bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermokinetics of reactions with liquid‐liquid phase separation
Authors: Baumann Christian , Becker† Friedrich
Published: July 2012

Isothermal heat flow calorimetry with controlled thermoelectric cooling resp. heating is applied to study the critical slowing down of the kinetics of reactions with liquid‐liquid phase separation. Results (i) on the acid‐catalysed hydrolysis of chloromethyloxirane by water, and (ii) on the ring‐opening and esterification of bromomethyloxirane by dichloroacetic acid in the presence of cyclohexane as an inert solvent, both at 298.15 K, are presented. The heat production w(t) which is proportional to the reaction rate ∂ζ/∂t (where ζ is the reaction coordinate) exhibits a funnel‐like fall‐off when the reaction path intersects the binodal curve at the critical solution point CP. From the shape of this part of the w(t) curves an average value of the critical exponent φ = 0.72±0.003, which refers to the change of ζ with t at constant T and p, is calculated. This result is to be compared with the theoretical value of φ = 0.708 obtained from a modification of the Griffiths and Wheeler theory of critical points in multi‐component systems to chemically reacting systems far from equilibrium by introducing ζ as an additional extensive variable. This allows the critical slowing down of the reaction rates to be interpreted as resulting from the divergence of the correlation length ζ resp. from the convergence of the transport phenomena to zero at the CP.
Computer simulations of crystal structures and elastic properties of cellulose
Authors: Marhöfer Richard J. , Reiling Stephan , Brickmann Jürgen
Published: July 2012

Structures and elastic properties (Young's moduli) of the different phases of cellulose I and II are studied on the basis of molecular dynamics (MD) simulations using a new force field for carbohydrates. The structural results for all phases as well as the Young's moduli for cellulose I and cellulose II with parallel chain arrangement are in very good agreement with experimental data whereas the calculated Young's modulus for cellulose II (with antiparallel chains) does not match the experimental error margin. Our results lead to the conclusion that cellulose II, as resulting from the mercerization, is arranged in parallel chains. Microscopically the transition from phase I to II can then easily be explained as a rearrangement of the exocyclic torsional angles.
Measuring the static dielectric constants of pure carbon dioxide and carbon dioxide mixed with ethanol and toluene at elevated pressures
Authors: Wesch A. , Dahmen N. , Ebert K. H.
Published: July 2012

Static dielectric constants (relative permittivities) of binary carbon dioxide/ethanol mixtures and carbon dioxide/toluene mixtures of various compositions at 10 MPa and 313 K, which were experimentally determined, are presented. In addition, the dielectric constant of pure carbon dioxide was measured at temperatures of 293, 313 and 336 K in the pressure range of up to 30 MPa. The measurements were conducted with a new type of sensor design. The dielectric constant of pure carbon dioxide shows a pronounced dependence on pressure close to the critical pressure. There is a nearly linear relationship between density and the dielectric constant. The dielectrical behavior of the mixtures of supercritical carbon dioxide with ethanol and toluene is qualitatively very similar to that of liquid benzene/ethanol and benzene/toluene mixtures at normal pressure.
Studying the energy hypersurface of multi‐minima systems — the threshold and the lid algorithm
Authors: Schön J. C.
Published: July 2012

A method is presented that allows the detailed study of regions of multi‐minima energy landscapes close to some deep‐lying minimum. Two versions are described the “lid algorithm” and the “threshold algorithm” for discrete and continuous landscapes, respectively. Applications of these algorithms to the travelling salesman problem (TSP), a two‐dimensional layer of Ne atoms, and the system Mg2F4 are sketched.
Study of the extrusion induced changes in the Li2O·2SiO2 glass structure by the methods of optical spectroscopy and radiation influences
Authors: Arbuzov V. I. , Carl G. , Durschang B. , Rüssel C.
Published: July 2012

The objects of investigation were Li2O·2SiO2 glass samples cutted out of the initial and the extruded glasses along and across the extrusion axis. All the glasses contained iron microimpurities (∼80 ppm). Some glasses were doped with Tb3+ ions the luminescence characteristics of which depend on average distances Tb3+ ‐Tb3+. It has been shown that the rates of radiation color centers accumulation as well as the rates of their isothermal annealing, on one hand, the absorption spectra of iron microimpurities, on the other hand, and finally the spectroscopic properties of terbium ions have been changing regularly when passing from extruded longitudinal sample to initial and finally to extruded transverse ones. All these results convincingly show that, under the conditions of extrusion, glass structure becomes less dense in its direction and more dense in the perpendicular one.
High‐temperature study of multivalent elements in glass‐forming melts: The particular case of iron
Authors: Tilquin J.‐Y. , Duveiller P. , Glibert J. , Claes P.
Published: July 2012

Linear sweep voltammograms and absorption spectra were recorded in Na2O·2.5 SiO2 containing about 1 wt% Fe at some temperatures ranging from 600°C to 1000°C. The reduction peaks which are observed in the melt at 600°C disappear at 1000°C. The absorption spectra obtained at high temperature are characteristic by a displacement of the charge transfer band and of the band related to octahedral Fe(II) towards higher wavelengths. No remarkable change in the Fe(III)‐Fe(II) redox equilibrium and in the coordination of the Fe(II) has been observed between 600°C and 1000°C.
Transport phenomena in oxidic glass‐forming melts
Authors: Pfeiffer Th. , Müller R. , Werner R. D.
Published: July 2012

