bluuebunny/crossref_metadata_2025_split huggingface dataset:
Density functional theory: Prediction of ellipsometric parameters and structural behaviour of pure fluid vapour‐liquid interfaces
Authors: Winkelmann J. 
Published: July 2012

AbstractBoth structural properties, like density profiles, interfacial behaviour and the corresponding phase equilibria are governed by the same intermolecular forces. The prediction of ellipsometric parameters requires a profile of the optical dielectric constant across the vapour‐liquid interface. This profile is provided by the Density Functional Theory (DFT). We apply the Local Density Approximation (LDA) to the DFT to a molecular system with a cut‐and‐shifted Lennard‐Jones potential. The potential parameters were determined by modelling surface tensions and saturated liquid densities of a pure fluid. The calculated density profile is then transformed into a profile of the refractive index across the interface. Using Crook's multilayer algorithm we predict the ellipsometric parameters Ψ, Δ and the ellipticity e as a function of the angle of incidence of a laser light beam over a range of temperatures. Predicted values for argon, cyclohexane and water are compared with experimental results.
Generation of ionic species by photolysis of polysilylenes in dilute solution
Authors: Eckhardt A. , Nešpůrek S. , Schnabel W. 
Published: July 2012

AbstractFrom flash photolysis studies (at λinc = 266 and 347 nm) involving the electrical conductivity and the optical absorption detection method it is inferred that poly(methylphenyl silylene), PMPSi, and poly(biphenylmethyl silylene), PBMSi, form ionic species in a monophotonic process in dilute tetrahydrofuran and dichloromethane solution at 295 K (ϕ(ion) ≈ 4×10−2, PBMSI and 1×10−2, PMPSi, in THF at λinc = 266 nm). The ionic species possess optical absorption bands between 350 and 500 nm with a maximum at about 380 nm. Both the photocurrent and the optical density at 380 nm decay with identical rates by a second order process (k2 ≈ 3 × 1011 dm3 mol−1 s−1, PBMSi in THF at 295 K).
Thermodynamics and separation of mixtures
Authors: Prausnitz J. M. 
Published: July 2012

AbstractThe classical foundation of chemical thermodynamics was essentially completed by J. W. Gibbs when he defined the chemical potential in 1875. However, this formulation gives only the point of departure for applying thermodynamics to a quantitative description of phase equilibria; these equilibria are required for design of separation operations that depend on transfer of molecules from one phase to another. To apply thermodynamics to real problems, we need meta‐thermodynamic information as supplied by molecular physics and physical chemistry. This need was recognized at the end of the 19th century by numerous outstanding scientists (e.g. Max Planck) who made efforts to establish practical theories for calculating chemical potentials; similar efforts continue today. To illustrate the applicability of thermodynamics, three examples are presented here: removal of hydrogen sulfide from gasified coal; precipitation of proteins from aqueous solution; separation of small and large solutes using a temperature‐sensitive hydrogel which exhibits a first‐order phase transition. Attention is given to the increasing role of molecular simulation that can provide not only quantitative thermodynamic “data” but, in addition, a qualitative “picture” of complex phenomena. Such pictures provide the first step for development of quantitative models.
On the stability and on the formation of two‐dimensional condensed equilibrium adsorbate films in an aprotic and in aqueous solution. Part III: Inversion from irreversible to reversible phase transition of adsorbed 2‐thiohydantoin at the interface mercury/water, sodiumfluoride due to coadsorbed polyethylenglycol
Authors: Schrettenbrunner M. B. , Chaiyasith P. , Baumgärtel H. , Retter U. 
Published: July 2012

AbstractThe following is an investigation on the influence of polyethylenglycol on the two‐dimensional phase transition of 2‐thiohydantoin. The two‐dimensional phase at the interface mercury/aqueous electrolyte has been characterised by measuring the potential as well as the time dependence of the differential capacitance.In the absence of polyethylenglycol, the irreversible phase transition is indicated by a discontinuous change of the capacitance with the potential. The condensed film is formed by two‐dimensional nucleation according to an exponential law and a constant growth rate.However in the presence of polyethylenglycol, the reversible phase transition is connected with a continuous change of the capacitance‐potential dependence. At the state of equilibrium different capacities have been observed. Different equilibrium states also can well be described theoretically as two‐dimensional instantaneous nucleation and growth of 2‐thiohydantoin, too. But in the case of reversible phase transitions of adsorbates a polyethylenglycol induced desorption being proportional to the area of 2‐thiohydantoin centres has to be considered.
Enthalpies and entropies of complexation of silver(I) and diaza‐18‐crown ethers in polar solvents
Authors: Burchard T. , Firman P. , Schneider H. , Cox B. G. 
Published: July 2012

