Hitoshi Ohtaki

Structure of water under subcritical and supercritical conditions studied by solution X-ray diffraction

Stuctures of water and aqueous electrolyte solutions under sub-and super-critical conditions studied mainly by X-ray diffraction and also by neutron diffraction are reviewed and the experimental results are compared with those reported by using computer simulations. Some Raman spectroscopic data are included for discussing the existence of hydrogen bonds in water at high temperature and high pressures (HTHPs).The authors propose a classification of supercritical water into three categories: (a) low density water, (b) medium density water, and (c) high density water, because density is a very important thermodynamic quantity to describe properties of sub- and super-critical water.From changes in the water–water intermolecular distance and the coordination number of water with temperature and pressure, and especially with density at HTHP, the authors conclude that the compact tetrahedral-like water structure is decomposed and long-distance water–water interactions increase with temperature and pressure, and they propose a model for water: under supercritical conditions water consists of small clusters, much smaller aggregates such as oligomers, and even monomeric gas-like water molecules.

Ionic Solvation in Aqueous and Nonaqueous Solutions

The history of studies on ionic solvation is briefly reviewed, and structural and dynamic properties of solvated ions in aqueous and nonaqueous solutions are discussed. An emphasis is placed on ionic solvation in nonaqueous mixed solvents in which preferential solvation of ions takes place. A new parameter for expressing the degree of preferential solvation of an ion is proposed.

X-RAY DIFFRACTION STUDIES ON THE STRUCTURE OF WATER AT HIGH TEMPERATURES AND PRESSURES

X-Ray diffraction measurements of liquid water at high temperatures and pressures (up to 200 °C and 200 MPa) are reported. A diffractometer with a transmission geometry and a cylindrical sample holder made of beryllium were used, with a specially designed high pressure unit. The data reduction procedures are described in detail. The analysis of the structure functions and radial distribution functions show that the O-O distances shorten due to pressure but they are elongated with increasing temperature. The average contraction rate is 0·02 pm MPa-1. The average coordination number of the first neighbour molecules does not change significantly with increasing temperature but slightly increases with increasing pressure. The density dependence of the distances and coordination are discussed. The expansion of water is proportional to the expansion of first neighbour distances in the medium density region only (0·95-1·01 g cm-3), while the coordination number produces a minimum in the medium density region. Th...

Effects of temperature and pressure on hydrogen bonds in water and in formamide

Abstract Effects of temperature and pressure on the hydrogen-bonding liquid structure of water and formamide are discussed on the basis of X-ray diffraction and NMR data measured at high temperatures and pressures. In the discussion of the water structure at high temperatures and high pressures, it is suggested that density can be a unique parameter to describe the water structure. Supercritical water has hydrogen bonds even at a low density of 0.7 g dm −3 . The distance between adjacent water molecules is kept practically unchanged at about 292 pm over a wide density region of ca. 0.95 g dm −3 through 0.7 g dm −3 , while the number of nearest neighbor water molecules decreases monotonously from 4.4 under ambient condition to 1.7 at the density of 0.7 g mol −3 at high temperature and pressure. On the other hand, in formamide, whose structure had been concluded to consist of a mixture of the head-and-tail-type-linear and ring-dimer structures, hydrogen bonds are broken with the elevation of temperature but are enhanced with pressure to form the more compact ring-dimer structure rather than the linear one.

Structural Studies on Saturated Aqueous Solutions of Manganese(II), Cobalt(II), and Nickel(II) Chlorides by X-ray Diffraction

As a part of our studies on crystallization processes of electrolytes, the structure of aqueous solutions of MCl2 (M = Mn, Co, Ni) equilibrated with hydrate crystals, MCl2 · mH2O (m = 6, 4, 2), was investigated by means of X-ray diffraction at 25, 40, 55, and 70°C. The complexes formed in MnCl2 solutions, were found to be mixed–ligand chloroaqua octahedral complexes of M2+ ions with the Mn—O and Mn—Cl distances of about 220 and 251 pm, respectively. The average number of Mn—Cl and Mn—O interactions increased from 1.2 to 1.9 and decreased from 4.8 to 4.1, respectively, with changing MnCl2 solutions from Mn25 (MnCl2 solution at 25°C) to Mn70 (MnCl2 solution at 70°C). In the octahedral species of Co2+, the Co—O and Co—Cl distances were found to be about 211 and 240 pm, respectively. With an increase in the saturated concentration by changing temperature from 25 to 70°C, the average coordination number of the Co—Cl contact per Co2+ increased from 0.5 to 1.2, and the average number of Co—O interactions decreased from 5.5 to 4.8. The structural analysis was carried out by taking into consideration the existence of the tetrahedral species in the solutions saturated at 40, 55, and 70°C, on the assumption of the existence of [CoCl4]2−. The Co—Cl distance was found to be 228 pm, while the number of Co—Cl interactions in the [CoCl4] complex was calculated to be 3.7 by the least-squares calculations. The Ni—O and Ni—Cl distances were estimated to be about 206 and 237 pm, respectively. The frequency factor n of the Ni—O and Ni—Cl interactions decreased monotonously from 5.6 to 5.0 and increased from 0.4 to 1.0, respectively, with increasing NiCl2 concentration. The n values of the Co—Cl and Ni—Cl interactions of the octahedral complexes increased sharply with concentration at higher concentrations. Comparing structures of the complexes in the saturated solutions and the hydrate crystals of these metal ions, we discussed a role of the complexing species on crystallization of the hydrates.

