Photolysis of diruthenium hexacarbonyl tetrahedrane compounds in Nujol glass matrices

The photochemistry of five diruthenium hexacarbonyl tetrahedrane compounds, Ru₂(CO)₆(μ-S₂C₆H₄) (1), Ru₂(CO)₆(μ-S₂C₂H₄) (2), Ru₂(CO)₆(μ-S₂C₃H₆) (3), Ru₂(CO)₆(μ-SCH₂CH₃)₂ (4), and Ru₂(CO)₆(μ-dmpz)₂, (5), where dmpz=3,5-dimethylpyrazolate, have been examined in frozen Nujol glasses at ca. 90 K. Compounds 1-4 are found to lose CO upon UV photolysis to form two isomeric photoproducts, while 5 is found to form one product almost exclusively. The various photoproducts are assigned to axial and equatorial CO-loss species on the basis of the spectra of analogous triphenylphosphine pentacarbonyl derivatives.     

Unanticipated guest motion during a phase transition in a ferroelastic inclusion compound

Urea inclusion compounds (UICs) have been used as tools to understand ferroelastic domain switching and molecular recognition during crystal growth. Although the vast majority of UICs contain helical arrangements of host H-bonds, those containing guests with the formula X(CH(2))(6)Y (X, Y = Br, Cl, CN, NC) adopt an alternative P2(1)/n packing mode in which the host molecules exist as stacked loops of urea hexamers. Such structures may be further separated into two classes, ones distorted away from hexagonal symmetry along [100] (Br(CH(2))(6)Br, Br(CH(2))(6)Cl, and Cl(CH(2))(6)Cl) and those distorted along [001] (e.g. NC(CH(2))(6)CN). In each of these systems, guests exist as equilibrium mixtures of gauche conformers whose populations control the direction and magnitude of the observed distortion. Such UICs are potentially ferroelastic, but the n-glide requires that domains are not related by a simple rotation-translation mechanism as in the helical systems. Ferroelastic (degenerate) domain reorientation would necessitate a large-scale reorganization of the urea framework and rupture of numerous H-bonds. Coupled with distortions of 2 to 10%, this mechanism-based barrier to domain switching has precluded observation of this phenomenon. To prepare ferroelastic UICs with minimal distortions from hexagonal symmetry, attempts were made to form solid solutions of UICs containing guests from the two classes. This failed, however: solid solution formation of the stacked loop form is usually possible within a series (e.g. with Cl(CH(2))(6)Cl and Br(CH(2))(6)Br), but not between series (e.g. Cl(CH(2))(6)Cl and NC(CH(2))(6)CN). Crystals of Cl(CH(2))(6)CN/urea, in which a single guest contains substituents from each class, are distorted along [001] by only 0.5% from hexagonal symmetry at 298 K and exhibit ferroelastic domain reorientation at high forces. At -66 degrees C, Cl(CH(2))(6)CN/urea undergoes a topotactic phase transition that is unexpectedly nontopochemical. The structure of the low-temperature phase, including the orientation of the methylene chain, closely matches the structures of UICs distorted by 10% along [100] (e.g. Cl(CH(2))(6)Cl/urea). In this transition, small conformation changes of guests give rise to large-scale guest translations of approximately 5.5 A down the channel axis, even though an analogous gauche-to-gauche jump is well established in closely related materials that adopt either high- or low-temperature forms (e.g. NC(CH(2))(6)CN/urea, Cl(CH(2))(6)Cl/urea). The large guest displacement during this transition explains the difficulty in preparing solid solutions of the P2(1)/n form with guests of formula X(CH(2))(6)Y from two different series (e.g. Cl(CH(2))(6)Cl and NC(CH(2))(6)CN). This failure arises not from the different orientations of guest-induced strain, but from preferential occupation of different sites along the channel by the two types of guests. The subtlety of this process and of the interactions involved highlights the difficulty in using simple considerations of isomorphism to design new materials.     

A bent-shape liquid crystal compound with antiferroelectric triclinic-monoclinic phase transition

Electro-optical and polarization current measurements on 1,3-phenylene-bis[4-(3-fluoro-4-decyloxyphenyliminomethyl) benzoate] (3F-10-O-PIMB) revealed a second order phase transition between two antiferroelectric ‘smectic banana’ phases. The observations show that the switching between the ferroelectric states in the higher temperature (HT) phase requires higher thresholds than in the lower temperature (LT) phase. It is hypothesized that the HT phase has a lower (triclinic, C₁) symmetry, than that of the LT phase (monoclinic, C₂). It is also shown that electric fields can induce transitions between different ‘smectic banana’ phases.     

