Thermodynamics of the polarizable stockmayer fluid

Pressure and internal energy of a fluid composed of polarizable Stockmayer molecules have been calculated by a molecular dynamics computation as well as by thermodynamic perturbation theory. It is found that the effect of molecular polarizability is underestimated by perturbation theory.     

Density maximum and polarizable models of water

To estimate accurately the density of water over a wide range of temperatures with a density maximum at 4?°C is one of the most stringent tests of molecular models. The shape of the curve influences the ability to describe critical properties and to predict the freezing temperature. While it was demonstrated that with a proper parameter fit nonpolarizable models can approximate this behavior accurately, it is much more difficult to do this for polarizable models. We provide a short overview of ρ-T diagrams for existing models, then we give an explanation of this difficulty. We present a version of the BK model [A. Baranyai and P. T. Kiss, J. Chem. Phys. 133, 144109 (2010); and ibid. 135, 234110 (2011)] which is capable to predict the density of water over a wide range of temperature. The BK model uses the charge-on-spring method with three Gaussian charges. Since the experimental dipole moment and the geometry is fixed, and the quadrupole moment is approximated by a least mean square procedure, parameters of the repulsion and dispersive attraction forces remained as free tools to match experimental properties. Relying on a simplified but plausible justification, the new version of the model uses repulsion and attraction as functions of the induced dipole moment of the molecule. The repulsive force increases, while the attractive force decreases with the size of the molecular dipole moment. At the same time dipole moment dependent dispersion forces are taking part in the polarization of the molecule. This scheme iterates well and, in addition to a reasonable density-temperature function, creates dipole distributions with accurate estimation of the dielectric constant of the liquid.     

Wavelet formulation of the polarizable continuum model

The first implementation of a wavelet discretization of the Integral Equation Formalism (IEF) for the Polarizable Continuum Model (PCM) is presented here. The method is based on the application of a general purpose wavelet solver on the cavity boundary to solve the integral equations of the IEF‐PCM problem. Wavelet methods provide attractive properties for the solution of the electrostatic problem at the cavity boundary: the system matrix is highly sparse and iterative solution schemes can be applied efficiently; the accuracy of the solver can be increased systematically and arbitrarily; for a given system, discretization error accuracy is achieved at a computational expense that scales linearly with the number of unknowns. The scaling of the computational time with the number of atoms N is formally quadratic but a N^1.5 scaling has been observed in practice. The current bottleneck is the evaluation of the potential integrals at the cavity boundary which scales linearly with the system size. To reduce this overhead, interpolation of the potential integrals on the cavity surface has been successfully used. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

A polarizable model for ethylene oxide

A series of interaction models for ethylene oxide are developed for use in molecular simulation of the thermal properties of both the gas and liquid phases. While it is possible to develop nonpolarizable models to accurately generate either the gas or liquid properties separately, it was not possible to do so using a single model for both phases. A polarizable, rigid all-atom model was developed that reproduces the temperature dependence of the second virial coefficient B(T) and the pressure of the liquid at ambient conditions. The model consists of Lennard-Jones and Coulomb interactions between intermolecular atomic sites plus a scalar polarizability located at the midpoint of the line joining the carbon sites. The electrostatic charges and the polarizability are set to match the experimentally determined dipole and quadrupole moments and the molecular polarizability.     

Collision rate constants for polarizable ions

Langevin described a model for the interaction between an ion and a neutral nearly a century ago and since then, many modifications have been introduced to adjust for specific circumstances. This work discusses the induced dipole-induced dipole interaction between an ion and a neutral without a permanent dipole and introduces an anisotropic adjustment. A point polarizable ion model (PPI) and an orientation dependent polarizable ion model (ODPI) are discussed and applied to systems where the ion is highly polarizable and the neutral is only weakly polarizable. Significant deviations from classical Langevin rate constants and significant differences between PPI and ODPI are observed.     

Diffusion of polarizable colloids in electromagnetic fields

Diffusion of polarizable colloids in electromagnetic fields is considered. Extensive and intensive thermodynamic formulations of magnetic and electric polarization energy is used to establish driving forces that generate diffusion of colloids. Effects of concentration, activity coefficient, and interfacial and polarization energies are formulated in terms of corresponding concentration dependent diffusion coefficients. Magnetic and electric diffusion coefficients are defined and it is shown that the former can be made comparable and even larger than ordinary diffusion coefficients of polarizable colloids. Magnetic diffusion coefficients are derived for mono- and polydisperse colloid mixtures relative to the fluid and then are transformed to the dispersion frame of reference. It is shown that ordinary and magnetic diffusion coefficients of polydisperse colloids can be resolved into four and three basic coefficients, respectively.     

