bluuebunny/crossref_metadata_2025 huggingface dataset:
Proton NMR studies on the structure of water in ionic and nonionic water‐in‐oil microemulsions
Authors: Seoud Omar A. El , Okano Laura T. , Novaki Luzia P. , Barlow Graham K.
Published: July 2012

The structure of water present in water‐in‐oil microemulsions of anionic magnesium bis(2‐ethylhexyl) sulfosuccinate in C6D6, cationic cetyldimethylbenzylammonium chloride in C6D6, cetyldimethyl‐3‐phenylpropylammonium chloride in chlorobenzene, and nonionic polyoxyethylene (4) dodecyl ether in n‐heptane was probed by 1H NMR. Chemical shifts of solubilized H2O‐D2O, and of surfactant discrete protons were determined, at fixed [water]/[surfactant] ratios, as a function of the deuterium content of solubilized water. Chemical shift data were used to calculate the so called “fractionation factor”, φ, of the aggregate‐solubilized water. Calculated φ for the different systems are unity, showing that micellar water, although different from bulk water, does not seem to coexist in “layers” of different degrees of structure, as suggested, e.g., by the two‐state water solubilization model.
Methylviologen redox reactions at semiconductor single crystal electrodes
Authors: Schoenmakers G. H. , Waagenaar R. , Kelly J. J.
Published: July 2012

The mechanism of the two‐step reduction of the methylviologen cation radical (MV2+) was studied at CdS, GaAs and InP single crystal electrodes. In aqueous solution the second step (MV+• → MV0) gives rise to layer formation which is not observed in mixed ethanol/water solutions. This layer has interesting electrical properties. The reoxidation of MV0 occurs by electron injection into the conduction band of all three semiconductors. The reoxidation of MV+• at InP is also a conduction band reaction and electroluminescence is observed with the p‐type semiconductor. Electron injection from MV+• into the conduction band of GaAs is not observed while the kinetics of the reaction at CdS depend critically on the state of the electrode surface and on the nature of the electrolyte solution.
Solvent dependence of electron‐transfer sensitized photolysis of onium salts
Authors: Eckert Gerd , Goez Martin , Maiwald Beate , Müller Uwe
Published: July 2012

The photoreactions of 9,10‐diphenylanthracene with two iodonium salts and a sulfonium salt in different solvents were investigated by fluorescence quenching, flash photolysis with UV/vis detection, measurements of the quantum yields of sensitizer decomposition, and photo‐CIDNP spectroscopy. Energy‐transfer sensitization is precluded by thermodynamics in these systems. In a solvent of lower polarity such as dimethoxyethane, the quenching rate constant is still about a third of the value in acetonitrile; the quantum yields of formation of radical cations and the quantum yields of sensitizer decomposition are decreased by about the same factor. In contrast, dramatic effects occur in the CIDNP spectra of the iodonium salts, which derive from two consecutive radicals pairs, the first containing the radical cation DPA± of the sensitizer and the radical On• of the onium salt, the second comprising DPA± and a phenyl radical produced by fragmentation of On•: In less polar solvents, the polarizations from the first pair are strongly reduced, and the polarizations from the second pair are larger by an order of magnitude. From a kinetic analysis, this effect was attributed to a higher rate of transformation of the first radical pair into the second in the less polar solvent. For the sulfonium salt, this transformation is faster than in the case of iodonium salts, so even in acetonitrile all polarizations stem from the second radical pair. S‐T±‐type CIDNP, which had been proposed in the literature for analogous chemical systems, could be excluded unambiguously, and all CIDNP effects could be explained by radical pair theory (S‐T0‐type CIDNP).
Fluorescence quenching of meso‐substituted anthracene derivatives by molecular oxygen
Authors: Wirp Christoph , Güsten Hans , Brauer Hans‐Dieter
Published: July 2012

Rate constants of fluorescence quenching by molecular oxygen, kSq, of a series of meso‐substituted anthracene derivatives have been determined in toluene and acetonitrile at room temperature. They adopt values between 0.4 and 3.4 × 1010M−1 s−1 in toluene and values between 0.7 and 5.5 × 1010M−1 s−1 in acetonitrile. The ratio k(toluene)/k (acetonitrile) in all cases nearly equals the inverse ratio of the solvent viscosities η(acetonitrile)q/η(toluene) = 0.61. Additionally, singlet oxygen (1O2) efficiencies for quenching of the singlet state ϕ and the triplet state ϕ have been measured in both solvents. No significant decrease of the efficiencies of 1O2 formation in the polar solvent compared to the non‐polar solvent has been observed for the meso‐substituted anthracenes. In both solvents ϕ varies approximately linear with 1/kSq. The weak dependence of (1 ‐ ϕ)k = k(3O2) on the free enthalpy of electron transfer suggests the participation of charge transfer in the quenching process, but small and almost identical slopes of Hammett‐plots of the rate constants of fluorescence quenching in both solvents indicate that there is only little contribution of charge transfer interactions if the rate constant of fluorescence quenching is below the diffusion limit.
HF infrared chemiluminescence in the reactions of fluorine atoms with cis‐Butene‐2, trans‐butene‐2 and 2,3‐d2‐butene‐2
Authors: Dehe Katharina , Heydtmann Horst
Published: July 2012

