bluuebunny/crossref_metadata_2025 huggingface dataset:
Interfacial effects on the dynamics of cooperatively rearranging regions
Authors: Streck C. , Yan X. , Richert R.
Published: July 2012

The glass‐forming liquid 2‐methyltetrahydrofuran is particularly well described by the theory of Adam and Gibbs as regards the coupling between molecular dynamics and configurational entropy near the glass transition temperature at Tg = 91 K. Therefore, the underlying physical picture of cooperatively rearranging regions captures the temperature dependence of orientational dynamics for the bulk liquid on the basis of the configurational entropy. We demonstrate that geometrical confinement of such a liquid to nanopores gives rise to a dynamical coupling of the liquid to the rigid pore wall resulting in a non‐ergodic regime for long time, whereas the short time dynamics remain unaffected.
A spectroelectrochemical study of the slow relaxation in polyaniline and its acceleration in composite films with poly(o‐phenylenediamine)
Authors: Malinauskas Albertas , Holze Rudolf
Published: July 2012

The kinetics of the slow relaxation, or “first cycle effect”, observed during the cathodic reduction of a polyaniline film, were investigated with UV‐vis spectroelectrochemistry. It was shown, that the slow cathodic reduction of a polyaniline layer, present initially in its emeraldine state, can be accelerated by placing an additional layer of poly(o‐phenylenediamine) between the substrate electrode and the polyaniline film. Furthermore, it was found, that the anodic oxidation of the leucoemeraldine form of polyaniline is also favoured by the inserted film of poly(o‐phenylenediamine). The observed acceleration of the slow relaxation by the intermediate film is discussed in the framework of a previously presented model, which takes into account anion movement in the polymer film proceeding during the redox processes in opposite directions for both polymers studied here.
Aggregation of cationic surfactants in D2O: A proton NMR study on effects of the structure of the headgroup
Authors: Bazito Reinaldo C. , El Seoud Omar A. , Barlow Graham K. , Halstead Tom K.
Published: July 2012

Aggregation of the following surfactants in D2O has been studied by 1H NMR spectroscopy at 500.13 MHz, 298 K: series I, dodecyltrialkylammonium bromides, alkyl group = methyl, ethyl, n‐propyl, and n‐butyl; series II, dodecyldimethyl (2‐hydroxyethyl)ammonium bromide, dodecylmethyl bis(2‐hydroxyethyl)ammonium bromide, and dodecyl tris(2‐hydroxyethyl)ammonium bromide. Chemical shifts of the surfactant discrete groups were measured at concentrations below and above the surfactant critical micelle concentration, c.m.c., whose value was determined by conductance measurements in H2O. Plots of observed chemical shifts versus [surfactant]are sigmoidal, and were fitted to a model based on the mass‐action law. Excellent fits were obtained for the discrete groups of all surfactants. The parameters determined by these fits were: the equilibrium constant for micelle formation, K, c.m.c., and the chemical shifts of the monomer, δmon and the micelle, δmic. 1H NMR‐based c.m.c. values are in excellent agreement with those which we determined by conductance measurements. Values of K increase as a function of increasing the size of the headgroup of series I surfactants, but the incremental contribution to the Gibbs free energy per CH2 group at the interface is small, its value increases from 0.13 kJ (moles of CH2)−1 to 0.29 kJ (moles of CH2)−1 in going from the trimethyl‐ to the tri‐n‐propyl, and the tri‐n‐butyl analogue, respectively. Micellization affects the chemical shifts of the groups at, or near to, the interface mainly because of differences in hydration of the surfactant molecule in its monomeric and aggregated forms. This conclusion is supported by comparing (δmic ‐ δmon) with (δCDCl[Text is not clear]‐δmon), where δCDCl3 is the chemical shift of the surfactant in CDCl3, i.e., in absence of hydration. For series II surfactants, the chemical shifts versus [surfactant]plots are qualitatively similar to those for series I, this shows that substitution of the 2‐hydroxyethyl group for the ethyl group in the hydrophilic part of the surfactant does not seem to change the properties of the formed aggregate.
Interfacial wetting transitions in ionic and metallic fluid mixtures
Authors: Freyland W. , Nattland D.
Published: July 2012

Recent experimental investigations of interfacial wetting phenomena in binary liquid mixtures characterized by Coulomb or screened Coulomb short range interactions are discussed. Wetting transitions occurring at the fluid‐wall and the fluid‐vapour interface are considered focusing on the examples of fluid alkalimetal‐alkalihalide mixtures and liquid gallium‐based binary alloys at elevated temperatures. Different spectroscopic methods have been employed to probe the respective interfaces, spectroscopic ellipsometry and second harmonic generation (SHG). In the case of K‐KCl solutions the SHG results exhibit changes characteristic of a prewetting transition in metalrich solutions implying a first order wetting transition at coexistence. A wetting transition is observed for liquid Ga‐Bi and Ga‐Pb alloys approaching the monotectic point at coexistence.
The influence of charge centres separation in complex structure ion on ionic‐strength dependence of equilibrium constants in aqueous solution
Authors: Sadofeev Igor G. , Mironov Igor V.
Published: July 2012

The acid‐base equilibria of oxalic, malonic and succinic acids, ethylenediamine, diethylenetriamine and glycine were studied potentiometrically (glass electrode) in NaClO4 (for polyamines also in NaCl) solution of various concentrations (0.01‐3 M), at 25°C. The dependence of the change of equilibrium constants with ionic strength on the distance between charge centres in ion is discussed. This dependence more markedly exhibits for ions with the shortest distances between charge centres. No differences in observed effects for different classes of ions were found. The simulation of the medium effects for the systems involving ions with separated charge centres was carried out on the basis of electrostatic notion with the data on the ion structure and the magnitudes of charges on atoms.
Kinetics of the reactions of bromine atoms with a series of aliphatic aldehydes at 298 K
Authors: Szilágyi I. , Imrik K. , Dóbé S. , Bérces T.
Published: July 2012

