Remarkable Temperature Dependence Research Articles

Electronic configuration of high-spin imidazole-ligated iron(II) octaethylporphyrinates.

The preparation and characterization of two new five-coordinate, imidazole-ligated, high-spin iron(II) octaethylporphyrinates is described. [Fe(OEP)(1,2-Me2Im)] and [Fe(OEP)(2-MeHIm)] have been characterized by X-ray structure determinations and temperature-dependent Mössbauer spectroscopy in zero and applied magnetic fields. The distinction between imidazole-ligated and other ligands in high-spin iron(II) porphyrinates, noted for a series of tetraarylporphyrinate derivatives (Hu, C.; Roth, A.; Ellison, M. K.; An, J.; Ellis, C. M.; Schiltz, C. E.; Scheidt, W. R. J. Am. Chem. Soc. 2005, 127, 5675), is seen here as well. The sign of the quadrupole-splitting constant is again negative, which is unique to the imidazole-ligated derivatives and suggests a distinct electronic structure. The derivatives again display a remarkable temperature dependence in the quadrupole splitting, which is also seen for deoxymyoglobin and -hemoglobin. Structural features for the two new derivatives are similar to those seen earlier, although the core conformations show somewhat more doming character.

Influence of Copolymer Configuration on the Phase Behavior of Ternary Blends

The influence of copolymer configuration on the phase behavior of various ternary polymer blends containing a crystallizable polyester, a noncrystallizable polyether, and an acrylic random copolymer of different chain configuration was investigated. In these ternary blends, the acrylic random copolymer is typically added to control rheological properties at elevated temperatures. In fact, the acrylic random copolymers composed of various compositions of MMA and nBMA were found to have different miscibility with polyester as well as polyether, leading to substantially different phase behavior of ternary blends. Remarkable temperature dependence was also found. The mean-field Flory-Huggins theory for the free energy of mixing, extended to ternary polymer blends, was adopted for predicting phase diagrams where the exact spinodal and binodal boundaries could be calculated. Phase diagrams of ternary blends, predicted by the Flory-Huggins formulations and related calculations, were in good agreement with experimental phase diagrams. The differences observed in the rheological processes of various ternary blends with different acrylic copolymers were directly related to changes in miscibility, associated phase behavior, and chain configuration.

Temperature dependence of magnetization in arrays of submicrometer Fe disks

We investigate the magnetization in the arrays of submicrometer-size disks fabricated from an epitaxial Fe layer grown on GaAs(001). The Fe disks having a layer thickness of 20nm undergo a transition between multiple and single magnetic domains when their diameter is about 100nm. Remarkable temperature dependence is found in the characteristics of the magnetization. (1) The saturation magnetization increases with decreasing temperature. While the low-temperature increase is more pronounced for smaller disk diameters, it is independent of the magnetic-domain state in the disks. (2) Magnetic hysteresis exhibits two-component-like behavior at low temperatures for ultrasmall disks. The additional component that emerges with lowering temperature gives rise to slow saturation of the magnetization, and is possibly responsible for the low-temperature increase of the saturation magnetization.

Remarkable temperature dependence of electric field-induced change in fluorescence spectra of pyrene doped in a polymer matrix

Electrofluorescence spectra as well as fluorescence spectra of pyrene doped in a polymer film have been measured at various temperatures in the region of 70–293 K. As the temperature decreases, fluorescence with a peak at 395 nm which gives a very different response to an applied electric field from other fluorescence emissions increases. Besides the structured monomer fluorescence with a peak at ∼380 nm and sandwich-type excimer fluorescence with a peak at ∼470 nm, three types of partially overlapped excimer fluorescence exist at low temperatures for pyrene doped in a PMMA film at high concentrations.

N,N,N-Tridentate iron(II) and vanadium(III) complexes

UV-Visible spectroscopic studies on methyl aluminoxane activated N,N,N-tridentate Iron(II) and Vanadium(III) complexes. The ethene oligomerization and polymerization activity of N , N , N -tridentate pyridine-diimine vanadium(III)- and iron(II)-complexes showed a remarkable time and temperature dependence after their activation with methyl aluminoxane (MAO). It was observed that the complex/MAO ratio and the time elapsed after MAO-addition to the catalyst precursor are critical factors for its overall performance. Different results were obtained from the same catalyst depending on the time that had passed from the time the catalyst was prepared until it was tested. Similarly. It was also found that an increase in reaction temperature decreased the catalytic activity. In order to investigate these observations, we employed UV–vis absorption spectroscopy on two representative vanadium(III)- and two iron(II)-based N , N , N -tridentate pyridine-diimine complexes activated with MAO. On time-scales of minutes to days, we have observed growths and decays of spectral band-systems, some of which are presumably related with chemical changes leading to active catalyst forms for ethene polymerization/oligomerization. Tentative explanations of the spectral changes that were observed for one of the vanadium systems under varying conditions of relative MAO concentrations are proposed. While the chemistry of the reactions of the complexes with MAO has still remained ill understood, the kinetic data based on spectral changes would be of empirical value to monitor and possibly correlate catalyst performances. This is the first glance into causes due to changes in the activated catalyst depending on concentration and time.

