In this article, recent studies on the electron transfer reaction and the proton transfer reaction in supercritical water were reviewed. Back electron transfer rate and the successive vibrational relaxation of p-nitroaniline dissolved in supercritical water were monitored by the ultrafast transient absorption spectroscopy. It was described how the changes of the dielectric constant and the hydrogen-bonding ability of water affect these phenomena. The proton transfer dynamics of 5-cynano-2-naphthol was studied by the time-resolved fluorescence spectroscopy. The reaction process was also strongly dependent on the hydrogen-bonding ability between water molecules.

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Water structure under high-pressure and high-temperature conditions has been revealed by X-ray diffraction and empirical potential structure refinement (EPSR) modeling. X-ray diffraction measurements of water at temperatures (298 K/30 MPa, 473 K/30 MPa and 573 K/30 MPa) and at high pressures (298 K/1 GPa, 473 K/0.35 GPa and 486 K/4 GPa) were performed on energy-dispersive type X-ray diffractometers in laboratory and in a synchrotron radiation facility, respectively. The EPSR modeling using the experimental diffraction data generated three dimensional structure of water including the position of hydrogen atoms. With decreasing density, the coordination number of water molecule decreases at high temperatures. Pressure bends the hydrogen bonds between water molecules and makes the water structure under high-pressure condition similar to that of simple liquid (Ar, Hg, etc.) The unique properties of water under extreme conditions (supercritical and high-pressure conditions) would originate from the change of hydrogen bonds between water molecules and the coordination number of water.

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Densities of dimethyl ether (DME)+ethanol liquid mixtures were measured at 20℃ under high pressure up to 40 MPa. Densities of DME+ethanol mixtures decreased with increasing DME and increased with increasing pressure at 20℃. The composition dependence of the excess molar volumes showed concaved shape and had a minimum around 50 mol% of DME at any pressures studied. The hydrogen bond between the oxygen of a DME molecule and the −OH group of an ethanol molecule makes an important role in the volume change in DME+ethanol liquid mixtures at 20℃ under high pressure.

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In this article, we have reviewed the results on pressure-induced Raman spectral changes of two aliphatic quaternary ammonium-based room temperature ionic liquids, N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate ([DEME][BF₄]) and N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide ([DEME][TFSI]) up to ～6 GPa. We focus on the behavior whether they crystallize or hold a liquid state at high pressures. Here we report the very unique behavior of ionic liquids under high pressure that [DEME][BF₄] form a superpressed (glassy) state upon compression, and the crystallizations occur by releasing the pressure on the superpressurized liquid. On the other hand, [DEME][TFSI] also enters the glassy state as well, but the results together with the visual observations show that no crystallization occurs upon decompression. We believe that the results provide new insights into the phase transition behavior of ionic liquids, except for the liquid to solid or solid to solid transitions upon compression.

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The solvation properties of some room-temperature ionic liquids (RTILs) and the solvent effect on bimolecular fluorescence quenching reaction have been examined at high pressures ranging from 0.1 to 300 MPa. It is found that Kamlet-Taft parameters (π^*, β) are sensitive to the type of anions, but not so dependent on the type of imidazolium cations consisting of RTILs. The pressure dependence of microviscosity is found to obey the empirical power-law equation. It is revealed that the rate constant for bimolecular fluorescence quenching reaction is significantly higher than theoretical rate constant of diffusion estimated from microviscosity, suggesting the occurrence of microscopic free space within RTILs which may facilitate the solute diffusion.

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In this article, my doctoral thesis, Behavior of hydrogen in the interior of the planets and satellites, was reviewed. I especially focus on the results of sound velocity measurements for iron hydride with the high resolution inelastic X-ray scattering here. The density evolution of the sound velocity of dhcp-FeH has been determined up to 70 GPa at room temperature from inelastic X-ray scattering and X-ray diffraction measurements. I found that the relationship between compressional wave velocity and density changes due to a magnetic transition from ferromagnetic to nonmagnetic FeH and that the nonmagnetic phase only follows the Birch's law. Assuming an ideal two-component mixing model, the hydrogen content of the Earth's inner core is obtained to be 0.30～0.37 wt% H, which corresponds to FeH_0.17～FeH_0.20.

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