bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular Motions in a Molecular Crystal: Tetrachloro‐m‐Xylene
Authors: Moura Ramos Joaquim J. , Sousa Rui J. Cândido , Correia Natália T. , Dionísio Madalena S. C.
Published: July 2012

In the present work we report a study of the molecular relaxation processes in tetrachloro‐m‐xylene in the solid state. The techniques of dielectric relaxation spectroscopy and thermally stimulated discharge currents were used in order to identify and characterise the different relaxation processes and it was found that a solid rotator phase appears on heating at a temperature of ∼−125°C. It was also observed by thermally stimulated currents that a higher temperature relaxation occurs above ∼−15°C. The attribution, at the molecular level, of this relaxation is not straightforward and it was suggested that it probably corresponds to the onset of the self‐diffusional motions in the solid.
The Role of Surface Oxides in the Electrooxidation of Methanol, Formic Acid and CO on Pt, Ru and Codeposited Pt‐Ru
Authors: Frelink T. , Visscher W. , Cox A. P. , van Veen J. A. R.
Published: July 2012

The growth of an oxide layer on Pt, Ru and codeposited Pt‐Ru was monitored ellipsometrically during the electrooxidation of CH3OH, HCOOH and adsorbed CO. In the potential region where these molecules or their intermediates are oxidized at the Pt‐Ru surface, the Ru‐oxide is no longer observed. This supports the bifunctional mechanism that has been proposed for the promoting role of Ru on Pt. PtO is not involved in the HCOOH and COads oxidation. The adsorbed CO layer on Pt and on Pt‐Ru could also be observed with ellipsometry.
The Role of Solvent Permittivity in Estimation of Electrolyte Activity Coefficients for Systems with Ion Pairing on the Basis of the Mean Spherical Approximation
Authors: Tikanen Alex C. , Fawcett W. Ronald
Published: July 2012

Mean ionic activity coefficient for aqueous solutions of CdCl2, CuCl2, MgCl2, Na2SO4, MgSO4 and CdSO4, systems for which dielectric relaxation data have recently been obtained, can be estimated accurately on the basis of a modified version of the mean spherical approximation (MSA) for electrolyte solutions with consideration of ion pairing. The modification makes use of the actual solution permittivity rather than the permittivity of the pure solvent. Reasonable ion pairing association constants, Kas, have been determined by fitting the theoretically determined mean ionic activity coefficients to experimentally measured values in the range of concentration for which the dielectric permittivity data exist using one additional adjustable parameter, namely, the restricted diameter for all species involved.
Solvatochromism in Pure Solvents: Effects of the Molecular Structure of the Probe
Authors: Novaki Luzia P. , El Seoud Omar A.
Published: July 2012

The following solvatochromic probes were synthesized: 1‐methyl‐8‐oxyquinolinium betaine (QB), sodium 1‐methyl‐8‐oxyquinolinium betaine‐5‐sulphonate (QBS), and 1‐methyl‐3‐oxypyridinium betaine (PB). Their solvatochromic behavior in 28 protic and aprotic solvents was investigated, and the data compared to those of 2,6‐diphenyl‐4‐(2,4,6‐triphenyl‐1‐pyridinio)‐1‐phenolate, (RB). Solvent polarity scales based on QB, QBS, and PB correlate linearly with the ET(30) scale of RB. The Taft‐Kamlet‐Abboud equation satisfactorily applies to the solvatochromic data of RB, QB, QBS and PB. The multiparameter regression coefficients (a) and (s) of the above mentioned equation show that the sensitivity of the probe to solvent dipolarity/polarizability, and hydrogen bond donation is clearly dependent on its molecular structure. This dependence was tested for a total of 26 probes of widely different structures. It is shown that this is a general behavior, i.e., log |a| and |s| are linearly dependent on the pKa and the dipole moment of the probe, respectively.
Time‐dependent processes in polyelectrolyte solutions
Authors: Reed Wayne F.
Published: July 2012

Most theoretical and experimental efforts in polyelectrolyte science have been aimed at understanding equilibrium properties. The purpose of this chapter is to call attention to the wide array of kinetic phenomena which occur in polyelectrolyte solutions on their way to equilibrium. Such time‐dependent processes include dissolution of dry powders, phase separation, aggregation, gelation, and degradation. Because equilibration times in polyelectrolyte solutions can be quite long, polyelectrolyte solutions may often be used and studied, wittingly or not, in non‐equilibrium states. After a brief review of some of the equilibrium properties of polyelectrolyte solutions, a few simple ideas and examples are given to illustrate non‐equilibrium processes. A larger emphasis is given to the degradation problem, simply because it is the area of time‐dependent phenomena where the author and co‐workers have so far made the most quantitative headway.
Viscoelastic properties of polyelectrolyte solutions
Authors: Noda Ichiro , Takahashi Yoshiaki
Published: July 2012

Viscoelastic properties of polyelectrolyte solutions were studied in the absence and presence of added salt over a wide range of polymer concentration in comparison with those of non‐ionic polymers in good solvents.

