bluuebunny/crossref_metadata_2025 huggingface dataset:
Intramolekulare Proteindynamik untersucht mit zeitaufgelöster Fourier Transform Infrarot‐Differenzspektroskopie
Authors: Gerwert Klaus
Published: March 2014

The intramolecular reactions of membrane proteins are investigated by the new developed time resolved FTIR difference spectroscopy technique. By this technique the kinetics of single groups in membrane proteins can be determined. As example the protonation kinetics of aspartic acids are shown. Simultaneously the absorbance change in the visible spectral region is measured at one wavelength. Thereby the IR‐Absorbance changes can be correlated to changes in the visible spectral region. The technique is applied to the lightdriven proton pump bacteriorhodopsin and to crystals of the photosynthetic reaction center of Rps. viridis.
Modulation of Gap Junctional Coupling in Pairs of Pancreatic Acinar Cells by cAMP, OAG and Protein Kinase C
Authors: Somogyi R. , Kolb H.‐A.
Published: March 2014

Junctional coupling was studied in murine pancreatic acinar cells using the double whole cell patch clamp method. Independent of the electrolyte composition of the pipette solution, cell pairs spontaneously uncoupled. Addition of 5 mM ATP and 0.1 mM cAMP to the pipette electrolyte was sufficient to stabilize electrical coupling within an observation time of over one hour. At stable cell‐to‐cell‐coupling superfusion of a cell pair by 1‐oleyl‐2‐acetylglycerol (OAG) containing bath solutions induced a progressive decrease of coupling after a delay of about 10 minutes. Under these conditions stepwise changes of the junctional conductance of about 45 pS and 90 pS could be identified. Similarity, addition of protein kinase C to the pipette solution reduced junctional coupling. An antagonistic effect of protein kinase A and protein kinase C dependent processes on the permeability of gap junctions is discussed.
The Helix‐M2 Model of the Ion Channel of the Nicotinic Acetylcholine Receptor
Authors: Hucho F.
Published: March 2014

Experimental data describing the ion channel which is part of the acetylcholine receptor are reviewed. The Helix M 2 model is discussed, which proposes that the channel wall is formed by homologous sequences of receptor subunits.
The Response of the Limulus Photoreceptor Cell to the Absorption of Single Photons
Authors: Stieve Hennig
Published: March 2014

The response of the visual cell of the horseshoe crab Limulus to light stimuli is described. Due to the absorption of a single photon, a rhodopsin molecule is light‐activated and starts a sequence of causal steps which lead to the generation of a relatively large elementary excitatory response, the “bump”. A bump is based on the concerted transient opening of a large number of sodium ion‐preferring channels through the cell membrane. The bump generating mechanism is discussed. – The “macroscopic” light response to higher stimulus intensities is believed to consist of a fused volley of bumps. Its intensity dependence is discussed. A conditioning pre‐illumination usually desensitizes the visual cell transiently. However, very weak pre‐illumination may cause an increase in sensitivity, “facilitation”.
Fluoreszenzkinetische Untersuchungen zum Mechanismus der cholinergen Reizung
Authors: Maelicke Alfred , Prinz Heino , Covarrubias Manual
Published: March 2014

Based on equilibrium binding and stopped‐flow kinetic studies with the fluorescent cholinergic agonist NBD‐5‐acylcholine and acetylcholine receptor from electrocytes of Electrophorus electricus or Torpedo marmorata, we have developed a minimal model of cholinergic excitation. During the initial phase of association, the two agonist binding sites per receptor monomer are rapidly filled. The resulting state of low agonist affinity of fully occupied receptor is “activatable” in the sense that low energy opening events of the receptor integral cation channel may occur. The “activatable” state is lost either by dissociation of agonist or, in the continuous presence of agonist, by conformational transitions to state(s) of high agonist affinity. As such conformational transitions are also induced in the presence of cholinergic antagonists, the existence of ligand‐induced conformational transitions does not suffice by itself to explain the phenomenon of (agonist‐specific) desensitization. Our data are in contrast to previously proposed specific allosteric models of ligand interaction of the receptor.
Electrically and Chemically Gated Ion Channels in Skeletal Muscle
Authors: Neumcke B. , Lönnendonker U. , Woll K. H.
Published: March 2014

The surface membrane of skeletal muscle fibres contains ionic channels which are controlled by the electric field and regulated by intracellular ions. In patch clamp experiments on adult frog and mouse muscle fibres we have identified the following types of channels: anion‐selective channels, Ca‐activated K channels and ATP‐sensitive K channels. Some properties of these ion channels are described and possible physiological functions are discussed.
On the Structure of the ATP‐Synthase from Chloroplasts
Authors: Boekema Egbert J. , Fromme Petra , Gräber Peter
Published: March 2014

The H+‐translocating ATPase (“ATP‐synthase, CF0F1) was isolated from chloroplasts and characterized biochemically and by high resolution electron microscopy. The ATP‐synthase has a hydrophilic part, CF1, and a hydrophobic part, CF0, with subunit composition I, II, III,12, IV. The top view projection of CF1 (i.e. vertical to the membrane) reveals a pseudohexagonal arrangement of the large subunits α3β3, which ara arranged in two layers. The α‐ and β‐subunits are connected by a stalk with the membrane‐integrated CF0 part. The shape and dimensions of CF0F1 are given and a model for the subunit arrangement is described.
Electric Field‐Driven ATP Synthesis Catalyzed by the Membrane‐Bound ATP‐Synthase from Chloroplasts
Authors: Bauermeister Hans , Schlodder Eberhard , Gräber Peter
Published: March 2014