The kinetics of the reactions of Br(2P3/2) with selected aldehydes, i.e. ethanal (l), propanal (2). 2‐methyl‐pro‐panal (3), 2,2‐dhethyl‐propanal (4) and trichloroacetaldehyde (5) were studied at 298±2 K. Rate constants for overall reactions were measured using the fast flow technique with resonance fluorescence detection of Br. Complementary determinations were carried out by the laser flash photolysis method. The following rate constants were obtained in the kinetic measurements (±2σ): k1=(2.1±0.2)×1012, k2=(4.3±0.4)×1012, k3=(6.3±1.4)×1012, k4=(8.5±0.8)×1012, k5≤1×109 all in cm3 mol−1 s−1 units.

The probable mechanism for the reactions of bromine atoms with aliphatic aldehydes has been discussed.
Quantized aggregation phenomena in II‐VI‐semiconductor colloids
Authors: Ptatschek V. , Schmidt T. , Lerch M. , Müller G. , Spanhel L. , Emmerling A. , Fricke J. , Foitzik A. H. , Langer E.
Published: July 2012

Highly concentrated 0.5 M II‐VI semiconductor quantum dot solutions for coating applications can be synthesized employing chalcogenolysis and condensation of functionalized cluster‐like cadmium and zinc ethoxy‐acetates. Furthermore, in nucleation studies on CdSe solutions, new magic clusters between 0.42 and 1.7 nm in size were found exhibiting sharp HOMO‐LUMO resonances (lowest absorption features) in the optical absorption spectra. High resolution small angle X‐ray scattering (SAXS) measurements performed on 1.7 and 3.4 nm CdSe clusters corroborate the size. Information on the intra‐cluster structure was hard to derive with respect to the small cluster size. These species could be Koch pyramids with a fractal dimension Df=2 as well as non‐fractal zincblende pyramids (additionally checked by XRD and HRTEM). In any case rather chain‐like (Df= 1) aggregates are formed. It further will be shown that in alcoholic CdSe sols the initially nucleated “seeds” are highly reactive. Their sharp HOMO‐LUMO transitions are found to be strongly modified by externally induced chemical reactions. For example, aminosilane capped 1.7 nm clusters decompose rapidly upon exposure to phosphines. After a period of few hours, they begin to re‐grow to their original size or they reorganize to give smaller 0.85 nm subunits depending on the P/N ratio. In contrast, 0.85 nm phosphine‐capped clusters double their size if exposed to amines. The last process liberates cadmium ions into the solution as found in complementary polarographic measurements.
UHV Model experiments of the semiconductor/electrolyte interface: Adsorption of I2on n‐ and p‐doped WSe2(0001)
Authors: Henrion O. , Jaegermann W.
Published: July 2012

The WSe2(0001)/I2semiconductor/electrolyte interface was simulated in Ultra‐high‐Vacuum (UHV) by adsorbing I2onto the van der Waals plane of the layered substrate at 100 K. The interface was characterized by photoelectron spectroscopy and surface photovoltage measurements. The interaction proved to be non‐reactive and I2adsorbs as molecular species. The electric potential distributions measured by UPS und XPS are in reasonable agreement to semiconductor/electrolyte interfaces. Iodine behaves as electron acceptor inducing strong band bending only for n‐doped and not for p‐doped samples. After correcting for source‐induced surface photovoltage effects the 2σulevel (LUMO) of adsorbed I2is identified of being responsible for establishing electronic equilibrium.
CO adsorption and hydrogenation on crystalline YBa2Cu3Ox thin films
Authors: Chen L. , Lin J. , Chen H. Y. , Xu S. , Li K. , Ong C. K. , Tan K. L. , Li J.
Published: July 2012

YBa2Cu3Ox (YBCO, x=6‐7) is shown to have catalytic activity for methanol synthesis from syngas in this work. In order to get more details about the nature of the reactive site and reaction mechanism, CO adsorption and hydrogenation on a high quality YBCO thin film were studied by using in situ Fourier transform infrared spectroscopy (FTIR), angle‐resolved X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction (XRD) and atomic force microscopy (AFM). Carbonate‐like species, aldehyde, formate and methoxy were detected as reaction intermediates. After the CO hydrogenation reaction Cu2+ ions were reduced to Cu1+, and the orthorhombic structures of YBCO was transformed to the tetragonal phase with the formation of carbonatelike complexes. The degradation of some YBCO to amorphous surface phases was also observed. Based on these studies, carbonate‐formate‐dioxymethylene‐methoxy (or carbonate‐aldehyde‐carnene‐methane) reaction scheme is proposed.
Chemical kinetics of phase boundaries in solids
Authors: Schmalzried H. , Janek J.
Published: July 2012

The kinetics of solid/solid interfaces is of major importance for the course of heterogeneous reactions in the solid state. Solid/solid interfaces are normally sources of elastic stress and thus may interact with outer surfaces. In addition, they can offer high diffusivity paths and thus represent two‐dimensional reaction media. Essential kinetic situations are defined and basic formal considerations are summarized. Corresponding experimental examples are presented in order to illustrate kinetic phenomena at static and moving boundaries. This includes questions of exchange fluxes, boundary‐controlled solid state reactions, morphology, nonlinear phenomena and reactions in boundaries.
NMR study of the dynamics and structure of a good glass former LiCl‐6H2O
Authors: Maurin P. O. , Jal J. F. , Dupuy‐Philon J. , Asahi N. , Kawamura J. , Kamiyama T. , Nakamura Y.
Published: July 2012

