Yoshiro Yasaka

Investigation of in Situ Oxalate Formation from 2,3-Pyrazinedicarboxylate under Hydrothermal Conditions Using Nuclear Magnetic Resonance Spectroscopy

We have investigated the assembly of a two-dimensional coordination polymer, Nd(2)(C(6)H(2)N(2)O(4))(2)(C(2)O(4))(H(2)O)(2), that has been prepared from the hydrothermal reaction of Nd(NO(3))(3)·6H(2)O and 2,3-pyrazinedicarboxylic acid (H(2)pzdc). In situ oxalate formation as observed in this system has been been investigated using (1)H and (13)C nuclear magnetic resonance spectroscopy, and a pathway for C(2)O(4)(2-) anion formation under hydrothermal conditions has been elucidated. The oxalate ligands found in Nd(2)(C(6)H(2)N(2)O(4))(2)(C(2)O(4))(H(2)O)(2) result from the oxidation of H(2)pzdc, which proceeds through intermediates, such as 2-pyrazinecarboxylic acid (2-pzca), 2-hydroxyacetamide, 3-amino-2-hydroxy-3-oxopropanoic acid, 2-hydroxymalonic acid, 2-oxoacetic acid (glyoxylic acid), and glycolic acid. The species are generated through a ring-opening that occurs via cleavage of the C-N bond of the pyrazine ring, followed by hydrolysis/oxidation of the resulting species.

Controlling the equilibrium of formic acid with hydrogen and carbon dioxide using ionic liquid.

The equilibrium for the reversible decomposition of formic acid into carbon dioxide and hydrogen is studied in the ionic liquid (IL) 1,3-dipropyl-2-methylimidazolium formate. The equilibrium is strongly favored to the formic acid side because of the strong solvation of formic acid in the IL through the strong Coulombic solute-solvent interactions. The comparison of the equilibrium constants in the IL and water has shown that the pressures required to transform hydrogen and carbon dioxide into formic acid can be reduced by a factor of approximately 100 by using the IL instead of water. The hydrogen transformation in such mild conditions can be a chemical basis for the hydrogen storage and transportation using formic acid.

Rotational dynamics of water and benzene controlled by anion field in ionic liquids: 1-butyl-3-methylimidazolium chloride and hexafluorophosphate

The rotational correlation time (tau(2R)) is determined for D(2)O (polar) and C(6)D(6) (apolar) in 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) and hexafluorophosphate ([bmim][PF(6)]) by measuring (2)H (D) nuclear magnetic resonance spin-lattice relaxation time (T(1)) in the temperature range from -20 to 110 degrees C. The tau(2R) ratio of water to benzene (tau(WB)) was used as a measure of solute-solvent attraction. tau(WB) is 0.73 and 0.52 in [bmim][Cl] and [bmim][PF(6)], respectively, whereas the molecular volume ratio is as small as 0.11. The slowdown of the water dynamics compared to the benzene dynamics in ionic liquids is interpreted by the Coulombic attractive interaction between the polar water molecule and the anion. As for the anion effect, the rotational dynamics of water solvated by Cl(-) is slower than that solvated by PF(6) (-), whereas the rotational dynamics of benzene is similar in the two ionic liquids. This is interpreted as an indication of the stronger solvation by the anion with a larger surface charge density. The slowdown of the water dynamics via Coulombic solvation is actually significant only at water concentrations lower than approximately 9 mol dm(-3) at room temperature, and it is indistinguishable at temperatures above approximately 100 degrees C. The quadrupolar coupling constants determined for D(2)O and C(6)D(6) in the ionic liquids were smaller by a factor of 2-3 than those in the pure liquid state.