AbstractThe enthalpies of complexation of Ag+ with diaza‐18‐crown ethers containing various combinations of donor atoms have been determined in polar organic solvents using calorimetric and potentiometric techniques. Enthalpies of complexation are also reported for two silver ion cryptates. The results are combined with previously determined free energies of complexation to obtain the corresponding entropies. In dipolar aprotic solvents, the enthalpies of transfer of the complexes are very similar to those of the free ligands, so that changes in enthalpies of complexation are dominated by the enthalpies of solvation of the silver cation. The thermodynamics of ligation, i.e., of complex formation between the gaseous silver ion and the diaza‐crown and cryptand ligands in solution, are calculated and, for the free energies, partitioned into contributions from each of the donor atoms (N, O, and S). Among the aprotic solvents, contributions of oxygen and sulphur atoms in the ligand bridges to the stabilities of diaza‐crown and cryptand complexes are additive and independent of solvent. In methanol, hydrogen bonding between methanol molecules and the free ligands reduces the magnitudes of both the free energies and enthalpies of formation of the silver complexes.
Approximate determination of rovibrational densities of states ρ(E,J) and numbers of states W(E,J)
Authors: Olzmann M. , Troe J. 
Published: July 2012

AbstractCompact approximate equations for rovibrational densities of states ρ(E,J) for harmonic oscillator‐rigid rotor molecular systems are derived. Linear molecules, spherical tops, prolate and oblate symmetric tops are considered. Vibrational densities of states are represented by the Whitten‐Rabinovitch approximation which then is convoluted with the contribution from the K‐rotor (for a symmetric top). Likewise, numbers of states W(E,J) are approximated for prolate symmetric top molecules. The derived expressions allow for quick calculations of specific rate constants k(E,J) of unimolecular reactions.
Brownian reorientation in a smectic phase with quadrupolar ordering
Authors: Schiller P. 
Published: July 2012

AbstractThe reorientation of molecules in smectic liquid crystalline phases around their long axes is accompanied with a dielectric dispersion in the high frequency region. This spinning motion is influenced by the anisotropy in the layers of smectic phases with tilted molecules and can be regarded as a Brownian process in a periodic potential. The solution of the corresponding Smoluchovsky equation allows to determine the two‐time correlation function for the transversal dipole moment and the dielectric susceptibility. It is found that a quadrupolar potential causes a pronounced deviation from the semicircle in a Cole‐Cole diagram, whereas the deviations are much smaller when a dipolar ordering is considered.
Kinetics and mechanisms of the reactions of O(1D)‐Atoms with CF3Br and CF2BrCF2Br in the gas phase and in solid argon matrices
Authors: Lorenzen‐Schmidt Heike , Weller Rolf , Schrems Otto 
Published: July 2012

AbstractThe rate coefficients and mechanisms for the reactions of O(1D) atoms with CF3Br (halon 1301) and CF2BrCF2Br (halon 2402) have been investigated in the gas phase at room temperature. The experiments have been carried out in a static photochemical reactor. Photolysis of ozone at 253.7 nm provided O(1D) atoms. The competitive reaction O(1D)+N2O has been chosen as internal standard to determine the rate coefficients. The course of the reactions was followed by FTIR spectroscopy. The measured data correspond to rate coefficients of (4.0±0.4)×10−11 cm3 s−1 for the reaction O(1D) + CF3Br and (8.8±1.2)×10−11 cm3 s−1 for the reaction O(1D) + CF2BrCF2Br.For further mechanistic studies, the reaction O(1D) + CF3Br has been investigated in solid Ar‐matrices at T = 14 K. Codeposition of O3/CF3Br/Ar mixtures and irradiation at 250 nm yielded F2CO as dominant reaction product. Experiments with 18O3 indicate that CF3OBr was formed during the course of the reaction. Our observations can be interpreted with the formation of an excited CF3OBr molecule which can be stabilized in the matrix cage or decompose rapidly to the final product F2CO. No further intermediates could be detected in the matrix experiments.
Electrochemical and Simultaneous Spectroscopic Study of Reduction Mechanism and Electronic Conduction During Electrodeposition of Tantalum in Molten Alkali Chlorides
Authors: Bachtler M. , Freyland W. , Voyiatzis G. A. , Papatheodorou G. N. 
Published: July 2012

