Asymmetry of current-voltage characteristics of ion-exchange membranes: Model of charge density of fixed groups linear by membrane thickness

The model of a finely porous membrane with a linear distribution of charges of its fixed groups along its depth is used in the work to explain the phenomenon of asymmetry of current-voltage characteristics of perfluorinated MF-4SK membranes surface–modified by polyaniline or halloysite nanotubes. A new exact analytical method of solution of a system of transport equations is suggested that is based on the assumption of the monotonous behavior of the electric potential inside the membrane and allows finding current- voltage characteristics (CVCs) of the membrane using a system of two implicit algebraic equations. Algebraic equations for limiting currents are obtained for the first time for different orientations of the anisotropic membrane in an electrodyalisis cell and existence of two inflection points in CVCs is explained when its less charge side is oriented towards the anode.     

Studies on the fixed charge density and permselectivity of parchment supported manganese ferrocyanide membrane

Summary Membrane potentials across parchment supported manganese ferrocyanide membrane for various electrolytes over a wide concentration range have been studied. It was found that the membrane potential increases with dilution of the external electrolyte solutions. The fixed charge density of the membrane has been evaluated byAltug andHair's method and comes out to be −0.06 N. The variation of the transport number of the counterions in the membrane phase and perm-selectivity of the membrane with dilution have also been studied.

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Zusammenfassung Es wurden Membranpotentiale von Manganferrocyanid-Membranen auf Pergamenttr?gern in L?sungen verschiedener Elektrolyte über weite Konzentrationsbereiche untersucht. Das Membranpotential nahm mit der zunehmenden Verdünnung zu. Die fixierte Ladungsdichte wurde nachAltug undHair ermittelt und betrug −0.06 N. Die ?nderung der überführungszahl der Gegenionen in die Membranphase und die Permselektivit?t der Membran wurden ebenfalls bestimmt.

     

Chemical bonding in view of electron charge density and kinetic energy density descriptors

Stalke's dilemma, stating that different chemical interpretations are obtained when one and the same density is interpreted either by means of natural bond orbital (NBO) and subsequent natural resonance theory (NRT) application or by the quantum theory of atoms in molecules (QTAIM), is reinvestigated. It is shown that within the framework of QTAIM, the question as to whether for a given molecule two atoms are bonded or not is only meaningful in the context of a well‐defined reference geometry. The localized‐orbital‐locator (LOL) is applied to map out patterns in covalent bonding interaction, and produces results that are consistent for a variety of reference geometries. Furthermore, LOL interpretations are in accord with NBO/NRT, and assist in an interpretation in terms of covalent bonding. © 2008 Wiley Periodicals, Inc.J Comput Chem, 2009.     

The effect of fixed charge modifications on electron capture dissociation

Electron capture dissociation (ECD) studies of two modified amyloid beta peptides (20-29 and 25-35) were performed to investigate the role of H* radicals in the ECD of peptide ions and the free-radical cascade (FRC) mechanism. 2,4,6-Trimethylpyridinium (TMP) was used as the fixed charge tag, which is postulated to both trap the originally formed radical upon electron capture and inhibit the H* generation. It was found that both the number and locations of the fixed charge groups influenced the backbone and side-chain cleavages of these peptides in ECD. In general, the frequency and extent of backbone cleavages decreased and those of side-chain cleavages increased with the addition of fixed charge tags. A singly labeled peptide with the tag group farther away from the protonated site experienced a smaller abundance decrease in backbone cleavage fragments than the one with the tag group closer to the protonated site. Despite the nonprotonated nature of all charge carriers in doubly labeled peptide ions, several c and z* ions were still observed in their ECD spectra. Thus, although H* transfer may be important for the NC(alpha) bond cleavage, there also exist other pathways, which would require a radical migration via H* abstraction through space or via an amide superbase mechanism. Finally, internal fragment ions were observed in the ECD of these linear peptides, indicating that the important role of the FRC in backbone cleavages is not limited to the ECD of cyclic peptides.     