When chloroplast vesicles, suspended in a conducting aqueous medium, are exposed to an external electric field pulse, the membrane‐bound ATP‐synthase can catalyze ATP synthesis similar to energization of the membrane by light pulses. Inhibitors which bind to the enzyme inhibit light‐induced ATP synthesis and external field‐induced ATP synthesis at the same concentration. With uncouplers, which increase the ion permeability of the membrane, a higher concentration is necessary to inhibit electric field‐induced ATP synthesis. This is due to the faster charging of the membrane capacity by an external electric field. The enzyme can exist in an oxidized and a reduced state. Both forms can catalyze field driven ATP synthesis, and the rates depend in a sigmoidal way on the field strength. For the same rate of ATP synthesis, the reduced form requires a lower field strength than the oxidized form.
Chromophore Location and Charge Displacement in Bacteriorhodopsin
Authors: Heyn M. P. , Braun D. , Dencher N. A. , Fahr A. , Holz M. , Lindau M. , Seiff F. , Wallat I. , Westerhausen J.
Published: March 2014

Using three selectively deuterated retinals, the in‐plane structure of the chromophore of bacteriorhodopsin was determined by neutron diffraction. The time‐resolved photovoltage of bacteriorhodopsin was analysed on the basis of distributed kinetics indicating the presence of three major phases in the charge displacement with time constants of 32 μs, 890 μs and 18 ms at 25°C, pH 7. Sixty to seventy percent of the relative charge displacement occurred in the slowest step. The current‐voltage characteristic of bacteriorhodopsin incorporated in planar lipid bilayers was measured and found to be distinctly non‐linear. Between −80 mV and +180 mV it can be approximated by a single exponential plus a constant. This behaviour can be explained in terms of a barrier model for the pump with the slowest step dominating the photocurrent and its voltage dependence. Combining the information from the current‐voltage dependence with the kinetic information, a stoichiometry of one proton pumped per bR cycling is obtained. There is no evidence for a reversal potential down to −160 mV.
Elementary Chemical Reactions in Gases and Liquids
Authors: Troe J.
Published: March 2014

This review serves as an introduction to the main topic of the Bunsentagung 1990. Representative examples of elementary chemical reactions in gases and in the gas‐liquid transition range are discussed and illustrated with respect to their temperature, pressure, and solvent dependences.
H + H2: The Current Status
Authors: Buchenau H. , Toennies J. P. , Arnold J. , Wolfrum J.
Published: March 2014

Results of experimental investigations on the thermal rate constants and the microscopic dynamics of the hydrogen exchange reaction, including the isotopic variants and the spectroscopy of the H3‐complex, are compared with recent predictions based on quasi‐classical, approximate and exact quantum, and variational transition state theory calculations.
Kinetics of Ion‐Molecule Reactions in Gases: Equilibrium and Nonequilibrium Theory with Applications
Authors: Stiller W. , Schuster R.
Published: March 2014

For calculating rate coefficients of ion‐molecule reactions the formalism of kinetic theory of weakly ionized diluted gaseous plasma is used. – Equilibrium rate coefficients of proton transfer reactions between ions and polar molecules show a weakly temperature‐dependent Arrhenius‐like behaviour. – Analogous nonequilibrium rate coefficients give a reaction‐induced kinetic isotope effect in reactions between small ions (H−, D−) with polar molecules (CH3CHO) in heavy carrier gases (xenon).
Reaktionen von H‐Atomen mit Cycloalkenen und 1‐Olefinen
Authors: Scherzer K. , Gebhardt J. , Olzmann M.
Published: March 2014

The reactions of hydrogen atoms with the cycloalkenes C5 to C8 and the n‐alkenes C2 to C8 have been studied at room‐temperature in a flow system. In the presence of an excess of H‐atoms two steps of chemical activation occur: 1) the addition of H‐atoms to the double bond and ii) the combination of the radicals formed with H‐atoms. The main processes are the stabilization (S) of the activated intermediates by collisions and the decomposition (D) via ring opening and degradation. It is shown that the ratio D/S decreases with increasing concentration and size of the hydrocarbon. To calculate the ratios D/S for the first and the second activation step the classical formalism for chemically activated systems is extended. A method based on the coupling of two conventional steady‐state master equations is presented to evaluate the contribution of either of the two reaction paths. It is shown that the second way, via activated alkanes, is more important under these conditions.
Formation of Solvated Ions by Electron Capture in van der Waals Aggregates
Authors: Oster Tatiana , Illenberger Eugen
Published: March 2014

We report on the formation of solvated anions in beams of molecular aggregates by resonant capture of low‐energy electrons. The initially generated negative ion can be effectively deactivated in the cluster environment (“built‐in 3‐body collision”). By this means anions which are normally inaccessible from the ground neutral state can be stabilized by intermolecular energy transfer. In these experiments halide ions solvated by H2O or CO2 are produced from mixed clusters of these molecules and CF3Cl or C2Cl4. The appearance of such products is found to depend strongly on the electronic state of the precursor anion.
Reactivity and Electronic Structures of Isolated Clusters
Authors: Rademann Klaus
Published: March 2014

Some aspects of recent advances in the study of isolated clusters and their size dependent properties are reviewed. Reliable techniques for cluster generation and detection are briefly discussed. Methods for studying physical properties and electronic structures of isolated clusters and cluster ions are addressed in the context of relevant fundamental questions. Progress concerned with the exploration of solvated organic clusters, the interrogation of transition metal cluster reactivities and the investigation of electronic structures of metal clusters is documented by selected examples. In some cases, relations between isolated cluster properties and condensed phase behaviour are indicated.
Elementary Reactions of NH(a1Δ) with Saturated Hydrocarbons in the Gas Phase
Authors: Hack W. , Rathmann K.
Published: March 2014

