Probing structural transition and guest dynamics of hydroquinone clathrates by temperature-dependent terahertz time-domain spectroscopy

The structural transition from hydroquinone clathrates to crystalline α-form hydroquinone was observed up to the range of 3 THz frequency as a function of temperatures. We found that all three hydroquinone clathrates, CO(2)-, CH(4)-, and CO(2)/CH(4)-loaded hydroquinone clathrates, transform into the α-form hydroquinone at around 102 ± 7 °C. The resonance peak of the CO(2)-loaded hydroquinone clathrate at 2.15 THz decreases with increasing temperature, indicating that CO(2) guest molecules are readily released from the host framework prior to the structural transformation. This reveals that the hydroquinone clathrates may transform into the stable α-form hydroquinone via the metastable form of guest-free clathrate, which depends on guest molecules enclathrated in the cages of the host frameworks. A strong resonance of the α-form hydroquinone at 1.18 THz gradually shifts to the low frequency with increasing temperature and shifts back to the high frequency with decreasing temperature.     

硝基胍H迁移异构化反应动力学的从头计算研究

Theoretical studies on the α- and β-forms nitroguanidine were carried out using ab initio theoretical methods, at the MP2/6-31G(d,p) level. The predicted geometrical parameters were in good agreement with the available theoretical values, which calculated by other author. The three C-N bond lengths in α-form nitroguanidine were different, the longest bond length was 1.430 A, the shortest was 1.283 A. But they were almost similar in β-form, the longest was 1.375 A, the shortest was 1.322 A. Therefore there were conjugative effects in β-form but not in α-form. The calculated results also show that the β-form is stable with respect to the α-form from energetically, lower 28.16 kJ/mol corrected with zero point vibrational energy. The transition-state for the unimolecular isomerization was conformed by the IRC calculation. The calculated energy barrier for the direct intramolecular hydrogen atom transfer isomerization process was 132.95 kJ/mol. The isomerization reaction, exothermal reaction, is a typical intramolecular hydrogen atom synfacial transfer reaction. Rate constants of the isomerization reaction were evaluated within the temperature range of 200-1773 K by the classical transition state theory. The rate constant was 1.99×10-11 s-1 and the equilibrium constant was 1.00×105 at 298 K. With the temperature increasing, the equilibrium value decayed and the reaction process was more difficult.     

Hysteretic spin crossover between a bisdithiazolyl radical and its hypervalent σ-dimer

The bisdithiazolyl radical 1a is dimorphic, existing in two distinct molecular and crystal modifications. The α-phase crystallizes in the tetragonal space group P4?2(1)m and consists of π-stacked radicals, tightly clustered about 4? points and running parallel to c. The β-phase belongs to the monoclinic space group P2(1)/c and, at ambient temperature and pressure, is composed of π-stacked dimers in which the radicals are linked laterally by hypervalent four-center six-electron S···S-S···S σ-bonds. Variable-temperature magnetic susceptibility χ measurements confirm that α-1a behaves as a Curie-Weiss paramagnet; the low-temperature variations in χ can be modeled in terms of a 1D Heisenberg chain of weakly coupled AFM S = (1)/(2) centers. The dimeric phase β-1a is essentially diamagnetic up to 380 K. Above this temperature there is a sharp hysteretic (T↑= 380 K, T↓ = 375 K) increase in χ and χT. Powder X-ray diffraction analysis of β-1a at 393 K has established that the phase transition corresponds to a dimer-to-radical conversion in which the hypervalent S···S-S···S σ-bond is cleaved. Variable-temperature and -pressure conductivity measurements indicate that α-1a behaves as a Mott insulator, but the ambient-temperature conductivity σ(RT) increases from near 10(-7) S cm(-1) at 0.5 GPa to near 10(-4) S cm(-1) at 5 GPa. The value of σ(RT) for β-1a (near 10(-4) S cm(-1) at 0.5 GPa) initially decreases with pressure as the phase change takes place, but beyond 1.5 GPa this trend reverses, and σ(RT) increases in a manner which parallels the behavior of α-1a. These changes in conductivity of β-1a are interpreted in terms of a pressure-induced dimer-to-radical phase change. High-pressure, ambient-temperature powder diffraction analysis of β-1a confirms such a transition between 0.65 and 0.98 GPa and establishes that the structural change involves rupture of the dimer in a manner akin to that observed at high temperature and ambient pressure. The response of the S···S-S···S σ-bond in β-1a to heat and pressure is compared to that of related dimers possessing S···Se-Se···S σ-bonds.     