Viscosities, electrical conductivities and Seebeck‐coefficients of a number of glass‐forming oxidic melts are presented and discussed. The data support the notion that glass melts do have a residual, temperature‐dependent structure that significantly determines mass, momentum and heat transport. It is shown that the use of Tg/T as reduced variable helps to develop a deeper insight in the underlying physics of these transport properties.
Solubility of gases in glass melts
Authors: Krämer Fritz W.
Published: July 2012

A short overview on gas solubility and solubility mechanism in oxide melts in given. Many gases of practical importance in glass melting like oxygen, nitrogen, carbon dioxide and sulfur dioxide and others dissolve in a glass forming melt both chemically by a chemical controlled process and physically by occupying the holes in the network structure. The chemical solubility at conditioning temperatures is orders of magnitude higher than the physical solubility and decreases with increasing temperatures. An exception is water whose chemical solubility increases with increasing temperature, and that is why water cannot be used as a fining agent. The physical solubility on the other hand increases slightly with increasing temperatures. At measuring physical solubilities mistakes can be made because of the omnipresent chemical solubility or because of impurities. Therefore the known relationship between physical inert gas solubilities and the size of the dissolving atoms or molecules was used to calculate the physical solubility of a lot of non‐inert gases for soda‐lime silica melts, for fused silica and for molten boric oxide.
A new approach on the coordination of al in non‐crystalline gels and glasses of the system Al2O3‐SiO2
Authors: Schmücker M. , Schneider H.
Published: July 2012

In the present study the idea of five‐fold coordinated aluminium in Al2O3‐rich aluminiumsilicate gels and glasses with compositions near to that of mullite (≈72 wt.% Al2O3, 28 wt.% SiO2) is re‐aximated. Analyses of 27AlMAS NMR spectroscopy and pair correlation functions on the basis of X‐ray diffraction data together with a critical review of crystallization mechanisms provide evidence for short‐range‐order similarities of mullite and corresponding gels and glasses. The study provides evidence for an occurrence of (Al/Si)O4‐tetrahedra and of AlO6‐octahedra in the gels and glasses. The (Al/Si)O4 to AlO6 ratios of non‐crystalline aluminium silicates are similar to that of mullite and comparable tricluster units of tetrahedra may occur in both, mullite and its non‐crystalline counterparts. There is less evidence that 5‐fold coordinated Al exists in aluminium‐silicate gels and glasses as was reported frequently in the literature.
Dynamics of lithium ions in monoconducting polymer electrolytes: A 7Li spin‐lattice‐relaxation study
Authors: Chung S. H. , Heitjans P. , Winter R. , Bzducha W. , Florjańczyk Z.
Published: July 2012

In monoconducting gel electrolytes prepared from polymeric lithium salt, poly(ethylene glycol), dimethylsulfoxide or propylene carbonate 7Li(I= 3/2) NMR spin‐lattice relaxation times (T1) at two resonance frequencies (24.5 and 77.5 MHz) have been measured for various compositions of the above system over the temperature region 0.75 to 2 Tg, with the glass transition temperature T″g≈200 K. The T1 data as a function of inverse temperature exhibit a cationic diffusion‐induced minimum. Both the temperature and the frequency of T1, indicate a change in the dynamical process at about 1.3 Tg.
Structural investigation of ferrimagnetic particles formed by glass crystallization
Authors: Hoell A. , Kranold R. , Lembke U. , Brückner R. , Müller R. , Görnert P. , Schüppel W.
Published: July 2012

By means of a definite heat treatment magnetite crystals were formed dispersed in a silicate glass, which initially contains Fe2O3 as a glass component. The specific saturation magnetization of the glass ceramic manufactured turns out to be dependent on the heat treatment conditions and shows a significant maximum for a certain temperature and treatment duration. Small angle X‐ray scattering, transmission electron microscopy, and X‐ray diffraction were used to obtain structural parameters. The correlations between size and packing density of the magnetite crystals and the magnetic properties are discussed.
Investigation of borosilicate electrode glasses using 11B SATRAS MAS NMR, 29Si MAS NMR and REDOR
Authors: Herzog K. , Peters J. , Thomas B. , Jäger C.
Published: July 2012

The structure of various hydrated and untreated glasses of the system Na2O‐B2O3‐SiO2 was investigated using multinuclear NMR. Two different BO4 units have been detected by 11B SATRAS (Satellite Transition Spectroscopy) NMR. Comparing the spectra of the untreated and hydrated glasses it was found, that nearly all BO3, units and one of the BO4 units are removed during the reaction of the glasses with hydrous solution. Furthermore, 1H‐11B REDOR (Rotational Echo Double Resonance) has been used to investigate the incorporation of protons in the hydrated glasses. New aspects containing the structural role of protons are obtained.
2D exchange NMR investigations of phosphate glass ceramic
Authors: Hartmann P. , Vogel J. , Jäger C
Published: July 2012

31P MAS and Exchange nuclear magnetic resonance (NMR) measurements were used to investigate the structure of P2O5‐CaO‐Na2O‐{Al2O3} glasses and glass ceramics. The progress of two‐dimensional (2D) Exchange NMR techniques is demonstrated.
The effect of alkyl groups on carbanion stability part II. Analysis of α‐ and β‐alkyl substitution on experimental and theoretical acidities
Authors: Balcioǧlu Nurettin , Sevin Fatma , Evin Özden (Özel) , Peynircioǧlu N. Bekir
Published: July 2012