The temperature dependences of the self‐diffusion coefficient and the spin‐lattice relaxation time have been studied by NMR measurements performed on a good glass former LiCl‐6H2O. The spin‐lattice relaxation time can be separated into two relaxation processes, i.e., inter‐ and intramolecular processes. We have related these relaxation processes to the translational diffusion which corresponds to the primary or α‐relaxation and to the local motions which are related to the β‐relaxation in glass forming systems. The results throw some light to the evolution with temperature of these relaxation processes in the LiCl‐6H2O system over the normal liquid, supercooled liquid and glass sates.
Diffusion on simple heterogenous surfaces
Authors: Nieto F. , Uebing C.
Published: July 2012

In this work we use the well‐known bivariate trap model with random topography in order to study the effects of surface heterogeneities on surface diffusion. Utilizing Monte Carlo simulations we characterize how the tracer, jump and chemical diffusion coefficients depend on temperature and concentration of deep traps, θ. In order to calculate the chemical diffusion coefficient we apply the fluctuation and the Kubo‐Green methods. Both methods are in good agreement at large values of θ only. The discrepancies are discussed and explained. The effects of heterogeneities are largely pronounced at low total coverage and low temperatures, where most of the adatoms are trapped by the deep traps. At high coverages the mobility of adatoms adsorbed on shallow traps dominate the diffusion behavior.
Thermodynamics and statistical mechanics of ammonia in zeolite NaZSM5
Authors: Boddenberg B. , Rakhmatkariev G. U. , Viets J.
Published: July 2012

The adsorption isotherm and the differential heat of adsorption of ammonia in zeolite NaZSM5 (Si/Al=32) were measured over a wide range of equilibrium pressure at the temperature 303 K. The obtained data can consistently be interpreted to be due to the formation of s‐meric (s = 1 − 6) ammonia/sodium cation complexes residing in the intersections of the straight and zig‐zag channels of the zeolite structure. The energies of formation and the potential energies of the s‐mers are reported.
Quantum cluster equilibrium theory of liquids part I: Molecular clusters and thermodynamics of liquid ammonia
Authors: Ludwig R. , Weinhold F. , Farrar T. C.
Published: July 2012

The quantum cluster equilibrium (QCE) theory is presented for liquid ammonia. The cluster equilibria that dictate phase composition are determined by the rigorous techniques of quantum statistical thermodynamics in the canonical ensemble, based on the ab initio partition function. The characteristic features of the supra‐molecular clusters which comprise the QCE model are discussed in terms of binding energies, geometries, cooperativity and charge transfer. Many possible equilibrium structures of ammonia are strongly disfavored for enthalpic and/or entropic reasons and do not show up in the QCE cluster populations. The numerical accuracy of the resulting QCE model is demonstrated by comparison with experimental data for thermodynamic properties of ammonia such as Clausius‐Clapeyron pressure dependence and specific heat.
Closed‐Loops of liquid‐liquid immiscibility in binary mixtures of equal sized molecules predicted with a simple theoretical equation of state
Authors: Yelash L. V. , Kraska T.
Published: July 2012

Several types of closed‐loop liquid‐liquid immiscibility behavior in binary mixtures have been found with a simple isotropic model fluid. The calculations are based on an equation of state composed by the Carnahan‐Starling hard sphere repulsion and the van der Waals‐attraction. The extension to binary mixtures is accomplished with the usual quadratic one‐fluid mixing rules. The results are presented in a global phase diagram as introduced by van Konynenburg and Scott. Several closed‐loop phase diagrams similar to those discovered by Boshkov ten years ago with a Lennard‐Jones equation of state have been found. The results are discussed in the context of the topological and physical origin of closed‐loop behavior in systems of isotropic mixtures.
Ambient temperature removal rate constants for singlet methylene with some nitrogen and sulphur compounds
Authors: Staker Warren S. , King Keith D. , Gutsche Greg J. , Lawrance Warren D.
Published: July 2012

The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene, 1CH2 (ā A1), with some nitrogen and sulphur compounds at ambient temperature (298±2 K). A measurement of the removal rate constant for 1CH2 with NH3 yielded a value of (3.33±0.19)xl0−10 cm3 molecule−1 s−1 in good agreement with the only previously reported measurement. Removal rate constants for the reactants CH3NO2, CH3CH2NO2, CH3CH2CH2NO2, CH3CN, CH3CH2CH2CN, CH2=CHCN, H2S, CH3CH2SH, CH3SCH3, CH3SSCH3, and CH3COSH were measured for the first time and found to be (3.23±0.28), (3.46±0.69), (3.55±1.28), (4.51±0.21), (3.64±0.41), (4.10±0.26), (3.87±0.22), (4.77±0.25), (4.53±0.23), (5.37±0.53), and (4.59±0.32)x10−10 cm3 molecule−1 s−1 respectively. The calculated removal efficiencies (removal rate constant relative to the Lennard‐Jones collision rate constant) are indicative of reaction being the dominant process leading to loss of 1CH2. Comparisons are drawn between the reactivities of the nitrogen and sulphur functional groups and their hydrocarbon analogues, and between the sulphur compounds and their isoelectronic oxygen analogues.
Fine‐Structure analysis of ClO fragments in the mode‐specific predissociation of chlorine dioxide
Authors: Delmdahl R. F. , Welcker S. , Gericke K. ‐H.
Published: July 2012