N,N,N-Tridentate iron(II) and vanadium(III) complexesPart III. UV–vis spectroscopic studies of reactions of ethene-oligomerization and polymerization catalysts with methyl aluminoxane cocatalyst

UV-Visible spectroscopic studies on methyl aluminoxane activated N,N,N-tridentate Iron(II) and Vanadium(III) complexes. The ethene oligomerization and polymerization activity of N , N , N -tridentate pyridine-diimine vanadium(III)- and iron(II)-complexes showed a remarkable time and temperature dependence after their activation with methyl aluminoxane (MAO). It was observed that the complex/MAO ratio and the time elapsed after MAO-addition to the catalyst precursor are critical factors for its overall performance. Different results were obtained from the same catalyst depending on the time that had passed from the time the catalyst was prepared until it was tested. Similarly. It was also found that an increase in reaction temperature decreased the catalytic activity. In order to investigate these observations, we employed UV–vis absorption spectroscopy on two representative vanadium(III)- and two iron(II)-based N , N , N -tridentate pyridine-diimine complexes activated with MAO. On time-scales of minutes to days, we have observed growths and decays of spectral band-systems, some of which are presumably related with chemical changes leading to active catalyst forms for ethene polymerization/oligomerization. Tentative explanations of the spectral changes that were observed for one of the vanadium systems under varying conditions of relative MAO concentrations are proposed. While the chemistry of the reactions of the complexes with MAO has still remained ill understood, the kinetic data based on spectral changes would be of empirical value to monitor and possibly correlate catalyst performances. This is the first glance into causes due to changes in the activated catalyst depending on concentration and time.

Catalytic Asymmetric Chlorination of β‐Keto Esters with Hypervalent Iodine Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Phase transition on (BEDO-TTF) (Cl2TCNQ): Spin dynamics observed by NMR measurements

13C NMR measurements were carried out to investigate the electronic state of Mixedstack organic charge-transfer complex (BEDO-TTF)(Cl 2 TCNQ). From the remarkable temperature dependence of the central shift and relaxation rate, we have confirmed that the transition at 110 K is from paramagnetic to nonmagnetic spin-singlet phase.

Catalytic Asymmetric Chlorination of β‐Keto Esters with Hypervalent Iodine Compounds

Abstract(Dichloroiodo)toluene (=dichloro(4‐methylphenyl)iodine; 2) was found to be a suitable chlorinating agent in the catalytic asymmetric chlorination of β‐keto esters 3 catalyzed by the [Ti(TADDOLato)] complex 1 (=bis(acetonitrile)dichloro[(4R,5R)‐2,2‐dimethyl‐α,α,α′,α′‐tetra(naphthalen‐1‐yl)‐1,3‐dioxolane‐4,5‐dimethanolato(2−)‐κO,κO′]titanium), whereby α‐chlorinated products were obtained in moderate to good yields and enantioselectivities of up to 71% (Scheme 2, Table 2). The enantioselectivity of the reaction shows a remarkable temperature dependence, the maximum selectivity being obtained at ca. 50°.

Perpendicular exchange bias and magnetic anisotropy in CoO/permalloy multilayers

For permalloy/CoO multilayers, after perpendicular field cooling, perpendicular exchange bias has been observed below room temperature. For out-of-plane hysteresis loops, the exchange field and the coercivity at fixed temperatures are proportional to ${1/t}_{\mathrm{FM}}$ and ${1/t}_{\mathrm{FM}}^{3/2}$ ${(t}_{\mathrm{FM}}=\mathrm{permalloy}$ layer thickness), respectively. At the same time, the remanent ratio increases with decreasing ${t}_{\mathrm{FM}}.$ The induced effective uniaxial anisotropy changes as a linear function of ${1/t}_{\mathrm{FM}},$ as a symbol of an interface anisotropy. The interface anisotropic energy increases with decreasing temperatures. For samples with small ${t}_{\mathrm{FM}},$ perpendicular anisotropy films can be established at low temperature. The established perpendicular magnetic anisotropy and its remarkable temperature dependence might facilitate perpendicular magnetic recording and magneto-optical storage techniques.