The elementary reaction of NH(a1Δ) with the saturated hydrocarbons:

was studied for R = CH4, C2H6, C3,H8, and n‐C4H10 at room temperature in the gas phase at low pressure (10 < p/mbar ≤ 40). NH(a) was produced by HN3 laser photolysis (λL = 248 nm) and detected by laser induced fluorescence. Time resolution was obtained from the variable delay between the photolysis pulse and the detecting laser pulse. – The rate constants (in cm3/mol s) were found to be:

independent of pressure in the above given range. – The product NH2(X̃) and the quenching pathway (NH(X)) were followed directly and quantitatively by laser induced fluorescence. The quenching pathway was found to be unimportant([NH(X)]α /[NH(a)]0 < 1%) for all alkanes. The fraction of NH2(X̃) formed, {[NH2(X̃)]α/[NH(a)]0} decreases with increasing chain length from (CH4) = 1.0; (C2H6) = (0.34 ± 0.06); C3H8 = (0.20 ± 0.02) to n‐C4H10 = (0.07 ± 0.02). These ratios did not change with pressure (10 ≤ p/mbar ≤ 40). – The increase in the rate constants for the reactions with H–(CH2)n – H with increasing n (1 ≤ n ≤ 4) and the decrease in the NH2 yield demonstrate that NH2 is not formed by abstraction but by an insertion mechanism.
Analysis of Radicals from Flames by Scavenging with Dimethyl Disulfide
Authors: Hausmann M. , Homann K. H.
Published: March 2014

Larger hydrocarbon radicals in flames, which are difficult to detect by optical methods, can be analyzed quantitatively by radical scavenging with dimethyl disulfide in the condensed phase. – Gas samples taken from a sooting ethyne/oxygen flame by a nozzle beam were frozen at liquid‐N2‐temperature on the inner surface of a hollow sphere simultaneously with the radical scavenger dimethyl disulfide. The latter was admitted in large excess as a gas and was condensed uniformly on the inner wall of the sphere. The mixture of condensable stable flame products and scavenging products in the excess CH3SSCH3 was separated and identified by GC/MS. – Larger hydrocarbons (CxHy, x ≥ 6) condensed quantitatively. H, CH3, C4H3 and the phenyl radical were scavenged as mono(methylthio) compounds; C2 and the carbenes CH2, C3H2, C5H2 yielded bis(methylthio) compounds; other bis(methylthio) compounds observed were C2nH2(SCH3)2 with n = 1, 2, 3 and 4. CH, C3H and C2H3 were identified by the respective tris(methylthio) compounds. Any consecutive reactions of radicals with stable flame products and their recombinations in the cold trap were suppressed by their scavenging reaction.
Energy and Charge Transfer in Clusters of Organic Molecules Following Resonant Electron Capture
Authors: Hashemi Reza , Kühn Alexander , Lotter Johannes , Illenberger Eugen
Published: March 2014

Negative ion formation following resonant electron capture by molecules and molecular aggregates is studied in beam experiments for the prototype systems CF3Cl, CF4 and C2F4. Analysis of the excess kinetic energy of the reaction products is performed in order to observe the change in decomposition dynamics on proceeding from single molecules to clusters. It is shown that electron capture by aggregates may ultimately generate monomeric anions in their relaxed geometries not accessible in the isolated compound. In some cases a relaxation energy of several eV has to be distributed among the target cluster. Fragment ions formed with remarkably high translational energy are effectively slowed down when arising from aggregates at the same attachment energy. Clusters of tetrafluoroethylene show an additional resonance at low attachment energies which exclusively decomposes into (C2F4)−n, n ≥ 2.
Gas‐Phase Chemistry of Isolated Metal Ions: Are Predictions on Catalytic Properties Possible Based on the Study of Simple Model Reactions?
Authors: Eller Karsten , Schwarz Helmut
Published: March 2014

The reactions of Ti+ through Zn+ with isopropyl isocyanate have been studied using Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. Cr+ –Zn+, in analogy to simple pyrolysis, afford only loss of propene and HNCO while Ti+ and V+ mainly give rise to a combined decarbonylation/demethanation to form acetonitrile complexes. The latter is strictly analogous to the heterogeneously catalyzed reaction of i‐PrNCO on Nix/Cα which presumably proceeds via surface‐bound nitrenes. Deuterium labeling is used to show that in the gas phase nitrenes are most likely also involved as reaction intermediates. It is proposed that the binding energies (BE's) of the metal ions to the intermediate nitrenes determine the course of the reaction as only metal ions with high D0(RN = M+) binding energies (Ti+, V+) arc able to produce CH3CN. Transcribed into heterogeneous catalysis this would imply that catalysts with high BE's to surface nitrenes should work out best in the formation of acetonitrile.
Activation Energies of the Proton‐exchange Reactions in Water Measured with the 1H‐NMR Spin Echo Technique
Authors: Pfeifer R. , Hertz H. G.
Published: March 2014

From the pH‐dependence of the transverse relaxation‐time T2 of protons in 17O‐enriched water we have determined the rate constants k1 und k2 of the two proton‐exchange processes (I) and (II)

magnified image

in the temperature range from +5°C to +60°C. – Modified Bloch equations including exchange between the different states of the proton‐septett are used to fit the experimental data yielding directly the rate constants k1 and k2. Classical and quantum mechanical relations connecting the limiting ionic equivalent conductivities λH+ and λOH− with the rate constants k1 and k2 are discussed.
Multiphoton Dissociation of Organic Molecules: Step by Step Investigation with Laser Tandem Mass Spectrometry
Authors: Boesl U. , Weinkauf R. , Walter K. , Weickhardt C. , Schlag E. W.
Published: March 2014