The photodissociation of OClO (2A2) ClO (X2πΩ,ν,J,Λ)+O (3P) in the 424‐450 nm region and in the 356‐371 nm region has been investigated by monitoring the ClO (X2πΩ,ν = 0,J,ν) fragments using two‐photon laser‐induced fluorescence. In the long‐wavelength regime the product vibrational excitation is clearly a function of the particular initial type of motion of OClO (2A2v1,v2,V3). This behavior is significantly reduced at shorter photolysis wavelengths. The population of the fragments rotational levels is observed to resemble Boltzmann distributions of around 1000 K. A population ratio of about P(2π3/2): P(2π1/2) = 3.5 is obtained for the two spin‐orbit states of ClO. The ν‐doublet states (A′) and (A″) of the ClO fragments are equally preferred in the dissociation process.
Hydrogen transfer in the porphin anion: A quantum dynamical study of vibrational effects
Authors: Brackhagen O. , Scheurer Ch. , Meyer R. , Limbach H.‐H.
Published: July 2012

The observed temperature‐dependent hydrogen transfer rate constants for the H, D, and T isotopic species of the porphin anion have been reproduced by two different quantum dynamical models. The approach is based on the master equation for the system‐state populations. The models involve adapted localization properties and a simplified interaction with a quantum mechanical heat bath. A one‐dimensional model describes a hindered circular transfer motion of the hydrogen atom between its four stable sites in the plane of the porphin ring framework. With five parameters that are common to the three isotopic species it yields agreement with the observed data. One of the parameters, the height of the fourfold potential energy barrier, predicts an acceptable estimate for the in‐plane N‐H wagging frequency. Interactions of the transfer motion with individual vibrations have been studied for a series of two‐dimensional model systems. There the circular transfer motion is combined with ring framework modes having properties similar to those obtained for porphin by Li and Zgierski [J. Phys. Chem. 95, 4268 (1991)]. For vibrational modes near or above 960 cm−1, strong interactions are found to be unlikely, as they would yield kinetic isotope effects and/or apparent activation energies in disagreement with the experimental data. For low frequency modes of the ring framework, however, sizeable couplings with the transfer cannot be ruled out. An interaction of the type suggested by quantum chemical results reported by Vangberg and Ghosh [J. Phys. Chem. B. 101, 1496 (1997)] has been included in a two‐dimensional model involving a rectangular displacement of the pyrrole rings. With low vibrational frequency, a very large coupling strength, and a barrier close to the predicted one, this model has also been found to be compatible with experiment.
NQR Studies of charge distribution in hydrogen bonded chloroacetates
Authors: Zdanowska‐Fraczek Maria
Published: July 2012

35Cl spectroscopy was applied to study the hydrogen bond symmetry as well as the isotope and pressure effects on it in a wide range of (2:1) salts of chloroacetic trichloroacetic and difluorochloroacetic acids. The NQR results were discussed qualitatively on the base of a variational correlated ground state wave function theory of the single hydrogen bond
A new view of the quantum dynamics for proton transfer along hydrogen bonds: Vibrational spectroscopy with neutrons
Authors: Fillaux F. , Nicolaï B. , Baron M. H. , Lautié A. , Tomkinson J. , Kearley G. J.
Published: July 2012

Inelastic neutron scattering is an ideal tool to observe proton transfer dynamics along hydrogen bonds in the quantum regime. “Tunnelling” transitions associated with the quantum transfer of a single proton have been observed for potassium hydrogen carbonate, KHCO3, and similar hydrogen bonded complexes in the solid state. More recently, the tunnelling transition was observed for a polypeptide: polyglycine II. The “tunnelling” bands are rather sharp. Therefore, the proton transfer is largely decoupled from the heavy atom dynamics. Force‐field calculations including INS intensities confirm that the proton dynamics are almost totally decoupled from those of the surrounding heavy atoms. This is in contrast to the “vibration assisted tunnelling” model that supposes a large modulation of the double minimum potential by the low frequency modes of the hydrogen bond. The double minimum potential functions governing the proton transfer are discussed.
Localization of hydrogen bond deuterons in proton sponges by dipolar solid state 15N NMR spectroscopy
Authors: López Concepción , Lorente Philippe , Claramunt Rosa María , Marín Javier , Foces‐Foces Concepcion , Llamas‐Saiz Antonio L. , Elguero Jose , Limbach Hans‐Heinrich
Published: July 2012

It is shown that dipolar solid state 15N NMR spectroscopy can be employed in order to estimate the positions of hydrogen bond deuterons in polycrystalline 15N labeled proton sponges containing an intramolecular N…H…N hydrogen bond. For singly 15N labeled 1, 8‐bis(dimethylamino)naphtalene. LPF6 (1‐h: LH, 1‐d: LD), an asymmetric hydrogen bond with cubic average distances between the two nitrogen atoms and the hydrogen bond deuteron of 1.19 Å and 1.47 Å are obtained. These values are in accordance with the N…H distances of 1.19(7) Å and 1.43(6) Å, obtained previously by x‐ray diffraction at 150 K.
Nuclear magnetic resonance of hydrogen bonded clusters between F− and (HF)n: Experiment and theory
Authors: Shenderovich Ilja G. , Smirnov Sergei N. , Denisov Gleb S. , Gindin Vladimir A. , Golubev Nikolai S. , Dunger Anita , Reibke Rebecca , Kirpekar Sheela , Malkina Olga L. , Limbach Hans‐Heinrich
Published: July 2012