A bent-shape liquid crystal compound with antiferroelectric triclinic-monoclinic phase transition

Electro-optical and polarization current measurements on 1,3-phenylene-bis[4-(3-fluoro-4-decyloxyphenyliminomethyl) benzoate] (3F-10-O-PIMB) revealed a second order phase transition between two antiferroelectric 'smectic banana' phases. The observations show that the switching between the ferroelectric states in the higher temperature (HT) phase requires higher thresholds than in the lower temperature (LT) phase. It is hypothesized that the HT phase has a lower (triclinic, C 1 ) symmetry, than that of the LT phase (monoclinic, C 2 ). It is also shown that electric fields can induce transitions between different 'smectic banana' phases.     

Pressure-induced structural phase transition in the half-Heusler compound CaAuBi

We report a structural phase transition of the ternary compound CaAuBi under pressure, from the known cubic half-Heusler phase to a hexagonal LiGaGe type phase, based on synchrotron X-ray diffraction patterns taken under pressures up to 18 GPa. We report lattice parameters and atomic coordinates, and perform total energy calculations for both the cubic and hexagonal phases under different pressures. Finally, we present a structure map that places CaAuBi in the context of related 18 electron XYZ ternary systems.     

Metamagnetic transition in the 75 K antiferromagnet Gd4Co2Mg3

Gd₄Co₂Mg₃ (Nd₄Co₂Mg₃ type; space group P2/m; a=754.0(4), b=374.1(1), c=822.5(3) pm and β=109.65(4)° as unit cell parameters) was synthesized from the elements by induction melting in a sealed tantalum tube. Its investigation by electrical resistivity, magnetization and specific heat measurements reveals an antiferromagnetic ordering at T_N=75(1) K. Moreover, this ternary compound presents a metamagnetic transition at low critical magnetic field (H_cr=0.93(2) T at 6 K) and exhibits a magnetic moment of 6.3(1) μ_B per Gd-atom at 6 K and H=4.6 T. Due to this transition the compound shows a moderate magnetocaloric effect; at 77 K the maximum of the magnetic entropy change is ΔS_M=−10.3(2) J/kg K for a field change of 0-4.6 T. This effect is compared to that reported previously for compounds exhibiting a magnetic transition in the same temperature range.     

Calorimetric study of phase transitions in the thiourea 1,1,2,2-tetrachloroethane clathrate compound

Heat capacities of the thiourea clathrate compound of 1,1,2,2-tetrachloroethane, {(NH₂)₂CS}₃(CHCl₂)₂, were measured at temperatures between 13 and 330 K. Two phase transitions were found. The enthalpy and entropy changes of the transition are 5940 J·mol^–1 and 28.1 JK^–1· mol^–1 for the one occurring at 224 K and 2756 J·mol^–1 and 11.3 JK^–1·mol^–1 for the other at 248 K. It is concluded from the transition entropy values that the guest molecules are orientionally disordered nearly to the same extent as in the neat liquid.Contribution No. 56 from the Microcalorimetry Research Center     

Temperature-induced phase transition in hydrogels of interpenetrating networks poly(N-isopropylmethacrylamide)/poly(N-isopropylacrylamide)

The combination of ¹H NMR spectroscopy, DSC, dynamic mechanical spectroscopy, and optical microscopy was used to investigate temperature-induced volume phase transition in hydrogels of interpenetrating networks (IPNs) poly(N-isopropylmethacrylamide)/poly(N-isopropylacrylamide) (PNIPMAm/PNIPAm) with various PNIPMAm content. In these IPNs, both networks are thermosensitive; such systems were not examined so far. All methods showed phase transition starting at 307 K, which is the volume phase transition temperature of PNIPAm, the major network component. Only the sample with the lowest content of PNIPAm (~54 %) shows two-step collapse transition, other samples with higher PNIPAm content show a single transition in NMR and DSC which indicates enhanced mutual entanglement of both components. In all samples, the phase transition results in substantial increase of both components of the shear modulus. Although the properties of all samples change with temperature in similar way, differences in dependence on the PNIPMAm content and the shape of the sample can be seen.     