In the semidilute region where the polymer concentration C is higher than the overlap concentration for viscosity C*v, but lower than C** (ca. 0.3 kg/dm3), the polymer concentration dependence of specific viscosity at zero‐shear rate η increases with increasing added‐salt concentration Cs and at high Cs it almost agrees with that of non‐ionic polymers in good solvents. In the concentrated and entangled region where C>C and C>C**, η more steeply increases with C even in salt‐free solutions, implying the screening of electrostatic interaction and the increase of local frictional coefficients as considered in concentrated non‐ionic polymer solutions.

In the dilute region where C is lower than the overlap concentration for steady‐state compliance C, the steady‐state compliance Je more strongly depends on molecular weight and C than in non‐ionic polymer solutions. In the semidilute region where C>C, but CC and C>C**, the polymer concentration dependence becomes similar to that of non‐ionic polymer solutions. In the concentrated and non‐entangled region where CC**, the frequency dependences of storage and loss moduli are well explained by the modified Rouse model like non‐ionic polymer systems in the same region.

The experimental results in the semidilute region are well explained by our theory of viscoelastic properties of polyelectrolyte solutions based on the reptation model with electrostatic interactions.
Viscometric behavior of dilute polyelectrolytes. Role of electrostatic interactions
Authors: Roure I. , Rinaudo M. , Milas M.
Published: July 2012

This paper deals with the investigation of the electroviscous effects on sodium hyaluronate aqueous solutions. Dilution of a polyelectrolyte in water or low salt concentration solvent gives a reduced viscosity passing through a maximum whose position corresponds to a polymer concentration directly related to the ionic concentration. Independently of the molecular weight when isoionic dilution is performed, the conformation is preserved and the intrinsic viscosity corresponding to the adopted ionic concentration is obtained as soon as the concentration range corresponds to the dilute domain.

At end the model previously proposed to take into account the role of the electrostatic interactions on the conformation is adopted to interpret, with a good agreement, the data of intrinsic viscosity versus ionic concentration.
Rigid‐rod polyelectrolytes based on poly(p‐phenylene sulfonic acid)
Authors: Rulkens R. , Wegner G. , Enkelmann V. , Schulze M.
Published: July 2012

The 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

The 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

Monte 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

The 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

The 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

Rate 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

The 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

Glass 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

We 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

Fe3+ 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

The 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

The 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

In 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

ATP 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

The 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

Fluorescence 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

In 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

The 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

We 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

Relaxation 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

Ab 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.
Photodissociation of HNCO: Three competing pathways
Authors: Droz‐Georget TH. , Zyrianov M. , Sanov A. , Reisler H.
Published: July 2012

The unimolecular decomposition of expansion‐cooled isocyanic acid (HNCO) via channels (1) 3NH + CO, (2) H + NCO, and (3) 1NH + CO [where 3NH and 1NH denote NH(X3σ−) and NH(a1 Δ), respectively] has been investigated following photoexcitation to the S1(1A ) state in two energy regimes: (i) in the region of the 1NH + CO threshold (41 700–45 500 cm−1; 240–220 nm), and (ii) ˜ 3200 cm−1 above D0(1NH + CO), at around 46000 cm−1 (217.6 nm). Several complementary experiments are presented: NCO, 3NH and 1NH photofragment yield spectra and relative 1NH/3NH branching ratios are obtained by laser induced fluorescence (LIF); photo‐fragment ion imaging is used to record CO angular recoil distributions, and 1NH rotational distributions correlated with specific CO (v, J) levels, HNCO excited to S1 undergoes complex dynamics reflecting simultaneous decomposition on several potential energy surfaces, and including internal conversion (IC) and intersystem crossing (ISC). In energy region (i), a progressive loss of structure in the 3NH yield spectrum is observed above the opening of channel (3), and is interpreted as the imprint of short‐time dynamics characteristic of the ISC step. State selectivity in the photodissociation is revealed by comparing the photofragment yield spectra of the three channels. In region (ii), product state distributions for channel (3) exhibit clear dynamical signatures, as expected for dissociation on S1. At low excess energies channel (2) derives from dissociation on S0, but the respective roles of S0 and S1 at higher energies are not well established yet. The results are discussed in terms of vibronic levels of mixed electronic character coupled directly or via radiationless decay to the various continua. The competition between the different processes depends sensitively on photolysis energy and excitation conditions.
Unimolecular dynamics in bimolecular ion‐molecule reactions
Authors: Craig Stephen L. , Brauman John I.
Published: July 2012

