solute electronic states associated with the solvent relaxation is described by a time-dependent RISM-SCF method, incorporating the time-dependent solute–solvent site–site radial distribution functions, which are derived from the surrogate linear response theory. Ab initio electronic structure calculations reveal that the 2 1A1 state is of ionic nature whose dipole moment is larger by 2.41 D than that of the ground state. It is found that the excited state dipole moment is enhanced in the solutions, which provides the red shift of ∼6000 cm−1 in the vertical excitation energy. The solvent relaxation further increases the charge polarization in solute, indicating the electronic state of excited C6H5CN is sensitive to the electrostatic field com...

Consequences of strong coupling between solvation and electronic structure in the excited state of a betaine dye.

The electronic ground and excited-state structures of the betaine dye molecule pyridinium- N-phenoxide [4-(1-pyridinio)phenolate] are investigated both in the gas phase and in aqueous solution, using the reference interaction site model self-consistent-field (RISM-SCF) procedure within a CASSCF framework. We obtain the total free energy profiles in both the ground and excited states with respect to variation in the torsion angle between the phenoxide and pyridinium rings. We analyze the effect of solvent on the variation of the solute dipole moment and on the charge transfer character in the excited state. In the gas phase, it is shown that the potential energy profile in the excited-state decreases monotonically toward a perpendicular ring orientation and the dipole moment decreases along with decreasing charge localization. In water, the free energy surface for twisting is better characterized as nearly flat along the same coordinate for sterically accessible angles. These results are analyzed in terms of contributions of the solvation free energy, the solute electronic energy, and their coupling. Correspondingly, the dependence of the charge transfer character on solute geometry and solvation are analyzed, and the important roles in the excitation and subsequent relaxation processes for the betaine dye are discussed. It is found that there is considerable solute electronic reorganization associated with the evolution of solvation in the excited state, and it is suggested that this reorganization may contribute significantly to the early time evolution of transient spectra following photoexcitation.

Solvent motions and solvation processes in a short-time regime: effects on excited-state intramolecular processes in solution.

We propose a method for treating equation of motions for atoms taking into account the inertial term with an interaction site model for capturing solvent dynamics attributed to solvent motions in a short-time regime, t < 100 fs. We show a prescription for solving the equation which governs the development of the fluctuation of solvent number density with the inertial term, and the results of the van Hove space-time correlation function of water are compared with those by a molecular dynamics simulation. Also, the procedure is applied to the study of solvation dynamics of the simplest betaine dye molecule pyridinium N-phenoxide in water in the excited state. It is shown that the coupling between solvation and a fast intramolecular reaction such as charge transfer is likely to play an important role in solvation dynamics of the simplest betaine, and the importance of tracking solvation process associated with electronic reorganization in the solute molecule in the short-time region is indicated.

Optimal charge and charge response determination through conformational space: global fitting scheme for representative charge and charge response kernel.

We propose a global fitting scheme derived in the least-squares sense to estimate the optimal partial charge and charge response kernel (CRK), partial differential(Q(a))/partial differential(V(b)), with the data collected from conformational space sampling. We applied the global fitting method to the 1-butanol system and show the performance and accuracy of our global fitting procedure. In addition, we chose 1-pentanol as the test system for electronic structure change through conformational change and applied the global fitting method to it. From our study, it is indicated that intramolecular polarization can be influenced by intramolecular hydrogen bonding, and it is shown that our global fitting method can correspond to such a situation. Also, the global fitting procedure was tested on a large molecular system, 1-dodecanol. We show the results of the application of our fitting method for the system needed to sample large sets of data over a large conformational space. It is indicated that the nonlocality in intramolecular polarization in the alkyl chain sequence can be observed and that the large fluctuation of CRKs through nonbonded interactions such as intramolecular hydrogen bonding, as seen in the 1-pentanol case, can appear in common. The global fitting scheme we propose can be used for building molecular models considering polarization effects explicitly, even in the case of target systems that include many conformers.