Reactive compatibilization in phase separated interpenetrating polymer networks

The effects of compatibilizing additives (monomethacrylic ester of ethylene glycol (MEG) and oligo-urethane-dimethacrylate (OUDM)) on the kinetics of interpenetrating polymer network (IPN) formation based on cross-linked polyurethane and linear polystyrene and its influence on the microphase separation, viscoelastic and thermophysical properties have been investigated. It was established, that various amounts (3-10 mass%) of the additive MEG and 20 mass% OUDM introduced into the initial reaction system prevent microphase separation in the IPN. In the course of the reaction the system undergoes no phase separation up to the end of reaction, as follows from the light scattering data. The viscoelastic properties of modified IPN are changed in such a way that instead of two relaxation maxima characteristic of phase-separated system, only one relaxation maximum is observed, what is result of the formation of compatible IPN system. The position of this relaxation transition depends on the system composition and on the reaction conditions.     

Calorimetric study of phase transitions in the thiourea carbon tetrachloride inclusion compound

Heat capacities of the inclusion compound (thiourea)_3.00CCl₄ have been measured in the temperature range 15–300 K. A first-order phase transition was found at 41.3 K and a second-order transition at 67.17 K. The enthalpy and entropy of the transition are 149 J mol^–1 and 3.7 J K^–1 mol^–1 for the former, and 241 J mol^–1 and 3.9 J K^–1 mol^–1 for the latter. A divergent expression C = A{(T _c –T)/T _c} ^– was fitted to the excess heat capacity of the upper phase transition. The best-fit parameters wereA = 7.4 J K^–1 mol^–1,T _c = 67.166 K and = 0.31. Possible types of molecular disorder in the high temperature phase are discussed in relation to the transition entropy and the molecular and site symmetries of the guest molecule. The heat capacity of the lowest temperature phase was unusually large and may indicate the existence of very low frequency vibrational modes or labile configurational excitation of the guest molecule. Standard thermodynamic functions were calculated from the heat capacity data and are tabulated in the appendix.Contribution No. 11 from the Microcalorimetry Research Center.     

相转移条件下过渡金属促进有机反应的一些新进展

本文综述了近年来，在相转移条件下，过渡金属催化氧化、还原、羰基化以及偶合反应及其在有机合成中应用新进展。     

Synthesis,structure and electrochemical properties of a polyoxometalate-templated compound with 2D twofold interpenetrating network

A new Keggin-type polyoxometalate-based compound {[Cu₂(L)₄(H₂O)₄](PW₁₁^VIW^VO₄₀)}·16H₂O (1) constructed from PW₁₁^VIW^VO₄₀ ⁴⁻, N,N′-bis(4-pyridylformyl) piperazine (L) and Cu(II) has been hydrothermally synthesized and structurally characterized by elemental analyses, IR and single-crystal X-ray diffraction analysis. Single-crystal X-ray diffraction analysis reveals that the semi-rigid piperazine-based ligands L coordinate to the Cu(II) atoms to constitute a two dimensional coordination network. The 2D (4, 4) cationic layers are stacked together in a perpendicular mode, resulting in the formation of twofold interpenetrating frameworks with large cavities. The PW₁₂ anions reside in the large cubic-like cavities, serving as non-coordinating templates. The compound 1 displays good electrocatalytic activity toward the reduction of nitrite in 1 M H₂SO₄ aqueous solution.     

Synthesis, structure and photoluminescent property of a novel potassic compound

A novel 3D coordination compound of K(H2TDA)(H20)(1)(H_3TDA=1H-1,2,3-triazole-4,5-dicarboxylic acid) has been prepared and characterized by IR spectroscopy,elemental analysis,ICP and single-crystal X-ray diffraction.Compound 1 displays strong fluorescent emission at room temperature.     

Mesomorphic phase transition behaviour of photopolymerisable liquid crystalline triphenylene ether compounds

This report discusses the preparation and the unusual mesomorphism of three homologues of photopolymerisable triphenylene ether compounds. These homologues showed ‘a cold crystallisation’ on heating differential scanning calorimetry curves, and this phenomenon is attenuated with increase methylene units, spacers between the triphenylene core and the terminal photopolymerisable acrylate group. Classical textures of hexagonal columnar (Col_h) mesophase were observed by polarising optical microscopy for all three photopolymerisable mesophase compounds described here. Also described is the mesomorphism of their intermediates, hydroxylalkoxytriphenylenes. In some cases, the discotic columnar hexagonal mesophases were confirmed by wide angle X-ray scattering techniques.     