We have measured the translational energy dependence of a series of chloride exchange reactions in the gas phase. The translational energy dependence varies with the changes in the potential energy surfaces across the series of reactions in a manner that is consistent with the predictions of statistical reaction rate theory. These results indicate that despite reports of non‐statistical behavior in simple SN2 reactions, the assumptions of unimolecular reaction rate theory (strong coupling, energy randomization) are valid for the complexes formed in many bimolecular ion‐molecule collisions.
Trajectory studies on the collisional relaxation of highly excited benzenes by mono‐ and polyatomic colliders
Authors: Lenzer T. , Luther K.
Published: July 2012

Collisional energy transfer (CET) of highly vibrationally excited benzene and hexafluorobenzene molecules has been investigated by quasiclassical trajectory calculations. Influences of the intramolecular potential reflected in molecular parameters are studied systematically in collisions with argon. It is found that the lower the vibrational frequencies the more efficient is CET. Intermolecular potential effects are investigated. At larger well depths (for benzene+argon) the first moment of energy transfer <ΔE> scales with ε and is not very sensitive to details of the interaction. However at small well depths ε (for benzene + helium) energy transfer is dominated by the exact shape of the intermolecular potential, especially of the repulsive part. Benzene‐benzene collisions are studied as an example for CET between polyatomics and large colliders. Excellent quantitative agreement with experiment is found for <ΔE> and its energy dependence. The results suggest that vibrational energy loss of the excited molecule is dominated by V‐V transfer and that the rotation is heated up more strongly with increasing ε, an effect also present for noble gas colliders.
Quantum scattering studies of collisional energy transfer from highly excited polyatomic molecules: A bend‐stretch model of He + CS2
Authors: Lendvay György , Schatz George C.
Published: July 2012

We present the results of an accurate quantum scattering study of collisional energy transfer in a bend‐stretch model of the He + CS2 system, considering energies up to 45 kcal/mol. These results are generated using a coupled channel method, with vibrational eigenfunctions obtained from a discrete variable representation method. Detailed comparisons with the results of classical trajectory' calculations are performed so as to assess classical/quantum correspondence for energy transfer moments, and for the energy transfer probability distribution function. We find good agreement of the energy averaged first moments over a wide range of molecular vibrational energies. The second moments, as well as <ΔE>up and <ΔE>down show less quantitative agreement. The quantum energy transfer distribution functions show considerable mode‐specific behavior, but the overall envelope is approximately exponential at large ∥ ΔE∥ with a broad spike near ∥ΔE∥ = 0. We analyze this behavior in terms of contributions from individual state‐to‐state transition probabilities. The corresponding classical distribution functions are very similar, showing better correspondence than was found for other models with smaller state densities.
Solvatochromism in binary solvent mixtures: Effects of the molecular structure of the probe
Authors: Novaki Luzia P. , El Seoud Omar A.
Published: July 2012

The solvatochromic behavior of 2,6‐diphenyl‐4‐(2,4,6‐triphenyl‐1‐pyridinio)‐1‐phenolate (RB), 1‐methyl‐8‐oxyquinolinium betaine (QB), sodium 1‐methyl‐8‐oxyquinolinium betaine‐5‐sulphonate (QBS), and 1‐methyl‐3‐oxypyridinium betaine (PB) was studied spectrophotometrically in mixtures of cyclohexane‐THF and cyclohexane‐1‐butanol. It was also studied in mixtures of water with acetone, acetonitrile, 1,2‐dimethoxyethane (DME), 1,4‐dioxane, and THF. Empirical parameters of solvent polarity, ET (in kcal/mol), were calculated from the position of the longest‐wavelength absorption band of the respective solvatochromic probe, and they were then correlated with the mole fraction of the more polar component of the binary solvent mixture (THF, 1‐butanol, or water). Although the non‐aqueous solvent mixtures behave ideally, plots of ET versus the mole fraction of THF, or 1‐butanol, are not linear for all probes, due to nonspecific (e.g., dipolar) interactions and specific solute‐solvent interactions (e.g., H‐bonding). The contribution of each type of interaction has been quantified and correlated with the structure of the solvatochromic probe molecule. For RB in cyclohexane‐THF mixtures, the deviation from linearity of the above mentioned plot is due to nonspecific probe dipole/solvent dipole interactions. For cyclohexane‐1‐butanol binary mixtures, both nonspecific and specific interactions contribute to the observed solvatochromism. For RB in binary aqueous mixtures, plots of ET versus the mole fraction of water are generally sigmoidal, and follow closely the changes of solvent structure, particularly in the microheterogeneous domain. The same plots for PB show a positive deviation from linearity, because this probe forms its own water‐rich solvation shell, whose composition is relatively insensitive to the nature of the organic co‐solvent. In all binary solvent mixtures, the solvatochromic behavior of QB is more or less similar to that of PB.
Gas phase addition kinetics of the tert‐butyl radical to oxygen
Authors: Dilger Herbert , Stolmár Martina , Tregenna‐Piggott Philip L.W. , Roduner Emil , Reid Ivan D.
Published: July 2012

