bluuebunny/crossref_metadata_2025_split huggingface dataset:
Rigid‐rod polyelectrolytes based on poly(p‐phenylene sulfonic acid)
Authors: Rulkens R. , Wegner G. , Enkelmann V. , Schulze M. 
Published: July 2012

AbstractThe synthesis and characterization of polyelectrolytes, composed of a shape persistent poly(p‐phenylene) backbone with regularly attached sulfonic acid‐ and n‐alkyl residues, is described. Polymers were obtained by Pd‐catalyzed coupling of substituted p‐phenylene‐bis‐boronic acid esters with 2,5‐dibromobenzenesulfonates or 4,4′‐dibromodiphenyl‐2,2′‐disulfonates, followed by saponification of the sulfonates. The polymers were found to have number average contour lengths (Ln) betwen 23 and 155 nm. The polymer sulfonic acids behave as strong acids and are soluble in water as their tetramethylammonium salts. The concentration dependence of the viscosity of aqueous solutions of the salts does not follow the behavior seen for conventional polyelectrolytes. The polymers show lyotropic behavior at low to moderate concentrations. Polymers with long alkyl substituents, if spread as monolayers on a Langmuir trough and subject to the Langmuir‐Blodgett technique, give LB‐multi‐layers, which have been characterized by X‐ray reflectometry. As a model compound, sodium 4,4″‐dimethyl‐1,1′:4′,1″‐terphenyl‐2′‐sulfonate was obtained as single crystal and its X‐ray structure is described.
Excluded volume effects of sulphobetaine polymers
Authors: Kato Tadaya , Takahashi Akira 
Published: July 2012

AbstractThe intrinsic viscosity, [η], radius of gyration, 〈S2〉1/2, and hydrodynamic radius, Rh were measured for the sulphobetaine type polyampholyte, poly(N‐3‐sulphopropyl)‐N‐methacrylooxyethyl‐N,N‐dimethyl ammonium betaine), in aqueous solution at various added‐salt concentration, Cs. The electrostatic expansion factors for polyampholyte effect, αe, were estimated for the sulphobetaine polymers over the wide range of molecular weights and Cs. It is concluded that the third‐power type equation for expansion parameters of both non‐ionic and the electrostatic excluded volume effect (attractive polyampholyte effect) can favorably explain the Cs dependence for the change of molecular dimensions in solution at intermediate ionic strength.
Charge reversal in electrical double layers — a balance between energy and entropy
Authors: Sjöström L. , Åkesson T. , Jönsson Bo 
Published: July 2012

AbstractMonte Carlo simulations and accurate analytical theories predict a charge reversal in an electric double layer under some specific conditions. One typical situation is for very highly charged aggregates in the presence of divalent counterions, where ionic correlations become important. Here, we reexamine the phenomenon in terms of a balance between energy and entropy contributions. High surface charge density, low temperature, low dielectric permittivity or multivalent counterions, all these factors increase the influence of the energetic term and favour a charge reversal. Previous studies have focused on the energy term and attempts to increase that. An alternative and equally valid way is to reduce the entropy term, which can be accomplished by replacing single atomic or molecular counterions by polyelectrolyte chains. The entropy of the charged chains is of course much lower than that of the small ions and the dominance of the energy term will again lead to a charge reversal, but in this case at a much lower and more realistic value of the electrostatic coupling.
The effect of polyvalent salt ions on the properties of annealed polyelectrolyte brushes
Authors: Birshtein T. M. , Zhulina E. B. 
Published: July 2012

AbstractThe scaling analysis of the equilibrium behaviour of polyelectrolyte brushes immersed in water‐salt solution is presented. The effect of polyvalent salt ions is investigated for the brushes with quenched and annealed charges. The nontrivial dependence of brush thickness on the ionic strength is predicted and discussed.
Adsorption of cationic starch on fibres from mechanical pulps
Authors: Wågberg Lars , Kolar Katarina 
Published: July 2012