For the reactions F+cis‐butene‐2 and F+trans‐butene‐2 identical vibrational distributions for the product molecule HF were determined: Nv=1 : Nv=2: Nv=3: Nv=4 = 0.26:0.33:0.26:0.15. Two reaction channels for each compound are responsible for this distribution. Isolated reaction channels were observed in the reaction system F+2,3‐d2‐butene‐2. The determined distributions for the latter reactions were N: N: N: N = 0.26:0.26:0.31:0.17 for HF and Nv=1, CD: Nv=2, CD: Nv=3, CD: Nv=4, CD = 0.23:0.38:0.31:0.08 for DF. For the latter system also rotational distributions are documented which indicate several microscopic channels. The comparison of F+d2‐butene‐2 with other F+RH reactions (RH = saturated hydrocarbon) shows a decrease in vibrational inversion due to the presence of a II‐bond system.
On the relative leaving group abilities of imidazole and the substituted phenoxide ions. The effect of the acyl group
Authors: Neuvonen Helmi
Published: July 2012

Rate coefficients for the imidazole‐catalysed hydrolysis of substituted phenyl acetates, chloroacetates and dichloroacetates have been determined in 40% (v/v) acetonitrile‐water. It is shown that the increasing electron‐withdrawal by the acyl group makes the general base‐catalysed nucleophilic reaction of imidazole detectable beside the uncatalysed nucleophilic attack of the amine. This is attributed to the acyl group‐induced change of the partitioning ratio of imidazole and the ester leaving group. A kinetic evidence is obtained of a change in the rate‐limiting step with increasing imidazole concentration for the imidazole‐catalysed reactions of 4‐chloro‐ and 4‐methoxyphenyl dichloroacetates.
Single molecule spectroscopy in solids: Quantum optics and spectral dynamics
Authors: Basché Th. , Bräuchle C.
Published: July 2012

In recent years different microscopic and spectroscopic techniques were successfully employed to detect the fluorescence signals of single dye molecules in the condensed phase. In low temperature solids the fluorescence excitation line of a single dopant molecule is a high resolution optical probe of its local nanoenvironment. As inhomogeneous contributions and ensemble averaging are absent, many effects hidden when looking at large populations of molecules become observable. Two important research areas in single molecule spectroscopy deal with the investigation of quantum optical effects and the spectral jumping behaviour of single molecule excitation lines caused by fluctuations of two‐level systems inherent to disordered solids. Rather than delivering a complete presentation, we will focus on some selected examples which emphasize the unique features of spectroscopy with a single absorber.
Reactivity of the oxide growth process on zr metal in aqueous potassium iodide and other alkali halide solutions
Authors: Ghoneim A. A. , Ameer M. A. , Saleh H. M. , El Heakal F. ‐Taib
Published: July 2012

The kinetics of growth of an oxide film on Zr electrode under open circuit conditions in Kl solutions were investigated using potential and impedance measurements. The effect of some parameters such as surface pre‐treatment, concentration and salt type, including alkali halide anions, on the reactivity of the oxide growth process were studied. In all cases the potential, capacitance and resistance were followed with time, and the oxide film was found to thicken continuously.

The iodide ion is assumed to participate in enhancing the formation of species which inhibits partially the growth of oxide film on Zr surface. The impedance diagrams of the oxide film/electrolyte interface is an almost capacitative one, with an impeded charge transfer reaction.
Systematic investigation of the global phase behavior of associating binary fluid mixtures: I. mixtures containing one self‐associating substance
Authors: Kraska T.
Published: July 2012

Global phase diagrams have been calculated for binary mixtures containing one self‐associating and one isotropic substance. The calculations have been made on the basis of an equation of state. This equation consists of the van der Waals equation as isotropic part and the association term of the Statistical Association Fluid Theory (SAFT) equation. The results are presented in two‐dimensional intersections of the global phase diagram. The influence of the self‐association on the binary behavior is discussed. Comparisons with similar investigations of the van der Waals, Redlich‐Kwong, and Carnahan‐Starling‐Redlich‐Kwong equations of state have been made. Further comparisons with recent calculations of binary water systems have been accomplished.
I. Glass Transition. Theoretical concepts on the glass transition of polymers and their test by computer simulation
Authors: Binder K.
Published: July 2012

Various organic molecules, in particular polymers, are extremely good glass formers and allow the study of supercooled melts near the glass transition in metastable equilibrium. Theories of the glass transition imply such an equilibrium (e.g. mode‐coupling theory, or Gibbs‐di Marzio theory) and can hence be tested in these systems.

Simplified lattice models for polymer melts (e.g. the bond fluctuation model) have been developed that can very efficiently be studied by Monte‐Carlo simulation, and although they fail to accurately describe the local structure, they describe many features of the experiments very well. In this model, the mechanism of the glass transition is a competition between the (energetically favorable) local stretching of effective bonds, and the (entropically favorable) dense packing of effective monomers (“geometric frustration”). It is shown that the model exhibits a dramatic slowing down upon cooling, and the properties of the frozen‐in glass distinctly depend on the cooling rate. While the Gibbs‐di Marzio theory predicts correctly the temperature region where the configurational entropy of the supercooled melt strongly decreases, the vanishing of the entropy is traced back to a severe underestimation of the entropy of the reference state (high temperature melt). The idealized version of the mode‐coupling theory (involving critical singularities above the freezing transition) is found to be not very useful either, as the singularities are strongly rounded. But the extended version of the mode‐coupling theory which describes this rounding, is in nice agreement with the simulations.
Progressive transformation from quartz to amorphous silica by shock pressure: Role of theα→βtransition
Authors: Chemarin C. , Panczer G. , Badjukov D. D. , Champagnon B.
Published: July 2012