Particle size effect on the new phase transition in a tridymite compound, CsCoPO4

A new phase transition (III–IV) was found at 311 K in CsCoPO₄ by DSC measurements. The crystal structure of all the phases, I–IV, in CsCoPO₄ was studied by synchrotron-radiation X-ray powder diffraction. The diffractometry revealed that CsCoPO₄ had the same crystal structure as that of corresponding phases in CsZnPO₄. An extremely large particle size effect was found on III–IV phase transition in CsCoPO₄; the phase transition enthalpy decreases with decreasing the particle size around 0.1 mm. Such large particle size effects had been also observed in CsZnPO₄. However, no III–IV phase transition was observed in the particle smaller than 0.1 mm of CsZnPO₄, while such a critical size was not found in CsCoPO₄.     

Single crystal X-ray diffraction study of a mixed-valence gold compound, Cs2AuAuCl6 under high pressures up to 18 GPa: Pressure-induced phase transition coupled with gold valence transition

We performed the single-crystal X-ray diffraction study of a perovskite-type gold mixed-valence compound, Cs₂Au^IAu^IIICl₆, under high pressures up to 18 GPa by using a diamond-anvil-cell with helium gas as an ideal hydrostatic pressure-transmitting medium. The lattice parameters and the variable atomic positional parameters were obtained with reasonable accuracy at various pressures. A structural phase transition at ca. 12.5 GPa from I4/mmm to Pm3m was found. The lattice parameters a₀ and c₀, denoted in the tetragonal cell setting, result in the relationship 2^1/2a₀=c₀, and the superstructure reflections h k l (l is odd), caused by the shift of the Cl ions from the midpoint of the Au ions, disappeared at pressures above the phase transition. Both elongated [Au^IIICl₆] and compressed [Au^ICl₆] octahedra in the low-pressure phase smoothly approach regular octahedra with increasing pressure. Above the structural phase transition at 12.5 GPa, all the [AuCl₆] octahedra are crystallographically equivalent, which shows that the tetragonal-to-cubic phase transition accompanies the valence transition from the Au^I/Au^III mixed-valence state to the Au^II single-valence state.     

Isotropic-to-nematic phase transition in a liquid-crystal droplet

In this paper, we focus on the isotropic-to-nematic phase transition in a liquid-crystal droplet. We present the results of an experiment to measure the growth of the nematic phase within an isotropic phase liquid-crystal droplet. Experimentally, we observe two primary phase transition regimes. At short time scales, our experimental results (R(t) approximately t0.51) show good agreement with a Stefan-type model of the evolution of the nematic phase within the isotropic phase of a liquid crystal. As time progresses, the growth of the nematic phase is restricted by increased confinement of the droplet boundary. During this stage of growth, the nematic phase grows at a slower rate of R(t) approximately t0.31. The slower growth at later stages might be due to a variety of factors such as confinement-induced latent heat reduction; a change of defect strength during its evolution; or interactions between the defect and the interface between the liquid crystal and oil or between adjacent defects. The presence of two growth regimes is also consistent with the molecular simulations of Bradac et al. (Bradac, Z.; Kralj, S.; Zumer, S. Phys. Rev. E 2002, 65, 021705) who identify an early stage domain regime and a late stage confinement regime. For the domain and confinement regimes, Bradac et al. (Bradac, Z.; Kralj, S.; Zumer, S. Phys. Rev. E 2002, 65, 021705) obtained growth exponents of 0.49 +/- 0.05 and 0.25 +/- 0.05. These are remarkably close to the values 0.51 and 0.31 observed in our experiments.     

Origin of pressure-induced insulator-to-metal transition in the van der Waals compound FePS3 from first-principles calculations

Pressure-induced insulator-to-metal transition (IMT) has been studied in the van der Waals compound iron thiophosphate (FePS₃) using first-principles calculations within the periodic linear combination of atomic orbitals method with hybrid Hartree–Fock-DFT B3LYP functional. Our calculations reproduce correctly the IMT at ∼15 GPa, which is accompanied by a reduction of the unit cell volume and of the vdW gap. We found from the detailed analysis of the projected density of states that the 3p states of phosphorus atoms contribute significantly at the bottom of the conduction band. As a result, the collapse of the band gap occurs due to changes in the electronic structure of FePS₃ induced by relative displacements of phosphorus or sulfur atoms along the c-axis direction under pressure.