AbstractThe present investigation, in which the adsorption of cationic potato and corn starch on fibres from Thermo Mechanical Pulps (TMP) has been studied, has been divided into two parts. In the first part, the adsorption of differently charged cationic potato starches onto the fibre fraction of a peroxide‐bleached TMP was investigated. The results show that the adsorption can be viewed as an ion‐exchange process in which the anionic charges on the fibres are neutralised by cationic charges on the starch. Experiments with different electrolyte concentrations show that the starch adsorption on this kind of fibre follows theories for polyelectrolyte adsorption. When Dissolved and Colloidal Substances (DCS) from the TMP is introduced into the experiments these conclusions are no longer valid since the complexation between the starch and the DCS becomes the dominating process. the results from the second part show the large influence of cellulosic fines and DCS on the amount of starch that can be adsorbed on the fibres. Since both fines and DCS have a higher charge density and a higher specific surface area than the fibres, they can adsorb significantly larger amounts of starch and may hence increase the total amount of starch adsorbed on the fibres, provided the fines and DSC can be retained to the fibres. The fines fraction is retained to the fibres by a bridging type of mechanism whereas the DCS‐fraction is retained through charge neutralisation of the charges on the DCS‐fraction by the charges on the starch. Potato starch was somewhat more efficient than corn starch in neutralising the effect of the DCS‐fraction.
Pressure effect on the quenching of singlet oxygen by amines in different solvents
Authors: Hild M. , Brauer H.‐D. 
Published: July 2012

AbstractRate constantskqof the quenching of singlet oxygen in its a1Δg‐state by five amines (piperazine (P), N,N′‐dimethylpiperazine (DMP), N‐methylpiperidine (MPP), quinuclidine (Q), N,N,N′,N′‐tetramethylphenylene‐1,4‐diamine (TMPD)) have been determined in solvents with different polarity in dependence on pressure up to 1200 or 1600 bar using time‐resolved phosphorescence techniques. In the case of the efficient O2(a1Δg) quencher TMPD in o‐dichlorobenzene and in benzonitrile at about 700 bar the transition is observed from the reaction‐controlled rate to the diffusion‐controlled rate limit. For all amines the highest negative value of the experimental activation volume was found in n‐hexane (−33.0 ≥ ΔV≥ −42.0 cm3· mol−1) and the lowest negative value in benzonitrile (−9.0 ≥ ΔV≥ −19.0 cm3mol−1). These results are consistent with the assumption that quenching of O2(a1Δg) by amines occurs via polar exciplexes with singlet and triplet multiplicity, respectively. The data were analysed taking into account contact complex formation. The reaction volume ΔVcof the formation of the contact complex was evaluated using the statistical model developed by Yoshimura and Nakahara. On the basis of Kirkwoods electrostriction model the dipole moments of the singlet exciplexes were calculated in the range between 5.7 (P) and 11.5 Debye (TMPD), indicating that only a partial charge transfer takes place in the quenching process.
The dependence of the electrode properties of glasses on their composition. The second approximation: Taking into account ion mobilities in the surface layers
Authors: Belyustin A. A. , Shultz M. M. 
Published: July 2012

AbstractThe electrode properties of glasses reflect the structural role of their components: network‐former or network‐modifier. The main evidence of this is the shape of the potential — pH curves and the potentiometric selectivity coefficient, KpotHM of the known Nicolsky equation. KpotHM is a product of two factors: the “energy factor” (the equilibrium constant of an exchange reaction of H+ and M+ ions between glass and solution, KexchHM) and the “dynamic factor” (ion mobility ratio in a glass surface layer, UM/UH).In the 1960s and for some time thereafter the energy factor was mainly used to explain the dependence of electrode properties of glasses on their composition (“the first approximation”). In this paper we consider the cases of the predominant effect of the dynamic factor on the electrode behavior of the glasses xLi2O‐xAl2O3‐0.25‐xB2O3‐(100‐2.25x) SiO2 and mixed‐alkali glasses 24% (Me′2O+Me''2O)‐12% Al2O3‐64% SiO2.
New glass ceramics with high fluoride conductivity
Authors: Goldammer Susanne , Kahnt Hanno 
Published: July 2012

AbstractGlass ceramics with fluoride conduction have been prepared by heat treatment of glasses of the system SiO2‐PbO‐PbF2. The influence of the annealing regime on the formation of crystalline phases and on the structure of the glass ceramics has been investigated. The conductivity has been studied at different stages of crystallization.Two different types of crystallization behaviour have been distinguished when glasses are annealed at about 20 K above Tg. Under these conditions only one single crystalline phase arises in the glass matrix. For glasses with up to 15 mol% PbF2 this crystalline phase is Pb3Si2O7 while at higher PbF2 content the crystalline phase is PbF2.Formation of Pb3Si2O7 causes drastical increase of conductivity due to the rise of fluorine concentration in the residual glass. The growth of PbF2 crystals yields the opposite effect. In some cases, PbF2 crystals built a continous network throughout the glass matrix. Then, conductivity takes place along grain boundaries and the total conductivity increases again.
Dynamical disorder in glasses studied by 2H and 1H NMR
Authors: Roggatz I. , Karle C. , Taupitz M. , Rössler E. 
Published: July 2012

