Remarkable Temperature Dependence Research Articles

ChemInform Abstract: Kinetics and Mechanism of Photoinduced and Thermal Proton‐Transfer Processes in o‐Hydroxybenzaldehyde and o‐Hydroxyacetophenone. A Remarkable Temperature Dependence of the Reaction Rate.

ChemInform Abstract: Kinetics and Mechanism of Photoinduced and Thermal Proton‐Transfer Processes in o‐Hydroxybenzaldehyde and o‐Hydroxyacetophenone. A Remarkable Temperature Dependence of the Reaction Rate.

IR Absorption and Raman spectroscopic study of reentrant-nematic liquid crystallineN-[4-(4-Octyloxy-benzoyloxy) Benzylidene]-4-Cyanoaniline (OBBC)

Abstract The IR absorption and Raman scattering of OBBC have been investigated in the solid, ReN, SmA and nematic phases. The intensity of the 2229 cm−1 band assigned to the C[tbnd]N stretching mode decreases with increasing temperature in the reentrant nematic phase; this is attributed to a change in the overlap of the molecules. While the IR bands at 1728 and 841 cm−1 also show a remarkable temperature dependence, Raman bands do not show significant temperature dependence in the liquid-crystal phases.

Kinetics and mechanism of photoinduced and thermal proton-transfer processes in o-hydroxybenzaldehyde and o-hydroxyacetophenone. A remarkable temperature dependence of the reaction rate

Time-resolved electronic emission and absorption spectroscopy of photoexcited solutions of o-hydroxybenzaldehyde (OHBA) and o-hydroxyacetophenone (OHAP) are reported. When these molecules are brought to their first excited single state an adiabatic excited-state phototautomerization reaction takes place and subsequently the tautomer in its lowest electronic triplet state and in its ground state may be observed as transient species. The return of the ground-state tautomer in solutions in aprotic solvents to the stable form of the molecule may be catalysed by concentrations of alcohols as small as 10–4 mol dm–3. The catalysis involves a protonation and a subsequent deprotonation step. The rate of the uncatalysed reaction in aprotic solvents exhibits a remarkable temperature dependence. The rate constant behaves in an Arrhenius manner, but with a negative activation energy in the high-temperature range and a normal positive activation energy in the low-temperature range. The explanation for this behaviour involves a fast unimolecular dynamic equilibrium between two forms of the ground-state tautomer. In one form there is an intramolecular hydrogen bond, and in the other form this bond is broken. Only the form with the hydrogen bond leads directly to the stable form of the molecule. In the other form of the tautomer the proton donor and acceptor sites are far apart as a consequence of rotational isomerization, which accompanies the breaking of the intramolecular hydrogen bond. It is shown that a dynamic equilibrium between two rotameric forms, as in the case of OHBA, may explain the small pre-exponential factors for the observed normal Arrhenius behaviour of a number of other intramolecular proton-transfer reactions.

Temperature and Pressure Dependence of Disaccommodation in a Manganese Zinc Ferrite Single Crystal

Disaccommodation in a Mn0.59Zn0.29Fe2.08O4 single crystal was studied in the temperature range between 270 and 300 K, and in the pressure range between 1 bar and 3 kbar. The obtained results were analyzed using a model in which the relaxation time τ is distributed with a spectrum of Gaussian type on a log τ scale. The activation energy determined from the temperature dependence of the relaxation time is 0.88 eV. The activation volume is about 2 Å3/atom and shows remarkable temperature dependence in the range studied.

Circular dichroism study of bacteriorhodopsin-lipid interaction

Delipidated bacteriorhodopsin purified from purple membrane of H. halobium was reconstituted with the circular dichroism active phospholipid. The observed circular dichroism spectra in the 450-700 nm region characteristic of bacteriorhodopsin showed the temperature dependence characterized by a midpoint at ca. 45 degrees C and this spectral change showed the disaggregation of bacteriorhodopsin trimer to monomer. The circular dichroism spectra in the 250-400 nm region characteristic of the azo chromophore of phospholipid exhibited a remarkable temperature dependence synchronized with the disaggregation of bacteriorhodopsin, suggesting that a large proportion of the phospholipid is present as boundary lipid.