Multiphoton ionization of molecules is connected with strong multiphoton dissociation of the molecular ions for high enough laser intensities. By varying the laser intensity, the degree of dissociation can be tuned from very soft (no fragmentation) to extremely hard. In earlier work we proposed and proved the “ladder‐switching” model to explain this process for excitation with nanosecond laser pulses. Recently, we developed a new laser tandem mass spectrometric technique which allows to investigate the fragmentation paths due to the “ladder‐switching” mechanism step by step. We applied this method to benzene cations and could verify the proposed tree of fragmentation pathways. Also a wavelength dependence of the fragmentation pattern has been detected and explained. Our new technique includes (i) a two stage ion source with laser ionization and second order space focus, (ii) secondary laser excitation and dissociation in this space focus and (iii) a reflectron time‐of‐flight analyzer.
Kinetik der Gasphasenreaktion NH2 + NO im Temperaturbereich 294–1025 K
Authors: Atakan B. , Wolfrum J. , Weller R.
Published: March 2014

Absolute rate coefficients for the reaction NH2 + NO were measured by time resolved observation of the OH radical formation at temperatures 294–1025 K in a heatable quartz reactor by means of the laser photolysis/laser induced fluorescence technique. The data can be represented by the expression: k1 = (7.9 ± 1.0) · 1015 T−(1.17 ± 0.25) cm3/mol s. The branching ratio β into the OH product channel was determined to be 0.1 ± 0.025 at 300 K; β increases with increasing temperature and reaches a value of 0.19 ± 0.05 at 1000 K.
Quenching of I(2P1/2) by O3
Authors: De Vöhringer Cecilia M. , Badini Raúl G. , Argüello Gustavo A. , Staricco Eduardo H.
Published: March 2014

The quenching of the spin‐orbit excited state of iodine atoms (I*) by ozone was measured in a flash photolysis system monitoring the resonant absorption of I* at 206.2 nm. A quenching rate constant of (1.1 ± 0.1) · 10−11 cm3 s−1 was obtained at 300 K. This high value implies a reactive channel leading to the production of IO + O2. Indirect experimental observations confirm the presence of the latter species after the flash. A comparison of the present results with the reaction involving the fundamental state of atomic iodine is made. A strong spin‐orbit effect is found. The rate constant for the quenching by O2 was also measured yielding a value of (2.7 ± 0.3) · 10−11 cm3 s−1.
LIF‐Measurements of the Dynamics of the Reactions HT (2.57 eV) + NO → OH + N and HT (1.86 eV) + O2 → OH + O
Authors: Jacobs A. , Schuler F. M. , Volpp H. R. , Wahl M. , Wolfrum J.
Published: March 2014

Translationally “hot” H atoms from photolysis of HBr and HCl at 193 nm, were used to investigate the reaction dynamics of H + NO → OH + N and H + O2 → OH + O. Nascent OH rotational and fine structure distributions were measured by LIF. For both reactions, a high degree of rotational excitation was found. Spin‐orbit states Π1/2 and Π3/2 were found to be equally populated. The lambda‐doublet distributions show a preference for the symmetrical OH‐Π:(A′) component. With a calibration method using H2O2 photolysis as a well defined source of OH radicals, absolute reactive cross sections at translational energies of 2.57 eV and 1.86 eV, respectively, could be determined: σH + NO = (0.01 ± 0.005) Å2 and σH + O2 = (0.34 ± 0.20) Å2.
The Reactions of the Cyclic Hydrocarbon Radicals c‐C3H5, c‐C5H9, and c‐C6H11 with O Atoms in the Gas Phase
Authors: Heinemann‐Fiedler P. , Hoyermann K. , Rohde G.
Published: March 2014

The production of the cyclic hydrocarbon radicals c‐C3H5, c‐C5H9, and c‐C6H11 by the reactions of c‐C3H6, c‐C5H10, and c‐C6H12 with F atoms and the reactions of the cyclic hydrocarbon radicals with O atoms were studied at low pressure (around 1 mbar) and low temperature (290–300 K) using the discharge flow method. Stable and labile species were detected by mass spectrometry combined with molecular beam sampling. The rate constants of the reactions c‐C3H5 + O (4), c‐C5H9 + O (5), and c‐C6H11 + O (6) were determined: k4 = 1.4·1014, k5 = 1.7 · 1014, k6 = 1.4 · 1014 cm3/mol · s. One main reaction route is interpretated by the formation of a highly excited adduct, decomposing to H + oxygen containing ring products; at high strain energy (reaction (4)) total ring cleavage, at low strain energy (reaction (6)) partial ring cleavage was observed. A second reaction route leads to OH + alkene. The rate constant for the reaction F + c‐C6H12 (3) was found as k3 = 5.7 · 1013 cm3/mol · s.
Comparison of the Reactions of Mono‐ and Polycyclic Aromatic Hydrocarbons with Oxygen Atoms
Authors: Frerichs H. , Tappe M. , Wagner H. Gg.
Published: March 2014

Newly obtained rate data for the reactions of the polycyclic aromatics biphenyl, methylnaphthalenes and phenanthrene with O (3P) are compared with the results of previous measurements for monocyclic aromatic hydrocarbons. These reactions were studied in an isothermal low pressure discharge flow system with mass spectrometric detection in the temperature range 298 K to 870 K, in an excess of O atoms over hydrocarbon. The obtained Arrhenius expressions and especially the activation energies for the reactions of mono‐ and polycyclic aromatic hydrocarbons with O atoms fit together well and strongly support as main reaction channel the addition of the electrophilic species to the aromatic ring
Interferometric Determination of the Rate Constant of the Isomerization of Cyclopropane to Propene in the Temperature Range between 690 K and 750 K
Authors: Hohm Uwe , Kerl Klaus
Published: March 2014