Structure and phase transitions in poly(diethylsiloxane)

The structure changes accompanying phase transitions in poly(diethylsiloxane) (PDES) have been studied by WAXS and SAXS techniques using oriented and isotropic samples. PDES may exist in two low-temperature modifications (the monoclinic α₁-form and presumably the “tetragonal” β₁-form) and two high-temperature modifications (the monoclinic α₂-form and the “tetragonal” β₂-form). In linear PDES the crystal - crystal transitions α₁–α₂ and β₁–β₂ occur near 214 and 206 K, respectively. At higher temperatures α₂ (280 K) and β₂ (290 K) forms transform into the mesomorphic phase α_m that gradually melts at 280–300 K giving an amorphous phase. According to x-ray and density data, α_m phase is also characterized by monoclinic structure slightly different from hexagonal packing.     

Phase transition behavior of a poly(oxyethylene-b-butylene adipate) ionomer with shape memory properties

A series of biodegradable poly(oxyethylene-b-butylene adipate) ionomers (POBAi) were prepared by two-step in situ polymerization using adipic acid, 1,4-butanediol and mixed monomers of bis(poly(oxyethylene)) sulfonated dimethyl fumarate. The chemical composition of these POBAi was ascertained by ¹H NMR spectroscopy. The objective of this study was to investigate the shape memory effect of POBAi containing ionomer compared to non ionic POBA. It was observed that POBA5.0i showed a good shape memory effect than that of POBA 2.5 mol% or none of ionic group due to much physical cross-linking point by rich ionic group. Stress-induced phase transition was investigated during the shape deformation and recovery process using a wide-angle X-ray diffractometer (WAXD). The POBA crystal phase transition from β- to α-form was observed in all POBA samples by either thermal treatment or physical drawing. The α-form crystal did not recover to the initial β-form during the recovery process because the monoclinic α-form crystal is structurally more stable than the orthorhombic β-form crystal.     

Solid-state polymerization of long-chain vinyl compounds. I. Effect of molecular arrangement on polymerizability of octadecyl methacrylate

γ-Ray-initiated postpolymerization of octadecyl methacrylate in polymorphic crystals and melt has been investigated to clarify the effect of molecular arrangement of the monomer on polymerizability. From thermal, x-ray, and infrared (IR) analyses this long-chain monomer exhibited three crystalline modifications that we refer to as α-, sub-α, and β-forms. The β-form (mp 28.7–29.7°C), which is obtainable from solution, is a stable state with triclinic chain packing. The α-form (mp 19.5–20.0°C), which is obtained first from the melt but transforms into β-form on storing, is a metastable state with hexagonal chain packing. The sub-α-form appears transiently in α→β transition. The polymerizability of octadecyl methacrylate in the β-form is extremely low, whereas the α-form can polymerize easily and the initial polymerization rate, saturated conversion, and polymer molecular weights increase with temperature. Polymerizability in the molten state at fairly high temperature is rather low, however. Thus maximum polymerizability is obtained just above the melting point of α-form. It has been found that particular orientation and suitable packing mode with some freedom of rotational motion of the monomer molecules in layered structure accelerate the polymerization reaction.     

EuPtP,Eu1–xGdxPtP und Eu0,5Sr0,5PtP: Strukturen und Zwischenvalenz

EuPtP, Eu_1–xGd_xPtP, and Eu_0.5Sr_0.5PtP: Structures and Intermediate Valence EuPtP with a strong temperature dependence of the Eu-valency undergoes two first order phase transitions at 235 K (α-/β-form) and 190 K (β-/γ-form). The compound crystallizes in a modified Ni₂In-type structure, the P-atoms (α-, β-EuPtP) and the Pt-atoms too (γ-EuPtP) are shifted towards one of two Eu-layers. Compounds of the series Eu_1–xGd_xPtP and Eu_0.5Sr_0.5PtP crystallize in the same type of structure as α- or β-EuPtP, but don't undergo with x > 0.05 the above mentioned phase transitions. They were studied by L_III—X-ray absorption and susceptibility measurements: The valence of Eu shows roughly the same temperature dependence as in EuPtP, only one magnetic order transition was found. The structural details and the behaviour of these compounds are discussed with a domain picture.     