AbstractWe have studied the reduction mechanism and electronic conduction during electrodeposition of tantalum in various TaCl5‐alkali chloride melts at temperatures up to 1100 K. This has been performed by simultaneous measurements of impedance and Raman spectra together with cyclic voltammograms. A detailed analysis of electrochemical and spectroscopic results yields the following predominant reduction steps during electrodeposition in pure alkali chloride melts: Ta(V)‐Ta(IV)‐Ta(0). The equilibrium potentials of the corresponding redox reactions are separated by 40 mV which is clearly resolved in impedance measurements. In LiCl/KCl‐eutectic melts a three step mechanism is found with Ta(III) being the intermediate species. In evaluating the impedance data we found indications for electronic contributions which are most prominent with added oxide impurities to the molten electrolyte. Direct evidence for an appreciable electronic conductivity has been obtained by electrochemical polarization measurements. These are reported here the first time for fused KCl‐TaCl5/Ta at 1050 K, where electronic conductivities up to 0.5ω −1cm−1 have been measured, i.e. of same order of magnitude as ionic conductivities.
A Multi‐Functional Electro‐Optical Molecular Device. The Photoelectrochemical Behavior of Spirobenzopyrans in Dimethylformamide
Authors: Zhi Jin Fang , Baba Ryo , Hashimoto Kazuhito , Fujishima Akira 
Published: July 2012

AbstractThe photoelectrochemical investigation of l,3,3‐trimethylspiro(2H‐l‐benzopyran‐2,2′‐indoline) (SP‐O) and l,3,3‐trimethyl‐6‐nitrospiro(2H‐l‐benzopyran‐2,2′‐indoline) (SP‐1) has been performed in a dimethylformamide (DMF) solution at room temperature and at a low temperature (‐42°C). Nitro‐substituted SP‐1 is electroactive in the experimental potential range of ‐1.8V to +0.8 V (vs. Ag/Ag, while no nitro‐substituted parent spirobenzopyran SP‐0 is lacking in electroactivity. In the case of SP‐1, nearly identical electrochemical behavior was found for the closed‐ring (SP) and open‐ring (MC) isomer at room temperature or at ‐42°C, suggesting that the ring form, i.e. open or closed, has no influence on the redox reaction of SP‐1. Moreover, reversible color changes were observed in the DMF solution of SP‐1 depending on reduction and oxidation potentials, namely, a change to yellow in the reduction and to deep violet in the oxidation. Both voltammetric and spectroelectrochemical evidences suggest the production of a nitrobenzopyran radical anion of closed‐ring (yellow) in a reduction and an open‐ring isomer MC (deep violet) in an oxidation either from SP and from MC of SP‐1 in the DMF solution. This behavior leads to the discovery of a novel photochromism and electrochromism “three state” conjugated system which may potentially be applicable to the construction of multi‐functional electro‐optical molecular devices.
On the Nature of the Nonanoic Acid Monolayer Adsorbed at the Hg/Aqueous Solution Interface: Influence of Conformational Freedom in Bulk Aqueous Solution
Authors: Bordi S. , Carlå M. , Passamonti P. , Fontanesi C. , Pelloni P. 
Published: July 2012

AbstractThe adsorption of the nonanoic acid at the electrified Hg/aqueous solution interface is studied by means of capacity and surface tension measurements. Comparison of the experimental rmax value with the calculated more favoured solvated conformation suggests that the acid is adsorbed in a partially coiled conformation. The adsorption free energy here determined, together with previously reported values found in the literature concerning other saturated acids and alcohols, is compared with reactivity and calculated molecular descriptors. The collected evidence suggests that the hydrophobic nature of the acid is responsible for the adsorption, while the electric field/molecule interaction, occurring at the electrified interface, governs the desorption. Lastly, some experimental (capacitive transients, effect of the temperature on the tensammetric curves) and theoretical findings are discussed which indicate that the adsorbed acid monolayer should be regarded as a usual, in the field of adsorption at electrodes, disordered network. Thus excluding the occurrence of a two‐dimensional phase transition involving the formation of a 2D solid‐like film.
Reaction Mechanism of the Iron(III)‐Catalyzed Autoxidation of Bisulfite in Aqueous Solution: Steady State Description for Benzene as Radical Scavenger
Authors: Warneck Peter , Ziajka Josef 
Published: July 2012