The transformation from crystallineα‐quartz to silica glass under shock pressure is followed, in the intermediate phase where both crystalline and amorphous SiO2are detected, by Raman microspectrometry. Experiments are carried out on samples recovered from increasing peak pressure. The evolution of the crystalline phase is compared to the evolution ofα‐quartz with a thermal annealing. No observation is made for aβ‐phase transformation before full amorphization of the material.
Phosphorescent glass
Authors: Bauerecker S. , Roch Th. , Cammenga H. K.
Published: July 2012

Using low‐melting enamel glasses it could be managed to develop a protecting glassy embedding for luminescent pigments. Such a lasting protection from greying ‐ especially against UV light in the presence of humidity ‐ offers new operational areas to luminescent pigments like the outdoor area. In the present work the luminescence spectra and decay functions of phosphorescent glasses are examined. For a ZnS pigment and a newly developed SrO‐aluminate pigment of high phosphorescence efficiency it could be shown that the emission spectra of the phosphorescent glasses do not essentially differ from those of the pure pigments. The phosphorescence efficiency of ZnS phosphorescent glass could be increased by more than 10 per cent by reducing the TiO2 content regarded as optimal up to now. First experiments show that an SrO‐aluminate pigment can also be embedded in a glassy matrix. Indeed, intensity losses of 70 per cent appeared. Here a high potential of improvement is to be assumed. Further investigations are necessary. In addition we have found that the phosphorescence decay functions of the samples under investigation depend on the excitation time. The phosphorescent glass can be universally applied as granules or as formed bodies and offers manifold applications.
II. Crystallization. Crystallization of glass forming melts
Authors: Gutzow I. , Dobreva A.
Published: July 2012

A survey on the kinetics of nucleation and the mechanisms of crystal growth is given. Theoretical and experimental developments verifying the more general, non steady state formulation of the theory of nucleation are discussed. It is argued that the experimentally observed transiency in the kinetics of nucleation in glass forming systems may be due to both non steady state time lag as well as to growth effects. In order to distinguish between both cases, an analysis of the temperature dependence of the induction times, t, and a comparison with the temperature dependence of the non steady state time lag and the rates of nucleation and growth are required. Experimental results on the mechanisms of crystal growth are provided.
Direct spectroscopic investigation of phase formation in glasses containing semiconductor microcrystals in correlation to their luminescence properties
Authors: Müller M. , Woggon U.
Published: July 2012

The formation of CdSe microcrystals in a glass matrix is studied by UV‐Vis in situ spectroscopy. Hereby the temperature range is found where nucleation takes place. The size distribution of the CdSe microcrystals depend on the growth rate, this means on growth temperature. The electronic properties which depend on the size of the CdSe quantum dots are investigated by photoluminescence and pump‐probe‐spectroscopy. The luminescence spectra give hints that more than one ‘family’ of CdSe microcrystals was grown during heat treatment.
Precipitation of hollandite whiskers within a tielite matrix via the sol‐gel process
Authors: Kohn Sandro , Jansen Martin
Published: July 2012

In the system K‐A1‐Ti‐O the precipitation of a aluminium titanate ceramic, penetrated by potassiumhollandite whiskers, was established via the sol‐gel process. The crystallization by internal precipitation of the two phases tielite Al2TiO5 and potasiumhollandite K2Al2Ti6O16 from the amorphous sol‐gel powder was examined by X‐ray powder diffractometry, 27Al‐MAS‐NMR, DTA/TG and SEM/EDX. It has been shown, that the hollandite phase crystallizes in the shape of a whisker beside the tielite matrix. Such hollandite crystals have potential as new ceramic whiskers for the purpose of the reinforcing ceramics. Thus a texture of high density, penetrated by whishers, is obtained.
AIPO4‐containing glass‐crystalline materials in the system Li2O‐Al2O3‐P2O5
Authors: Jęczmionek L. , Kucharski J. , Wasylak J.
Published: July 2012

Publication concerns the crystallization of glasses in the system Li2O‐Al2O3‐P2O5. The aim of this study was to determine the area of the occurrence of glasses in the system Li2O‐Al2O3‐P2O5 to examine their tendency for crystallization of AlPO4 phase and others in the process of thermal treatment. Applying the method X‐ray diffraction the crystallization mechanism of AlPO4 (cristobalite and tridymite form) and other accompanying phases, and the influence of the addition of small amount of SiO2 on the crystallization of these glasses was examined. The structure of glasses and glass‐crystalline materials obtained were defined by means of infrared spectra. AlPO4 glass‐crystalline materials have satisfying resistant properties to use them as coats on steel.
Directed crystallization of glasses in the system SiO2‐CaO‐MgO+(ZnO+Li2O) nucleated with MoO3 and WO3
Authors: Wasylak J. , Kucharski J.
Published: July 2012

In the glass‐crystalline materials obtained through the directed nucleation, the main principal is to determine the optimum quantity of nucleating catalyst needed to bring about fine‐crystalline phase in the glass during heating. The methods of choosing the catalyst are very well known, but sometimes some of them are labour‐consuming and complicated equipment are demanded.

In this work a simple and quick method of choosing the catalyst to produce glass‐crystalline materials was examined. The method is elaborated on the DTA analysis curves basis. Investigations were made on the system SiO2‐CaO‐MgO+(ZnO+Li2O) nucleated with molybdenum and tungsten oxides.