High-pressure x-ray diffraction and Raman spectroscopy of ice VIII

In situ high-pressure/low-temperature synchrotron x-ray diffraction and optical Raman spectroscopy were used to examine the structural properties, equation of state, and vibrational dynamics of ice VIII. The x-ray measurements show that the pressure-volume relations remain smooth up to 23 GPa at 80 K. Although there is no evidence for structural changes to at least 14 GPa, the unit-cell axial ratio ca undergoes changes at 10-14 GPa. Raman measurements carried out at 80 K show that the nu(Tz)A(1g)+nuT(x,y)E(g) lattice modes for the Raman spectra of ice VIII in the lower-frequency regions (50-800 cm(-1)) disappear at around 10 GPa, and then a new peak of approximately 150 cm(-1) appears at 14 GPa. The combined data provide evidence for a transition beginning near 10 GPa. The results are consistent with recent synchrotron far-IR measurements and theoretical calculations. The decompressed phase recovered at ambient pressure transforms to low-density amorphous ice when heated to approximately 125 K.     

Chemical-clathrate hybrid hydrogen storage: storage in both guest and host

Hydrogen storage from two independent sources of the same material represents a novel approach to the hydrogen storage problem, yielding storage capacities greater than either of the individual constituents. Here we report a novel hydrogen storage scheme in which recoverable hydrogen is stored molecularly within clathrate cavities as well as chemically in the clathrate host material. X-ray diffraction and Raman spectroscopic measurements confirm the formation of beta-hydroquinone (beta-HQ) clathrate with molecular hydrogen. Hydrogen within the beta-HQ clathrate vibrates at considerably lower frequency than hydrogen in the free gaseous phase and rotates nondegenerately with splitting comparable to the rotational constant. Compared with water-based clathrate hydrate phases, the beta-HQ+H2 clathrate shows remarkable stability over a range of p-T conditions. Subsequent to clathrate decomposition, the host HQ was used to directly power a PEM fuel cell. With one H2 molecule per cavity, 0.61 wt % hydrogen may be stored in the beta-HQ clathrate cavities. When this amount is combined with complete dehydrogenation of the host hydroxyl hydrogens, the maximum hydrogen storage capacity increases nearly 300% to 2.43 wt %.     

Temperature dependent Raman scattering study of l-ascorbic acid

Temperature dependent Raman study of l-ascorbic acid has been performed from 15 to 418 K. Changes in the wavenumber vs. temperature plots for some internal modes were interpreted as conformational molecular change and the discontinuity in the wavenumber vs. temperature plots along with the appearance of a new vibrational mode in the temperature range 200-270 K suggests that l-ascorbic acid undergoes a structural phase transition. For temperatures higher than 300 K, no relevant modification was observed on the Raman spectra thus indicating a stable structure at high temperatures. Additionally, a correlation between OH stretching wavenumber and the behavior of hydrogen bond is also made.     

Effect of molecular weight on conformation of poly(β-benzyl L-aspartate) in films

In order to study the effect of the molecular weight on the crystallinity and conformational changes of poly(β-benzyl L aspartate) in films, a previous study on high molecular weight samples has been extended to included polymers of low molecular weight, about 3.3 × 10³. Films were prepared from chloroform solution by quick or slow evaporation at room temperature. The conformation and the thermal behavior were studied by means of infrared spectroscopy and differential scanning calorimetry. All films dried quickly are composed of polymer in the left-handed α-helical form. All samples studied which have molecular weights above 2.3 × 10⁴ are similar in crystallinity and the left-handed α-helices in them crystallize to ω-helices during slow evaporation. In the low molecular weight region, however, the left-handed α-helices reverse to right-handed α-helices during slow evaporation, and the right-handed α-helices, in turn, reverse and crystallize to highly ordered ω-helices upon heat treatment, although there is some simultaneous conversion to the β-form. The transition temperatures of the quick-dried films for conversion from the left-handed α-helix to the ω-helix and from the ω-helix to the β-form increase linearly with increasing molecular weight up to about 2 × 10⁴, but no large molecular weight dependence is observed beyond that region.     