AbstractThe Fe(III)‐catalyzed oxidation of bisulfite in aqueous solution was studied at pH = 3 in the presence of benzene by monitoring the rise of hydrogen ion activity and phenol product concentration as a function of time, keeping [HS03−/[Fetot]>>l. After a brief initial period, during which product concentrations rose rapidly and Fe(III) was largely converted toward Fe(II), the reaction underwent a period of slower growth characterized by a quasistationary state and nearly constant [Fe(II)]/[Fe(III)] ratio until toward the end of the reaction, when most of the S(IV) was consumed, Fe(II) was reoxidized toward Fe(III). In two sets of experiments either the concentration of benzene or that of [Fe(III)]o was varied in ranges of 0.3–2.2 mmol dm−3 and 8.5–51 μmol dm−3, respectively, while keeping the other parameters constant ([HSO3−]o = 2 mmol dm−3). The data were analyzed by means of a steady state expression developed for the time period of nearly constant [Fe(II)]/[Fe(III)] ratio on the basis of a radical‐driven chain oxidation mechanism presented previously. The data display the behavior predicted by the steady state equation; this allowed the determination of ratios of rate coefficients for reaction pairs: SO4− reacting with HSO3− and benzene, k11/k12 = 0.31±0.04; S05− reacting with Fe2+ and HSO3−, k3/k10 = 54.1±1.7; HSO5− reacting with Fe2+ and HSO3−, k4/k16 = 2.2±0.2; and the branching ratio for the reaction of SO5− with HSO3− leading to (a) SO3− and (b) SO4− radicals, k10b/k10≤0.04. The experimental results largely support the proposed reaction mechanism.
Transport of Mass and Moving Phase Boundaries Observed by Neutron Transmission through Compacted Zeolite NaX
Authors: Stockmeyer Rolf , Strothmann Helmut 
Published: July 2012

AbstractThe density C(x, t) of adsorbates (water, cyclohexane) at moving liquid/gas boundaries in samples of compacted zeolite NaX has been observed by neutron transmission experiments with a resolution dx = 0.2 cm, dt = 30 s, over a distance of 20 cm and during a time of observation up to 400 hours. The front of humidity migrates, driven by capillary forces, from the wet to the dry side of the sample with an initial velocity of about 10−6 ms−1. In terms of a capillary model the absorbate moves with a pseudo‐diffusion‐coefficient in the order of 10−7 m2s−1. The velocity slows down to 10−7 m s−1 if the amount of adsorbate is limited. A slow process of redistribution of the fluid takes place which does not smoothen the profile of the density of mobile molecules at the liquid/gas boundary in the porous solid, as one would expect in the case of diffusive motions of molecules at gradients in a homogeneous phase of an adsorbate. The step of density observed at the moving liquid/gas boundary is formed according to a dispersion/diffusion coefficient D(C) of about 10−9 m2s−1.
Diffusion of the Trimethyl‐Cyclohexadienyl Radical Adsorbed on Silica: Redfield Theory and Results of μSR Experiments
Authors: Schwager Martina , Dilger Herbert , Roduner Emil , Reid Ivan D. 
Published: July 2012

AbstractThe 2,4,6‐trimethyl cyclohexadienyl radical adsorbed on a fully hydroxylated silica surface was investigated using the time‐differential and time‐integrated muon spin resonance techniques near avoided crossings of magnetic energy levels in high longitudinal magnetic fields. Information about surface diffusion is obtained from the relaxation resulting from the partial averaging of the hyperfine anisotropy when the adsorbed species diffuses over the surface of a sphere. A Redfield formalism is developed to analyse the stochastic motion in terms of diffusion coefficients. The results agree with those of a stochastic Liouville approach reported previously. Diffusion of the trimethyl‐substituted radical is described by the same Arrhenius frequency factor but a considerably higher activation energy than that of the unsubstituted analogue.
Nuclear Magnetic Resonance Investigation of Intramolecular Motions in Methylpyranosides
Authors: Latanowicz L. , Reynhardt E. C. , Utrecht R. , Medycki W. 
Published: July 2012

