A historical review is given on the proton tunneling model of phase transitions in hydrogen-bonded crystals making the main reference to KDP type ferroelectrics. The model has been shown to be inconsistent with experimental results in Japan. This procedure is reviewed in the present paper and will be in other papers in this issue. The validity of the tunnneling model in the present stage of study on structural phase transitions is discussed.
A review is given of the deuteration effect on the crystal structure, in particular on the hydrogen-bond system, and its relation to the phase transition. The correlation between hydrogen-bond parameters and some examples of peculiar phase diagrams between deuterium content and temperature are discussed in the context of the geometric isotope effect. Similarity and difference between the deuteration effect and the negative pressure effect are discussed in the case of KH2PO4 and squaric acid (H2C4O4) . It is pointed out that the hydrogen bond is not necessarily the softest part of the crystal as usually assumed, and pressure effect is responded by rotation of the PO4 or C4O4 group as well. The case of (NH4) 3H (SO4) 2 is discussed as an example where a surprising similarity between deuteration and positive pressure effect is seen in their phase diagrams.
In view of the importance of the ice rule imposed on the proton configurations in hydrogen-bonded crystals, a cluster theory is developed and applied to the phase transitions in the mixed crystals of ferroelectrics and antiferroelectrics Rb1-x (NH4) xH2PO4. At low temperatures, protons rearrange themselves to fulfill the ice rule condition, and produce the glass structure resulting from the competition between two kinds of proton ordering. It is shown that the correlation among hydrogen bonds under the ice rule constraint leads the system to the new ordered state.
Although the proton-tunneling model is an elegant idea to explain an isotope effect on the ferroelectric phase transition temperature of hydrogen-bonded ferroelectrics such as KDP and DKDP, there is no evidence of proton tunneling phenomena in these crystals from the experimental point of view. The dynamical mechanism of ferroelectric phase transition in this kind of crystals is order-disorder type of distorted PO4 tetrahedrons and the proton-tunneling idea has been broken down. The isotope effect can be explained in association with the difference of the zero point vibration energy between H and D atoms in an asymmetric potential.
Phase transitions in ice are summarized in aspect of the hydrogen bond and its network Recent works and developments in phase transition from Ih to the ordered phase, phase transitions between high pressure phases: VII, VIII and X, and amorphous-amorphous phase transition are centrally reviewed.
High pressure Raman scattering measurements are performed on hydrogen-bonded molecular system, crystal of squaric acid H2C4O4. Specific intramolecular vibrational modes of squaric dianion C4O42- are found to disappear under pressures due to vanishment of the molecular distortion. This indicates that application of presure deformed the proton potential from double-minimum to single-minimum type. We fourther found a novel quantum paraelectric state, where deformed dianions can not show any ordering down to the lowest temperature, in the pressure range of 3.0-4.5 GPa.
In K3H (SO4) 2-K3D (SO4) 2 mixed crystals, internal v2 mode frequencies of DSO4 show abrupt splitting at the phase transition temperature, while those of HSO4 split continuously. The length of D-hydrogen bond is concluded to be independent of the D concentration. This conclusion is supported by the measurement of the characteristic time τc-1 of deuteron motion; gradients of the temperature dependence of τc-1 are almost the same for any concentration of D studied.
Developments of studies of hydrogen transfer dynamics and tunneling effect in a variety of hydrogen bonds in solids are reviewed. Particular attention was paid for interactions between the hydrogen transfer motion and the surrounding of the hydrogen bond. Those are phonon-assisted tunneling, information propagation between the two interacting NHO hydrogen bonds linked by π-electronic molecular frame, and a novel correlation effect on proton transfer in the proton/deuteron mixed system.
The similarity in the masses of the proton and the neutron, and the extremely large scattering cross-section for hydrogen make neutron an ideal probe for studying the dynamics of hydrogen in materials. With neutron scattering it is possible to measure the excitation energies of hydrogen as well as the intensity maps I (Q, ε) at those excitations in a wide Q-range. The measurements of the intensity map I (Q, ε) directly probe the shape of the hydrogen wave function, and can give us the further novel information for the exact hydrogen potential.This new method was applied to determined the hydrogen potential in the hydrogen bond for the first time.
The present status of our understanding of the mechanism of the phase transition from a ferroelectric phase (or an antiferroelectric phase) to a paraelectric phase in hydrogen bonded crystals is reviewed. Discussions are focused on the isotope effects on the phase transition temperature and hydrogen bond lengths. The mechanism of proton transfer is also discussed.
Recent investigations on the dynamics of the proton forming hydrogen bond in solids are reported. It is emphasized that simple tunneling picture of proton within a static double minimum potential does not properly describe the properties of hydrogen bond. Instead, protonic polaron state is proposed to describe the ground state of the system. Experimental results of neutron spectroscopy are shown to be censistent with this new picture.