The thermally induced isomerization of cyclopropane (C) to propene (P) was studied for the first time with an interferometric technique, using HeNe‐laser radiation of vacuum‐wavelength λ = 632.99 nm. The investigations have been performed in the temperature and pressure ranges between 690 K and 750 K and between 7 · 104 Pa and 9 · 104 Pa, respectively. By recording the isothermal and nearly isobaric increase of the refractive index nM (t) and therefore of the polarizability αM (t) = xC(t)αC + xP(t)αP of the mixture with increasing reaction time t, one can obtain the mole fraction xC(t) = (αM(t) – αP)/(αC – αP) = exp(–k ∞ t) of cyclopropane. Assuming that the high pressure limit of this reaction is already achieved, a usual Arrhenius plot ln(kx/s−1) = ln(A/s−1) – EA/RT with the high‐pressure rate constant kx yields for the frequency factor ln(A/s−1) = 34.32(60) and for the activation energy EA = 267.5(3.6) kJmol−1. These values, also somewhat smaller, are in fair agreement with data published in the literature.
Influence of the Density of Surface Hydroxyl Groups on TiO2 Photocatalytic Activities
Authors: Kobayakawa Koichi , Nakazawa Yoshitake , Ikeda Masato , Sato Yuichi , Fujishima Akira
Published: March 2014

The photocatalytic activities of titanium dioxide for the reduction of silver ions and the oxidation of isopropanol were evaluated by using titanium dioxide with the various amount of surface hydroxyl groups per unit surface area. The photocatalytic activities increased with increasing concentration of the surface hydroxyl groups on the catalyst and then decreased from an inflection point. The mechanism was discussed from the stand point of the surface hydroxyl radical formation and their interaction.
Thermodynamic Studies of the Absorption of Hydrogen by Pd – Ti(Zr) Solid Solution Alloys
Authors: Sakamoto Y. , Miyagawa U. , Hamamoto E. , Chen F. L. , Flanagan T. B. , McNicholl R.‐A.
Published: March 2014

The solubility of hydrogen in Pd – Ti and Pd – Zr solid solution alloys containing up to 6.0 at.% Ti and Zr has been measured at temperatures between 273 K and 433 K and hydrogen pressures up to 1000 Torr. The relative partial molar enthalpy, ΔH0H, at infinite dilution for Pd – Ti alloys shows a minor minimum around 4 at.% Ti, in spite of the Pd lattice contraction on Ti substitution. For Pd – Zr alloys the ΔH0Hvalues increase in exothermicity regularly with Zr content. The relative partial molar entropy, ΔS0H, at infinite dilution in both alloy series decreases with the solute contents. The apparent H‐H attractive interactions,g1, in Pd – Ti(Zr) alloys decrease with alloying metal content, and the trend of theg1value with Ti content is dissimilar to those found for other lattice contracted alloys. The stability of the β‐phase hydride in Pd – Ti alloys decreases with Ti content and the β‐phase hydride in Pd – Zr alloys also becomes less stable with Zr content, albeit by a small amount, despite being a lattice expanded alloy system. Solvus compositions at the αmaxphase boundary in both Pd – Ti and Pd – Zr alloys are almost equal at the same alloying compositions of each alloy series, although increasing with alloying metal content in both cases.
Copper Catalyzed Aryl Dehalogenation Reactions and their Inhibition
Authors: Lippert Th. , Wokaun A. , Lenoir D.
Published: March 2014

The aryl coupling reaction of halobenzenes over alumina‐supported copper catalysts has been studied by FTIR spectroscopy. Time‐dependent changes in the spectra were recorded after stepwise addition of bromobenzene to the carrier gas stream. The coupling product, biphenyl, as produced by the copper‐catalyzed “Ullmann type I” reaction, as well as phenol and phenolate species, have been observed. It was found that the thermal pretreatment of the supported catalyst has a pronounced influence on the rate of the catalytic bromobenzene conversion, and on the distribution of side products observed. Novel approaches to inhibit the catalytic dehydrogenation of aryl halides by ethanolamine addition to the catalyst have been studied for the investigated model reaction. The inhibitory action is due to both site blocking and irreversible deactivation of the copper surface due to nitride formation.
Ultrasonic Attenuation of an Isobutyric Acid/Water Mixture of Critical Composition
Authors: Belkoura L. , Calenbuhr V. , Müller‐Kirschbaum Th. , Woermann D.
Published: March 2014

Ultrasonic attenuation data (frequency range 10 MHz < f < 45 MHz) of a binary isobutyric acid(COOH)/H2O mixture and a pseudo‐binary isobutyric acid(COOH)/H2O, D2O mixture of critical composition are analyzed in terms of the Ferrell‐Bhattacharjee attenuation function F(Ω). The system specific parameter ω0 necessary to calculate the temperature and frequency dependence of the universal reduced variable Ω is determined by static and dynamic light scattering experiments. It is found that F(Ω) fails to describe the ultrasonic attenuation in both systems. The data do not scale as expected theoretically and indicate a temperature and frequency dependent background attenuation caused by chemical processes. The background attenuation (α/f2)b,T reaches considerable values at frequencies f < 20 MHz and at temperatures away from the critical ((α/f2)b,T/(α/f2)T of the order of 0.1). The chemical background attenuation reflects parts of the broad spectrum of low frequency relaxation processes (frequency at the principle relaxation time f1 ≃ 2 MHz) found by Kaatze et al. (J. Chem. Phys. 93, 4955 (1989)) in a broadband ultrasonic attenuation study in a critical isobutyric acid(COOH)/H2O mixture.
Dynamic Investigation in a Nematic Mixture with High Glass‐Transition Temperature
Authors: Kresse Horst , Ernst Sabine , Wedler Wolfgang , Demus Dietrich , Kremer Frieder
Published: March 2014