Influence of different amounts of catalyst on crystallization process of different particle size glass samples was determined using characteristic points on DTA curve.
Compositional influence on shape and position of the first sharp diffraction peak (FSDP) in silicate and germanate glasses
Authors: Göttlicher J. , Pentinghaus H. J.
Published: July 2012

Silicate and germanate glasses studied by X‐Ray diffraction methods (WAXS) show characteristic features concerning the first peak of their scattering functions (FSDP: first sharp diffraction peak). Shapes and positions of the FSDPs develop systematically with composition. The shape of the FSDPs can only be described by introducing up to three peaks. We assume that the glass networks are heterogeneous in the scale of medium range order with respect to their chemistry and/or topology and these regions are mainly connected coherently.
Preparation and spectroscopy of lead‐tin fluorophosphate glass doped with TPPS and TPPS‐Sn
Authors: Orihara Yuichi , Uo Motohiro , Inoue Hiroyuki , Makishima Akio , Tani Toshio
Published: July 2012

Tetraphenyl‐porphine‐tetrasulfonic acid (TPPS), which is a well known dye for photochemical hole burning (PHB), and its Sn complex (TPPS‐Sn) were incorporated into a low melting SnF2‐P2O5‐SnO‐PbF2 glass system with a conventional melting method. From the absorption and luminescence spectroscopy, it was found that in the glass the above organic dyes were not decomposed and the original structure of TPPS‐Sn was maintained, however the structure of TPPS changed from that in the aqueous solutions. The excitation spectra and the extraction of the dyes suggest that TPPS reacted with Sn during melting, and TPPS‐Sn. TPPS and/or TPPS‐Sn with one pyrrole reduced were produced. This is supported by the result of PHB measurement.
Formation of amorphous and nano‐crystalline carbon in C+ implanted silica glass
Authors: Herrmann U. , Wendler E. , Dunken H. H. , Wesch W. , Seifert F.
Published: July 2012

Rutherford backscattering spectrometry, VIS‐NIR transmission of IR reflection measurements as well as Raman investigations have been used to characterize C+ implanted (30 keV, 0.5×1016cm‐2) silica glass. Distribution of implanted ions can be described by a Gaussian profile with a maximum at about 80 nm. C+ ions penetrate to a depth of 180 nm in the substrate material. Transmission spectra in the VIS‐NIR region were simulated with the dielectric functions of substrate glass and amorphous carbon using a multilayer model. Evaluation of the IR reflectance spectra indicates the creation of CO and a damage of the glass structure up to a depth of about 300 nm. Raman measurements of implanted samples show the aggregation of amorphous carbon. Anneling of samples for one hour at 700 or 1000°C in nitrogen atmosphere leads to the formation of nano‐crystalline carbon precipitates. Furthermore, the relaxation of the glass structure and the existence of Si‐C bonds has been observed after heat treatment.
Photoluminescence of a soda lime glass exchanged with silver ions
Authors: Villegas M. A. , Paje S. E. , Llopis J. , Fern Navarro J. M. Ández
Published: July 2012

A commercial soda‐lime glass subjected to ion exchange with Ag+ at different temperatures and times is studied by photoluminescence (PL), optical absorption (OA) and transmission electron microscopy (TEM). Some samples were reheated in air and in a reducing atmosphere (N2 ‐ H2) to favour the reduction of the Ag+ ions. The evolution of the silver colloids with the exchange parameters is monitored by means of the caracteristic absorption band at 420 nm.

PL spectra reveal that the samples exchanged with silver show a broad emission band centered around 600 nm upon excitation in the nearby ultraviolet, which intensity is very sensitive to temperature and exchange time. The excitation spectra exhibit a broad band with three peaks at about 360, 395 and 445 nm related to different types of silver centres.
On the incorporation of copper ions in heavy metal fluoride glasses
Authors: Scholz G. , Stößer R. , Sebastian S. , Grande T. , Aasland S. , Nofz M.
Published: July 2012

Cu2+ ions incorporated into (ZBLAN) fluoride glasses reflect via distributions of spin coupling parameters local randomness of the glass matrix. Cw‐X‐band ESR measurements at 4.2 K applying different values of the microwave power as well as computer simulations have shown that even at very low copper concentrations (<0.6 mol%) the Cu2+ ions form point defects coexisting and weakly interacting with magnetically coupled species. The tendency of Cu2+ ions to form aggregates is remarkedly higher than in oxide glasses which corresponds to a nonuniform distribution of the paramagnetic centres in the glass matrix. After cyclic heating and cooling processes, recrystallization of the matrix represented by lowering the line widths, and resolving hyperfine splittings in the g∥ region could be observed.
Relationships between glass structure and spectroscopic properties of Eu3+ and Tb3+ doped glasses
Authors: Ebendorff‐Heidepriem H. , Ehrt D.
Published: July 2012

Fluoride phosphate and phosphate glasses were doped with Eu3+ and Tb3+ ions. The relationships between glass structure and spectroscopic properties of the doped ions were of interest to obtain information about the local glass structure around rare earth ions.

Nephelauxetic shift of Tb3+ f→d absorption band, Eu3+ fluorescence intensity ratio of 5D0→7F2 to 5D0→7F1 as well as electron‐phonon coupling strength and phonon energy determined from Eu3+ phonon sideband spectroscopy were studied in dependence on phosphate content of the host glasses. Tb3+ fluorescence intensity ratio of 5D4→7FJ to 5D3→7FJ (J= 0,1,2… 6) transitions is affected by Tb3+ concentration and OH content of the glasses.

