Chihiro Wakai

Structural study of supercritical water. I. Nuclear magnetic resonance spectroscopy

The proton chemical shift of water is measured at temperatures up to 400°C and densities of 0.19, 0.29, 0.41, 0.49, and 0.60g/cm3. The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas. The chemical shift is related to the average number of hydrogen bonds in which a water molecule is involved. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is more than one for a water molecule in the supercritical densities. The density and temperature dependence of the chemical shift at supercritical temperatures are analyzed on the basis of statistical thermodynamics. It is shown that the hydrogen bonding is spatially more inhomogeneous at lower densities.

Collective rotational dynamics in ionic liquids: A computational and experimental study of 1-butyl-3-methyl-imidazolium tetrafluoroborate

The aim of this study is the analysis of the rotational motion in ionic liquids, in particular, 1-butyl-3-methyl-imidazolium tetrafluoroborate. By comparing single-particle and collective motion it is found that the Madden-Kivelson relation is fairly fulfilled in long-term simulation studies (>100 ns), i.e., the collective reorientation can be predicted by the corresponding single-particle property and the static dipolar correlation factor, GK. Furthermore, simulated reorientation is in accordance with hydrodynamic theories yielding hydrodynamic radii comparable to van der Waals radii. Since viscosity is the central quantity entering hydrodynamic formulas, we calculated and measured the viscosity of our system in order to have two independent cycles of hydrodynamic evaluation, a computational and an experimental one. While the static dielectric constant agrees with dielectric reflectance experiment, the hydrodynamic radii derived from the experiments are much lower as a consequence of enhanced rotational motion. Even more, a considerable dynamic broadening is observed in the experiments.

Structural study of supercritical water. II. Computer simulations

The proton chemical shift of supercritical water is analyzed by computer simulations with emphasis on its relationship to the number of hydrogen bonds per water molecule and the dipole moment of a water molecule. The chemical shift is shown to be proportional to the number of hydrogen bonds, and the dipole moment of a water molecule at supercritical states is estimated within the simple point charge (SPC)-like and TIP4P-like frameworks of the water intermolecular potential model. The dipole moment can then be used to construct an effective potential model suitable for simulating supercritical water. The radial and orientational correlations in supercritical water are examined using the effective potential model.

A new high-temperature multinuclear-magnetic-resonance probe and the self-diffusion of light and heavy water in sub- and supercritical conditions

A high-resolution nuclear-magnetic-resonance probe (500 MHz for 1H) has been developed for multinuclear pulsed-field-gradient spin-echo diffusion measurements at high temperatures up to 400 degrees C. The convection effect on the self-diffusion measurement is minimized by achieving the homogeneous temperature distributions of +/-1 and +/-2 degrees C, respectively, at 250 and 400 degrees C. The high temperature homogeneity is attained by using the solid-state heating system composed of a ceramic (AlN) with high thermal conductivity comparable with that of metal aluminium. The self-diffusion coefficients D for light (1H2O) and heavy (2H2O) water are distinguishably measured at subcritical temperatures of 30-350 degrees C with intervals of 10-25 degrees C on the liquid-vapor coexisting curve and at a supercritical temperature of 400 degrees C as a function of water density between 0.071 and 0.251 gcm3. The D value obtained for 1H2O is 10%-20% smaller than those previously reported because of the absence of the convection effect. At 400 degrees C, the D value for 1H2O is increased by a factor of 3.7 as the water density is reduced from 0.251 to 0.071 gcm3. The isotope ratio D(1H2O)D(2H2O) decreases from 1.23 to approximately 1.0 as the temperature increases from 30 to 400 degrees C. The linear hydrodynamic relationship between the self-diffusion coefficient divided by the temperature and the inverse viscosity does not hold. The effective hydrodynamic radius of water is not constant but increases with the temperature elevation in subcritical water.

Pressure dependencies of rotational, translational, and viscous friction coefficients in water‐d2, acetonitrile‐d3, acetonitrile, chloroform, and benzene