AbstractProton spin‐lattice relaxation times T1 and T1 as well as the proton second moment have been measured for polycrystalline methyl α‐D‐glucoside, methyl β‐D‐glucoside and methyl β‐D‐galactoside over a wide temperature range. Analyses of the experimental data have yielded information about intramolecular reorientations in the solid state. C3 reorientations of methyl groups take place in all the samples. Potential energy calculations have shown that the CH2OH group can execute trans‐gauche jumps between two potential minima separated by 120. Analyses of the T1 and second moment data have confirmed the presence of this conformational motion. A weak relaxation mechanism, related to the dynamics of an OH group, has been revealed in methyl β‐D‐galactoside.
Investigation of Double Layer Structure at Platinum in Acid Electrolytes by Modulated IR‐Spectroscopy
Authors: Probst J. , Thull R. 
Published: July 2012

AbstractThe electrochemically modulated infrared spectroscopy (EMIRS) is a highly sensitive in‐situ technique to obtain vibrational spectra of species adsorbed on metal electrodes. The contribution discusses the influence of several experimental parameters as the electrode potential, the frequency and amplitude of the superposed potential modulation and the pH value of the electrolyte on the spectra of water molecules within the Helmholtz Double Layer at a platinum surface. The spectra contain significant information about conformational changes of water molecules, which are coadsorbed by atomic hydrogen on the platinum surface. The comparison of experimental results to those of theoretical calculations with Lagrange formalism indicates that, modulating the potential of a positively charged electrode, a complex molecule structure is varied at the surface.
Charakterisierung von elektrokatalytisch hergestellten Folien und Schichten aus Silizium
Authors: Dürrler Michael , Grüniger Hans Rudolf , Rys Paul , Kiess Helmut , Rehwald Walther 
Published: July 2012

AbstractSilicon foils, prepared by electrocatalysis, consist of microcrystalline, porous silicon. The size of the crystallites ranges between 15 and 20 nm, whereas the average pore size is about 60 nm. The density of this material is 65% of that of crystalline silicon. It is p‐type and could be made n‐type by neutron activation.Electrical and optical measurements indicate that the charge carrier mobility is limited by the grain boundaries. The grain boundaries give, in addition, rise to strong recombination of charge carriers. The luminescence of the material was investigated in a few experiments, and attempts were made to fabricate solar cells from electrocatalytically produced silicon foils with the pn‐junctions prepared by neutron activation. The luminescence showed a peak at about 570 nm. The open circuit voltages obtained with solar cells were in the range between 120 and 200 mV. Further experiments to increase the intensity of the luminescence and to improve the quality of the cells could not be made within the framework of this project. It is, however, expected that further experimentation leads to a better understanding of the properties of the material and to applications.
Catalysis by Manganese Oxide Monolayers Part 1: Alumina and Magnesia Supports
Authors: Ma J. , Chuah G. K. , Jaenicke S. , Gopalakrishnan R. , Tan K. L. 
Published: July 2012

AbstractManganese oxide deposited on alumina and magnesia by different preparation methods has been studied with respect to its surface structure and reactivity. The controlled deposition of mono‐atomic layers is effected by a grafting reaction between surface hydroxyl groups and manganese ethoxide. Repeated grafting allows multiple layers to be built up in a controlled manner. The properties of these catalysts differ from those prepared by conventional wet impregnation techniques and mechanical mixing. Test reactions involving carbon monoxide oxidation and isopropanol decomposition reveal significant differences in the activity and selectivity of the catalysts prepared by grafting, impregnation and mechanical mixing. The manganese oxide grafted to alumina shows bulk‐like properties after a full monolayer is built up. XPS studies show that the grafting reaction leads to continuous films of the manganate covering the entire surface with monolayer thickness, whereas impregnation most likely results in small crystallites which cover only part of the surface. The calcination temperature is an important parameter determining the oxidation state of the manganese and the crystallinity of the oxide. Ammonia and isopropanol temperature‐programmed desorption indicates that the acid/base and redox properties of the catalysts are considerably modified by a strong interaction between the support and the overlayer.