AbstractWe review 2H and also some 1H nuclear magnetic resonance (NMR) results on mobile guest molecules dissolved in organic high and low molecular weight glasses. The guest molecules such as benzene exhibit high symmetry and perform a rotational jump process between equivalent orientations in the solid state. Characteristic two‐phase NMR spectra and a pronounced non‐exponential 2H spin‐lattice relaxation are observed. The 1H spin‐lattice relaxation is exponential, and shows a non‐Arrhenius temperature dependence. All these features can be well described over a large temperature range (10–200K) by assuming a broad distribution of activation energies and entropies. The distribution of activation energies displays significant differences for the various guest/host systems. We think the approach chosen here can also be applied to describe β‐processes or internal processes in neat glasses.
On the nature and role of Fe3+ ions in oxide and fluoride glasses
Authors: Stößer R. , Scholz G. , Nofz M. , Grande T. , Aasland S. 
Published: July 2012

AbstractFe3+ doped oxide and fluoride glasses were investigated by ESR spectroscopy at 4.2 K. For the first time, the resulting complex spectra could be completely reproduced starting from a diagonalization of the S = 5/2 spin‐hamiltonian, including the appropriate zero‐field splitting components D and E, and using distributions for the fourth order spin coupling parameters. By careful analysis of the differentiated and integrated experimental and calculated spectra it could be shown, that if only Fe3+ point defects are present in glasses, the main ESR intensity covers the range of g'∼4.3. Since all experimental spectra yield remarkable contributions in the range of g'∼2 there is evidence for the existence of magnetically interacting clusters in the glasses. Differences between Fe3+ doped oxide and fluoride glasses essentially concern the concentration dependent spectral pattern and the low‐temperature spin relaxation behaviour.
On the intermediate range structure in phosphate glasses
Authors: Walter G. , Hoppe U. , Baade T. , Kranold R. , Stachel D. 
Published: July 2012

AbstractThe first prominent peak observed at s1 in the total X‐ray structure factors, S(s), of metaphosphate glasses containing the metal ions Li, Na, K, Ag, Ba, Sr, Cd, Pb, and of barium phosphate glasses with BaO mol% concentrations ranging from 2.5 to 56.5 has been studied to extract information on the intermediate range structure. The period, 27π/s1, of the quasi‐periodic real‐space electron density fluctuations is shown to be correlated with the ionic volume fractions of the glasses.
The determination of transition state properties in the ethane and ethene ion dissociation
Authors: Mähnert Joachim , Güthe Felix , Weitzel Karl‐Michael 
Published: July 2012

AbstractThe dissociation of ethane and ethene ions has been investigated by means of the ZEKE‐PEPICO technique. The breakdown curves for the lowest reaction channel ‐ in both molecular ions the H2 loss reaction ‐ has been measured at temperatures of 300 K and close to 0 Kelvin. The observed shift of the breakdown curves at the two temperatures is 12 meV in the case of ethane ions and 8 meV in the case of ethene ions. These numbers can essentially be explained by the difference in the vibrational energy at the two temperatures. In both cases it is concluded that the dissociation energy E0(J) does not depend significantly on the rotational angular momentum of the molecules which is expected for reactions governed by a tight transition state. These findings are discussed in terms of the energetic, kinetic, and mechanistic information available for these reactions.
Oxygen evolution from water. Potential metal ligands in photosynthetic water oxidation identified by site‐directed mutagenesis of Synechocystis sp. PCC 6803
Authors: Bowlby Neil R. , Sithole Idah , Babcock Gerald T. , McIntosh Lee 
Published: July 2012