Liquid state 1H and 19F NMR experiments in the temperature range between 110 and 150 K have been performed on mixtures of tetrabutylammonium fluoride with HF dissolved in a 1:2 mixture of CDF3 and CDF2Cl. Under these conditions hydrogen bonded complexes between F− and a varying number of HF molecules were observed in the slow proton and hydrogen bond exchange regime. At low HF concentrations the well known hydrogen bifluoride ion [FHF]− is observed, exhibiting a strong symmetric H‐bond. At higher HF concentrations the species [F(HF)2]−, [F(HF)3]− are formed and a species to which we assign the structure [F(HF)4]−. The spectra indicate a central fluoride anion which forms multiple hydrogen bonds to HF. With increasing number of HF units the hydrogen bond protons shift towards the terminal fluorine's. The optimized gas‐phase geometries of [F(HF)n]−, n = 1 to 4, calculated using ab initio methods confirm the D∞h, C2v, D3h and Td symmetries of these ions. For the first time, both one‐bond couplings between a hydrogen bond proton and the two heavy atoms of a hydrogen bridge, here 1JHF and 1JHF where |1JHF|≥|1JHF'|, as well as a two‐bond coupling between the heavy atoms, here 2JFF, have been observed. The analysis of the differential width of various multiplet components gives evidence for the signs of these constants, i.e. 1JHF and 2JSF>0, and 1JHF|. <0. Ab initio calculations of NMR chemical shifts and the scalar coupling constants using the Density Functional formalism and the Multi‐configuration Complete Active Space method show a reasonable agreement with the experimental parameters and confirm the covalent character of the hydrogen bonds studied.
Formation, stability and fragmentation of biomolecular clusters in a supersonic jet investigated with nano‐ and femtosecond laser pulses
Authors: Meffert Anja , Grotemeyer Jürgen
Published: July 2012

Gas phase reactions have often been discussed to play a role in the MALDI ionisation mechanism. Therefore the cluster formation of several typical MALDI matrix substances like ferulic acid (= 4‐hydroxy‐3‐methoxy‐cinnamic acid), vanillic acid (= 4‐hydroxy‐3‐methoxy‐benzoic acid), sinapic acid (= 3,5‐dimethoxy‐4‐hydroxy‐cinnamic acid) and 2,5‐dihydroxy‐benzoic acid was investigated after laser desorption into a supersonic beam of argon by means of time‐of‐flight mass spectrometry. Not only the formation of homogeneous matrix clusters but also that of heterogeneous clusters with some small amino acids was studied. The different neutral clusters formed in the supersonic expansion were ionised by a multiphoton process employing either nano‐ or femtosecond laser pulses. Ionisation conditions were chosen to avoid direct ionisation of the small, non aromatic amino acids. Consequently the ionisation process of the neutral clusters is determined by the MALDI matrix compounds provided with an aromatic chromophore.

Strong differences in the detection of cluster ions due to the laser pulse length employed for multiphoton ionisation were observed. Only femtosecond activation led to mass spectra with intense signals of the cluster ions. In addition in the case of femtosecond ionisation protonated amino acids were detected in the mass spectra. As direct ionisation of the free amino acids is not possible under the chosen ionisation conditions because they lack an adequate chromophore these protonated amino acids are assumed to be formed via an intracluster proton transfer in the heterogeneous dimer and subsequent decay of the ionised cluster (dissociative proton transfer). Such well‐known processes for heterogeneous clusters consisting of a substituted aromatic molecule and small polar solvent molecules may be involved in the MALDI ionisation process.
Pathways for the photochemical hydrogen abstraction by n, π*‐excited states
Authors: Nau Werner M.
Published: July 2012

As a result of recent experimental and theoretical investigations, new aspects of the photochemical hydrogen abstraction by n, π*‐excited states have emerged. The absolute reactivity of singlet‐excited n, π* states exceeds that of the corresponding triplet states, but the singlet reaction turns out to be chemically much less efficient than the triplet reaction. Hence, radiationless deactivation of the singlet states represents the dominant pathway for interaction with hydrogen donors. The fraction of singlet encounters undergoing deactivation appears to increase with decreasing reaction exothermicity. Hence, when relatively inert solvents like methanol or chloroform act as hydrogen donors towards singlet‐excited azoalkanes, the reaction becomes entirely inefficient. This constitutes an example of a novel fluorescence quenching mechanism, which is referred to as an “aborted” hydrogen abstraction. The inefficiency of the singlet reaction can be rationalized, making use of the most recent theoretical findings, in terms of the occurrence of a conical intersection, i.e., a real surface crossing, rather than the previously implicated avoided crossing. The conical intersection provides an efficient channel for nonadiabatic return to the ground‐state reactants and, thus, serves to account for the observed inefficiency of product formation from the singlet state. In general, the conical intersection appears to follow the transition state for hydrogen abstraction, but the consequences of a situation, where the conical intersection precedes the transition state are also discussed. The latter becomes reasonable for endothermic hydrogen abstractions, for which one would then expect a unit efficiency of singlet deactivation. The absolute reactivities in hydrogen abstractions, the involvement of CT interactions and tunneling, and the behavior of different n, π* chromophores (azoalkanes versus ketones) are also discussed in view of the most recent experimental data. General rules for photochemical hydrogen abstractions by n, π*‐excited states are expressed.
Vibration‐assisted intermolecular hydrogen tunneling in photoreactive doped molecular crystals: Effect of temperature and pressure
Authors: Prass B. , Stehlik D. , Chan I. Y. , Trakhtenberg L. I. , Klochikhin V. L.
Published: July 2012

The pressure and temperature dependence of the reaction rate of the photochemical intermolecular H‐transfer reaction in acridine‐doped fluorene single crystals exhibit all typical features of solid state H‐tunneling. Independent Raman data including the pressure dependence have identified three low energy inter‐ and intramolecular vibrational fluctuations as reaction promoting modes. The mode identification allows a more subtle analysis of the H‐transfer reaction rate behavior on the basis of recent theoretical concepts. Surprisingly, the complete data set k (P, T) can be satisfactorily reproduced by an average one frequency mode model with parameters consistent with an average of the parameters calculated for the individual reaction promoting modes.
D. Proton transfer in complex systems, liquids and biological systems: Proton transfer reactions at the surface of Ice. Heterogeneous reactions involved in stratospheric ozone depletion
Authors: Bianco Roberto , Gertner Bradley J. , Hynes James T.
Published: July 2012