Wörter

Dielectric measurements have been performed on oriented samples of a nematic mixture with a glass‐transition temperature of about 285 K in a frequency range between 0.1 and 106 Hz and on unoriented, quenched samples of this mixture from 156 K up to 300 K. Rotations around the short and long molecular axis gave a Vogel‐Fulcher‐Tammann‐behaviour; the seperate rotation of polar end‐groups caused the appearance of a high‐frequency‐Arrhenius‐like mechanism which was observable at very low temperatures.
Electrostatic Interactions of Bodies Bearing Thin Double‐Layers II. Exact Numerical Solutions
Authors: Adamczyk Z. , Belouschek P. , Lorenz D.
Published: March 2014

A new numerical algorythm was used to calculate the electrostatic interaction forces between regularly shaped particles bearing thin double‐layers immersed in an arbitrary electrolyte. Our numerical procedure enabled the solution of the nonlinear one‐dimensional Poisson‐Boltzmann (PB) equation in which the excluded volume effect is quantitatively considered for an arbitrary surface charge. These one‐dimensional solutions (parallel‐plate system) were then integrated according to the modified Derjaguin summation method giving the electrostatic force between bodies either convex or concave in the vicinity of their contact area. Calculations were performed for plates and particles bearing similar surface charge (homointeractions) for simple electrolytes and electrolyte mixtures with constant charge and constant potential boundary conditions. From these solutions the range of validity of previously derived analytical approximations (Part I [16]) was estimated. It was shown that the excluded volume effect plays a significant role for β > 10−2 increasing the surface potential of plates (constant charge model) at all separations as well as the absolute value of the interaction force. At the critical separation h̄* the force remains finite in accordance with our previous analytical equations. A significant role of trace amounts of higher valency electrolytes decreasing considerably the interaction force was confirmed.
High‐Frequency Dielectric Dispersions in Dilute Water‐in‐Oil Microemulsions
Authors: Cametti C. , Codastefano P. , Tartaglia P.
Published: March 2014

A set of measurements of the dielectric constant of a three‐component microemulsion system sodium bis (2‐ethylhexyl) sulfosuccinate‐water‐decane as a function of frequency (106 to 109 Hz) and temperature (5 to 35°C) and for two values of the volume fraction of the dispersed phase (0.025 and 0.050) has been made. The frequency dependence of the dielectric constant clearly shows a low‐frequency dispersion and a high‐frequency one, both describable in terms of a Cole‐Cole type relaxation. The high‐frequency dispersion is characterized by a relaxation time of the order of 10−9 s and a dielectric increment of few units, both largely independent of temperature and composition. We try to identify the origin of the high‐frequency dielectric dispersion and to relate it to the dynamics of the surfactant polar heads within the single droplet.
Ergodic and Nonergodic Relaxation Timescales for Metastable RNA Folding
Authors: Fernández Ariel , Belinky Alejandro
Published: March 2014

We investigate the relaxation of metastable RNA secondary structure in the limit of very long random chains. We make use of a Monte‐Carlo simulation which handles nonequilibrium refolding events occurring as a random uncorrelated primary sequence is being generated by sequential incorporation of nucleotides. We provide evidence confirming the existence of two different relaxation timescales with rates of divergence identical respectively to the ergodic and nonergodic relaxation timescales for spin glasses approaching the thermodynamic limit. These results substantiate the concept of substate in biopolymer folding, as introduced previously in the context of proteins.
New Possibility for Metastable RNA Folding of Biological Significance: A Physico‐Chemical View at Biological Regulation and Control
Authors: Fernández Ariel
Published: March 2014

We assess the importance of short‐lived nonequilibrium RNA folding in biological regulation. As an illustration, we explain the radical difference in pause site patterns during the synthesis of MDV‐1 RNA, depending on the nature of the process involved. The two situations of interest are: a) Replication by Qß‐replicase, a template‐instructed synthesis, and b) Transcription performed by E. Coli RNA polymerase. We start by establishing the possibility of soliton‐induced tautomerization of uracil which occurs exclusively in transcription. This tautomeric form of uracil forms base‐pairing with guanine and leads to a unique metastable folding of RNA. The assessment of this fact enables us to prove that in both replication and transcription, the pauses in the process are due to refolding events occurring in the replica chain as it is being synthesized and in the upstream portion of the template strand.
Ternary Diffusion and Onsager's Reciprocity Relations in Aqueous ZnCl2 + KCl and CdCl2 + KCl Revisited
Authors: Spallek M. , Hertz H. G. , Käshammer S. , Weingärtner H.
Published: March 2014

We report on e.m.f. data in the ternary electrolyte systems ZnCl2 + KCl + H2O and CdCl2 + KCl + H2O. In conjunction with diffusion data reported previously, these results enable a reassessment of a previous analysis of Onsager's reciprocity relations for ternary diffusion in these systems. The results eliminate contradictions to these relations observed in previous work.
The Determination of Rate Constants for the Reactions of Some Alkenes with the NO3 Radical
Authors: Benter Th. , Becker E. , Wille U. , Rahman M. M. , Schindler R. N.
Published: March 2014

The kinetics of the reaction of NO3 radicals with five alkenes was investigated at room temperature in a discharge fast flow system with mass spectrometric detection. NO3 radicals were produced by hydrogen abstraction from HNO3 with F atoms. The 1:1 stoichiometry of the reactions was confirmed. Rate constants were determined by measuring the decay of NO3 and/or of alkene under pseudo‐first‐order conditions. The following values of rate constants are reported: trans‐2‐butene (3.9 ± 0.3)·10−13, cis‐2‐butene (3.8±0.2)·10−13, iso‐butene (3.9±0.4)·10−13, 2‐methyl‐2‐butene (8.4±0.6)·10−12, 2,3‐dimethyl‐2‐butene (4.1±0.4)·10−11 in units of cm3 molecule−1 s−1. The results are compared with literature data.
In Situ Investigation of Duplex Semiconducting Films on Tin
Authors: Metikoš‐Huković M. , Šeruga M. , Ferina S.
Published: March 2014