Interpretation of the temperature-dependent color of blue copper protein mutants

The electronic absorption spectrum of the mutant of the blue copper protein amicyanin with a pseudoazurin loop (AmiPse) shows a remarkable temperature dependence. The absorption band at ≈460 nm increases at low temperature while the transition at ≈600 nm is not much affected by a variation of the temperature. An approximate density functional theory (DFT) study of the active site model [CuII(imidazole)2(SCH3)(S(CH3)2)]+ (protein backbone and solvation neglected) leads to two local minimum structures (axial and rhomb) which both have a geometry close to that typical for blue copper proteins. One (rhomb) has two structurally different histidine donors, and this geometry is also found in most experimental type 1 structures. The two forms axial and rhomb are distortional isomers and are energetically almost degenerate. The temperature dependence of the spectrum of AmiPse is interpreted with a temperature-dependent change of the relative population of the two local minimum structures with slightly different energy. The 460 nm transition is believed to be due to preferential population of the structure rhomb; this is in agreement with the published assignment of the high energy transition, based on thorough spectroscopic and computational studies. Consequences of a perturbation of the “gas phase” structures axial and rhomb by the protein and solvation are also discussed on the basis of published, experimentally observed structures and spectroscopic data.

Effect of substrate temperature on microstructure and deuterium retention of molybdenum co-deposition with oxygen

The formation of co-deposition has a large impact on plasma surface interactions. It has been reported that co-deposition of Mo and O changed the surface properties of the TRIAM-1M tokamak and affected the confinement of the plasma. In this work, the influence of the substrate temperature on the crystallographic structure and hydrogen retention of the Mo–O co-deposition was studied by means of transmission electron microscopy and thermal desorption spectroscopy. Microstructures and hydrogen retention of the depositions showed a remarkable substrate temperature dependence. The deposition formed at high temperatures (>673 K) was found to be Fe 2MoO 4, which is the oxide of deposited elements and substrate elements. For the deposition formed and implanted at high temperatures (>573 K), the retention of deuterium is negligibly small. These results indicate that the retention of the hydrogen isotope is expected to become quite small by optimizing the wall temperature, even if the peculiar material is formed on the plasma-facing surface.

Bi 초전도 박막의 부착계수 해석

BSCCO thin films are fabricated by an ion beam sputtering method, and sticking coefficients of the respective elements are evaluated. The sticking coefficient of Bi element in BSCCO film formation was observed to show a unique temperature dependence; it was almost a constant value of 0.49 below about 730<TEX>$^{\circ}C$</TEX> and decreased linearly over about 730<TEX>$^{\circ}C$</TEX> In contrast, Sr and Ca, displayed no such remarkable temperature dependence. This behavior of the sticking coefficient was explained consistently on the basis of the evaporation and sublimation processes of Bi<TEX>$\sub$</TEX>2/O<TEX>$\sub$</TEX>3/. It was concluded that Bi(2212) thin film constructs from the partial melted Bi(2201) phase with the aid of the liquid phase of Bi<TEX>$\sub$</TEX>2/O<TEX>$\sub$</TEX>3/.

EPR study of the structural phase transitions in chromium doped thallium indium disulphide

The results of low-temperature electron paramagnetic resonance (EPR) investigations of Cr 3+ doped TlInS 2 single crystals in the temperature range 5–300 K are presented. Remarkable temperature dependences of the linewidth and resonance field of the main absorption peak in EPR spectra of Cr 3+ ion were observed. These peculiarities are attributed to the well-known successive phase transitions between 200 and 220 K. Considerable changes of EPR spectra were observed at the temperatures below 80 K, which are associated with strong line broadening, splitting, and appearance of additional peaks. Such transformations are considered as results of tetragonal distortions of InS 4 tetrahedra. An active group responsible for the previously reported phase transitions at the temperatures 216, 204 and 79 K has been determined.