Two heterogeneous reactions on an ice surface that are relevant in connection with stratospheric ozone depletion are discussed theoretically: the hydrolysis of chlorine nitrate, ClONO2, to produce HOC1 and HNO3, and the acid ionization of hydrochloric acid HC1. For the former, a recently proposed proton transfer mechanism actively involving the ice lattice is supported by examination of the ClONO2 hydrolysis energetics when proton transfer is prevented. For the latter, a previously proposed picture of facile HCl ionization at, but not on, the ice surface is supported by the results of a quantum treatment of the proton and of a model investigation of the free energetics of ionization atop the ice surface.
Quantum dynamical entanglement of protons in liquid water: New Raman light scattering and inelastic neutron scattering experiments
Authors: Chatzidimitriou‐Dreismann C. A. , Abdul‐Redah T. , Streffer R. M. F.
Published: July 2012

Recent theoretical work indicated that the short‐time quantum entanglement between closely lying protons (or other particles) in condensed matter may be experimentally accessible, even at room temperature. This effect has been predicted to cause the appearance of certain “anomalous” components in the scattering intensity, the “anomaly” being caused by the fact that an external field cannot interact properly with domains containing quantum correlated protons (or H‐bonds). First experimental evidence for this novel effect has been provided with the aid of Raman light scattering from liquid H2O/D2O mixtures. Very recently, associated deep inelastic (Compton) neutron scattering experiments on liquid H2O/D2O mixtures gave an independent confirmation of the considered effect. The results of both experiments, which have found so far no conventional interpretation, provide direct evidence for short‐lived quantum entanglement of adjacent protons (and deuterons) in condensed systems atT≈︁ 293 K. In addition, we present first results of two new experiments concerning: (1) Raman light scattering on solutions of NaCl in H2O/D2O; (2) deep inelastic neutron scattering on solutions of urea (H2N‐CO‐NH2) in H2O/D2O.
Proton self‐exchange of para‐substituted N,N‐dimethylanilines and their conjugate acids in aprotic solvents
Authors: Goez Martin , Heun Rainer
Published: July 2012

The intermolecular proton transfer between the title compounds was investigated by dynamic NMR spectroscopy. The mechanism was found to be direct proton transfer, other transfer pathways being excluded by the experimental conditions (aprotic solvents with an extremely low water content of about 5×10−4M). It was shown that the observed exchange rates are identical with the rates of the proton transfer proper; association or dissociation pre‐ or postequilibria are not rate determining. Magnitude and solvent dependence of the kinetic isotope effects, as well as the influence of the solvent polarity on the intrinsic activation barrier ΔG‡int indicate considerable solvent reorganization during the transfer. These observations, as well as correlations between the activation parameters of proton self‐exchange and electron self‐exchange of the substrates studied provide some evidence that inner‐sphere and outer‐sphere contributions to ΔG‡int might be separable.
The contributions of secondary proton bridges to the catalytic power of the serine proteases
Authors: Bibbs Jeffrey A. , Garoutte Michael P. , Wang Bing , Tittel Paul D. , Schowen K. Barbara , Schowen Richard L.
Published: July 2012

The curvature of proton‐inventory functions (rate constants in mixtures of protium and deuterium oxides) for hydrolysis of substrates of varying structure by trypsins from four different sources has been used as a signal of transition‐state stabilization by a short, strong hydrogen bonds (“low‐barrier hydrogen bonds”) at the histidine‐aspartate site of the catalytic triad. Previous estimates suggested contributions as large as 85 kJ mol−1 but the current findings are consistent with contributions of no more than a few kJ mol−1.
Quantum dynamics of proton transfer processes in enzymatic reactions. Simulations of phospholipase A2
Authors: Bala P. , Grochowski P. , Lesyng B. , Mccammon J. A.
Published: July 2012

For the past few years we have been developing a theoretical model capable of describing quantum dynamical processes which occur in molecular systems and in enzymatic active sites. This, in particular, includes proton transfer processes.

Our most advanced studies have been performed for phospholipase A2, an enzyme which hydrolyzes phospholipids. The potential energy function for the active site is computed using an Approximate Valence Bond (AVB) method. The dynamics of the key proton in the enzyme's active site is described either by the classical molecular dynamics (MD/AVB model) or by the time‐dependent Schroedinger equation. The dynamics of the remaining atoms of the enzyme are described using classical MD. The coupling between the quantum proton and the classical atoms is accomplished via Hellmann‐Feynman forces, as well as the time‐dependence of the potential energy function in the Schroedinger equation (QCMD/AVB model).

The quantum proton transfer from the water molecule to histidine is followed by a nucleophilic attack of the created OH− group. The processes are simulated using the parallelized QCMD code.
Structural investigations of fluid carbon tetrafluoride by statistical mechanical calculations and Monte Carlo simulation
Authors: Bassen A. , Waldner I. , Bertagnolli H.
Published: July 2012

Statistical mechanical calculations with the extended reference interaction site model (XRISM) and Monte Carlo simulations are performed on fluid CF4. The parameters of a five‐center (12‐6) Lennard Jones potential with added point charges are deduced. The results are compared with total atom pair correlation functions recently determined by neutron diffraction experiments at three supercritical densities. Apart from the good agreement between experimental and theoretical results, we found that depending on the method used, different potentials are able to describe the investigated system. The atom pair correlation functions and angular distribution functions indicate a completely disordered arrangement of the molecules in fluid CF4 with no significant short‐range orientational order except for very close distances. The comparison of the results with former Reverse Monte Carlo (RMC) simulations allows the conclusion that the RMC method is an effective tool, equivalent to the well established Monte Carlo and XRISM methods, in determining the microscopic structure of fluid CF4, but without the exact knowledge of intermolecular potentials.
Diffusive motion of water molecules in harmotome studied by neutron scattering
Authors: Stockmeyer Rolf
Published: July 2012