Multivariational analyses were performed to separate the contribution of electronic and steric effects of alkyl groups on the experimental hydrocarbon acidities of four different sets of compounds, consisting of hydrocarbons (SET‐I), acetophenone derivatives (SET‐II), and acetone CH3COCHR1R2 derivatives, where the acidity of the protons on the alkyl substituted site, (SET‐III), and the acidity of the methyl protons are both investigated (SET‐IV). Significant regression was obtained upon backward elimination for the first and last set. Due to the colinearity of the parameters in SETs‐II and ‐III the significance of the correlation is not so high. As expected, the gas phase acidity of the hydrocarbons in SET‐I is very sensitive to the polarizability and electronegativity of the alkyl substituents. The substituent have similar effects on the solution acidities of the ketone derivatives in SETs‐II, ‐III, though suppressed due to solvation, unlike the cases in SET‐II and SET‐III, the effects of the substituents in SET‐IV, where the substituents are relatively far away from the carbanion center generated are strongly reduced and reversed. This different behavior may be attributed to the domination of the solvent effects. Theoretical acidities of these compounds were calculated using the semiempirical methods such as MNDO, AM1 and PM3. The results are, quantitatively and qualitatively, unsatisfactory for the gas phase, as well as for the solution acidities. Multivariational analysis of substituent effects shows the insufficiency of the present parametrization to reflect the substituent effect properly. It seems that all these semiempirical methods are useless in prediction of carbon acidities, without reparametrization using carefully selected sets of compounds.
Reactions of sodium/antimony‐clusters with molecular chlorine in a flow‐tube‐reactor coupled to a time‐of‐flight mass spectrometer
Authors: Hartmann A. , Castleman A. W. , Weil K. G.
Published: July 2012

A gas aggregation technique is used to produce NaxSby, clusters. After passing a flow‐tube reactor, the clusters are ionized with unfocused high fluence KrF‐excimer laser radiation and then mass analyzed in a time‐of‐flight mass spectrometer. Dramatic changes are observed in the cluster ion distribution upon adding high purity chlorine gas into the flow tube. As a result of reactions with Cl2, all NaxSby, clusters disappear while pure antimony cluster ions arise. The distribution patterns of the produced Sbx+ clusters are the same as those observed in experiments with pure antimony.
Role of electrolyte/polymer interface on the quantitative interpretation of in‐situ ellipsometry of electroactive polymer films
Authors: El‐Rahman H. A. Abd , Kudelka S. , Schultze J. W.
Published: July 2012

A new in‐situ ellipsometric procedure has been used for determination of the optical properties and thickness of polyaniline films as a function of. the redox state in sulphuric and p‐toluenesulfonic acids. The results could be interpreted successfully only when the optical properties of the electrolyte/polymer was considered variable with the redox state. Most probably for the first time, the study shows, that the apparent refractive index of electrolyte varies with the redox state in a similar manner to the optical properties and thickness of the polymer film. In this case, for all calculations a one layer model was sufficient. The origin of such observation is attributed to the change in electrolyte composition at the polymer surface and heterogeneity of the film when oxidized.
Density functional theory: Structure and interfacial properties of binary mixtures
Authors: Wadewitz T. , Winkelmann J.
Published: July 2012

Based on Density Functional Theory (DFT) of inhomogeneous fluid mixtures we model the liquid‐vapour interface of some binary mixtures. Attractive contributions to the Helmholtz free energy are treated by a mean‐field approximation (MF). The local density approximation (LDA) is applied to the repulsive reference system described by a hard sphere equation of state. A cut‐off and shifted Lennard‐Jones‐12‐6 potential divided according to the WCA‐prescription is used as interaction pair potential. Binary Lennard‐Jones parameter are introduced and fitted to experimental liquid‐vapour compositions to describe interactions between different species. The calculations give good results in the case of simple fluid mixtures like Ar‐N2 and CO‐N2. Similar results are obtained for the nonpolar mixture n‐heptane‐toluene. The predicted surface tensions γ agree very well with experimental data. Computed partial density profiles clearly show the enrichment of the component with the lower surface tension in the interphase. The relative Gibbs surface excess concentration is calculated from partial density profiles and compared with experimental values. The influence of activity coefficients on Gibbs adsorption is discussed.
Measurement and thermodynamic evaluation of phase equilibria in the Fe‐Ti‐O system
Authors: Eriksson Gunnar , Pelton Arthur D. , Woermann E. , Ender A.
Published: July 2012

The stability of pseudobrookite type solid solutions in the binary section FeTi2O5‐Ti3O5 is investigated in the temperature range 1060° to 1360°C by the quench method combined with chemical analysis and X‐ray diffractometry. These data are then combined with all other available phase equilibrium and thermodynamic data in order to obtain a set of optimized thermodynamic equations for the phases in the FeTiO3‐TiO2‐Ti2O3‐Fe system. Models for the ilmenite and pseudobrookite solutions are developed. The good agreement between the calculated phase equilibria and the measurements indicates the essential correctness of the thermodynamic data and the phase diagrams.
Primary reactions of oxygenic photosynthesis
Authors: Witt H. T.
Published: July 2012

Functional and structural aspects of the mechanism of oxygenic photosynthesis are described and documented by examples: (1) Coupling of two Photosystems I and II for the electron transfer from water to NADP+. (2) Vectorial electron transfer and transmembrane charge separation as the primary act of light energy conversion. (3) Functioning of the reaction centers I and II in the photosystems and the role of the pool of plastoquinones. (4) Proton translocation and use of the proton potential for ATP formation. (5) Approaches to the mechanism of water oxidation. (6) Organization of the proteins and their cofactors as well as the molecular structure of Photosystem I. (7) Light energy migration in the antenna system.
Comparison of electron paramagnetic resonance lineshape, orientation and power saturation of the TyrD radical from spinach and the green alga Scenedesmus obliquus
Authors: Schille Hilmar , Luzzolin Lucia , Dittme Jens , Dau Holger
Published: July 2012