Temperature-dependent electronic structure and magnetic stability of thin ferromagnetic films

We study correlation effects and temperature dependencies in the electronic structure of thin ferromagnetic local-moment films. In a first step the Kondo-lattice model is investigated as a candidate for a proper representation of local-moment ferromagnets. Magnetic and electronic key quantities as the Curie temperature and the quasiparticle density of states are derived with previously tested many-body procedures. It is shown that the magnetic properties can be interpreted exclusively in terms of the temperature-dependent electronic quasiparticle structure. An extended RKKY theory leads to effective Heisenberg exchange integrals, which turn out to be functionals of the conduction electron self-energy, getting therewith a remarkable temperature and band occupation dependence. In a second step, the model studies are combined with tight-binding-LMTO bandstructure calculations in order to get for real ferromagnetic films quasiparticle densities of states and quasiparticle bandstructures. The proposed method avoids the double-counting of relevant interactions and takes into account the correct symmetry of the atomic orbitals. Special results are given for thin ferromagnetic EuO (1 0 0) films. The Curie temperature T C of the EuO film turns out to be strongly thickness-dependent, starting from a very low value (≃15 K) for the monolayer and reaching the bulk value at about 30 layers. For a 20-layer film we predict the existence of a surface state, the temperature behaviour of which can lead to a surface halfmetal–insulator transition.

Synthesis of Novel Silicon-Containing Amphiphilic Diblock Copolymers and Their Self-Assembly Formation in Solution and at Air/Water Interface

Amphiphilic block copolymers, poly(1,1-dimethylsilacyclobutane-block-2-hydroxyethyl methacrylate) (poly(DMSB-b-HEMA)), poly(1,1-diethylsilacyclobutane-b-2-hydroxyethyl methacrylate) (poly-(DESB-b-HEMA)), and poly(1,1-dibutylsilacyclobutane-block-2-hydroxyethyl methacrylate) (poly(DBSB-b-HEMA), were synthesized by a living anionic polymerization, and their self-assembled structures were investigated. Small-angle X-ray scattering (SAXS) analysis suggested that the block copolymers formed micelles in methanol and that the aggregation number of the micelles increased with the increase of polysilacyclobutane (polySB) content and number of carbon atoms in the alkyl groups on silicon. A remarkable temperature dependence of the micelle structure of poly(DMSB-b-HEMA) was observed. Small-angle neutron scattering (SANS) revealed that the block copolymer formed spherical micelle with a core-shell structure above room temperature and a disklike micelle below room temperature. The surface pressure-area per molecule (π-A) isotherm of spread copolymer indicated that poly(SB-b-HEMA)s formed a thin layered film on the water surface. The π-A isotherm of the poly(DMSB-b-HEMA) spread layer also exhibited a characteristic temperature dependence. Direct X-ray reflectivity (XR) measurement on the water surface indicated that the block copolymers formed a monolayer with uniform thickness and smooth interfaces at medium surface pressure around 30 mN/m. In the case of poly(DBSB-b-HEMA), on the other hand, it was supposed that another layer with a large thickness appeared in the monolayer at a higher surface pressure than 35 mN/m. Atomic force microscopy (AFM) measurement of poly(DBSB-b-HEMA) film deposited on a glass plate at that surface pressure supported the existence of a thicker layer in addition to monolayer.

Molecular and Cellular Physiology of the Dissociation of Atrial Natriuretic Peptide from Guanylyl Cyclase A Receptors

Guanylyl cyclase subtype A (GCA) is the main receptor that mediates the effects of atrial natriuretic peptide (ANP) in the regulation of plasma volume and blood pressure. The dynamics of the dissociation of ANP from GCA were investigated in cultured Chinese hamster ovary (CHO) cells stably transfected with wild-type (WT) or mutant GCA receptors. The rate of dissociation of specifically bound (125)I-ANP-(1-28) from intact CHOGCAWT cells at 37 degrees C was extremely rapid (K(off) = 0.49 +/- 0.02 min(-1)), whereas in isolated membranes prepared from these cells, the dissociation at 37 degrees C was >10-fold slower (K(off) = 0.035 +/- 0.006 min(-1)). The dissociation of ANP from CHOGCAWT cells showed remarkable temperature dependence. Between 22 and 37 degrees C, K(off) increased approximately 8 times, whereas between 4 and 22 degrees C, it increased only 1.5 times. Total deletion of the cytoplasmic domain or of the catalytic guanylyl cyclase sequence within this domain abolished ANP-induced increases in cGMP, dramatically slowed receptor-ligand dissociation by at least 10-fold, and abolished the temperature dependence of the dissociation of ANP. Deletion of the kinase-like domain led to maximal constitutive activation of guanylyl cyclase, markedly decreased K(off) to 0.064 +/- 0.006 min(-1), and also abolished the temperature dependence of dissociation. Substitution of Ser(506) by Ala and particularly the double substitution of Gly(505) and Ser(506) by Ala within the kinase-like domain markedly reduced ANP-induced increases in cGMP, whereas K(off) decreased modestly (albeit significantly) to 0.36 +/- 0.03 and 0.24 +/- 0.02 min(-1), respectively. As a whole, the results demonstrate for the first time that temperature per se or ATP alone cannot account for rapid GCA receptor-ligand dissociation under physiological conditions and suggest that ligand dissociation is modulated in part by the interaction of still unidentified cytosolic factors with the cytoplasmic domain of GCA.