The quasielastic neutron scattering from H2O and D2O in harmotome, a natural zeolite with the unit cell content Ba2(AlO2)4 (SiO2)12nH2O, has been measured at temperatures between 294 and 423 K with loadings of n = 2, 4, and 12 water molecules per unit cell. From the observed coherent scattering relaxation times for D‐D correlations from 2 to 11 ps were deduced. Fitting the incoherent quasielastic peaks from H2O to a two‐site jump model yielded residence times of the protons in the range from 4 to 6 ps.
Magnetic susceptibility and proton Knight shift in Pd‐Ag and Pd‐(rare earth) solid solution alloy hydrides
Authors: Sinsel Bernd , Anton Herbert , Schmidt Peter C. , Weiden Norbert , Weiss Alarich
Published: July 2012

The magnetic susceptibility χ and the1H‐Knight shiftKHfor the ternary hydrides Pd0.6Ag0.4Hr(0.05 ≤r≤ 0.16), Pd0.7Ag0.3Hr(0.08 ≤r≤ 0.26), and Pd0.936(RE)0.064H0REGd (0.04 ≤r≤ 0.47), Dy (0.06≤r≤0.50), and Lu (0.02 ≤r≤ 0.59) are studied as a function of the hydrogen contentusing high resolution1H‐NMR spectroscopy. Additionally, the temperature dependence of χ for the hydrogen free host alloys Pd0.936(RE)0.064is measured using a SQUID magnetometer and the local f magnetic moments of Gd and Dy are determined. In all systems considered, χ is decreasing with increasingrandKHis negative and increasing with increasingr.The various contributions to χ andKHare discussed on the basis of augmented spherical wave (ASW) supercell band structure calculations.
Electronic conductivity of gadolinia doped ceria
Authors: Lübke S. , Wiemhöfer H.‐D.
Published: July 2012

The electronic conductivity of gadolinia doped ceria (Ce0.8Gd0.2O1.8) was measured as a function of oxygen activity (aOz=1 corresponding to a gas with oxygen partial pressure pO2= 1.013 bar) in the range from aOz≈︁101 to aO2≈︁10−13 for temperatures between 600 and 750°C. The Hebb‐Wagner polarization technique was used with an encapsulated Pt‐microcontact in N2 atmosphere and a Cu2O/CuO reference electrode. For instance, a minimum value of 3.10−7 S/cm was found for the electronic conductivity at 600°C and an oxygen activity of 9.10−7. The minimum is shifted to higher activity with increasing temperature. The activity dependence of the electronic conductivity in the n‐type and p‐type range agrees well with the theoretically expected values (p‐type range σp≈︁a
, n‐type range σn≈︁a
). Extrapolation of the measured electronic conductivities, the electrolytic domain boundaries of Gd‐doped CeO2 were obtained as log aO2= 13 to log aO2=‐24.7 at 600°C and log aO2=11 to log aO2=‐20.1 at 700°C (the oxygen ion conductivity being 4.05.10−3 S/cm at 600°C and 1.29.10−2 S/cm at 700°C).

The activation energy was determined from the temperature dependence of the electronic conductivity giving EA,e=(2.59±0.07) eV for electrons in the n‐type range and EA,h=(1.16±0.04) eV for the holes in the p‐type range. The migration enthalpy of electrons was Hmig,e = 0.49 eV.
Photophysics and photochemistry of trans‐4‐nitrostilbenes and trans‐2,4‐dinitrostilbenes: Effect of intramolecular charge transfer
Authors: Görner Helmut
Published: July 2012

The photophysical and photochemical properties of trans‐R‐2′,4′‐dinitrostilbene (II) and a series of trans‐R‐2′,4′‐dinitrostilbenes (II‐R, R: 2‐NO2, 3‐NO2, 4‐Br, 4‐F, 4‐Me, 4‐C3H7, 4‐OCH2C6H5, 3,4,5‐(OMe)3, 4‐OEt, 4‐NMe2, 4‐NEt2) were studied in solution as a function of solvent polarity and temperature. The quantum yield of fluorescence (Φf) is very small for all II‐R at 25°C. At −196°C Φf is moderate for several derivatives in 2‐methyltetrahydrofuran (MTHF) but strongly enhanced for those bearing electron donating substituents. For the latter compounds the quantum yield of trans → cis photoisomerization is low, but for the other compounds Φt → c is substantial (0.2‐0.5 in toluene or MTHF at room temperature). The triplet state absorbs typically in a broad spectral range; its lifetime (τT) lies in the 20‐200 ns range and is longer for R = 4‐NEt2; at −196°C τT of all II‐R approaches milliseconds. The results are compared with those of trans‐4‐R‐4′‐nitrostilbenes (I‐R, R: NO2, H, OMe, NH2, NMe2, NEt2). Phosphorescence of singlet molecular oxygen was observed for several mono‐ and dinitrostilbenes at room temperature. Generally, the quantum yield of singlet oxygen formation is much smaller than that of intersystem crossing into the triplet state. The triplet mechanism accounts for trans→cis photoisomerization and the contribution of this pathway is lowered by intramolecular electron transfer to the nitro group(s). The similarities and differences between I‐R and II‐R type compounds and the effects of intramolecular charge transfer are discussed.
A heuristic approach to the analysis of Mössbauer‐Spectra: The ‘simulated annealing’‐algorithm
Authors: Lankau T. , Bieber S. , Nagorny K.
Published: July 2012