AbstractIn order to identify amino acids that could serve as ligands to the manganese cluster, which comprises the active site of the oxygen‐evolving complex (OEC), and the calcium ion that affects the stability of the OEC, we have generated a series of site‐directed mutants in the D1 polypeptide of the PS II reaction center. Our approach has been to emphasize characterization of mutants with reduced PS II activity rather than those that lack PS II activity altogether in order to address questions about the effects of the mutations on the functional properties of the Mn ensemble. A number of mutants have been isolated with impaired PS II activity and a characterization of in vivo photosynthesis from mutant and WT strains is presented. All of the mutants described in this report produce nearly WT levels of PS II in the thylakoid membrane. Non‐conservative mutation at Asp‐103 (DN103D1), Glu‐104 (EQ104D1) or Glu‐333 (EQ333D1) results in moderate (∼ 50%) inhibition of oxygen evolution, whereas mutation at Glu‐65 (EQ65D1) or His‐337 (HF337D1) results in cells that retain only 10 to 20% of WT activity. In vivo fluorescence induction kinetics are consistent with a lesion on the donor side of PS II in each of the mutants tested. We conclude that Glu‐65 and His‐337 are directly involved in binding manganese or calcium, and that Asp‐103 and Glu‐104 may be involved in calcium binding but are not manganese ligands.
ATP‐synthesis. Rotatory catalysis by F‐ATPase: Real‐time recording of intersubunit rotation
Authors: Junge W. , Sabber D. , Engelbrecht S. 
Published: July 2012

AbstractATP synthase (F‐ATPase) is an ubiquitous enzyme in photosynthesis and respiration of prokaryotic and eukaryotic organisms. It couples proton translocation through its membrane portion, F0, to the synthesis of the “energy carrier molecule” ATP at the peripheral portion, F1 [1]. Three cooperative reaction sites are distributed with trigonal symmetry over the hexagonal array of (αβ)3 in F1 [2, 3]. It has been proposed that the endergonic release of spontaneously formed ATP [4, 5] might involve mechanical energy transduction [6, 7] through the proton driven rotation of subunit γ within (αβ)3 [2, 8, 9]. We recorded the putative intersubunit rotation in real time [10]. Applying polarized absorption relaxation after photobleaching (PARAP) to immobilized F1 with eosinlabeled γ, we observed the rotational motion of γ relative to immobilized (αβ)3 in the time range of 100 ms, compatible with the rate of ATP hydrolysis by immobilized F1. Its angular domain of at least 200 degrees favours a rotatory tri‐site mechanism of catalysis with γ acting as a crankshaft in the center of (αβ)3.
Frontier orbitals of porphyrin electron donors in biomimetic model compounds — partial lifting of orbital degeneracy in asymmetric porphyrins studied by EPR on anion radicals
Authors: Huber Martina , Fuhs Michael 
Published: July 2012

AbstractThe electronic structure of three porphyrins is investigated by EPR. These porphyrins are used as photoinduced electron donors in biomimetic model compounds for the light induced charge separation in photosynthesis. For efficiency of the model compound the distribution of the LUMO and HOMO over the porphyrin are important. Thus, cation radicals probing the HOMO, and anion radicals probing the LUMO are studied. The effect of lowering the porphyrin symmetry on the Jahn‐Teller distortions in anion radicals (J. Seth, D. F. Bocian; J. Am. Chem. Soc. 116, 143–153 (1994)) are studied. Jahn‐Teller effects severely limit the information obtainable by EPR. Strong perturbations to the symmetry in 2,8,12,18‐tetrabutyl‐3,7,13,17‐tetramethyl‐5,15‐diphenylpor‐phyrinato‐magnesium cause sufficient lifting of degeneracy to allow to determine details of the electronic structure in the anion radical. The cation radical of this porphyrin is found to have an A1u, or a mixed A1u/A2u ground state. On the basis of both results an optimized design of model compounds based on this porphyrin is discussed quantitatively: linking of an electron acceptor to the phenyl group bearing positions of this porphyrin should result higher efficiency than linking at the other two meso positions.
Conformational control of electron transfer states: Induction of molecular photodiode behaviour
Authors: Rettig Wolfgang , Maus Michael , Lapouyade René 
Published: July 2012

AbstractFluorescence rate constants of various donor‐acceptor biphenyl model compounds were compared with their corresponding absorption spectra in solvents of different polarity and analysed with respect to their molecular conformations in ground and excited states. For a pretwisted derivative, the typical behaviour of a molecular photodiode was observed i. e. allowed access to the excited state but forbidden return to the ground state. This is interpreted by assuming a structural relaxation towards an orbitally‐decoupled conformation. Implications regarding similar observations for the photosynthetic reaction center are discussed.
Transport across Boundary Layers in Ionic Crystals Part I: General Formalism and Conception
Authors: Jamnik J. , Maier J. 
Published: July 2012