With increasing phosphate content of the glasses, the covalency between rare earth ions and surrounding ligands increases due to substitution of fluoride ions by oxygen ions having higher electron polarizability. This effect leads to increasing nephelauxetic shift, Ω2 parameter and electron‐phonon coupling strength. In fluoride phosphate glasses, the diphosphate groups dominate the spectroscopic properties of the rare earth ions. The Tb3+ fluorescence intensity ratio of 5D4→7FJ to 5D3→7FJ increases with increasing Tb3+ concentration and OH content, respectively, due to acceleration of cross‐relaxation between Tb3+ ions and due to energy transfer from Tb3+ ions to OH groups.
Structure and cation sensitivity of silicate glasses ‐ a review
Authors: Thomas B. , Scholz K. , Herzog K.
Published: July 2012

Silicate glasses of different compositions can be used as materials for ion sensitive electrodes. We have investigated glasses of the systems Na2O‐SiO2, Na2O‐RO‐SiO2 (R = Ba, Sr, Ca, Mg, Be, Cd, Zn), and Na2O‐R2O3‐SiO2 (R = Ga, Al, B) in a wide composition range to find relationships between the electrochemical properties (electrode sensitivities and selectivities) and the microstructures of the compact glasses and their hydrated layers. Various NMR spectroscopic techniques have been used (MAS, CP‐MAS, CRAMPS, SATRAS, REDOR) for probing the glass structure. Principal Component Analysis was found to be useful to test possible correlations between the composition, the glass structure parameters and electrode properties.
Electroluminescence with organic compounds
Authors: Salbeck Josef
Published: July 2012

Some fundamental aspects of basic principles and applications of electroluminescence, based on organic low molecular weight compounds and polymeric compounds will be presented. The basic working principles and the basic construction of electroluminescent devices are described. Some factors affecting the efficiencies and lifetime or organic electroluminescent devices are reviewed, describing recent developments in dye‐based organic hetero‐junction and polymer systems. The distinctive advantages of these organic LEDs are high efficiency of all colors and potential ease of fabrication, offering the possibility of applications in flat‐panel displays. No attempt is made in the present article to review the whole field, but instead this interesting field is described by focusing on a few selected topics, and by describing few representative results.
Thermal contributions in infrared laser induced grating experiments at high pressure
Authors: Dreizler A. , Latzel H. , Dreier T. , Koch A. , Wolfrum J.
Published: July 2012

Degenerate four wave mixing (DFWM) and laser induced thermal grating spectroscopy (LITGS) have been used to detect ethylene and methanol in mixtures with nitrogen, argon or helium in the pressure range between 0.005 and 18 bar. The grating forming step for both techniques was accomplished via absorption of radiation in the interference region from two beams from a single longitudinal mode CO2 laser. For DFWM the same CO2 laser and for LITGS a diode pumped Nd: YAG laser were employed to read out the transient grating. Using photo thermal deflection spectroscopy (PDS) as an independent spectroscopic method thermal effects were confirmed for total pressures above 0.1 bar. For both DFWM and LITGS signal intensities increased with buffer gas pressure due to collision induced transfer of optical excitation energy into translational degrees of freedom. The temporal behaviour of the IR‐grating generation and thermalization processes and the advantages of hydrocarbon trace species detection from thermal gratings at high pressures is discussed.
A differential anomalous X‐ray scattering study of lead(II)acetate trihydrate in 2‐methoxyethanol
Authors: Beudert R. , Batschkowitsch S. , Bertagnolli H. , Gutwerk D. , Kolb U. , Weigelt J.
Published: July 2012

The results of a differential anomalous X‐ray scattering (DAS) study on lead(II)acetate trihydrate, dissolved in 2‐methoxyethanol, are reported. Anomalous X‐ray scattering at two energies below the L3‐edge of lead (Pb) was used to determine the local environment of Pb and to get more precise information on the complex molecular structure of the liquid. The differential scattering intensities and their Fourier transform, the differential atom pair correlation function (DACPF), were obtained after an intricate data evolution procedure. The analysis of the DACPF provides strong evidence for a dimeric structure of lead(II)acetate trihydrate in the methoxyethanolic solution via lead ‐ oxygen ‐ lead correlation.
Photoelectrochemical investigations of complex formation phenomena on oriented n‐pyrite (FeS2) crystal surfaces
Authors: Büker K. , Alonso‐Vante N. , Tributsch H.
Published: July 2012

Quantum yields of illuminated single crystalline n‐pyrite (FeS2)/electrolyte junctions were measured at λ = 632.8 nm and found to be significantly higher in the presence of negatively charged redox species, such as I− and [Fe(CN)6]4− in contrast to species like Fe2+. Higher photocurrents and photovoltages at (100) n‐pyrite rotating disc electrodes in comparison with (111) electrodes fabricated from the same single crystal are obtained. Further, (100) n‐pyrite electrode surfaces were treated with pyrazine and 4,4'‐bipyridine. A significant decrease of dark current and a shift of the equilibrium potential in 0.5 M FeSO4 is ascribed to chemical complex formation between the electron donating ligands and the Lewis acidic Fe(II) centres of the (100) n‐pyrite surface. In the case of pyrazine treatment an enhancement of photocurrent and photovoltage could be observed. The observed phenomena are discussed in terms of coordinated chemical surface complex formation and charge transfer properties.
Pressure effect on the ene reaction of singlet oxygen with tetramethylethylene in different solvents
Authors: Hild M. , Brauer H.‐D.
Published: July 2012