Adsorption characteristics of bottle-brush polymers on silica: effect of side chain and charge density

The adsorption behavior of bottle-brush polymers with different charge/PEO ratio on silica was studied using optical reflectometry and QCM-D. The results obtained under different solution conditions clearly demonstrate the existence of two distinct adsorption mechanisms depending on the ratio of charge/PEO. In the case of low-charge density brush polymers (0-10 mol %), the adsorption occurs predominantly through the PEO side chains. However, the presence of a small amount of charge along the backbone (as low as 2 mol %) increases the adsorption significantly above that of the uncharged bottle-brush polymer in pure water. As the charge density of the brush polymers is increased to 25 mol % or larger the adsorption occurs predominantly through electrostatic interactions. The adsorbed layer structure was studied by measuring the layer dissipation using QCM-D. The adsorbed layer formed by the uncharged brush polymer dissipates only a small amount of energy that indicates that the brush lie along the surface, the scenario in which the maximum number of PEO side chains interact with the surface. The adsorbed layers formed by the low-charge density brush polymers (2-10 mol %) in water are more extended, which results in large energy dissipation, whereas those formed by the high-charge density brush polymers (50-100 mol %) have their backbone relatively flat on the surface and the energy dissipation is again low.     

Constraint-based analysis of a physics-guided kinetic energy density expansion

An approach guided by physical consistency in determining the general forms of D-dimensional kinetic energy density functionals (KEDF) has been demonstrated previously, producing an expansion which contains the majority of the known one-point KEDF forms. It has also been shown that any noninteracting KEDF must necessarily have a homogeneity degree of 2 in coordinate scaling, and that the ratio of the collective KED to electron density must approach the ionization energy as $r\to \infty$. This article demonstrates that the scaling condition is already satisfied in the general expansion despite not being conceived with the scaling as a constraint, and that the second condition places a restriction on the expansion terms of the KED. The discussion is extended as well for some known KEDs for comparison.     

The effect of vibrational motion on the electron charge difference density

The effect of the internal vibrational motion on the electron difference density has been studied for the hydrogen molecular ion H⁺₂. The effect is quite small although there are differences between the static and dynamic densities near the nuclei.     

Electrical potentials of two identical particles with fixed surface charge density in a salt-free medium

The electrical potentials of two identical planar, cylindrical, and spherical particles immersed in a salt-free dispersion are solved analytically by a perturbation approach for the case of constant surface charge density. The system under consideration simulates, for example, micelles, where the ionic species in the liquid phase come mainly from the dissociation of the functional groups on the droplet surface. We show that for planar particles, the present zero-order perturbation solution is exact, and for cylindrical and spherical particles, the first-order perturbation solution provides sufficiently accurate results, with an averaged percentage deviation on the order of 1% under typical conditions. In general, the higher the surface charge density, the higher the valence of counterions, the smaller the separation distance between two particles, and the smaller the curvature of particle surface, the better the performance of the perturbation solution.     

Dielectric properties of ion-exchange beads of sulfonic acid type dispersed in aqueous solutions — effect of fixed charge density

Ion-exchange beads with different densities of fixed charges were prepared by sulfoethylation of dextran gel beads. The relative permittivities and the electrical conductivities of the ion-exchange beads in a sodium form were evaluated by a dielectric technique consisting of the following two procedures proposed in a previous study (2): (a) Dielectric measurements for densely packed sediments of the ion-exchange beads; (b) Analysis of the observed dielectric relaxations by means of a theoretical equation of interfacial polarization for suspensions of spherical particles. The deduced permittivities of the ionexchange beads in equilibrium with water were about 60, which is lower than those of outer aqueous phases. The deduced conductirities were of the order of 1 to 10 mS cm^–1 and were increased reasonably with the fixed charge density. Closer consideration on these deduced values indicates that no specific interaction exists between the counter ions and the fixed charges irrespective of changes in fixed charge density and temperature.     