Photon correlation spectroscopy in micellar solutions of sodium and potassium oleate

Photon correlation spectroscopy was applied to concentrated micellar solutions of sodium and potassium oleate. In rod-like micellar phases the relaxation times of the concentration fluctuation were measured as a function of temperature. A remarkable temperature dependence of the relaxation time was found in the isotropic phase of rod-like micelles. This suggests that the rotational diffusion is strongly reduced by the entanglement effect. The thermal expansion coefficient of rod-like micelles was determined.

Polarized Low Temperature Luminescence of Single Crystal Pentacarbonylpyridinotungsten(0)-Complexes

Polarized luminescence spectra of single crystal W (CO )5-X (with X = pyridine, 4-methylpyridine, 4-t-butylpyridine, 3,4-dimethylpyridine, 4-am inopyridine, 3-acetylpyridine, 3,5-dibromopyridine, 3-benzoylpyridine, 4-formylpyridine) at low temperatures (1.9 K≦ T ≦ 80 K) are reported. Depending on the nature of the pyridine substituent the emitting state is either a ligand field (LF) state or a metal to ligand charge transfer (CT) state. This results in drastic differences in both bandshape and temperature behaviour of the luminescence spectra. The CT-luminescence is structureless and is not influenced by temperature variation, whereas the LF-lum inescence spectra exhibit in most cases a distinct vibrational fine structure at low temperatures and show a remarkable temperature dependence. The emission spectra of single crystal W (CO )5-3,4-dim ethylpyridine and W (CO )5-pyridine can be strongly influenced by magnetic fields (H ≦ 6 T). The results are explained within a model assuming molecular C4v symmetry and regarding spin orbit coupling.

Temperature Dependence of the Far Infrared Active Lattice Modes of Solid Nitrous Oxide

Infrared active lattice modes of solid N 2 O were precisely measured in temperature range from 1.8 to 130K. Remarkable temperature dependence of the absorption bandwidths was obtained and explained by means of the perturbation theory of the anharmonic crystal. The results suggest ways in which the lattice vibration energy is dissipated. No information related to suggestion of the dipole ordering was found by the spectroscopic studies.

Acoustic mode Grüneisen parameters near the λ point of NH 4Cl and NH 4Br

Acoustic mode Grüneisen parameters of NH 4Cl and NH 4Br were calculated with the experimental values of second and third order elastic constants obtained by means of ultrasonic method near the λ phase transition temperature. It is found that in NH 4Cl longitudinal acoustic mode Grüneisen parameter depends on temperature, while in NH 4Br transverse acoustic mode Grüneisen parameters show remarkable temperature dependence. Elastic anharmonicity of NH 4Cl behaves quite differently from that of NH 4Br.

Luminescence of Bi 3+-activated LaOBr, a system with emission from different states

The luminescence properties of LaOBr-Bi 3+ are reported. Two luminescent centres are observed. These are ascribed to isolated Bi 3+ ions and to a second BiOBr-rich phase. The emission of the isolated Bi 3+ ion shows a remarkable temperature dependence. At low temperatures the emission is in the UV region. At higher temperatures the UV emission disappears and an emission in the visible region appears. At still higher temperatures the visible emission disappears and the UV emission reappears. The emission in the visible region is ascribed to a charge-transfer transition.

Soliton-Like Excitations and Electronic Structure in Quasi One-Dimensional Conductors. II. Numerical Calculations

Numerical calculations are performed for the soliton-like excitations proposed in the preceding paper around the estimated magnitudes of parameters. The antisymmetric soliton exists in the wide range of parameters but only in the limited range of δ Q / Q 0 which changes with temperature. The excitation energy and the displacement amplitude of this soliton show remarkable temperature dependence. The symmetric soliton can exist in the limited range of parameters and the above temperature dependence is more moderate than the antisymmetric soliton.