Rate constantskRof the ene reaction of singlet oxygen (1Δg) with tetramethylethylene (TME) have been determined in six solvents with different polarity at elevated pressures up to 1200 bar using time‐resolved phosphorescence technique. Experimental activation volumes, ΔV, resulted in the range between −20.7 cm3· mol−1(in benzonitrile) and −35.1 cm3· mol−1(in n‐hexane). These results were analysed taking into account contact complex formation as the initial step. The reaction volume ΔVcof the formation of the contact complex was evaluated using the statistical model developed by Yoshimura and Nakahara. The data obtained confirm the assumption of a polar intermediate resembling a perepoxide. The intrinsic activation volume ΔVwas found to be −14.5 cm3· mol−1and the dipole moment of the transition state was estimated to be μ≠= 3.4 Debye.
Ionic motions and phase transitions in solid trimethylethylammonium tetrafluoroborate studied by 1H, 19F, and 14N NMR, powder X‐ray diffraction, and differential scanning calorimetry
Authors: Ono Hiroshi , Ikeda Ryuichi , Ishida Hiroyuki
Published: July 2012

1H, 19F, and 14N NMR, powder X‐ray diffraction, and differential scanning calorimetry (DSC) were studied for trimethylethylammonium tetrafluoroborate, (CH3)3NC2H5BF4. Five solid phases, named I, II, III, IV, and V in the order of decreasing temperature, were obtained in the temperature range 77‐630 K. The transition temperatures and the corresponding enthalpy changes determined by DSC were 222 K (0.16 kJ mol−1), 239 K (1.8 kJ mol−1), 293 K (2.1 kJ mol−1), and 362 K (4.5 kJ mol−1). X‐ray powder patterns showed that Phase I and II form an NaCl‐type cubic lattice (a = 10.14 Å, Z = 4) and a tetragonal lattice (a = 8.59, c = 6.24 Å, Z = 2), respectively. The NMR study revealed that the cationic orientation in Phase I and II is dynamically disordered. Moreover, since the self‐diffusion of anions and the isotropic rotation of cations were observed in Phase I, this phase can be considered as a mesophase very close to the plastic phase. In Phases II, III, IV, and V we have detected the isotropic reorientation of anions, the anisotropic tumbling of cations, the C'3 reorientation of the whole (CH3)3N group about C‐N bond axis, and the C3 reorientation of the CH3 groups. The motional parameters were evaluated for these cationic and anionic motions.
Fermi resonance structure and femtosecond quantum dynamics of a chiral molecule from the analysis of vibrational overtone spectra of CHBrClF
Authors: Beil Andreas , Luckhaus David , Quack Martin
Published: July 2012

The rovibrational spectra of bromochlorofluoromethane (CHBrClF) were measured at intermediate (0.1 cm−1) and high resolution (0.001 cm−1) between the far infrared (20 cm−1) and the visible range of the spectrum (14000 cm−1) by interferometric Fourier transform techniques and diode laser techniques in stationary samples and supersonic jets. All fundamentals and numerous combinations and overtone transitions could be assigned and their band positions and integrated strengths were determined. A quantitative interpretation of the complex multiple resonance structure of the CH chromophore spectra (about 50 assigned bands) is given in terms of an effective anharmonic CH‐stretching‐bending Hamiltonian and in terms of vibrational variational calculations on a 3‐dimensional grid using ab initio (MP2 and B3LYP) and empirically adjusted potential hypersurfaces for the CH chromophore normal coordinate subspace. The systematic analysis of anharmonic resonances of the alkyl CH chromophore in symmetric top (C3v) and asymmetric top (Cs) molecules is herewith extended to chiral molecules. We provide a detailed analysis of the chiral Cs symmetry breaking coupling in the effective Hamiltonian (coupling constant |k′sab| =(20 ± 10) cm−1) and in the potential (Csab and further terms) and their relationship. The implications for the femtosecond intramolecular dynamics for CHBrClF are discussed in terms of quantum dynamical results for time‐dependent populations, survival probabilities, wavepackets and also time‐dependent symmetries and their violations (formal a′ and a′′ species for vibrational Cs and l quantum number for vibrational C∞v symmetry). We have also developed an improved synthesis of CHBrClF, which is suitable for the preparation of large quantities (several mol) of the pure (racemic) compound, and have demonstrated enantiomer separation by gas chromatography.
Intramolecular dynamics of small molecules ‐ empirical evaluation of adiabatic and nonadiabatic corrections
Authors: Ogilvie J. F.
Published: July 2012

From published analyses of vibration‐rotational spectra of diatomic and linear triatomic molecules, we evaluated the term coefficients Ykl, and various Zkl in terms of mechanical (related to vibration and rotation of the nuclei about the centre of mass), adiabatic, and nonadiabatic vibrational and rotational effects (as the associated electrons follow imperfectly the nuclei). We propose generalisations about relative orders of magnitudes of mechanical, adiabatic and nonadiabatic effects that pertain to intramolecular dynamics of free small molecules, and discuss the importance of taking into account all these effects in either comparing results deduced from experiment with those drawn from computations of molecular electronic structure or deducing structural information from spectral parameters.
Homogeneous ice nucleation observed in single levitated micro droplets
Authors: Krämer B. , Schwell M. , Hübner O. , Vortisch H. , Leisner T. , Rühl E. , Baumgärtel H. , Wöste L.
Published: July 2012

Homogeneous ice nucleation rates in single isolated micro droplets of water are determined under various atmospheric conditions inside an electrodynamic Paul‐trap. The droplet size is measured by angle‐resolved detection of the scattered light from a Helium‐Neon laser. The freezing process is detected by a sudden change of the depolarization ratio which is an indication for the formation of nonspherical particles.

The experimentally determined homogeneous ice nucleation rates JLS are JLS = (2.2±0.4)·108 cm−3 s−1 at T = (−37.2±0.15)°C and JLS = (5.6±1.0)·107 cm−3 s−1 at T = (−37.7±0.15)°C.