Nuclear magnetic resonance study on rotational dynamics of water and benzene in a series of ionic liquids: Anion and cation effects

The rotational correlation times (τ(2R)) for polar water (D(2)O) molecule and apolar benzene (C(6)D(6)) molecule were determined in ionic liquids (ILs) by means of the (2)H (D) NMR spin-lattice relaxation time (T(1)) measurements. The solvent IL was systematically varied to elucidate the anion and cation effects separately. Five species, bis(trifluoromethylsulfonyl)imide (TFSI(-)), trifluoromethylsulfonate (TfO(-)), hexafluorophosphate (PF(6)(-)), chloride (Cl(-)), and formate (HCOO(-)), were examined for the anion effect against a fixed cation species of 1-butyl-3-methyl-imidazolium (bmim(+)). Four species, bmim(+), N-methyl-N-butylpyrrolidinium (bmpy(+)), N,N,N-trimethyl-N-propylammonium (N(1,1,1,3)(+)), and P,P,P-trihexyl-P-tetradecylphosphonium (P(6,6,6,14)(+)), were employed for the cation effect against a fixed anion species of TFSI(-). The τ(2R) ratio of water to benzene, expressed as τ(W/B), was used as a probe to characterize the strength of Coulombic solute-solvent interaction in ILs beyond the hydrodynamic limit based on the excluded-volume effect. The τ(W/B) value was found to strongly depend on the anion species, and the solute dynamics are sensitive not only to the size but also to the chemical structure of the component anion. The cation effect was rather weak, in contrast. The largest and most hydrophobic P(6,6,6,14)(+) cation was exceptional and a large τ(W/B) was observed, indicating a unique solvation structure in [P(6,6,6,14)(+)]-based ILs.

Communication: Exploring the reorientation of benzene in an ionic liquid via molecular dynamics: Effect of temperature and solvent effective charge on the slow dynamics

The rotational time correlation function (RTCF) of solute benzene molecules in the ionic liquid (1-butyl-3-methylimidazolium chloride) has been studied using classical molecular dynamics simulation. The effect of solvent charge on the functional form of RTCF was investigated by comparing four force fields for the solvent where the total charge on the anion and the cation was set to ±1e, ±0.7e, ±0.5e, and 0, respectively. For all three charged solvent models, the RTCF exhibits a long-time tail where the relaxation rate exhibits a significant slowdown. This feature is strengthened by higher solvent charges as well as lower temperatures, indicating the influence of the strong Coulombic fields arising from the solvent charges. The long-time tail is caused by the extraordinarily slow solvent structural relaxation of ionic liquids compared to the time scale of their local vibrational and librational dynamics.

Universality of Viscosity Dependence of Translational Diffusion Coefficients of Carbon Monoxide, Diphenylacetylene, and Diphenylcyclopropenone in Ionic Liquids under Various Conditions

Translational diffusion coefficients of diphenylcyclopropenone (DPCP), diphenylacetylene (DPA), and carbon monoxide (CO) in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIm][NTf2]) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIm][NTf2]) were determined by the transient grating (TG) spectroscopy under pressure from 0.1 to 200 MPa at 298 K and from 298 to 373 K under 0.1 MPa. Diffusion coefficients of these molecules at high temperatures in tributylmethylphosphonium bis(trifluoromethanesulfonyl)imide ([P4441][NTf2]), and tetraoctylphosphonium bis(trifluoromethanesulfonyl)imide ([P8888][NTf2]), and also in the mixtures of [BMIm][NTf2], N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([Pp13][NTf2]), and trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)imide ([P66614][NTf2]) with ethanol or chloroform have been determined. Diffusion coefficients except in ILs of phosphonium cations were well scaled by the power law of T/η, i.e., (T/η)(P), where T and η are the absolute temperature and the viscosity, irrespective of the solvent species, pressure and temperature, and the compositions of mixtures. The values of the exponent P were smaller for the smaller size of the molecules. On the other hand, the diffusion coefficients in ILs of phosphonium cations with longer alkyl chains were larger than the values expected from the correlation obtained by other ILs and conventional liquids. The deviation becomes larger with increasing the number of carbon atoms of alkyl-chain of cation, and with decreasing the molecular size of diffusing molecules. The molecular size dependence of the diffusion coefficient was correlated by the ratio of the volume of the solute to that of the solvent as demonstrated by the preceding work (Kaintz et al., J. Phys. Chem. B 2013 , 117 , 11697 ). Diffusion coefficients have been well correlated with the power laws of both T/η and the relative volume of the solute to the solvent.