Viscous flow and jump dynamics in molecular supercooled liquids. II. Rotations.

The rotational dynamics of a supercooled model liquid of rigid A-B dumbbells interacting via a Lennard-Jones potential is investigated along one single isobar. The time-temperature superposition principle, one key prediction of mode-coupling theory (MCT), was studied for the orientational correlation functions C(l). In agreement with previous studies we found that the scaling of C(l) in a narrow region at long times is better at high-l values. However, on a wider time interval the scaling works fairly better at low-l values. Consistently, we observed the remarkable temperature dependence of the rotational correlation time tau(1) as a power law in T-T(c) over more than three orders of magnitude and the increasing deviations from that law on increasing l (T(c) is the MCT critical temperature). For 0.7<T<2, good agreement with the diffusion model is found. For lower temperatures the agreement becomes poorer, and the results are also only partially accounted for by the jump-rotation model. The angular Van Hove function shows that in this region a meaningful fraction of the sample reorientates by jumps of about 180 degrees. The distribution of the waiting times in the angular sites cuts exponentially at long times. At lower temperatures it decays at short times as t(xi-1), with xi=0.34+/-0.04 at T=0.5, in analogy with the translational case. The breakdown of the Debye-Stokes-Einstein relation is observed at lower temperatures, where the rotational correlation times diverge more weakly than the viscosity.

Meta–Insulator Phenomena in Strongly Correlated Oxides. The Vacancy-Doped Titanate Perovksites, Nd1−xTiO3 and Sm1−xTiO3

Electrical transport, both resistivity and thermopower, and heat capacity data are reported for two systems of cation vacancy-doped titanate perovskites, Nd1−xTiO3 and Sm1−xTiO3. In the former case the range of x is from 0.33 to 0.00 and for the latter from 0.17 to 0.00. Thus, the nominal carrier concentration, n, can be varied from 0 (x=0.33) to 1.0 (x=0.00) electrons per Ti atom. For the Nd series two distinct metal–insulator transitions (MIT) are seen, one obtained by electron doping the charge-transfer insulator, CTI (x=0.33), which occurs at x∼ 0.20, and the other by hole doping the Mott–Hubbard insulator, MHI (x=0.00), which is found near x∼0.10. Beginning near the CTI composition, x=0.30, evidence is found for variable range hopping from both resistivity and thermopower data. Mott metallization occurs near x=0.20 and Fermi-liquid (FL) behavior (T2 dependence of the resistivity) is seen between x=0.20 and 0.10. The coefficient of the T2 term increases with decreasing x (increasing n) and attains a large value, 3.0×10−8 Ω-cmK−2, for x∼0.12, which is more than 10 times larger than values seen in other titanate systems. Correspondingly large values for the heat capacity γ are observed. The x=0.10 sample shows a remarkable resistivity temperature dependence with a maximum at ∼175 K and minimum at ∼50 K followed by an upturn at the lowest temperatures. This is similar to behavior seen for Kondo or valence-fluctuating materials and is qualitatively consistent with existing theory, which predicts a Kondo-like transition state between the FL and MHI regimes. The x=0.05 and ∼0.00 are hole-doped antiferromagnetic semiconductors but thermopower data for the former shows a change from p- to n-type carriers just near 300 K. Sm1−xTiO3 is even more unusual as only the p-type semiconducting and the Kondo-like conducting regimes are seen. Comparisons are drawn with related Ln1−xAxTiO3 and Ln1−x TiO3 series and the role of correlation is emphasized.

Ortho–para conversion in solid hydrogen catalyzed by O 2 impurities

The ortho–para conversion in solid H 2 doped with less than 0.1% of O 2 impurities is studied by pulsed NMR between 1.5 and 10.9 K. The rate of O 2-catalyzed conversion (CC) reveals remarkable temperature dependence with three distinct regions. For T>8 K the CC rate is almost temperature independent and is limited by the conversion time τ cc =10.5 s of the ortho-H 2 neighbors nearest to an O 2 impurity. Within 4.6<T<7 K the ortho-H 2 diffusion (proceeding, most probably, via tunneling to nearby vacancy positions) determines the CC rate, resulting in an activation-type variation with temperature. However, at T<4 K the temperature dependence of the rate becomes much weaker again, and that may be ascribed to another mechanism of ortho-H 2 diffusion.