TyrD is a redox‐active amino acid of the Photosystem II (PS II) D2‐protein. Upon oxidation it forms an unusually stable neutral radical with characteristic EPR properties. We prepared highly active BBY particles of spinach and the green alga Scenedesmus obliquus. The X‐band EPR spectra are similar with a peak to trough linewidth of 19.3 G for spinach and 19.0 G for Scenedesmus. Power saturation experiments were performed at 20 K. For unknown reasons, the signal of spinach satures slightly faster than that of the alga. Partially oriented PS II membrane particles exhibit comparable orientation‐dependent four‐line spectra. This indicates that the orientation of TyrD with respect to the membrane normal is similar in both species.
The pH‐dependence of the photosystem II fluorescence: Cooperative transition to a quenching state
Authors: Heinze Ilona , Dau Holger
Published: July 2012

Thylakoid membranes were isolated from various cultures of the green alga Scenedesmus obliquus which differed in the extent of the observable‐high‐energy‐state quenching of chlorophyll fluorescence (qE). In uncoupled thylakoid membranes the pH‐dependence of the fluorescence parameters FM and F0 exhibits two distinct phases: (1) for pH‐values ranging from 6.5 to 5.7, acidification results in a pronounced decrease of FM and F0; (2) for pH‐values below 5.7, acidification results in a slight decrease of FM and a significant increase of F0. The extent of the first phase of F and F quenching corresponds to the extent of qE in intact cells; the variability of qE of intact cells is not explainable by a pK‐shift. Simulations on basis of the reversible radical pair model using pH‐dependent molecular rate constants indicate the occurrence of a cooperative transition to a state with an enhanced rate of non‐radiative decay of excited chlorophyll‐singlet states (Hill coefficient n1 ≥ 4, pK1 ≈︁ 6.1 for all cultures). For pH‐values below 5.7, a pH effect on Photosystem II electron transfer reactions is involved (n2 ≈︁ 1, pK2 ≥ 4.5).
Evidence for delocalization of the triplet state 3P680 in the D1D2cytb559‐complex of photosystem II
Authors: Kamlowski A. , Frankemöller L. , van der Est A. , Stehlik D. , Holzwart A. R.
Published: July 2012

Spin polarized transient ESR spectra of 3P680 in D1D2cytb559‐complexes of Photosystem II are studied as a function of temperature. The spin polarization as well as the rise time of the ESR signals are characteristic of triplet formation via recombination from the primary radical pair P+•680Pheo−•. Below 100K the well‐known spectrum of 3P680 is observed with zero‐field splitting (zfs) parameters associated with the triplet state of monomeric chlorophyll‐a (Chl). Between 100 K and 200 K the spectral pattern changes drastically and above 200 K a considerably narrowed triplet spectrum (i.e. reduced zfs parameters) is observed which suggests a delocalization of the triplet excitation. When the temperature‐dependent spectra are normalized to the same triplet yield, isosbestic points are observed indicating that the spectra are weighted sums of the limiting case spectra at low and high temperature (10 K and 250 K, respectively). This is taken as evidence that a transition between two triplet states of different electronic origin occurs. The coefficients of this superposition follow a sigmoidal temperature dependence which suggests a distribution of activation energies for the triplet delocalization process in accordance with a glass‐like distribution of protein matrix conformations. The narrowing of the spectrum at high temperature can be interpreted as thermally activated delocalization of the triplet excitation over at least two Chl units with different orientations with respect to the PS II reaction center, viz. one oriented with its plane tilted 30° with respect to the membrane plane and the other with its plane perpendicular. The similarities and differences in the cofactor arrangement of PS II and purple bacteria reaction center are discussed.
Photophysical and photochemical properties of porphyrin aggregates
Authors: Siggel U. , Bindig U. , Endisch C , Komatsu T. , Tsuchida E. , Voigt J. , Fuhrhop J.‐H.
Published: July 2012

In aqueous media substituted protoporphyrins and amphiphilic tetraphenylporphyrins aggregate to fibers, tubules, helical ribbons and sheets. Six different types are presented, characterized by the excitonic shifts in the electronic absorption spectra. On excitation, the aggregates from octopusporphyrin show fluorescence and population of the triplet state. In other aggregates fluorescence, triplet state or both are effectively quenched.
X‐and K‐band transient EPR of the light induced radical ion pairs in photosynthetic model systems
Authors: van der Est A. J. , Fuechsle G. , Stehlik D. , Wasielewski M. R.
Published: July 2012

The light induced electron transfer in the complex 2‐tetraalkylphenylenediamine‐Zinc porphyrin‐2‐naphtho‐quinone (TAPD‐ZnP‐NQ) is investigated using transient EPR at X‐band (9 GHz) and K‐band (24 GHz). Spin polarized spectra are presented for the radical pair TAPD+•NQ−• which is generated by light excitation of the complex at 30 K in 2‐methyl‐tetrahydrofuran and the liquid crystal BDH E7. In the liquid crystal, the complex is partially ordered and the spectra depend on the orientation of the sample with respect to the magnetic field. A qualitative interpretation of the spectra on the basis of the coupled, correlated radical pair (CCRP) model is presented. The spectra of the partially oriented samples clearly show that: (i) the exchange interaction, J, makes a substantial contribution to the coupling between the radicals and is of the same order of magnitude as the dipole‐dipole coupling (ii) when this coupling is taken into account, the experimental spectra are consistent with the expected geometry of the complex. This system is one of the few cases in which the two couplings are of similar magnitude and allows their influence on the polarization pattern to be investigated. The value of J≈0.1 mT obtained from the spectra is at least two orders of magnitude larger than that found in the analogous radical pair, P+•Q−•, in photosynthetic reaction centres, although the average distance between the spins in the two systems differ at most by a factor of 1.5. Possible reasons for this difference are discussed.
Novel methodology for topological studies of photosynthetic membrane protein complexes
Authors: Vater J. , Heinze K. , Friedrich B. , Kablitz R , Blokesch A. , Irrgang K.‐D. , Thiede B. , Salnikow J.
Published: July 2012