ESR- und ENDOR-Untersuchungen von p-Benzosemichinon-Diarylthallium-Ionenpaaren / ESR- and ENDO R-Investigations of p-Benzosemiquinone-Diarylthallium Ion Pairs

Diarylthallium hydroxides react very smoothly in organic solvents with hydroquinones to stable paramagnetic complexes. This reaction has been investigated with two ligands and several organometallic compounds in 8 different solvents. In basic solutions clearly two paramagnetic species can be detected by ESR spectroscopy. One component exhibits a large doublet splitting assigned to the thallium counterion and the second radical does not show any metal coupling. The relative concentrations of both radicals depend strongly on the solvent and the temperature. Therefore, a slow equilibrium betw een a contact ion pair and a solvent separated ion pair is assumed. The thermodynamic parameters are determined to ⊿H = 0.8 kcal/mol, ⊿G293 = -0.1 kcal/mol, ⊿S = 3.04 cl/mol. The proton and thallium coupling constants as well as the g-value of the contact ion pair show a rem arkable temperature dependence. The signs of proton splitting constants are determined by ENDOR-TRIPLE investigations. The change of the electronic structure of the radical anion by formation of a contact ion pair is discussed. For interpretation of these results we assumed a fast equilibrium between different conformations of the contact ion pair according to behaviour of comparable aroxyl radicals. Therefore, the systems investigated allow a simultaneous observation of solvent separated and different contact ion pairs.

Magnetic Susceptibilities of Liquid Na-Sn Alloys

The magnetic susceptibility of liquid Na-Sn alloys has been measured as a function of temperature and concentration. The curve of susceptibility isotherm exhibits a distinct minimum around the composition of Na 4 Sn 3 near the liquidus temperature. At the same composition, the remarkable temperature dependence has also been observed, and the minimum point in the susceptibility isotherm shifts from Na 4 Sn 3 to NaSn at higher temperatures. This large temperature variation has been explained in terms of the partial dissociation of the compound-forming liquid alloy.

Magnetic susceptibilities of liquid NaIVB alloys

The magnetic susceptibility of liquid NaIVb alloys has been measured as a function of temperature and concentration. The curve of susceptibility isotherm of liquid NaSn alloys exhibits a distinct minimum around the stoichiometric composition of Na 4Sn 3 at liquids temperature. At the same composition, a remarkable temperature dependence has also been observed, and the minimum composition in the susceptibility isotherm shifts from Na 4Sn 3 to NaSn at high temperature. This large temperature variation has been interpreted in terms of partial dissociation. While the curve of susceptibility isotherm of liquid NaPb shows a slightly negative deviation from the linearly interpolated line. The maximum deviations occurs at the composition of the Na 4Pb.

Viscoelastic properties of erythrocyte membranes in high-frequency electric fields

The high deformability of erythrocytes which is essential for their transport through the capillaries depends critically on their discoid shape and on the elasticity of the plasma membrane, which may be determined by interactions of the cytoskeleton, the lipid/protein leaflet and the glycocalyx. Although techniques exist for measurement of the static elastic properties of erythrocytes, the cells are continuously deformed in vivo, the stress varying within periods of a few seconds. Thus dynamic elastic behaviour is essential for their physiological function. We present here a novel means of measuring the dynamic elastic constants of the red cell based on the transient deformation of individual cells in an inhomogeneous high-frequency (HF) electric field. By microscopy it is possible to record cellular elongations as small as 200 nm occurring within time scales of 1 ms. A main advantage is that the cellular response is linear and thus can be more readily interpreted theoretically. We have observed a creep function consisting of two exponentials with response times of 0.1 s and 1 s, which can be described in terms of a simple viscoelastic model. A remarkable temperature dependence of the membrane elasticity between 25 degrees C and 15 degrees C is observed for freshly drawn cells but not for trypsinized ones.

Theoretical Study of Electrical Resistivity in Actinide Metals

A possible explanation is given for the anomalous electrical resistivities of Pu and PuAl 2 which show maxima at low temperatures where the susceptibilities show no remarkable temperature dependence. We study a mechanism in which the s-wave conduction electrons mixed with the f electrons by the s-f hybridization are scattered by the spin fluctuations of the f electrons via tlte f-f Coulomb interaction. The decrease in the resistivity at higher temperatures is due to the breakdown of the s-f hybridization (dehybridization) owing to the thermally enhanced spin fluctuations. The dehybridization effect and the temperature dependence of the resistivity strongly depend on the width and the structure of the f band. For favorable structure of the f band the calculated resistivity shows a maximum.