Molecular motion and phase transitions of clathrate hydrates**

Abstract

Heat capacities and complex dielectric permittivities of three clathrate hydrates of type II, encaging tetrahydrofuran (THF), acetone (Ac), and trimethylene oxide (TMO), were measured at low temperatures. The heat capacity measurement was done in the temperature range 13–300 K by using an adiabatic calorimeter with a built-in cryorefrigerator. The permittivities were measured in the temperature range 20–260 K and in the frequency range 20 Hz-1 MHz. For pure samples, with a glass transition due to freezing out of water, reorientational motion of the host lattice was observed calorimetrically at 85 K for THF and at 90 K for Ac hydrates, respectively. Spontaneous temperature drift rates of the calorimetric cell were measured under adiabatic conditions to derive the characteristic time for enthalpy relaxation. The enthalpy relaxation times thus derived were well correlated in an Arrhenius plot with the dielectric relaxation times derived from the dielectric relaxation of orientation polarization. The situation is the same as hexagonal ice which has a similar four co-ordinated hydrogen-bonded network.     

Physical properties and structure of silk. VI. Conformational changes in silk fibroin induced by immersion in water at 2 to 130°c

Silk fibroin films in the random-coil and β-form conformations were immersed in water at temperatures from 2 to 130°C, and conformational changes were followed by x-ray diffraction, infrared spectroscopy and differential scanning calorimetry. On treatment with water below 60°C, the random-coil conformation is converted to the α form and above 70°C to mixtures of the α and β conformations. The β-form content increases as the immersion temperature is raised. The β form is not affected by immersion in water in the temperature range studied.     

Calorimetric and relaxation properties of xylitol-water mixtures

We present the first broadband dielectric spectroscopy (BDS) and differential scanning calorimetry study of supercooled xylitol-water mixtures in the whole concentration range and in wide frequency (10(-2)-10(6) Hz) and temperature (120-365 K) ranges. The calorimetric glass transition, T(g), decreases from 247 K for pure xylitol to about 181 K at a water concentration of approximately 37 wt. %. At water concentrations in the range 29-35 wt. % a plentiful calorimetric behaviour is observed. In addition to the glass transition, almost simultaneous crystallization and melting events occurring around 230-240 K. At higher water concentrations ice is formed during cooling and the glass transition temperature increases to a steady value of about 200 K for all higher water concentrations. This T(g) corresponds to an unfrozen xylitol-water solution containing 20 wt. % water. In addition to the true glass transition we also observed a glass transition-like feature at 220 K for all the ice containing samples. However, this feature is more likely due to ice dissolution [A. Inaba and O. Andersson, Thermochim. Acta, 461, 44 (2007)]. In the case of the BDS measurements the presence of water clearly has an effect on both the cooperative α-relaxation and the secondary β-relaxation. The α-relaxation shows a non-Arrhenius temperature dependence and becomes faster with increasing concentration of water. The fragility of the solutions, determined by the temperature dependence of the α-relaxation close to the dynamic glass transition, decreases with increasing water content up to about 26 wt. % water, where ice starts to form. This decrease in fragility with increasing water content is most likely caused by the increasing density of hydrogen bonds, forming a network-like structure in the deeply supercooled regime. The intensity of the secondary β-relaxation of xylitol decreases noticeably already at a water content of 2 wt. %, and at a water content above 5 wt. % it has been replaced by a considerably stronger water (w) relaxation at about the same frequency. However, the similarities in time scale and activation energy between the w-relaxation and the β-relaxation of xylitol at water contents below 13 wt. % suggest that the w-relaxation is governed, in some way, by the β-relaxation of xylitol, since clusters of water molecules are rare at these water concentrations. At higher water concentrations the intensity and relaxation rate of the w-relaxation increase rapidly with increasing water content (up to the concentration where ice starts to form), most likely due to a rapid increase of small water clusters where an increasing number of water molecules interacting with other water molecules.     