Novel techniques have‐been developed for the investigation of photosynthetic membrane protein complexes from the thylakoid membrane of spinach. The protein components of photosystem II were fractionated by a solid phase extraction procedure using Chromabond cartridges. The cytochrome b6f complex was dissociated with high concentrations of the nonionic detergent n‐dodecyl‐β‐D‐maltoside. Subsequently the subunits of both membrane protein complexes were separated by high resolution reversed phase HPLC. Nearest neighbour relationships were investigated in the cytochrome b6f complex by molecular cross‐linking with o‐phthaldialdehyde. A methodology has been developed to identify the cross‐linking sites on the molecular level by tryptic digestion of the protein conjugates and separation of the fragments by high resolution HPLC. Matrix Assisted Laser Desorption Ionization‐Mass Spectrometry (MALDI‐MS) was used as a sensitive tool to identify the protein components and crosslinked peptide fragments of photosystem II and the cytochrome b6f complex.
GEQUAC, an excess Gibbs energy model for simultaneous description of associating and non‐associating liquid mixtures
Authors: Egner K. , Gaube J. , Pfennig A.
Published: July 2012

A physically founded model for the excess Gibbs energy, GEQUAC (Group‐surface Explicit QUAsiChemical theory), presented here, allows the description of associating as well as non‐associating liquid mixtures. This is achieved by explicit accounting for the different poles of the molecular surface between which strong interactions take place in a mixture. The model is tested against results of Monte‐Carlo computer simulations performed in lattice systems. Comparison of GE, HE and TSE determined from GEQUAC, computer simulations, and experiment shows that the thermodynamic behaviour of mixtures containing polar as well as hydrogen‐bonding components can be modeled.
Thermodynamic properties of the molten Tl‐GeTe4 system
Authors: Nakamura Yoshio , Kamiyama Takashi , Itoh Shinji
Published: July 2012

From the electromotive force method, the chemical potential or activity of Tl in the molten Tl‐GeTe4 system has been measured as a function of composition and temperature. The chemical potential of Tl has then been separated in terms of partial entropy and enthalpy, the both quantities showing an abrupt change from negative to positive values around the stoichiometric composition corresponding to Tl8GeTe4. The integral molar entropy and enthalpy, ΔSm and ΔHm, show sharp minima at the composition Tl8GeTe4 where the electrical conductivity has a sharp minimum and the thermoelectric power changes its sign. From these results it is suggested that the strong interaction between Tl and Te leads to form a chemical entity Tl2Te and the observed thermodynamic properties can be interpreted as a mixture of Tl2Te and unbounded atomic Ge and Te in the GeTe4‐rich side and or atomic Ge and Tl in the Tl‐rich side.
Dielectric constants, refractive indexes and polarizations for 1‐Alcohol +Heptane mixtures at 298.15 and 308.15 K
Authors: Sastry N. V. , Valand M. K.
Published: July 2012

New experimental data on dielectric constants, refractive indexes for the binary mixtures of heptane + 1‐pentanol, + 1‐hexanol, + 1‐heptanol, + 1‐octanol, 1‐decanol and + 1‐dodecanol at 298.15 and 308.15 K are reported. The molar and orientation polarizations were calculated for the pure and mixture components. The excess dielectric functions were estimated. The variation of correlation factor and free energy of mixing has been examined over the whole range of 1‐alcohol mole fractions for all the binary mixtures. The results are interpreted in terms of molecular interactions.
Observation of the nucleophilic displacement (SN2) reaction F− + CH3Br → CH3F + Br− Induced by dissociative electron capture in binary van der waals clusters
Authors: Lehmann Lars , Matejcik Stefan , Illenberger Eugen
Published: July 2012

We report on the SN2 reaction F− + CH3Br → FCH3 + Br− in binary clusters composed of CH3Br and C2F6 in a crossed electron/molecular beam experiment. The reaction is induced by resonance dissociative electron attachment to the C2F6 component where the nucleophile F− is generated at an energy (around 3.8 eV) where CH3Br does not capture electrons to form Br−. The results indicate that the cross section for the SN2 reaction is independent of the total excess energy which can be varied between 1.7 and 5 eV in the present system.
Variable reaction coordinate direct RRKM theory
Authors: Klippenstein Stephen J. , Allen Wesley D.
Published: July 2012

