We have prepared various ionic liquids containing cationic organometallic complexes, which exhibit intriguing physical properties and chemical reactivities. This review describes molecular design, preparation, and crystal structures of organometallic ionic liquids and related materials. Two topics are discussed, in which X-ray crystallography played an important role: magnetic-field effects on the crystallization of ferrocenium ionic liquids, and odd-even effects on the melting phenomenon of cobaltocenium ionic liquids.
Here we review our recent works on thermal phase behaviors of ionic liquids, 1-alkyl-3-methylimidazolium hexafluorophosphates ([Cnmim]PF6, n=1~4). Their complex thermal phase behaviors observed in calorimetric measurements were investigated at the molecular level using combined techniques of Raman spectroscopy, quantum chemical calculations, X-ray analyses, and nuclear magnetic resonance spectroscopy. It was demonstrated that the conformational flexibility of the side chain in the cation played a key role for thermal phase behaviors of some ILs as represented by [C4mim]PF6, nevertheless, which was not always the case for others as evidenced in [C1mim]PF6 results.
This article reviews our structural works on simple molecular glasses prepared by a low-temperature vapor-deposition (VD) technique. Following the general interest in simple molecular glasses, the glass-forming ability and required cooling rate are described for popular simple molecules. Then the necessity of the VD method and the actual VD-type cryostat used for our X-ray diffraction experiment are presented. The neutron diffraction data of CS2 and CCl4 glasses and the X-ray diffraction data of CO2 glass are demonstrated. The CO2 glass is the simplest glass with bulk quantity whose structure was investigated by the diffraction method. The significant orientational correlation between neighboring molecules was found in all of the glasses taken up in this article.
We review our neutron scattering studies on hierarchical structure and dynamics of imidazolium-based ionic liquids. In the diffraction patters, a peak reflecting the nanoscale segregation of polar and nonpolar domains appeared at around Q=0.3 Å-1, while a peak due to the ionic correlation at Q=0.8~1.0 Å-1 depending on the anion size. The relaxation of nanostructure, ionic diffusion, imidazolium relaxation and alkyl reorientation were separately observed by means of quasielastic neutron scattering techniques. We have also investigated anion and cation effects on the above motions and revealed that the hierarchical dynamics of the ionic liquids is mainly governed by the coulombic interaction between the polar part of cations and anions.
Nano-confinement of water (“water pocket”) in room-temperature ionic liquid (RTILs) was experimentally proved by a complementary use of small angle X-ray and neutron scattering. The size of the water pocket was estimated to be 2 ~ 3 nm. The water pocket is loosely packed and size-tunable in the RTIL. Hydrogen bonding water of the water pocket is different from that of bulk water. The water pocket is modified by intrinsic nanodomains in the RTILs. With reflecting slow dynamic fluctuations on the nanoscale, the polar/non-polar nanodomains in the RTILs are utilized for loosely packed nano-confinement in bioscience.
We studied a state of solvent water in the glassy dilute glycerol aqueous solutions from a viewpoint of water polyamorphism. We examined the pressure-induced polyamorphic transition of the glassy samples that relates to the transition between the low- and high-density amorphous ices, LDA and HDA. The polyamorphic behavior of the solvent water depends on the solute concentration. The state of solvent water during the polyamorphic transition may be the state that the LDA-like and HDA-like solvent waters coexist. This finding provides insight into the dynamic behavior of aqueous solutions at low temperatures.
Liquid structures and transport properties of ionic liquids are determined by the intermolecular interactions between ions. The ion dependence of the interactions between ions and self-diffusion coefficients of ions in ionic liquids was studied by ab initio molecular orbital calculations and molecular dynamics simulations. The relationship between the interaction between ions and the liquid structure and transport properties of ionic liquids were discussed.
Thermodynamic anomaly of water is expected to be ascribable to the critical phenomena associated with the second（liquid-liquid）critical point, whose existence is, however, hardly proven directly. This is because the（hypothesized）point is thought to exist below the crystal homogeneous temperature, where no liquid bulk water can be probed. We have revealed that tin tetraiodide has quite a similar polyamorphic nature, but its second critical point is highly considered to lie in an accessible region.
The first sharp diffraction peak (FSDP) reflects the intermediate-range order in disordered systems. Here we start from the FSDP in elemental liquid, liquid phosphorus, and we glance similarities between the pressure dependence of the FSDP in SiO2 glass and that in a polymer [isotactic poly(4-methyl-1-pentene)] in its melted state. These FSDP are related to voids. In contrast, some first peaks in polymers are related to nanophase separation.