First principles calculation for K3H (SO4) 2 is performed to investigate the origin of the isotope effect in hydrogen-bonded (anti) ferroelectric materials by taking account of the zero-point oscillation effects of the proton and the deuteron. The calculated proton potential surface is not a double-well-type, but it makes the positions of the hydrogen and the deuterium different. The shrinkage of the dipole moment due to the zero-point oscillation of the proton is also important.
In this report, the cooperation of electron and proton in organic-inorganic hybrid systems is discussed. Particularly, the experimental results of the X-ray diffraction measurements on the 1-D metallic charge-transfer crystal of [Pd (H2-xEDAG) (HEDAG) ] TCNQ (EDAG=ethylenediaminoglyoxime, x=ca. 0.7) were focused. Below the metal-insulator (M-I) phase transition (180 K), commensurate superlattice reflections were observed with a period of c*/2, c*/3, and c*/12 along the 1-D axis, c. The full X-ray diffraction analyses of temperature-dependent reflections ignoring the superlattice spots show remarkable anomalies of the temperature factors below 180 K for several atoms which associate with the intermolecular H-bonds between donor-molecules. This is a result of cooperation between the order-disorder transition of the 1-D H-bond intermolecular superlattice and the M-I transition of the 1-D electronic state in TCNQ acceptor stacks. These experimental results support a model of purely 1-D metallic hole-conduction introduced by partial deprotonation of H-bonds in the ligands of [Pd (H2-xEDAG) (HEDAG) ] .
The hydrogen bond plays important roles in the reactions of molecular crystals. If the reactive group of the molecule is hydrogen bonded with the neighboring molecules in the crystal, the hydrogen bond severely influences the reactivity of the molecule, because it (1) changes the electronic state of the reactive group and/or (2) inhibits the migration of the group in the process of reaction. Even if the hydrogen bond has no connection with the reactive group, it strengthens the crystal lattice and accelerates the photoreactions since not only the energy to destroy the crystalline lattice is unnecessary but also light can easily penetrate into the crystal. The hydrogen migration in the solid-state reactions can be directly observed by neutron diffraction when only the peculiar hydrogen atom considered can be replaced with the deuterium atom.
Depolarized low-frequency Raman spectra below 250 cm-1 in liquid water have been well interpreted by a decomposition of one Cole-Cole type relaxation mode and two intermolecular vibration modes of tetrahedrally coordinated structure due to hydrogen bonds. High-frequency Raman spectra between 300 cm-1 and 4000 cm-1 in liquid water have been well explained by a molecular vibration pattern of temporal C2v distorted tetrahedral-like structure around oxygen atom. This new interpretation is alternative to a conventional one.
In liquid state water molecules form the random and percolated network of hydrogen bonds. In order to understand the collective motion of this network, the local radial distribution function was observed with the molecular dynamics simulation. The molecule alternately goes through the structured period and the destructured period. By comparing with the neutron scattering data it is shown that the cluster of the structured molecules resembles to low-density amorphous (LDA) ice, and the cluster of the destructured molecules resembles to the high-density amorphous (HDA) ice. Simulated liquid water, therefore, is a composite of the LDA-like clusters and the HDA-like clusters. The collective network rearrangement motion at the room temperature can be regarded as a molecular scale precursor of the hypothetical transition between high-density liquid (HDL) water and low-density liquid (LDL) water in the supercooled region.
Hydration structures surrounding protein molecules have great influences on folding, stability, physical properties and functions of the molecules. In spite of this fact, X-ray protein crystallography have not sufficiently contributed on hydration structure analyses of proteins. Recent progress in cryogenic techniques enables us to analyze hydration structures of proteins. Cryogenic analyses have provided the structural information on hydration patterns around hydrophobic residues and network structures formed by hydration water molecules. New insights into the hydration structures of proteins have been given by cryogenic X-ray crystallography.
Neutron quasi-Laue diffraction data (2 Å resolution) from tetragonal hen egg-white lysozyme were collected in ten days with neutron imaging plates. The positions of 960 hydrogen atoms in the molecule and 157 bound water molecules, were determined. The distinctive nature of hydrogen atoms and bound water molecules in the lysozyme molecule was discussed.
Since a vitamin E radical produced by the antioxidant reaction of vitamin E causes damage to vital functions, ubiquinol and vitamin C regenerate a vitamin E molecule from a vitamin E radical in vivo. The kinetic studies of the regeneration reactions have been carried out in solutions by means of stopped-flow spectroscopy. Substantial deuterium kinetic-isotope-effects on the second-order rate constants have been observed in both the reactions of ubiquinol and vitamin C. Furthermore, a deviation from a linear relationship in the Arrhenius plot has been observed in the reaction of ubiquinol. These results suggest that the tunneling effect plays an important role in the regeneration reactions in various tissues and mitochondria.
Recent discovery of several functional nucleic acids has changed our concept about their roles in biological system, and oppened a new field of RNA world. Except for the Watson-Crick base pairings, non-canonical hydrogen bondings have been found between bases, between base and ribose, and between riboses. They are participated in specific roles for folding the stem parts in the characteristic tertiary structure, and to realize a special function on it. In this article, several structural motifs will be disscussed with some examples.