Molecular rotational friction coefficients (ζ) were determined for neat water‐d2, neat acetonitrile‐d3, neat acetonitrile, a 15% solution of chloroform‐d1 in chloroform, and a 3% solution of benzene‐d6 in benzene by measuring 2H and 14N nuclear magnetic resonance spin–lattice relaxation times as a function of pressure (0.1–300 MPa). The pressure dependencies of the rotational ζ values were obtained from the single‐body rotational correlation times for deuterated molecules in each liquid. The pressure dependencies were compared with those of the translational and viscous ζ values derived, respectively, from the known self‐diffusion coefficients and viscosities. In such simple molecular liquids as chloroform and benzene, the translational and viscous ζ values had almost the same pressure coefficient or activation volume, whereas the rotational ζ values had considerably smaller pressure coefficients. The fractional viscosity (η) exponent α in the phenomenological linear relation between ζ and ηα was 0.9 for the translational ζ in acetonitrile and 0.4–0.6 for the rotational ζ in acetonitrile (tumbling motion), chloroform, and benzene. Water was found to be exceptional because the pressure dependence of ζ depended more strongly on the modes of molecular motions. The deviation of the viscosity exponent from unity clearly indicates a breakdown of the Stokes–Einstein–Debye law with respect to pressure variations. The viscosity exponent is not universal, but specific to intermolecular interactions and therefore dependent on the liquid structure.Molecular rotational friction coefficients (ζ) were determined for neat water‐d2, neat acetonitrile‐d3, neat acetonitrile, a 15% solution of chloroform‐d1 in chloroform, and a 3% solution of benzene‐d6 in benzene by measuring 2H and 14N nuclear magnetic resonance spin–lattice relaxation times as a function of pressure (0.1–300 MPa). The pressure dependencies of the rotational ζ values were obtained from the single‐body rotational correlation times for deuterated molecules in each liquid. The pressure dependencies were compared with those of the translational and viscous ζ values derived, respectively, from the known self‐diffusion coefficients and viscosities. In such simple molecular liquids as chloroform and benzene, the translational and viscous ζ values had almost the same pressure coefficient or activation volume, whereas the rotational ζ values had considerably smaller pressure coefficients. The fractional viscosity (η) exponent α in the phenomenological linear relation between ζ and ηα was 0.9 for ...

Attractive potential effect on the self-diffusion coefficients of a solitary water molecule in organic solvents

1H-Fourier-transform pulsed field-gradient spin-echo NMR method is applied at 30 °C to measure the self-diffusion coefficients of water, acetonitrile, and benzene in dilute organic solutions of carbon tetrachloride, benzene, chloroform, dichloromethane, acetonitrile, acetone, and water. The dependence of the translational friction coefficients ζT of the apolar solute molecule, benzene, on solvent viscosity η is linear as predicted by the simple hydrodynamic theory. The η dependence of ζT of the polar solute molecule, acetonitrile, is linear, though the slope is different in apolar and polar solvents. The η dependence of ζT of the hydrogen-bonding solute molecule, water, is nonlinear and dependent on the strength of the solvent as a proton acceptor. The breakdown of the hydrodynamic theory is discussed in terms of the attractive part of the solute–solvent interaction. The role of the attractive interaction is illustrated through a linear relationship between the proton chemical shift of water and the resid...

Structure and dynamics of water: from ambient to supercritical

Abstract Our recent works on supercritical water are reviewed. In order to elucidate the hydrogen bonding state of supercritical water, the proton chemical shift of the water proton is measured at temperatures up to 400 °C and densities of 0.19, 0.29, 0.41, 0.49, and 0.60 g/cm3. The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas. The chemical shift is then related to the average number of hydrogen bonds in which a water molecule is involved. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one for a water molecule at the densities larger than the critical. The density dependence of the chemical shift at supercritical temperatures is analyzed on the basis of statistical thermodynamics. It is shown that the hydrogen bonding is spatially more inhomogeneous at lower densities. The dipole moment of water at supercritical states is also estimated from the number of hydrogen bonds. The dynamical counterpart of our structural study of supercritical water has been performed by NMR relaxation measurements. Using D2O, we measured the spin-lattice relaxation time and determined the reorientational relaxation time as a function of the water density and temperature. It is then found that while the reorientational relaxation time decreases rapidly with the temperature in the subcritical condition, it is a weak function of the density in the supercritical conditions.

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.

Inertial and attractive potential effects on rotation of solitary water molecules in apolar and polar solvents

Deuteron nuclear magnetic resonance spin–lattice relaxation times T1 have been measured for solitary water molecules (D2O) at very low concentrations (8–70 mM) in a series of solvents over a wide range of temperatures; the solvents studied are carbon tetrachloride (10 to 40 °C), benzene (10 to 60 °C), chloroform (−40 to 50 °C), acetonitrile (−40 to 50 °C), and acetone (−40 to 50 °C). The orientational correlation times τ2R for D2O with extremely small moments of inertia (I) in CCl4, C6H6, CHCl3, CH3CN, and (CH3)2CO at 30 °C are, respectively, 96, 224, 230, 625, and 826 fs, which are all by far smaller than the bulk value (2210 fs). The τ2R value is proportional to the proton chemical shift of water in each solvent which is taken as a measure of the strength of solute–solvent interactions in the short range. On the other hand, τ2R is almost inversely proportional to solvent viscosity in disagreement with the simple hydrodynamic friction model. The correlation time (τ*) scaled by the free rotator correlatio...

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.