High-pressure vibrational spectroscopy of sulfur dioxide

Solid sulfur dioxide was investigated by vibrational spectroscopy over a broad pressure and temperature range, extending to 32.5 GPa at 75-300 K in diamond anvil cells. Synchrotron infrared spectra provided the first measurements of the pressure dependence of the lattice modes in the far-IR region. Below 17.5 GPa, two fundamentals exhibit splittings enhanced by pressure. The asymmetric stretching mode of SO(2) exhibits a remarkable pressure-induced softening. The observations are consistent with the ambient pressure Raman measurements indicating that SO(2) crystallizes in an acentric cell, but are inconsistent with a previously proposed interpretation that the structure of the high-pressure phase consists of (SO(2))(3) clusters. Dramatic changes in the Raman spectra are found above 17.5 GPa at room temperature. These indicate major changes in structure and possible formation of SO(2) clustering with an enlarged unit cell. The behavior at low temperature differs from that at room temperature. These findings provide constraints on the phase diagram of sulfur dioxide.     

Crystalline Organization in Syndiotactic Polystyrene Gels and Aerogels

The crystalline structure of syndiotactic polystyrene gels and aerogels has been investigated by using x-ray diffraction. Results show that, depending on the solvent, the crystalline structure of the junction zones of the gels is a clathrate phase or the solvent free orthorhombic β-form. For aerogels obtained from gels with a clathrate phase, the aerogel crystalline phase consists of the nanoporous δ-form while for aerogels obtained from gels with the β-form, the original crystalline structure is maintained.     

Vibrational spectra of methane clathrate hydrates from molecular dynamics simulation

Molecular dynamics simulations were performed on methane clathrate hydrates at ambient conditions. Thermal expansion results over the temperature range 60-300 K show that the unit cell volume increases with temperature in agreement with experiment. Power spectra were obtained at 273 K from velocity autocorrelation functions for selected atoms, and normal modes were assigned. The spectra were further classified according to individual atom types, allowing the assignment of contributions from methane molecules located in small and large cages within the structure I unit cell. The symmetric C-H stretch of methane in the small cages occurs at a higher frequency than for methane located in the large cages, with a peak separation of 14 cm(-1). Additionally, we determined that the symmetric C-H stretch in methane gas occurs at the same frequency as methane in the large cages. Results of molecular dynamics simulations indicate the use of power spectra obtained from the velocity autocorrelation function is a reliable method to investigate the vibrational behavior of guest molecules in clathrate hydrates.     

Radiation-induced emission from golden hamster embryo cells

Emission from high-energy-electron-irradiated golden hamster embryo (GHE) cells has been studied over the temperature range 12–300 K both by a one-shot-single-photon-counting method and by photocurrent measurements with an oscilloscope. Emission from the irradiated phosphate buffered saline (PBS) also has been studied. The emission spectra from PBS at 12 and 77 K show a maximum around 330 and 380 nm, respectively, which are the same spectra as those from irradiated pure H₂O. The emission from irradiated GHE consists of the new band at 480 nm in addition to the emission from H₂O. The 480 nm emission is observed at the temperature range of 12–300 K, though the emission at 300 K is much lower than that at low temperature. The 480 nm emission is ascribed to the transition from excited organic substances in GHE cells. The intensity of 480 nm emission at 300 K increases linearly with increasing irradiation-dose in the range of 11–600 Gy.     

Temperature effects on the IR spectra of crystalliine amino acids,dipeptides, and polyamino acids. I. Glycine

The IR spectra of the α, β, and γ polymorphic modifications of pure glycine (without diluents) have been studied in the temperature range 93–433 K; changes in the IR spectra caused by variations of temperature are correlated with previously obtained diffraction data about polymorphic transitions, anisotropic compression of structure, and changes in hydrogen bond parameters.     

Drastic change of the ferromagnetic properties of the ternary germanide GdTiGe through hydrogen insertion

Hydrogen absorption of the CeFeSi- and CeScSi-type forms of GdTiGe was performed. Before hydrogenation they show an antiferromagnetic transition at around 412 K and a ferromagnetic transition at 376 K, respectively. Hydrogenation of both forms leads to formation of the same hydride GdTiGeH which crystallizes with a filled CeScSi-type structure where all the [Gd(4)] tetrahedra are filled by hydrogen. This hydride is paramagnetic in the temperature range 4-300 K. The slightly negative value of the paramagnetic Curie temperature θ(p) confirms that all ferromagnetic interactions were destroyed in the case of the CeScSi-type form. From first-principles calculations with the PAW GGA methodology, the localization of hydrogen within the [Gd(4)] tetrahedra was confirmed through energetic stabilization. It was also seen that the energy changes significantly with volume, indicating the itinerant (delocalized) role of the electrons in the magnetism.