AbstractIn a sequence of papers we will systematically address the problem of mass and charge transport through boundaries from the standpoint of irreversible thermodynamics and kinetics. Special emphasis is put on the transport through space charge regions. As a model material we consider a binary crystal (MX1+δ) exhibiting ionic and/or electronic conduction and thus allowing for changes in the nonstoichiometry as well. A typical situation is a mixed conductor sandwiched between two partially reversible electrodes. The boundaries are assumed to be morphologically stable. Driving forces considered are differences in chemical and/or electrical potential. They are assumed to be sufficiently small to allow for a linear response treatment. No reference to ad hoc assumptions such as the local electroneutrality condition is made. We also allow for compositional changes in the material. The continuum approach will be presented in the framework of linear irreversible thermodynamics. The introduction of virtual electric fields significantly facilitates the formalism with respect to electrochemical relationships, boundary conditions and analytical treatment. The instationary state will be formally tackled in Fourier space. A conceptual discretisation is generally of advantage – in particular in cases in which the continuum problem can only be solved numerically – since it takes account of the atomistic situation in a kinetic framework. This paper sets out the fundamental equations which will be applied to specific examples. One application already reveals quantitatively the important result that space charge regions necessarily appear in weakly disordered materials during a chemical diffusion even if one starts from a space – charge free initial situation. The steady state concentration profiles are derived.
The Structural Properties of Ion Clusters for Unequal Sized Ions
Authors: Kraska T. 
Published: July 2012

AbstractThe structure of ion clusters have been calculated for the nonrestricted primitive model in the low density limit. The investigated systems are symmetric with respect to the charge but not with respect to the ion diameter. The influence of the size ratio on the ion cluster structure has been investigated as a function of the temperature for cluster ranging from the dimer to the octamer. In a certain range of ion diameter ratios all structural results are independent of the size ratio. Outside this range both quantitative and qualitative have been found depending on the cluster size. Comparisons with experimentally obtained cluster structures taken from the literature have been accomplished. Furthermore the temperature dependence of the numerically obtained structural properties have been fitted to analytical functions.
Quantitative Molecular Dynamics Simulation of High Pressure Adsorption Isotherms of Methane on Graphite
Authors: Aydt E. M. , Hentschke R. 
Published: July 2012

AbstractWe perform computer simulations of methane in a slit shaped graphite pore using the molecular dynamics technique. From the equilibrium density profile across the pore we determine the adsorption isotherms in the pressure range between 20 and 200 bar for the two temperatures −20° and 50°C. The simulated surface excess concentration is found to be in good agreement with the experiment.
Deuterium and Carbon‐13 NMR Study of Molecular Rotational Diffusion in Two Star‐Hydrocarbon Oils
Authors: Luff S. H. 
Published: July 2012

AbstractRelaxation rates of carbon‐13 magnetization of the C9 carbon positions in two poly‐n‐octyl star‐hydrocarbon oils, 9‐n‐octyl‐n‐heptadecane and 9,10‐di‐n‐octyl‐n‐octadecane, and of deuterium magnetization in 9‐n‐octyl‐n‐heptadecane‐9d, and 9,10‐di‐n‐octyl‐n‐octadecane‐9d1, labelled with deuterium at the same carbon position, have been measured over wide ranges of temperature and at two field strengths. In this paper simulation of the observed relaxation rates within the assumptions of the Lipari and Szabo picture of molecular rotational diffusion, and dipolar and quadrupolar relaxation mechanisms, determined molecular rotational diffusion constants, activation energies, and Lipari and Szabo order parameters and correlation times for internal molecular motion at the labelled positions. The appropriate quadrupolar coupling constants could also be determined from the measured data. The effect of the rate of internal molecular isomerization was discussed and it was shown that in general this had to be included in the theoretical picture in order to derive a consistent set of motional parameters that generated all observable relaxation rates of magnetization.
Potential energy surfaces for CH bond cleavage reactions
Authors: Harding Lawrence B. 
Published: July 2012

AbstractAb initio, multi‐reference, configuration interaction calculations are reported for the reactions.
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Two equivalent, barrier‐less paths are found for the CH3 + H recombination, two inequivalent, barrier‐less paths are found for the CH2F+H and CHF2 + H recombinations (depending on which side of the radical the hydrogen atom approaches) and only one barrier‐less path is found for the CF3 + H recombination. The minimum energy path for H atom approaching CF3 from the concave side is predicted to have a barrier of 27 kcal/mol. Both the minimum energy path energies and the transitional frequencies as a function of RCH for all four reactions are predicted to be quite similar.