Various aspects of the direct implementation of the variable reaction coordinate formalism for evaluating the kinetics of barrierless reactions are considered. A new discussion of the decoupling of the conserved and transitional modes within this formalism is provided with sample results for NCNO suggesting an optimum simple procedure for implementing this decoupling. The effort involved in replacing analytic potentials with direct ab initio determinations in the Monte Carlo‐based evaluation of the transitional mode partition functions is illustrated through sample calculations for Cl− +CH3Cl, CN + O2, CN + NO, and 1CH2+CO. Some procedures for improving the convergence properties of such direct statistical evaluations are also suggested. A brief summary is given of a procedure for directly evaluating the effects of anharmonicities and certain rovibrational couplings on the density of states for the complex. The results from our previous direct variable reaction coordinate RRKM theory study for the dissociation of singlet ketene are slightly revised on the basis of the improved decoupling scheme. The results for both the dissociation rate constant and vibrational distributions are in quantitative agreement (i.e., within 20% throughout the energy ranges considered) with the corresponding recent experimental results. Finally, a possible procedure for making greater use of the ab initio data, and perhaps also obtaining improved convergence, is summarized.
On nascent angular‐momentum distributions in complex‐forming bimolecular reactions
Authors: Olzmann M.
Published: July 2012

A detailed modelling of a complex‐forming bimolecular reaction requires the knowledge of the nascent energy and angular‐momentum distribution of the intermediate complex. A method is presented to calculate the distribution of the orbital angular momenta as a result of reactive collisions in a radial potential. For the case of thermal collisions, analytical solutions are obtained. Reactions of photochemically generated species are also considered. The method is applied to the reaction of CH4 with O+2 and O(1D), respectively.
Picosecond kinetics of trans‐cis‐photoisomerisations: From jet‐cooled molecules to compressed solutions
Authors: Schroeder Jörg
Published: July 2012

The understanding of photoisomerisation reactions involving large amplitude motion is discussed with special emphasis on the merits of systematic studies over wide ranges of physical conditions. It appears that the friction β in such cases is proportional to macroscopic solvent viscosity η, and the original Kramers theory is sufficient to describe the effect of friction in the range from intermediate to strong damping, i.e. from low viscosity fluid to compressed high viscosity liquid solution. This one‐dimensional model may be connected to multidimensional unimolecular rate theory to obtain a representation of the density dependence of the rate coefficient down to the thermal collisionless regime. The apparent failure of this approach in the case of trans‐stilbene can be traced to modifications of the excited state PES by the solvent. This interpretation is consistent over the entire range of physical conditions studied, starting at the jet‐cooled isolated molecule and solvent cluster and ending in compressed liquid solution at very high viscosities. It is shown that detailed information about the PES is urgently needed to provide safer ground for a profound understanding of the photoisomerisation dynamics.
In situ Raman spectroscopy for the study of high‐pressure reactions
Authors: Kessler W. , Luft G. , Zeiß W.
Published: July 2012

An optical high‐pressure cell has been developed for spectroscopic investigations of rapid high‐pressure reactions. It can be coupled with commercial Raman spectrometer. The measuring cell is suitable for use at pressures of up to 2000 bar and temperatures of up to 200°. The advantages of in situ Raman spectroscopy are demonstrated in the course of an investigation of the decomposition of tertiary butyl peroxypivalate in solution. This organic peroxide is a standard initiator for the radical high‐pressure polymerisation of olefins for which it is used at pressures of 1300 to 3000 bar and temperatures of 160 to 190°. The rate of decomposition was measured within a wide pressure and temperature range. From the results the half‐life, the frequency factor, the energy of activation and the activation volume of the peroxide decomposition reaction were determined. The data thus obtained were compared with data in the literature which had been obtained by other methods.
Strain‐birefringence of a polyelectrolyte gel
Authors: Hahn O. , Woermann D.
Published: July 2012

Results of measurements of the stress‐optical coefficient and the Young elasticity modulus of ‐SO3− and ‐CH2‐SO3− polyelectrolyte gels are reported. Upon stretching the gels become birefringent. The birefringence is assumed to be caused by a partial orientation of optical anisotropic structures formed by highly cross‐linked regions which are embedded in nonbirefringent regions with a lower concentration of cross links. These results give support to a proposed structural model of the gels which takes into account small‐angle and ultrasmall‐angle X‐ray scattering data as well as information about the transport properties of the gels in form of membranes.
Adsorption and selective oxidation of H2S on alumina powders: In‐situ UV‐VIS studies in a differential reactor
Authors: Melsheimer J. , Lee J.K. , Schlögl R. , Böhm M.C.
Published: July 2012

In‐situ UV‐VIS spectroscopy in transmission and reflection mode as well as on‐line GC and TPD MS were performed on pressed and powdered wafers made from industrial aluminas in different microreactor cells at 400 K. These investigations were carried out on activated samples using a continuous gas flow of He containing either H2S (1%) or SO2 (0.5%) or H2S (1%) with SO2 (0.5%) or O2 (0.5 and 5.0%).

The adsorbed reactants lead to an absorption band at 290 nm (H2S and SO2). This absorption band could be attributed to chemisorbed species.

Using a differential microreactor for transmission spectroscopy, a conversion of ca. 10% was measured in the steady state and an attribution of the different visible absorption bands to

to

(H2S oxidation by O2) and

to

(modified Claus reaction, 2H2S + SO2  (3/x) Sx + 3H2O) seems plausible. The species S may be responsible for the stationary conversion during the modified Claus reaction.
Intracluster electron and proton transfer in ionic ortho‐xylene/dimethylamine aggregates
Authors: Ring S. , Koch S. , Baumgärtel H.
Published: July 2012