The ‘Simulated Annealing’ algorithm is used to analyse Mössbauer spectra with distributed hyperfine structure parameters. The distribution is described by a set of points. No initial assumptions regarding the distribution function are therefore necessary to run the program successfully. The algorithm can be easily extended to handle more than one distribution.
Conductometric study of LiCIO4 and Bu4NBPh4 in propylene carbonate‐toluene mixtures
Authors: Moumouzias George , Ritzoulis George
Published: July 2012

The electrolytic conductances of LiClO4 and BU4NBPh4 in Propylene carbonate‐Toluene mixtures at 25, 30 and 35°C have been measured. Limiting equivalent conductance, association constant and distance parameter of ions were calculated by means of Lee‐Wheaton equation. Gibbs energy of ion‐pair formation due to non‐Coulombic interaction, conductive transport activation energy, Walden product and Stokes radius are also evaluated.
Real time kinetics and thermochemistry of the uptake of HCl, HBr and HI on water ice in the temperature range 190 to 210 K
Authors: Flückiger Benoît , Thielmann Axel , Gutzwiller Lukas , Rossi Michel J.
Published: July 2012

The real time kinetics of adsorption and desorption of HCl on ice have been studied in a very low pressure reactor (Knudsen cell). We used two complementary techniques, namely pulsed valve or transient supersaturation and steady state experiments. The kinetics of adsorption, following a first order rate law, has been found to depend on the number of HCl molecules in a pulse for transient supersaturation and on the HCl partial pressure or flow rate in steady state experiments. The uptake coefficient γ ranges from 0.34 to 0.22 in the temperature range 190 to 210 K for dry adsorption leading to a quasi‐liquid state of HCl on ice and from 0.31 to 0.13 for adsorption leading to a liquid solution of HCl. In both cases a negative temperature dependence for uptake of HCl on ice of −1.8±0.5 and −3.1±0.5 kcal/mol has been observed. In addition, the fraction of HCl that could be retrieved by desorption after dosage with HCl depended strongly on the thermodynamic state of the HCl/ice substrate as well as on the temperature and varied in the range from 10 to 80%. We determined the rate of irreversible surface‐to‐bulk loss, with a limiting rate of 1.0·1013 molecule s−1 cm−2 at 200 K. In addition, we studied the kinetics of adsorption of HBr and HI on ice resulting in γ values similar to HCl. However, ice in the range 190 to 210 K did not support any measurable vapor pressure of both HBr and HI after adsorption of up to several formal monolayers in contrast to HCl.
A network of characteristic functions describing the dynamics of adsorbate‐adsorbent systems
Authors: Rödenbeck Christian , Kärger Jörg , Hahn Karsten
Published: July 2012

Adsorbate‐adsorbent systems characterized by a stationary particle exchange with their surrounding are considered. Depending on the considered processes, like, e.g., tracer exchange or particle conversion, the behaviour of such a system may be described by a number of characteristic functions (tracer exchange curves, effectiveness factor, residence time distribution, transit time distribution, and others) whose particular shape is determined by the properties of the underlying transport mechanisms. It is shown that, under very general assumptions (mainly stationarity), there are one‐to‐one relations between these characteristic functions. Owing to these relations, the behaviour in a given process may be determined from that in a completely different one, without the need of any explicit information about the underlying transport mechanisms. The paper derives a great variety of relations (including equations for the analysis of a number of frequently used experimental setups) and discusses the information content as well as general properties of the characteristic functions.
Solvatochromism in supercritical fluids
Authors: Maiwald Michael , Schneider Gerhard M.
Published: July 2012

Solvatochromic shifts of the five probe indicators 2‐nitroaniline, 4‐nitroaniline, 4‐nitroanisole, 4‐nitrophenol, and N,N‐dimethyl‐4‐nitroaniline have been used to specify the solvatochromic solubility parameters π* and β of the five pure fluids CO2, N2O, CClF3, SF6, and NH3 according to Kamlet, Abboud, and Taft by means of UV/VIS spectroscopy. Three isotherms, nearcritical and supercritical, were examined, for the first time from zero pressure up to 100 MPa. Except NH3 with β≈︁0.70 (293 K), 0.65 (333 K), and 0.55 (413 K) at 100 MPa, respectively, all other fluids do not show significant basicity at any temperature and pressure conditions. Plotting π* vs reduced density of any fluid results in steeply increasing curves with maximum enhanced π* data at reduced densities of about 0.6 which continuously level out and finally rise again asymptotically as straight lines.
Finite size dependence of the tracer surface diffusion coefficient near a second order phase transition
Authors: Nieto F. , Uebing C.
Published: July 2012

We investigate the finite size dependences of the tracer surface diffusion coefficient in the vicinity of a second order phase transition. For this purpose, we use a lattice gas model of repulsively interacting particles on a square lattice. For all lattice sizes L studied, the tracer diffusion coefficient, Dt, is a smooth function of temperature. There are no anomalies in the direct temperature dependence of Dt near the critical temperature Tc. However, its derivative, dDt/d(1/kBT), exhibits cusp‐like anomalies which are largely pronounced for large lattice sizes.
Influence of the preparation conditions on the properties of electropolymerised polypyrrole
Authors: Brandl Volker , Holze Rudolf
Published: July 2012

Polymerisation conditions have a remarkable effect on the various material properties of polypyrrole. Care has to be taken to ensure identical conditions for electrochemical experiments when films electropolymerised differently are examined in comparison. The effect of a variety of experimental parameters on redox capacity, conductivity and electrooptic behaviour has been examined. Based on experimental evidence presented here and reported elsewhere explanations are proposed. A choice of optimum conditions suitable for obtaining a tailored polymer can be derived.