The structure of liquid formamide studied by means of X-ray diffraction and NMR at high temperatures and high pressures

Abstract The structure of liquid formamide has been investigated by means of X-ray diffraction and NMR at high temperatures and high pressures (HTHP). The temperatures were fixed at 298 K and 373 K, and the pressures employed were 0.1, 20, and 40 MPa for the X-ray diffraction studies, while the NMR measurements were carried out over the temperature range from 300 K to 395 K at 0.1 MPa and over the pressure range from 0.1 to 200 MPa at 298 K. Formamide has a network structure consisting of a mixed ring and head-and-tail-type linear structures through N-H⋯O hydrogen bonds at 298 K and 0.1 MPa, and the ring structure, which has weaker hydrogen bonds than the linear structure, is broken with increasing temperature to change to the linear-type structure. On the other hand, the linear structure changes to the ring structure, having a more compact molecular arrangement than the linear one, with the increase in pressure.

STRUCTURAL STUDIES ON PREFERENTIAL SOLVATION OF SILVER(I) ION BY ACETONITRILE OVER N, N-DIMETHYLFORMAMIDE IN THEIR MIXTURES

Abstract The solvation structure of the silver(1) ion in neat N,N -dimethylformamide (DMF), neat acetonitrile (AN) and their mixtures has been determined by the X-ray diffraction method at 25 °C. In neat DMF and AN, the silver(1) ion forms the tetrahedral [Ag(dmf) 4 ]+ and [Ag(an) 4 ]+ complexes with the Ague5f8O and Ague5f8N bond lengths of 233(1) and 226(1) pm, respectively. The tetrahedral [Ag(dmf) 2 (an) 2 ]+ complex is the predominant species in 3:1 and 2:1 DMF:AN molar mixtures.In the 1:2 DMF-AN mixture, the tetrahedral [Ag(dmf)(an) 3 ]+ complex is formed as the main species. The silver(1) ions are present as an equilibrium mixture of [Ag(dmf) 2 (an) 2 ]+ and [Ag(dmf)(an) 3 ]+ (ca. 1:1) in the 1:1 DMF-AN mixture.Thus, a preferential solvation of the silver(1) ion by AN occurs in DMF-AN mixtures, as expected from their solvation energies in each solvent. The Ague5f8O bond length in the [Ag(dmf) 2 (an) 2 ]+ and [Ag(dmf)(an)3]+ complexes is 237(3) and 241(3) pm, respectively, and increases with the number of AN molecules within the complexes. On the other hand, the AgN bond length remains practically unchanged by the coordination of DMF molecules; 224(2) pm in [Ag(dmf) 2 (an) 2 ]+ and 324(1) pm in [Ag(dmf)(an) 3 ]+.

Development of modern solution chemistry: a search of new fields

Abstract In order to investigate new fields and how one can approach a new field of chemistry, studies carried out by Nobel laureates are surveyed from the historical point of view and are classified into five categories. A brief discussion shows how they created new fields of chemistry and how one can open a gate to a new world. A short history of the development of solution chemistry is reviewed in connection with a view for the development of theories and methodologies used for investigations in solution chemistry. New fields and new methodologies have often been discovered by extension and combination of existing theories and methodologies with a new vision recognising existing chemistry from another side of the traditional face of chemistry. A recently developed method to determine the structure of short-lived reaction intermediates in solution, which has not been determinable by traditional analytical methods, is shown as an example. This is a method combining a stopped-flow technique and the EXAFS method. Some results obtained for metal substitution reactions of porphyrin complexes are given, and emphasis is placed on an effective combination of different methodologies to create a new method of investigation.

Application of a compact synchrotron radiation facility to studies on the structure of solvated chloride and iodide ions in various solvents

A beamline equipped on the compact superconductive synchrotron radiation (SR) facility at Ritsumeikan University was designed for performing extended X-ray absorption fine structure (EXAFS) analyses of solutions and solids. The introduction of three different crystals, Si(220), Ge(220), and InSb(111), as the monochromator of the EXAFS spectrometer covers the K-edge absorption energy range of elements with low atomic numbers from silicon (atomic number 14) through zinc (30). The LIII-edge of the iodide ion could be measured by Si(220) and Ge(220). It is, however, not possible to measure the absorption of the bromide ion with this equipment. Using this spectrometer, the solvation structure of Cl- and I- ions was investigated in water, methanol, and ethanol under atmospheric pressure.

Perspective of solution chemistry of Japan — The historical background and recent trends

Abstract The development of solution chemistry in Japan is reviewed in connection with the history of solution chemistry in Europe and in the United States. Studies on solution chemistry from molecular aspects, as well as those by means of thermodynamics and statistical thermodynamics, are discussed from experimental and theoretical sides. Recent trends in solution chemistry, especially those in Japan, are overviewed from static and dynamic points of view.