Heteroclusters of ortho‐xylene (OX) and dimethylamine (DMA) were studied by resonance‐enhanced two‐photon ionization spectroscopy (R2PI). The obtained mass spectra consist mainly of reaction products from dissociative electron and dissociative proton transfer reactions in the ionic aggregates. In the 1:1 aggregate, a competition between both reaction pathways is observed. Proton transfer, being the favoured reaction pathway in the 1:1 aggregate after resonance enhanced two‐photon ionization, is suppressed when going to higher clusters as a consequence of structural and energetic changes in the aggregates. Due to the high reactivity of the ionic aggregates, intermolecular fragmentation is only observed after intracluster reaction.
A dynamic 15N NMR study of kinetic hydrogen/deuterium isotope and tunnel effects on the triple proton transfer in crystalline 3,5‐dimethylpyrazole
Authors: Aguilar‐Parrilla Francisco , Limbach Hans‐Heinrich , Klein Oliver , Elguero José
Published: July 2012

Using dynamic solid state 15N CPMAS NMR spectroscopy (CP≡cross polarization, MAS≡magic angle spinning) the kinetics of the degenerate intermolecular triple proton and deuteron transfer in the cyclic trimers of 15N‐labeled polycrystalline 3,5‐dimethylpyrazole (DMP) have been studied in a wide temperature range. At high temperatures, rate constants of the various isotopic HHH, HHD, HDD, and DDD transfer reactions are obtained in the millisecond timescale by lineshape analysis of partially deuterated doubly 15N‐labeled DMP. At low temperatures, the kinetics were followed by magnetization transfer methods in the laboratory frame. In order to suppress artifacts arising from 15N‐spin diffusion these experiments were performed on singly 15N labeled DMP, partially diluted in the non‐labeled material. As multiple kinetic hydrogen/deuterium isotope effects on triple proton transfer reactions have not yet been studied these effects are modeled theoretically for the single barrier case involving a concerted proton motion and for the triple barrier case where the three protons are transferred stepwise. The experimental kinetic isotope effects obey the rule of the geometric mean, i.e. kHHH/kHHD ≈︁ kHHD/kHDD ≈︁ kHDD/kDDD ≈︁ 3.6 i.e. ≈︁ kHHH/kDDD ≈︁ 47 at 300 K, which is indicative of a single barrier where all hydrons loose zero‐point energy in the transition state. At low temperatures, strong deviations from an Arrhenius behavior are observed for all isotopic reactions, indicating incoherent triple hydron tunneling processes, which are described in terms of a modified Bell tunneling model.
Pressure/composition isotherms of proton conducting SrYb0.05Zr0.95O2.975/H2O by means of nuclear resonance reaction analysis
Authors: Hempelmann R. , Eschenbaum J. , Altmayer M. , Groß B. , Grambole D. , Herrmann F. , Nagengast D. , Krauser J. , Weidinger A.
Published: July 2012

Films of SrYb0.05Zr0.095O2.975 were prepared by sol‐gel processing. Water vapour pressure/hydrogen composition isotherms were recorded ex‐situ comprising pressures between 0 and 84.5 kPa and temperatures between 873 and 1073 K. Nuclear Resonance Reaction Analysis (NRRA) was used to determine the hydrogen concentration and, in addition, depth profiles of hydrogen concentration. The chemical potential of interstitial hydrogen in oxides could be determined; from its temperature dependence thermodynamical constants were calculated. We observe two hydrogen fractions with a site energy difference of 20 kJ/mol. A size effect of the thermodynamic properties is observed below a thickness of 350 nm: nanofilms exhibit higher solubility than bulk material.
NMR studies of catalytic reaction of methanol absorbed in trisodium 12‐molybdophosphate crystal
Authors: Ishimaru Shin'Icbi , Ikeda Ryuichi , Nakamura Nobuo
Published: July 2012

The molecular dynamics and the catalytic reaction of methanol absorbed in 12‐heteropoly compound Na3PMo12O40 were studied by 1H, 2H, 13C, and 31P solid state wideline and/or MAS NMR spectra. Stoichiometric compounds Na3PMO12O40·nCH3OH where n = 6 and 9 were obtained under constant partial pressures. The presence of two kinds of methanol molecules, mobile and almost rigid, was revealed in crystals. Production of dimethylether and reduction of heteropoly anions were observed in the specimen heat‐treated at 200°C. We succeeded to detect an ester‐like species between heteropoly anion and methanol which can be assigned to an intermediate product of the reaction. Based on this finding we proposed a microscopic reaction mechanism of the catalytic reaction.
Colloidal assemblies used as templates to control the size, shape and self organization of nanoparticles
Authors: Pileni M. P.
Published: July 2012

In this paper we show that the use of colloidal assemblies as templates favors the control of the size and shape of nanoparticles. As expected theoretically, the change in size and shape of copper metal nanosized particles induces changes in their optical properties. Cylindrical copper metal particles having the same size and shape can be obtained in various regions of the phase diagram when the template is made of interconnected cylinders.

Self‐assembly of silver and silver sulfide nanoparticles is reported. Monolayers of particles organized in a hexagonal network are formed over very large domains. Small or large aggregates can also be produced and, in these aggregates, the particles are highly organized and form pseudo‐crystals with a face‐centered cubic structure for various particle sizes.
Chemistry of nanosized silica‐coated metal particles‐EM‐study
Authors: Giersig Michael , Liz‐Marzán Luis M. , Ung Thearith , Su Dangsheng , Mulvaney Paul
Published: July 2012

Colloidal gold and silver are difficult to manipulate. The surfaces are normally coated with organic acids or polymers, and conventional methods for thin metal oxide coatings normally result in clumping and heterocoagulation. We have recently developed a method to silica‐coated gold and silver particles which avoids any particle coalescence. The method can be applied to a wide range of nanoparticles [1,2]. In this report we present some results from studies into chemical reactions of nanosized coated metal particles using high resolution transmission electron microscopy. The chemical reactions were carried out on silica‐coated silver and gold nanoparticles in aqueous solution. The reaction of a single Ag@SiO2 nanoparticle with I2 to form AgI was monitored directly by transmission electron microscopy. As well the dissolution of Au cores by cyanide ion led to the creation of monodisperse, hollow, nanosized silica particles.
Structure determination and homogeneous optical properties of CdS/HgS quantum dots
Authors: Mews A. , Banin U. , Kadavanich A. V. , Alivisatos A. P.
Published: July 2012