The kinetics of the addition reaction of O2 to the tert‐butyl radical has been investigated at 1.5 bar as a function of temperature (241–462 K) using the muon spin relaxation technique in longitudinal magnetic fields. The data are represented by kch = 43+88‐26 X 10−10T−0.96±0.21 cm3 molecules−1 s−1, or alternatively by an Arrhenius activation energy of −2.4(0.7) kJ mol−1 and an apparent frequency factor of 68(19)x10−13 cm3 molecules−1 s−1. The results of these temperature‐dependent measurements agree well with an experimental literature value at room temperature, but disagree with a more recent estimate by a factor of six.
Surface‐embedded oxygen: Electronic structure of Ag(111) and Cu(poly) oxidized at atmospheric pressure
Authors: Schedel‐Niedrig Th. , Schlögl R. , Bao X. , Muhler M.
Published: July 2012

We investigated the surface electronic structure of an oxidized Ag(111) single crystal and a polycrystalline Cu foil prepared under conditions of pressure and temperature (1000 mbar and 900 K) far beyond those used in usual UHV studies. It has recently been shown that a strongly bound oxygen species (Oγ) apart from oxygen atoms chemisorbed on the surface (Oα) or oxygen atoms dissolved in the bulk (Oβ) can be prepared on silver surfaces. Using core‐level photoemission we show these three atomic oxygen species, Oα, Oβ and Oγ, exhibit characteristic spectral properties. X‐ray absorption spectroscopy at the O K‐edge indicates that the formation of the strongly bound Oγ species is accompanied by a strong hybridization between O2p and Ag4d states and that also the Ag5sp states are important for the metal‐to‐oxygen covalent interaction. The reaction with methanol was used to selectively remove the surface‐embedded atomic species and to study the effect of the strongly bound oxygen Oγ species on the valence electronic structure of silver. This separation of the oxygen species did not work for the more reactive copper‐oxygen system. The chemical bonding interaction of the novel Oγ species with silver is discussed in terms of the ligand‐field theory emphasizing the covalent character of the interaction. For copper, the spectroscopic analysis is complicated by the existence of a stable oxide Cu2O with similar bonding properties of the oxygen than postulated for the surface‐embedded species in silver.
Optical spectroscopy on Se8 clusters and Se chains embedded in zeolite matrices
Authors: Goldbach A. , Saboungi M.‐L.
Published: July 2012

Incorporation of Se into the supercages of zeolite Y leads to formation of Se8 rings and distorted Se chains with a ratio which is dictated by the nature of the zeolitic counter cations. Both rings and chains give rise to characteristic Raman and UV/vis absorption bands. In particular low frequency Raman bands are attributed to phonon localization in ordered segments of distorted chains.
Pore condensation in novel highly ordered mesoporous silica
Authors: Groß Stephan , Findenegg Gerhard H.
Published: July 2012

The pore condensation of SF6 in two novel ceramic silica materials, obtained by templating lyotropic phases of nonionic surfactants and of amphiphilic block copolymers, has been investigated over a temperature range from below the triple point up to the critical point of the fluid. One of the materials represent an ordered array of cylindrical pores (mean pore size 29 Å); the sorption isotherms for this material exhibit no hysteresis in the entire experimental temperature range. For the other material, which is believed to constitute a more complex pore system (mean pore size 34 Å), a pronounced hysteresis loop is seen near the triple point temperature but not at higher reduced temperatures. For both materials pore condensation disappears well below the bulk critical temperature. The experimental isotherms are used to determine the coexistence curve and the critical temperature of the fluid in the confined pore space.
Inelastic light scattering and phonon‐confinement in nanocrystalline Y2O3
Authors: Bruch Ch. , Krüger J. K. , Unruh H.‐G. , Krauss W. , Zimmermeier B. , Beck Ch. , Hempelmann R.
Published: July 2012

Nanocrystalline powders of Y2O3 with grain sizes in the range between 7 and 40 nm were prepared by the micro‐emulsion technique and by condensation from an inert gas atmosphere. The cubic respectively monoclinic powders were examined by means of Brillouin scattering (consolidated samples) and micro‐Raman scattering (powders and consolidated samples).