These values are much more precise as determined by measurements inside a cloud expansion chamber. The method of single particle observation eliminates complications arising from these nucleation rate measurements. The results underline the theory that one single ice germ of certain size is sufficient to induce the freezing of the whole droplet. The depolarization ratio of the scattered intensity in the parallel and perpendicular polarization plane Is/Ip from the frozen particles exhibits a broad distribution with a mean value of Is/Ip = 0.3.
Trapping and charge stabilization in chlorosome containing bacteria: Comparative study on Chloroflexus aurantiacus and Chlorobium limicola
Authors: Schmidt Kristiane , Trissl Hans‐Wilhelm
Published: July 2012

Trapping and charge stabilization in intact cells of Chloroflexus aurantiacus and Chlorobium limicola were studied by means of photovoltage kinetics and fluorescence emission spectra upon excitation in the chlorosome. In Chloroflexus two electrogenic phases with time constants of 100 ps for trapping (P+ H− formation) from the core‐complexes and 530 ps for charge stabilization (QA reduction) were found. In Chlorobium there were also two electrogenic phases of 105 ps and 650 ps. The later phase most likely represents the charge stabilization on the iron‐sulfur center, Fx. A comparison of stationary fluorescence spectra of both species indicates that the trapping time from the core pigments in Chlorobium is significantly faster than in Chloroflexus. This proves the existence of at least two intermediary acceptors between the primary donor (P 840) and Fx in the Chlorobium reaction center. In both species the photovoltage signal in the low excitation limit appeared after a lag phase of 55 to 65 ps, which is ascribed to the energy transfer from the chlorosome to the core pigments.
Primary charge separation. The primary processes of bacterial photosynthesis — ultrafast reactions for the optimum use of light energy
Authors: Zinth W. , Arlt T. , Wachtveitl J.
Published: July 2012

Femtosecond spectroscopy is used to study the molecular mechanisms of the primary electron transfer. Data on native and mutated reaction centers of Rhodopseudomonas viridis show, that primary electron transfer is an ultrafast stepwise reaction. The electron is transferred via a chain of pigments: In a first reaction step the electron is transported from the special pair to the accessory bacteriochlorophyll with a time constant of ≈ 3 ps. A second, faster reaction carries the electron with 0.65 ps to a bacteriopheophytin. Experiments on mutated reaction centers with strongly modified reaction times yield additional information on energetics, reorganisation energies and electronic coupling of the reaction center. A consistent theoretical treatment of the data shows that standard non‐adiabatic theory describes well the primary electron transfer process. It also reveals that the realised reaction parameters optimise the reaction centers for highest possible quantum yield.
Effects of zwitterionic detergents on the primary donor of bacterial reaction centers
Authors: Müh Frank , Rautter Joachim , Lubitz Wolfgang
Published: July 2012

The effects of zwitterionic detergents on the spectroscopic properties of the primary electron donor P of photosynthetic reaction centers (RCs) from the purple nonsulfur bacterium Rhodobacter sphaeroides are investigated. When RCs are solubilized with N‐(n‐alkyl)‐N,N‐dimethyl‐3‐ammonio‐1‐propanesulfonates (sulfobetaines) the maximum of the characteristic near‐infrared absorption of P, λmax, is shifted to lower wavelengths at room temperature as compared with nonionic detergents. While the range of the blueshift (λmax = 850 – 862 nm) is identical for different sulfobetaines, the dependence of the shift on the detergent/RC ratio varies systematically with the number of carbon atoms in the n‐alkyl‐chain. N‐lauryl‐N,N‐dimethyl‐amine‐N‐oxide (LDAO), a zwitterionic detergent with a less polar head group, is only able to induce blueshifts of λmax down to 859 nm at much higher detergent/RC ratios. ENDOR experiments performed on the primary donor radical cation P+• reveal that two distinct states of P exist in such samples. Thus the optical bandshifts are caused by transitions between two spectroscopic species, called P866 and P850. We interpret the detergent effect as an electrostatic influence of the detergent head groups on the equilibrium between two protein conformations which in turn affect the structure of the primary donor.
ENDOR and pulsed EPR studies of photosynthetic reaction centers: Protein‐cofactor interactions
Authors: Lendzian F. , Rautter J. , Käß H. , Gardine A. , Lubitz W.
Published: July 2012

The influence of specific protein‐cofactor interactions on the electronic structure of the primary donor cation radical P+• and the acceptor anion radicals Q−•A and QB−•B in wild type and mutant reaction centers of photosynthetic bacteria is investigated by ENDOR and Pulsed EPR techniques. The results show that hydrogen bonds to the primary donor have a strong effect on the distribution of the unpaired electron over the two BChl halves of the special pair, P+•. A correlation between the rate of reduction of P+• by cytochrome C2 and the spin density distribution within the dimer is found. ESEEM made it possible to detect 14N nuclear quadrupole resonances for Q−•A and Q−•B that are assigned to nitrogen atoms of ligating amino acids (histidines and peptide backbone). The deduced hyperfine couplings indicate significant delocalization of spin density onto these residues. This indicates an active role of these ligands in the electron transfer from QA to QB.
Magnetic investigations on a valence‐delocalized dinuclear Fe(II)‐Fe(III) complex
Authors: Saal C , Mohanta S. , Nag K. , Dutta S. K. , Werner R. , Haase W. , Duin E. , Johnson M. K.
Published: July 2012