Combining a polarizable force-field and a coarse-grained polarizable solvent model. II. Accounting for hydrophobic effects

A revised and improved version of our efficient polarizable force-field/coarse grained solvent combined approach (Masella, Borgis, and Cuniasse, J. Comput. Chem. 2008, 29, 1707) is described. The polarizable pseudo-particle solvent model represents the macroscopic solvent polarization by induced dipoles placed on mobile pseudo-particles. In this study, we propose a new formulation of the energy term handling the nonelectrostatic interactions among the pseudo-particles. This term is now able to reproduce the energetic and structural response of liquid water due to the presence of a hydrophobic spherical cavity. Accordingly, the parameters of the energy term handling the nonpolar solute/solvent interactions have been refined to reproduce the free-solvation energy of small solutes, based on a standard thermodynamic integration scheme. The reliability of this new approach has been checked for the properties of solvated methane and of the solvated methane dimer, as well as by performing 10 × 20 ns molecular dynamics (MD) trajectories for three solvated proteins. A long-time stability of the protein structures along the trajectories is observed. Moreover, our method still provides a measure of the protein solvation thermodynamic at the same accuracy as standard Poisson-Boltzman continuum methods. These results show the relevance of our approach and its applicability to massively coupled MD schemes to accurately and intensively explore solvated macromolecule potential energy surfaces.     

Thermogravimetry. Fast methods for evaluation of kinetic parameters

The paper discusses three fast methods for determination of the reaction order, as follows: the single-point method proposed by Kissinger, Horowitz and Metzger, an original two-point method, and the three-point method suggested by a Gorbachev's paper. These methods cannot elucidate the reaction mechanism, but they can help in the rapid derivation of the apparent kinetic parametersn andE.

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Zusammenfassung Es wurden drei Methoden zur Schnellbestimmung von Reaktionsordnungen ausgewertet: eine Einpunktmethode von Kissinger, Horowitz und Metzger, eine Zweipunktmethode und eine Dreipunktmethode nach einer Anregung von Gorbachev. Mittels dieser Methoden kann man zwar nicht den Reaktionsmechanismus bestimmen, wohl aber eine schnelle Bestimmung der kinetischen Scheinparametern undE durchführen.

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: , , , , . , n .

     

Ab initio based polarizable force field parametrization

Experimental and simulation studies of anion-water systems have pointed out the importance of molecular polarization for many phenomena ranging from hydrogen-bond dynamics to water interfaces structure. The study of such systems at molecular level is usually made with classical molecular dynamics simulations. Structural and dynamical features are deeply influenced by molecular and ionic polarizability, which parametrization in classical force field has been an object of long-standing efforts. Although when classical models are compared to ab initio calculations at condensed phase, it is found that the water dipole moments are underestimated by approximately 30%, while the anion shows an overpolarization at short distances. A model for chloride-water polarizable interaction is parametrized here, making use of Car-Parrinello simulations at condensed phase. The results hint to an innovative approach in polarizable force fields development, based on ab initio simulations, which do not suffer for the mentioned drawbacks. The method is general and can be applied to the modeling of different systems ranging from biomolecular to solid state simulations.     

Amperometry with two polarizable electrodes. XVI

Summary Conditions for the magnesium determination in alkaline solution by EDTA titration with biamperometric indication were examined. The most preferable pH range was found to be above 9.6. A reliable determination can be carried out in the presence of Ca, Ni, Cu and Co up to the ratio of 11. In the case of higher concentrations of Cu and Ni masking with potassium cyanide can be used. In this case, however, a change of the titration curves occurs.A new method for the determination of water-hardness by biamperometric indication is proposed. It is rapid and accurate and meets the requirements of routine and research work (error: ±1%).

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Zusammenfassung Die Bedingungen für die Magnesiumbestimmung in alkalischer Lösung durch Titration mit ÄDTA bei biamperometrischer Endpunktsanzeige wurden untersucht. Der günstigste pH-Bereich liegt über 9,6. Zuverlässige Bestimmungen können in Gegenwart von Ca, Ni, Cu und Co bis zum Verhältnis 11 ausgeführt werden. Höhere Konzentrationen an Cu oder Ni können mit KCN maskiert werden, wobei allerdings eine Umkehrung der Kurven auftritt.Ein Verfahren zur Wasserhärtebestimmung mit biamperometrischer Indikation wird vorgeschlagen, das schnell durchzuführen ist und genaue Werte liefert (Fehler ±1%).

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Part XV: Vorlíek, J., and P. Peták: Microchem. J. 12, 466–471 (1967).     

Surface properties of the polarizable Baranyai-Kiss water model

The water surface properties using the Baranyai-Kiss (BK) model [A. Baranyai and P. T. Kiss, J. Chem. Phys. 133, 144109 (2010)] are studied by molecular dynamics simulation, and compared to popular rigid water potentials, namely to the extended simple point charge (SPC/E) and the transferable interaction potential with 4 points (TIP4P) models. The BK potential is a polarizable model of water with three Gaussian charges. The negative charge is connected to its field-free position by a classical harmonic spring, and mechanical equilibrium is established between this spring force and the force due to the charge distribution of the system. The aim of this study is, on the one hand, to test the surface properties of the new model, and on the other hand, to identify differences between the models listed above. The obtained results reveal that the BK model reproduces very well a number of properties corresponding to liquid-vapor equilibrium, such as the coexisting liquid and vapor densities, saturated vapor pressure or surface tension. Further, this model reproduces excellently the critical point of water even in comparison with a large number of widely used polarizable and nonpolarizable models. The structural properties of the liquid surface of BK water turns out to be very similar to that of the SPC/E model, while the surface of TIP4P water is found to be somewhat less ordered. This finding is related to the fact that the critical temperature of the TIP4P model is lower than that of either SPC/E or BK.     