Water as an in Situ NMR Indicator for Impurity Acids in Ionic Liquids

A sensitive in situ NMR spectroscopic method for detecting acids contaminating ionic liquids (ILs) has been developed. The chemical shift and the spectral width of water added to ILs were used as indicators to measure the impurity acid level. Owing to the high resolution power of NMR, the detection limit is below the level of 10(-3) mol kg(-1). A new method is applicable to a number of commonly used ILs such as the imidazolium- and ammonium-based ILs except for those composed of acidic cations or anions. The method was utilized to monitor the purification efficiency in the recrystallization of a typical hydrophilic IL, 1-butyl-3-methylimidazolium methanesulfonate from acetone. It was demonstrated that impurity acids can be almost perfectly removed by single or double recrystallization.

Polarity and Nonpolarity of Ionic Liquids Viewed from the Rotational Dynamics of Carbon Monoxide.

The rotational dynamics of carbon monoxide (CO) in a molten salt, ionic liquids (ILs), and alkanes were investigated by (17)O NMR T1 measurements using labeled C(17)O. The molten salt and the studied ILs have the bis(trifluoromethanesulfonyl)imide anion ([NTf2](-)) in common. In hexane near room temperature, the rotational relaxation times are close to the values predicted from the slip boundary condition in the Stokes-Einstein-Debye (SED) theory. However, in contradiction to the theoretical prediction, the rotational relaxation times decrease as the value of η/T increases, where η and T are the viscosity and absolute temperature, respectively. In other alkanes and ILs used in this study, the rotational relaxation times are much faster than those predicted by SED, and show a unique dependence on the number of alkyl carbons. For the same value of η/T, the CO rotational relaxation times in ILs composed of short-alkyl-chain-length imidazolium cations (1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium) are close to those for a molten salt (Cs[NTf2]). On the other hand, the rotational relaxation times in ILs composed of long-chain-length imidazolium (1-methyl-3-octylimidazolium) and phosphonium (tributylmethylphosphonium and tetraoctylphosphonium) cations are much shorter than the SED predictions. This deviation from theory increases as the alkyl chain length increases. We also found that the rotational relaxation times in dodecane and squalane are similar to those in ILs with a similar number of alkyl carbons. These results are discussed in terms of heterogeneous solvation and in comparison with the translational diffusion of CO in ILs.

Frequency-domain investigation of the ionic mobility of triflate salts in tetrahydrofuran.

The frequency-dependent molar conductivities of two triflate salts, tetrabutylammonium triflate (TBATf) and lithium triflate (LiTf), in tetrahydrofuran are measured in the microwave frequency domain at the concentrations where the direct-current molar conductivity increases with concentration. The relaxation frequency of the conductivity of TBATf increases with concentration as was demonstrated by a simulation and theoretical calculation on a simple model system. However, the low-frequency side of the relaxation of the conductivity of LiTf grows with increasing concentration, suggesting the presence of large aggregates such as triple ions. The molar conductivities of both salts at 20 GHz are about an order of magnitude smaller than those predicted by the Nernst-Einstein relationship, indicating the importance of the picosecond or faster dynamics in the determination of the absolute value of the conductivity.

SO2 capture by ionic liquid and spectroscopic speciation of sulfur(IV) therein

The absorption of equimolar sulfur dioxide (SO2) by tributyloctylphosphonium bicarbonate ([P4448]HCO3) resulted in the formation of a corresponding bisulfite ionic liquid ([P4448][bisulfite]) accompanied by carbon dioxide (CO2) release. The liquid formed absorbed an additional 0.6 equivalents of SO2. The speciation of sulfur(IV) in the SO2-loaded ionic liquid was performed using Raman and NMR spectroscopies. The two known isomeric forms of bisulfite ion in aqueous systems were identified while the condensation of bisulfite anion was suppressed in [P4448][bisulfite]. The isomer with the proton bonded to the sulfur atom (HSO3−) was more abundant than the one with the proton bonded to the oxygen atom (HOSO2−). The isomeric exchange rate was much slower in the IL than in water as distinguished by 1H NMR. When excess SO2 was absorbed by [P4448][bisulfite], the presence of molecular SO2 and HS2O5− were suggested by Raman bands as an indication of concerted physisorption and chemisorption.