The magnetic properties of a valence delocalized mixed valent Fe(II)‐Fe(III) complex have been investigated. On the basis of magnetic susceptibility measurements and NIR absorption spectroscopy the double exchange parameter was determined to B = 940cm−1 while for the isotropic exchange interaction an antiferromagnetic contribution of, J = −100 cm−1 was obtained. The results were further discussed with regard to vibronic interaction. For the vibronic‐interaction parameter A only an upper bound of 2500 cm−1 could be given.
Self‐organized electron transfer as a possible element in the photosynthetic process
Authors: Pohlmann L. , Tributsch H.
Published: July 2012

As energy fluxes in several systems far from thermodynamic equilibrium can be largely enhanced under nonlinear conditions permitting local entropy export, this should also be possible on a more microscopic scale for biological electron transfer processes. As a precondition a molecular feedback loop must exist, which transfers a small fraction of the energy, which drives the reaction, through the surrounding medium to assist the electron transfer in a non‐linear way. This can lead to microscopic self‐organization, i. e. to a temporary dissipative structure which may significantly improve electron transfer through a non‐equilibrium electron distribution and a decrease of the activation barrier. The problem is theoretically modelled as a Kramers approach in which the friction term may become active. A formula for “self‐organized” electron transfer is obtained which yields — for finite activation barriers — significantly higher rates than the Marcus formula. The presented model may help to explain how biological catalysts have evolved the ability of generating favourable energy barriers for multi‐electron transfer near the thermodynamic overall potential.
On the Chemical Interpretation of the One‐Electron Density Matrix of Some Ionic Solids: LiH, LiF, and LiFHF
Authors: Asthalter Tanja , Weyrich Wolf
Published: July 2012

Two‐dimensional cuts through one‐electron density matrices of three ionic solids in the position representation were calculated from restricted Hartree–Fock wavefunctions. The basis‐set quality was assessed by comparison of calculated Compton profiles/reciprocal form factors with experimental ones. The off‐diagonal regions of the density matrix contain the main information about chemical bonding in the crystal. The decomposition of the total density matrix into band contributions shows that the valence band always dominates in those regions. Furthermore, one‐electron density matrices of small subunits of the crystal and of symmetrically orthogonalised crystal wavefunctions were calculated. Their similarity and differences with respect to the solid are discussed.
Intermetallic Phases at the Cathodically Polarized Interface Platinum/Polycrystalline Yttria Stabilized Zirconia
Authors: Barbi G. B. , Chiodelli G. , Magistris A. , Manara A.
Published: July 2012

A deep cathodic polarization has been applied to a Platinum/Polycrystalline Yttria Stabilized Zirconia interface under low oxygen pressure. In the temperature range 973–1273 K the Electromotive Force recorded after the current cut off did not show any rest potential at a level corresponding to nucleation of a zirconium metal phase, likely because such process is kinetically unfavoured with respect to that of formation of intermetallic phases, whose stability introduces a limitation of the local oxygen chemical potential depression.

Two rest potentials were found at less negative potentials which levels correspond to intermetallic phases which very high stability can be accounted for d–d orbitals bonding, as described by the Engel‐Brewer correlation.

In some EMF decays it was observed the occurrence of a third rest potential, located at ∼ −450 mV (reference pure oxygen) that could be attributed to a transitory presence of a Pt–Zr–O phase.
In Situ Determination of Acidity Level in Concentrated Phosphoric Acid Solutions by Potentiometric Method at Imposed Current Intensity
Authors: Hammouti B. , Oudda H. , Elmaslout A. , Benayada A. , Bessiere J.
Published: July 2012

The device constituted by two past electrodes: ferrocene and orthochloranil at fixed current potentiometry, can be used as a good sensor for the in‐situ determination of phosphoric acid concentration (2.0–14.7 M). Good agreement was observed regarding the values obtained with our method and the spectrophotometric values in synthetic phosphoric media. Different factors affecting the variation of potential at imposed current such as current intensity, temperature, stirring rate of solution and H+ ion concentrations are studied. The potential variations measured by our device between ferrocene and orthochloranil electrodes have the same values as those obtained by the voltamperometric curves. A new acidity function Ri(H), which represents the determination of the H+ activity has been introduced and compared to Strehlow Ro(H) and Hammet Ho functions. A good agreement has been observed between the Ro(H) and Ri(H) functions.
Self‐Diffusion Coefficient and Apparent Molar Volume Studies of Crown Ethers in Aqueous (D2O) and CDCl3 Solutions
Authors: Patil K. J. , Heil S. R. , Holz M. , Zeidler M. D.
Published: July 2012

We have measured the self‐diffusion coefficient and density properties of 15‐crown‐5 and 18‐crown‐6 ethers in aqueous (D2O) and CDCl3 solutions in the range of 0 to 6 m concentration of ethers. Some measurements for 1 m crown in D2O with various concentrations of alkali‐metal bromides are also reported. It has been observed that self‐diffusion coefficients of the crowns are small and decrease with increase in concentration of the crown both in D2O and CDCl3 solutions. The density data in D2O were used to obtain the apparent molar volumes of the crown φv as a function of concentration. It has been observed that φv goes through a minimum at about 2 m of crown concentration. Similar measurements and calculations of 1 m crowns in D2O in various salt solutions indicate that D varies peculiarly as a function of salt concentration. In case of KBr solutions, D increases initially and goes through a flat maximum at about 1–2 m of KBr concentration. The apparent molar volumes of the salts calculated in 1 m 18‐crown‐6 and 1 m 15‐crown‐5 solutions exhibit anomalous dependence on salt concentration. These results are explained on the basis of solute‐solute, solute‐solvent, ion‐crown and ion‐ion interactions in solution phase.