The passive film on tin formed under dynamic conditions represents a duplex layer of the same oxidation state, with a stepwise concentration profile of two semiconducting or almost insulating oxide films in series. Their electrochemical properties are limited by their electronic properties. The electrode capacity (C) and photopotential (Eph) were measured in situ during oxide formation and reduction on tin in slightly alkaline solution. Analysis showed that the changing electronic properties of the oxide formed have a great impact on Eph and C. The formation of the inner SnO2 layer takes place homogeneously all over the surface with constant thickness. Schottky‐Mott type behaviour was found, indicating a surface donor concentration of 1015 cm−3 and a flat band potential vs. S.C.E., Efb, of −0.95V. For the outer layer the changes of electronic properties with increasing film thickness, as well as ageing processes, limit the validity of all electronic models with constant parameters. The oxide/hydroxide ratio increases with anodic polarization. The reaction is favoured by a free‐energy decrease. At higher potentials, especially under illumination, the electrode is covered with a very stable SnO2 species which is difficult to reduce.
Dynamic Behaviour of Ternary Microemulsions Consisting of Didodecyldimethylammoniumbromide, Water and Hydrocarbon
Authors: Lenz U. , Hoffmann H.
Published: March 2014

Ternary micromulsions containing water, dodecane and the surfactant didodecyldimethylammoniumbromide (DDABr) have been studied with flow and electric birefringence methods as well as with pressure jump relaxation techniques and conductivity measurements. Previous measurements had shown, that the isotropic L2‐phase can be subdivided into an area with spherical droplets and an area with interconnected water channels. Unexpectedly, a large birefringence was found within the region of the spheres and only small amplitudes in the other subregion. Kinetically measured relaxation times agree well with the rotation times which were determined from the birefringence methods. It is supposed that the water droplets are of ellipsoidal shape. The rotational diffusion time constants were determined from the birefringence signals. Both techniques gave the same results. The rotation time constants and the amplitudes of the birefringence signals increase with increasing water content. For large W/S ratios no large changes in the birefringence signals were observed in the percolation region of the conductivity. The measurements furthermore show that the lifetimes of both the globules and the channels are short and in the range of μs.
Synthetic and Biological Polymers in Deionized Solution. 3. Expansion of Water‐soluble Neutral Polymers and Polymers Having a Small Amount of Ionizable Groups
Authors: Okubo Tsuneo
Published: March 2014

Viscosities of water‐soluble neutral polymers and the polymers having a small amount of carboxylic acid groups are measured in deionized solution and in the presence of a small amount of sodium chloride. These experiments are made in order to examine whether the electrical double layers are formed or not around the linear‐type neutral polymers in the deionized solution. The neutral polymers examined are poly(vinylpyrrolidone)s (PVP) of molecular weights of 3.6 · 105, 4 · 104 and 1 · 104, poly(vinylalcohol) (PVA) of molecular weight of 8.8 · 104, poly(ethyleneglycol), hydroxypropyl cellulose, and poly(acrylamide) having no carboxylic acid groups. The reduced viscosities of PVP having the highest molecular weight (3.6 · 105) in the deionized solution are slightly higher than those in the presence of sodium chloride. This supports that the polymer chain is expanded slightly by the electrical double layers formed around the polymer. It is highly plausible that the ionization effect occurs by the adsorption of ionic species on the polymer. For PVP of molecular weight of 4 · 104 and PVA, viscosities in the deionized solutions are higher, but very slightly, than those in the presence of sodium chloride. The viscosities of other neutral polymers are quite independent of the ionic concentration. The deionized polymers of poly(acrylamide‐co‐acrylic acid), which have a small amount of ionizable groups, are unexpectedly highly stretched by the electrostatic intersegment repulsive interaction because of the very long Debye‐screening length.
NMR and NIR Studies of the Tautomerism of 5,10,15,20‐Tetraphenylporphyrin including Kinetic HH/HD/DD Isotope and Solid State Effects
Authors: Schlabach Martin , Wehrle Bernd , Rumpel Helmut , Braun Jürgen , Scherer Gerd , Limbach Hans‐Heinrich
Published: March 2014

The liquid and solid state tautomerism of 5,10,15,20‐tetraphenylporphyrin‐15N4 (meso‐tetraphenylporphyrin, TPP) has been studied by dynamic NMR spectroscopy and by NIR spectroscopy. The kinetic HH/HD/DD isotope effects on the liquid state tautomerism were reinvestigated in the temperature region between 200 K and 300 K. It was confirmed that the kinetic HH/HD isotope effects are large and the HD/DD isotope effects small. At 298 K the values 9.7 and 1.8 were obtained. The Arrhenius curve of the HH reaction is non‐linear indicating that the proton transfer takes place by thermally activated tunneling at low temperatures. The kinetic isotope effects are consistent with a stepwise proton transfer pathway involving intermediate tautomeric states in which protons are bound to adjacent nitrogen atoms. Solid state NMR experiments were performed on triclinic TPP, where the reaction barrier is higher compared to TPP in the liquid state. In addition, the degeneracy of the tautomerism in the solid state is lost due to intermolecular interactions. The latter conclusion is supported by the finding that for tetragonal mixed crystals of [(TPP—14N4—Ni)0.1(TPP—15N4—H2)0.9] the degeneracy of the tautomerism and its barrier is equal to the values found for the liquid solution. The NIR measurements show the presence of NH‐stretching overtones in the region of about 6450 cm−1. The anharmonicity of this vibration is of the usual order, which indicates that the reaction coordinate of proton transfer is not identical with the normal modes of the NH‐stretching vibrations, but rather a combination of various vibrational modes including the stretching, bending and skeletal modes. As a consequence, there will be a large number of vibrational states from which tunneling might occur.
Fullerene ions and their relation to PAH and soot in low‐pressure hydrocarbon flames
Authors: Baum Th. , Löffler S. , Löffler Ph. , Weilmünster P. , Homann K.‐H.
Published: March 2014