The structural and optical properties of heterogeneous semiconductor nanoparticles consisting of CdS and HgS are investigated by High Resolution Electron Microscopy (HRTEM) and selective spectroscopy like Hole Burning (HB) and Fluorescence Line Narrowing (FLN). The HRTEM study shows that epitaxy is possible in nanocrystals, provided the crystallites have well defined faceted shapes to begin with. From the HB‐ and FLN experiments homogeneous absorption and fluorescence spectra are calculated. It could be shown that the absorption is coupled to HgS‐like phonons (250 cm−1) whereas the emission frequency is closer to the LO phonon frequency of CdS.
Photosensitization of composite metal oxide semiconductor films
Authors: Bedja I. , Hotchandani S. , Kamat P. V.
Published: July 2012

New approach of surface modification consisting on mixing SnO2 and TiO2 colloids and packing them as a composite film (SnO2‐TiO2) on a conducting glass surface has led to the increased sensitized photocurrent quantum yields. The photoinjected electrons from the sensitizer into TiO2 are quickly transferred into lower lying conduction band of SnO2 carriers thus promoting the charge separation. High internal photocurrent quantum yield for CdS film as sensitizer, approaching unity (φ = 95%), has been obtained. In addition, composite SnO2‐TiO2 nanocrystalline films improve the charge separation in adsorbed bis(2,2′‐bipyridine)(2,2′‐bipyridine‐4,4′‐dicarboxylic acid) ruthenium complex (Ru(II)) too. Higher internal photocurrent quantum yield (φ = 92%) has been obtained for Ru(II) by replacing SnO2 carriers against composite SnO2‐TiO2 particles in the film. On the other hand, the later composition does not show any increase in the photovoltage.
Synthesis, morphology and optical properties of thiol‐stabilized CdTe nanoclusters in aqueous solution
Authors: Rogach A. L. , Katsikas L. , Kornowski A. , Su D. , Eychmüller A. , Weller H.
Published: July 2012

Thiol‐stabilized CdTe nanoclusters with sizes ranging from 1.3 to 2.4 nm have been prepared in aqueous solution and separated as oxidize‐stable powders. Investigation of CdTe nanoclusters by means of X‐ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) indicates the cubic (zincblende phase) CdTe crystalline structure. Pronounced electronic transitions in the absorption spectra are shifted dramatically towards higher energies in comparison with bulk CdTe as a consequence of quantum confinement in nanosized CdTe particles. Two kinds of size‐ and surface‐dependent photoluminescence (“excitonic” and trapped) of CdTe nanoclusters in aqueous solution were observed. A further growth of CdTe nanoclusters leads to a shift of both absorption edge and “excitonic” photoluminescence band towards longer wavelengths.
Colloids and polymers: Amphiphilic block copolymers
Authors: Förster Stephan
Published: July 2012

Amphiphilic block copolymers self‐assemble into a variety of mesoscopic structures comprising micellar and ordered continuous topologies. Size and topology of these structures can be controlled by variation of block lengths. Their stability allows the preparation of well‐defined polymer/inorganic hybrid materials. Quantum‐size semiconductor colloids as well as nanometer‐sized noble metal colloids can be prepared within block copolymer microdomains. Enhanced stability of such colloids allows to use them as efficient catalysts.
Synthesis, structure and properties of coagulated ultrathin membranes
Authors: Rehage Heinz , Achenbach Barbara , Kaplan Adnan
Published: July 2012

The Coulomb interactions between charged surfactants and multivalent counter‐ions can be used to form coagulated cross‐linked membranes at the surface of water. We have systematically measured the kinetics of the two‐dimensional gelation and the rheological properties of the stabilized films. The membranes, thus prepared, exhibit striking rubber‐elastic features. The results of Brewster‐Angle‐Microscopy indicate that these films consist of disc‐like aggregates, having typical diameters of about 10 μm. These particles are in close contact and form fractal network clusters. The shape of these structures is in general agreement with aggregates calculated from percolation theories.
Nanocrystalline materials: Nanocrystalline metals and oxides I: Pulsed electrodeposition
Authors: Natter H. , Schmelzer M. , Janßen S. , Hempelmann R.
Published: July 2012

Three preparation routes are presented which, in a simple and cheap way, allow to produce rather large amounts of nanocrystalline solids with controlled particle sizes and narrow particle size distributions: the pulsed electrodeposition for nanocrystalline metals and alloys (this contribution), and the precipitation in microemulsions of nanocrystalline metal fluorides as well as the sol‐gel type hydrolysis in microemulsions for nanocrystalline metal oxides (next contribution).

By means of pulsed electrodeposition we have prepared n‐Pd, n‐Cu, n‐Ni, n‐Co and n‐Fe as well as n‐Ni‐Cu and n‐Ni‐Fe alloys. The particle sizes are analysed preferentially using scattering methods; further characterisations concern chemical analyses and density or porosity measurements, respectively. The thermal stability is remarkable; grain growth is not detectable below 150°C.

As an example of a particular physical property we present the very fast hydrogen diffusion in nanocrystalline Pd as investigated by means of quasielastic neutron scattering, an experimental technique which requires large amounts of sample.