Measurements of the elastic properties of quasi‐isotropic pellets yield a longitudinal modulus reduced down to 25% compared with that of the monocrystal and point to optical inhomogeneities in the range of the wavelength of the light. With decreasing dimensions of the crystallites a shift of the characteristic Raman lines to lower frequencies and a distinct broadening of the lines are observed. A trial is made to interpret these features by the phonon‐confinement model of Richter et al. [1]. A possibility to characterize the nanocrystalline powders by means of Raman spectroscopy is discussed.
Investigation of the complex conductivity of nanocrystalline Y2O3‐stabilized zirconia
Authors: Mondal P. , Hahn H.
Published: July 2012

Nanocrystalline ZrO2‐ and Y2O3‐powders with grain sizes from 5‐20 nm were prepared by the Inert Gas Condensation method. The powders were mixed in different fractions, pressed into green bodies and sintered to ceramic samples with 35‐49 nm average grain size. The phase, grain size and specific surface area of the powders and ceramic samples was characterized by X‐ray diffraction and nitrogen adsorption. Densities from 82 to 93% of the single crystal density were determined using the Archimedes principle. The complex conductivity was measured for frequencies ranging from 20 Hz to 3 MHz at temperatures between 80 and 1100 K. The lattice relaxation, grain boundary relaxation and the contact process could be resolved and identified. The dc‐conductivity for the lattice and the grain boundaries were deduced from the data. The activation energies for dc‐conductivity were determined for the lattice and the grain boundaries as 0.85 ± 0.05 and 1.0 ± 0.1 eV, respectively.
Corrosion of gold by alkane thiols
Authors: Edinger K. , Grunze M. , Wöll Ch.
Published: July 2012

STM images of self assembled alkanethiol films on gold surfaces show holes which have been identified as vacancies in the topmost gold layer. We show that this vacancy formation is accompanied by a corrosion process of gold in ethanolic thiol solutions. The amount of dissolved gold from polycrystalline substrates as a function of immersion time, thiol concentration, chain length and solvent has been investigated by Atomic Absorption Spectroscopy (AAS). The results suggest that the thiol molecule itself is the active oxidant. In case of a 1 mmol solution of docosanethiol in ethanol a gold concentration corresponding to 6% of a gold monolayer has been detected after an immersion time of 24 h. By using hexane as a solvent the amount of dissolved gold is reduced to less than 0.2%, indicating a strong solvent (e.g. polarity) dependence for the corrosion process. The kinetics of the corrosion process have been measured and compared to kinetic data related to the formation of an ordered thiol film.
Hydrogen solubility and thermodynamics of dissolved hydrogen in Pd1‐xTbx(x = 0.05 and 0.08) solid solution alloys
Authors: Narayanan S. Shankara , Ramaprabhu Sundara
Published: July 2012

The hydrogen absorption and the thermodynamics of dissolved hydrogen in Pd1‐xTbx(x= 0.05 and 0.08) solid solution alloys have been determined from the pressure‐composition‐temperature relation in the ranges 5 ßP/mbar ß 1000 and 473 ßT/K ß 873 using a pressure reduction method. The hydrogen solubility is found to increase with increase of xTb.The partial molar enthalpy of hydrogen solution at infinite dilution (ΔH°H) and the partial molar excess entropy of dissolved hydrogen at infinite dilution (ΔSEH°) decrease with increase of terbium content. The chemical potential of dissolved hydrogen at infinite dilution (ΔH°H) decreases with increase of xTbindicating that the stability of dissolved hydrogen increases with increase of xTb. The Pd1‐xTbx‐H systems fall on the correlation between the ionic diameter (dRE) of the substitutent RE metals when in solution in Pd and the exothermicity of hydrogen absorption for the Pd1‐xREx‐H (RE = Er, Ho, Dy, Y, Gd and Sm;x= 0.05 and 0.075) systems.