Fast centroid molecular dynamics: a force-matching approach for the predetermination of the effective centroid forces

A fast centroid molecular dynamics (CMD) methodology is proposed in which the effective centroid forces are predetermined through a force-matching algorithm applied to a standard path integral molecular dynamics simulation. The resulting method greatly reduces the computational cost of generating centroid trajectories, thus extending the applicability of CMD. The method is applied to the study of liquid para-hydrogen at two state points and liquid ortho-deuterium at one state point. The static and dynamical results are compared to those obtained from full adiabatic CMD simulations and found to be in excellent agreement for all three systems; the transport properties are also compared to experiment and found to have a similar level of agreement.     

Phase behavior of polarizable spherocylinders in external fields

Applied electric fields are known to induce significant changes in the properties of systems of polarizable molecules or particles. For rod-shaped molecules, the field-induced behavior can be rather surprising, as in the case of the negative electric birefringence of concentrated solutions of rodlike polyelectrolytes. We have investigated the interplay of shape anisotropy and field-induced anisotropy in molecular dynamics simulations of systems of polarizable soft spherocylinders in an electric field, in the limit of infinitely anisotropic polarizability, taking full account of mutual induction effects. We find a novel crystalline structure (K(2)) in the high-field limit, whose formation is driven by interactions between induced dipoles. For high pressures, the phase diagram exhibits a polar nematic phase between the hexagonal close-packed crystal phase and the K(2) phase. We also compare this system with an analogous system of spherocylinders with permanent electric dipoles and find that qualitatively similar behavior is obtained in the limit of strong coupling of the permanent dipoles to the external field.     

Theoretical evaluation of the efficiency of catalysts with the aid of deforming forces

Conclusions A method for calculating the deforming forces appearing as a result of the action of a catalyst on a reactant has been proposed. It has been shown that the magnitudes and directions of the deforming forces correlate with the acceptor properties of the catalyst and can be used for comparing the effects of different catalysts on a reactant. The greater sensitivity of the forces in comparison to the traditional parameters E _AB ^R has been demonstrated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheksaya, No. 8, pp. 1758–1765, August, 1982.We thank S. G. Gagarin and V. A. Shlyapochnikov for their valuable remarks and advice.     

Simulating polarizable molecular ionic liquids with Drude oscillators

The Drude oscillator model is applied to the molecular ionic liquid 1-ethyl-3-methyl-imidazolium triflate. The range of manageable Drude charges is tested. The strength of the polarizability is systematically varied from 0% to 100%. The influence on the structure, single particle dynamics, and collective dielectric properties is investigated. The generalized dielectric constant can be decomposed into a dielectric permittivity, a dielectric conductivity, and an optical dielectric constant ?(∞). The major part of the static generalized dielectric constant comes from the collective rotation of the ions, i.e., the dielectric permittivity. The translational contribution from the dielectric conductivity is about 58% of the dielectric permittivity. For the evaluation of the optical dielectric contribution, the computational dielectric theory was adapted to the case of heterogeneous polarizabilities. In case of 100% polarizability, it reaches a value of approximately 2.     

Extremely strongly polarizable hydrogen bonds in purine solutions

IR spectra of aqueous purine solutions are plotted for various degrees of protonation. With increasing protonation a continuous absorption occurs. This shows that extremely strongly polarizable NH⁺…N hydrogen bonds are formed between the purine molecules in the range of 0–50% protonation.     

A polarizable ellipsoidal force field for halogen bonds

The anisotropic effects and short‐range quantum effects are essential characters in the formation of halogen bonds. Since there are an array of applications of halogen bonds and much difficulty in modeling them in classical force fields, the current research reports solely the polarizable ellipsoidal force field (PEff) for halogen bonds. The anisotropic charge distribution was represented with the combination of a negative charged sphere and a positively charged ellipsoid. The polarization energy was incorporated by the induced dipole model. The resulting force field is “physically motivated,” which includes separate, explicit terms to account for the electrostatic, repulsion/dispersion, and polarization interaction. Furthermore, it is largely compatible with existing, standard simulation packages. The fitted parameters are transferable and compatible with the general AMBER force field. This PEff model could correctly reproduces the potential energy surface of halogen bonds at MP2 level. Finally, the prediction of the halogen bond properties of human Cathepsin L (hcatL) has been found to be in excellent qualitative agreement with the cocrystal structures. © 2013 Wiley Periodicals, Inc.