Controlling hydrogel properties by tuning non-covalent interactions in a charge complementary multicomponent system

Mixing small molecule gelators is a promising route to prepare useful and exciting materials that cannot be accessed from any of the individual components. Here, we describe pH-triggered hydrogelation by mixing of two non-gelling amphiphiles. The intermolecular interactions among the molecules can be tuned either by controlling the degree of ionization of the components or by a preparative pathway, which enables us to control material properties such as gel strength, gel stiffness, thermal stability, and an unusual shrinking/swelling behaviour.

The properties of a charge complementary multicomponent gel can be tuned either by pH change or by varying the preparative pathway.     

Vibrational analysis of a rate-slowing conformational kinetic isotope effect

An enthalpy-entropy approach to analyzing a rate-slowing conformational kinetic isotope effect (CKIE) in a deuterated doubly-bridged biaryl system is described. The computed isotope effect (k_H/k_D?=?1.075, 368?K) agrees well with the measured value (k_H/k_D?=?1.06, 368?K). The rate-slowing (normal isotope effect) nature of the computed CKIE is shown to originate from a vibrational entropy contribution defined by the twenty lowest frequency normal modes in the ground state and transition state structures. This normal entropy contribution is offset by an inverse vibrational enthalpy contribution, which also arises from the twenty lowest frequency normal modes. Zero point vibrational energy contributions are found to be relatively small when all normal modes are considered. Analysis of the H_ZPE, H_vib, and S_vib energy terms arising from the low frequency vibrational modes reveals their signs and magnitudes are determined by larger vibrational energy differences in the labeled and unlabeled ground state structures.     

Cavity effect amplification in the recognition of dicarboxylic acids by initial ditopic H-bond formation followed by kinetic trapping

Di[dihydroxotin(IV)] Tr?ger's base bis-porphyrin 1, a host molecule with two internal and two external guest interaction sites, binds 相似文献   

Transient kinetic studies involving exciplex formation. Methods of analysis

In this paper, rate parameters are extracted from fluorescene decay data in the framework of Feedback-Type Kinetics (k₈ < k₄). The exciplex dissociation (k₄) and fluorescene decay rate constants (k₈) have been obtained for the five following systems: For this task we have added to the straight line method of Ware which is valid in certain cases only at high perturber concentrations, a new but similar Taylor series method applicable at low concentrations. Thereafter, a safer and more general method of kinetic analysis based on a quasi-linear χ² minimization procedure has been developed and applies to data in any range of concentration.     

The charge density distribution in a model compound of the catalytic triad in serine proteases

Combined low temperature (28(1) K) X-ray and neutron diffraction measurements were carried out on the co-crystallised complex of betaine, imidazole, and picric acid (1). The experimental charge density was determined and compared with ab initio theoretical calculations at the B3LYP/6-311G(d,p) level of theory. The complex serves as a model for the active site in, for example, the serine protease class of enzymes, the so-called catalytic triad. The crystal contains three short strong N-H...O hydrogen bonds (HBs) with dN...O < 2.7 A. The three HBs have energies above 13 kcalmol(-1), although the hydrogen atoms are firmly localized in the "nitrogen wells". This suggests that low-barrier hydrogen bonding in catalytic enzyme reactions may be a sufficient, but not a necessary, condition for obtaining transition-state stabilization. Structural analysis (e.g., covalent N-H bond lengthening) indicates that the hydrogen bond between H3A and 08 of imidazole and betaine respectively (HB2) is slightly stronger than the bond between H1A and O1A of imidazole and picric acid (HB1), although HB1 is shorter than HB2: (dN...O(HB1)= 2.614(1) A, dN...O(HB2) = 2.684(1) A, dH...O(HB1) = 1.630(1) A, dH...O(HB2)= 1.635(1) A, dN-H(HB1) = 1.046(1) A, dN-H(HB2) = 1.057(1) A). Furthermore, the charge density analysis reveals that HB2 has a larger covalent character than HB1, with considerable polarization of the density towards the acceptor atom. The Gatti and Bader source function (S) is introduced to the analysis of strong HBs. The source function is found to be a sensitive measure for the nature of a hydrogen bond, and comparison with low-barrier and single-well hydrogen bonding systems (e.g., benzoylacetone and nitromalonamide) shows that the low-barrier hydrogen bond (LBHB) state is characterized by an enormously increased hydrogen atom source contribution to the bond critical point in the HB. In this context, HB2 can be characterized as intermediate between localized HBs and delocalized LBHBs.     