Positive and negative fullerene ions from C32 to over C600 have been analysed in low‐pressure premixed flames of acetylene and benzene with oxygen. Mass spectra were taken through a molecular beam sampling system by a linear time‐of‐flight mass spectrometer (TOF‐MS) and a reflectron TOF‐MS. Fullerenes are mainly formed during the beginning of soot formation, and less in the oxidation zone or under nonsooting conditions in benzene flames. When soot is present, the whole spectrum of fullerene ions is formed, whereas in the oxidation zone mainly smaller fullerenes, preferentially C60, C70 and C50 occur. — Polycyclic aromatic hydrocarbons (PAH) are the precursors of fullerenes, both in the presence of soot and under homogeneous conditions. However, hydrogenated fractions of carbon cages as intermediates could not be detected. This is attributed to fast unimolecular reactions and to formation reactions of fullerenes in young soot particles. On account of correlations with concentration profiles of large PAH ions, on the one hand, and with the soot particle mass distributions, on the other, possible mechanisms of fullerene formation are discussed. Fullerenes do not grow further, as do the soot particles, but they are oxidized at high temperature. Thermal decomposition was not observed. From comparison of fullerene formation with the two fuels and under different burning conditions it is concluded that negative fullerene ions are formed by electron attachment to neutral fullerenes and that for positive fullerene ions there is hardly any ionisation equilibrium.
Kinetics and mechanism of formation of iron(ii) complex with 8‐hydroxy‐7‐(2‐thiazolyl)azo‐5‐quinolinesulfonic acid
Authors: Noritake Masaki , Jitsukawa Koichiro , Einaga Hisahiko
Published: March 2014

Thermodynamic stability and kinetics and mechanism of formation of iron(II) complex with 8‐hydroxy‐7‐(2‐thiazolyl)azo‐5‐quinoline‐sulfonic acid (THQS) have been studied spectrophotometrically in an aqueous 0.10 mol dm−3 sodium chloride solution at 25°C. The iron(II) complex with coordinated 8‐quinolinolato moiety of THQS forms with the stability constant (β11) of logβ11/mol−1 dm3 = 8.52 ± 0.20. This complex exhibits a metal‐to‐ligand charge‐transfer band in the visible region. Stopped‐flow kinetics revealed biphasic processes with a faster formation of the complex with coordinated 2‐thiazolylazophenolato moiety followed by a slower linkage isomerization of this intermediate to the final, more stable complex with coordinated 8‐quinolinolato moiety. The reaction pathways and concomitant rate constants are elucidated and determined; the reaction mechanism is discussed in detail.
Contact‐complex formation reaction between solvent molecules in square‐well fluids 2. Two‐ and three‐dimensional fluids
Authors: Yoshimura Y.
Published: March 2014

The contact‐complex formation reaction between solvent molecules in square‐well fluids is studied for the two‐ and three‐dimensional system. The effect of the third body at the second‐virial limit is expressed by the following three factors: (1) the excluded volume effect, (2) the desolvation effect, and (3) the „glue effect”︁, where this effect means that the third body behaves like a glue binding a pair of molecules. The density derivative of the equilibrium constant of the contact‐complex formation, at the second‐virial limit, goes through a minimum with increasing temperature in both the two‐ and three‐dimensional fluids, whereas it increases monotonically in the one‐dimensional fluid. However, a Monte Carlo simulation shows that the difference due to the dimensionality of the system is rapidly smeared out with increasing density. The equilibrium constants of the contact‐complex formation in the two‐ and three‐dimensional square‐well fluids have qualitatively the same temperature and density dependence as in the one‐dimensional fluid at densities about 0.5 times higher than their critical densities
The reduction of hypohalites at GaAs electrodes: Charge‐transfer and recombination processes
Authors: Minks B. P. , Kelly J. J.
Published: March 2014

Results obtained from electrical impedance spectroscopy at n‐GaAs in the dark with Fe(CN)63‐, ClO− and BrO− in alkaline solution indicate that the cathodic reduction current is due mainly to electron‐hole recombination at the surface. The same conclusion also holds for the recombination of photo‐generated holes at illuminated n‐GaAs in indifferent electrolyte. Electroluminescence measurements showed that surface recombination with the hypohalites is more effective than with Fe(CN)63‐. Quantum efficiency measurements at p‐GaAs revealed that the second charge transfer step of the hypohalite reduction can involve either the capture of an electron by or the injection of a hole from a surface intermediate. This result leads to a slight revision of a mechanism previously proposed for the reduction of ClO− and BrO− at GaAs.
Interactions of different nonylphenolepolyethyleneglycoles (NPG) with iron electrodes
Authors: Fahr T. , Reinhard G.
Published: March 2014

Nonylphenolepolyethyleneglycoles (NPG) are characterized by means of electrochemical methods. We made electrochemical impedance spectroscopy and current density‐potential measurements at superpure iron electrodes in KNO3 solution with and without addition of NPG. In order to acquire some knowledge about the electrical potential‐distance profile at the electrode surface of iron, we made Zeta potential measurements at dispersed α‐Fe2O3 particles. Adsorption phenomena of NPG at electrode surfaces free of any oxide layer were studied by a.c. polarography (tensammetry) at the dropping mercury electrode. It is shown that NPG can inhibit the corrosion reaction of superpure iron in neutral solutions as combined anodic/cathodic inhibitors, but increase the cathodic process in weak acid solutions.