Positron annihilation as a probe of electronic density in the two-dimensional conductor 1T-TAS2

Tantalum disulfide is a layered compound with quasi-two-dimensional electronic conduction. The particular topology of the 2D-Fermi surface causes the onset of charge density waves which drive a metal-to-insulator phase transition in two steps. Three standard positron annihilation techniques have been used to sample the electronic density and momentum distributions in this compound as a function of temperature from 4 K to 400 K. The positron lifetime is 261(2)ps at 300 K (single component) and does not change very much with temperature. It increases to 293(2)ps in a sample containing 1% of irradiation induced defects. The anisotropy of the momentum distribution is clearly visible both in angular correlation and Doppler broadening experiments. This anisotropy is reduced but remains when positrons are trapped by irradiation defects. The momentum distributions change significantly with temperature, especially when the charge density wave phase transitions are crossed. An interpretation taking into account the possible trapping of positrons by charge density waves is proposed. The remarkable opposite temperature dependences of the positron data obtained in the in-layer and transverse directions are attributed to electron localization in a preferential orientation. Finally, an attempt is made to have access to the topology of the Fermi surface of this material by mean of the projected density of states along different axes. This procedure demonstrates that even with standard positron techniques, insight on an unusual Fermi surface can be obtained Emploi de trois techniques d'annihilation de positons pour souder la densite electronique et les distributions d'impulsion entre 4 et 400 K. Mise en evidence de l'anisotropie de la distribution d'impulsion dans les experiences de correlation angulaire ou d'elargissement Doppler, anisotropie qui diminue quand les positons sont pieges sur les defauts d'irradiation et varie de facon significative au passage des transitions de phase liees aux ondes de densite de charge. Variations thermiques remarquables et opposees selon la direction des feuillets ou perpendiculairement. Renseignements sur la surface de Fermi

ChemInform Abstract: SYNTHESIS OF UNSUBSTITUTED THIA CROWN ETHERS VIA INTRAMOLECULAR CYCLIZATION

AbstractDurch intramolekulare Cyclisierung der Diole (I) werden Thia‐15‐Krone‐5 (IIa) und Thia‐18‐Krone‐6 (IIb) dargestellt.

Temperature dependence of the internal bias in TGS-Alanine crystals near Tc

Abstract The temperature, driving field amplitude and frequency dependence of hysteresis loops of triglycine sulfate doped with L-alanine has been examined in the neighborhood of the Curie temperature, Tc. The magnitude of the internal bias associated with L-alanine content shows a remarkable temperature dependence near Tc, decreasing by more than two orders of magnitude within ±1°C of Tc. A semiquantitative interpretation of this behavior in terms of the interaction between orientable (glycine) dipoles and fixed (L-alanine) dipoles is given

Ion-induced high density excitation in ionic crystals

The intrinsic luminescence from alkali halides such as NaCl, KBr and KI under 500 keV to 2 MeV H + and He + ion bombardments has been measured at temperatures from 6 K to 100 K. It is found that the intrinsic luminescence yield per unit energy deposition decreases with the increase of the LET (linear energy transfer). The intrinsic luminescence fron NaCl and KBr induced by ion bombardments showed an anomalous temperature dependence. As the temperature increases, the yield of the ion-induced luminescence increases below the thermal quenching temperature, while no increase is observed for X-ray bombardments. The ion-induced luminescence yield for KI did not show this remarkable temperature dependence. The number of emitted photons per exciton is estimated to be less than ∼ 10 −3 and this value is much less than yield for X-ray bombardments. These phenomena do not depend on the intensity of the beam current. The LET and temperature effects on the intrinsic luminescence are ascribed to the enhancement of the non-radiative decay due to exciton-exciton interaction within an ion track.