Determination of the charge distribution of methane by a method of density constraints

The molecular charge distribution of methane is expressed in terms of an orthonormal set of molecular orbitals which are determined solely by imposing a set of constraints on the derived one-electron charge density, the constraints being that the charge density reproduce the experimental expectation values of a set of one-electron operators. The molecular orbitals are expanded in terms of an SCF set of atomic orbitals on carbon and a single 1s STO on each hydrogen. The derived charge distribution is found to be equal to the SCF Hartree-Fock distribution in its prediction of one-electron expectation values. The energy, as determined by the associated wave function, is –40.156 a.u. This energy value is comparable to that obtained in a SCF LCAO MO calculation with a similar basis set and is 0.048 a.u. above the best calculated value of the Hartree-Fock limit.

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Zusammenfassung Die molekulare Ladungsverteilung des Methans wird mit Hilfe eines orthogonalen Satzes von Molekülorbitalen ausgedrückt, die lediglich durch eine Reihe von Nebenbedingungen an die Einelektronen-Ladungsdichte bestimmt sind. Die Nebenbedingungen bestehen darin, da\ die Ladungsdichte die experimentellen Erwartungswerte eines Satzes von Einelektronen-Operatoren reproduzieren soll. Die Molekülorbitale werden nach einem Satz von atomaren SCF-Orbitalen am Kohlenstoff sowie einem einzigen 1s STO an jedem Wasserstoffatom entwickelt. Man findet, da\ die erhaltene Ladungsverteilung der SCF-Hartree-Fock-Verteilung bezüglich der Bestimmung von Einelektronen-Erwartungswerten gleichkommt. Die Energie, die aus der zugehörigen Wellenfunktion bestimmt wird, ist –40,156 a.u. Dieser Energiewert ist mit demjenigen, der in SCF LCAO MO-Berechnungen mit einem Ähnlichen Basissatz bestimmt wird, vergleichbar und liegt 0,048 a.u. über dem besten berechneten Wert der Hartree-Fock-Grenze.

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Résumé La distribution de charge moléculaire du méthane est exprimée au moyen d'un ensemble orthonormé d'orbitales moléculaires déterminées uniquement à l'aide d'une série de contraintes sur les densités de charge obtenues, à savoir que ces densités reproduisent les valeurs expérimentales des valeurs moyennes de différents opérateurs monoélectroniques. Les orbitales moléculaires sont développées en orbitales atomiques SCF sur le carbone et en orbitale de Slater 1s sur chaque hydrogène. La distribution de charge obtenue est trouvée égale à la distribution SCF Hartree-Fock dans les prévisions des valeurs moyennes d'opérateurs monoélectroniques. L'énergie, déterminée à l'aide de la fonction d'onde associée, est –40.156 u.a. Cette valeur de l'énergie est comparable à celle obtenue dans un calcul SCF LCAO MO avec une base similaire et est située à 0,048 u.a. au dessus de la meilleure évaluation de la limite Hartree-Fock.

     

Calculation procedure for the kinetic analysis of a reaction with a thermal effect

A calculation procedure for the numerical analysis of a non-isothermal reaction taking place at constant pressure is described. The procedure is used to calculate the temperature dependence of the ignition delay of H₂ oxidation using a 44-reaction mechanism.

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Описывается алгоритм численного решения прямой кинетической задачи с учетом теплового эффекто реакции, протекающей при постоянном давлении. Применение метода иллюстрируется расчетом температурной зависимости задержки воспламенения водорода для кинетической схемы, включающей 44 элементарные стадии.

     

Topology of charge density of flucytosine and related molecules and characteristics of their bond charge distributions

The molecular charge distribution of flucytosine (4-amino-5-fluoro-2-pyrimidone), uracil, 5-fluorouracil, and thymine was studied by means of density functional theory calculations (DFT). The resulting distributions were analyzed by means of the atoms in molecules (AIM) theory. Bonds were characterized through vectors formed with the charge density value, its Laplacian, and the bond ellipticity calculated at the bond critical point (BCP). Within each set of C=O, C-H, and N-H bonds, these vectors showed little dispersion. C-C bonds formed three different subsets, one with a significant degree of double bonding, a second corresponding to single bonds with a finite ellipticity produced by hyperconjugation, and a third one formed by a pure single bond. In N-C bonds, a decrease in bond length (an increase in double bond character) was not reflected as an increase in their ellipticity, as in all C-C bonds studied. It was also found that substitution influenced the N-C, C-O, and C-C bond ellipticity much more than density and its Laplacian at the BCP. The Laplacian of charge density pointed to the existence of both bonding and nonbonding maxima in the valence shell charge concentration of N, O, and F, while only bonding ones were found for the C atoms. The nonbonding maxima related to the sites for electrophilic attack and H bonding in O and N, while sites of nucleophilic attack were suggested by the holes in the valence shell of the C atoms of the carbonyl groups.     

Studies with model membranes. X. Evaluation of the thermodynamically effective fixed charge density and permselectivity of mercuric and cupric iodide parchment-supported membranes

Thermodynamically effective fixed charge densities of mercuric and cupric iodide parchment supported membranes were estimated by methods of Teorell, Meyer, and Sievers; Altug and Hair; and the most recent one of Kobatake and co-workers based on the thermodynamics of irreversible processes. The two limiting forms of Kobatake's equation for dilute and concentrated ranges gave identical values of charge densities. It is interesting to note that these two values of limiting cases are closer to the Teorell-Meyer-Sievers and Altug-Hair values. The theoretical predictions for membrane potential by the Kobatake equation were borne out quite satisfactorily by experimental results obtained with both the membranes.     

Topological analysis of the electronic charge density in the ethene protonation reaction catalyzed by acidic zeolite

In the present work, the distribution of the electronic charge density in the ethene protonation reaction by a zeolite acid site is studied within the framework of the density functional theory and the atoms in molecules (AIM) theory. The key electronic effects such as topological distribution of the charge density involved in the reaction are presented and discussed. The results are obtained at B3LYP/6-31G(**) level theory. Attention is focused on topological parameters such as electron density, its Laplacian, kinetic energy density, potential energy density, and electronic energy density at the bond critical points (BCP) in all bonds involved in the interaction zone, in the reactants, pi-complex, transition state, and alkoxy product. In addition, the topological atomic properties are determined on the selected atoms in the course of the reaction (average electron population, N(Omega), atomic net charge, q(Omega), atomic energy, E(Omega), atomic volume, v(Omega), and first moment of the atomic charge distribution, M(Omega)) and their changes are analyzed exhaustively. The topological study clearly shows that the ethene interaction with the acid site of the zeolite cluster, T5-OH, in the ethene adsorbed, is dominated by a strong O-H...pi interaction with some degree of covalence. AIM analysis based on DFT calculation for the transition state (TS) shows that the hydrogen atom from the acid site in the zeolitic fragment is connected to the carbon atom by a covalent bond with some contribution of electrostatic interaction and to the oxygen atom by closed shell interaction with some contribution of covalent character. The C-O bond formed in the alkoxy product can be defined as a weaker shared interaction. Our results show that in the transition state, the dominant interactions are partially electrostatic and partially covalent in nature, in which the covalent contribution increases as the concentration and accumulation of the charge density along the bond path between the nuclei linked increases.