A Constant Potential Molecular Dynamics Simulation Study of the Atomic‐Scale Structure of Water Surfaces Near Electrodes

Novel and technologically important processes and phenomena arise at water surfaces in the presence of electric fields. However, experimental measurements on water surfaces are challenging, and the results are scarce and inconclusive. In this work, the constant potential molecular dynamics method, in which the electrode charges are allowed to fluctuate to keep the electric potential fixed, was implemented in the study of a near‐electrode water surface systems. This simulation system was set up with a vapor/liquid‐water/vapor slab and two electrodes under different sets of applied electrostatic potential, yielding i) a detailed characterization of the external E‐field dependent electrostatic potential/density/dipole moment density profiles, and ii) the relationship between the water surface width and the applied electrode voltage differences which has been rarely reported. The adjustments in the number density profiles in the vicinity of water surfaces due to external E‐fields were observed, while the capillary interfacial widths for the surfaces near both cathode and anode were found with different increment rates under increasing E‐fields. By examining dipole density profiles across the water surfaces, we found that external E‐field induced polarization occurs in both bulk and surface regimes, yet the surface polarization densities vary asymmetrically with respect to the increasing E‐fields. Detailed discussions were carried out to explain the correlation between water surface tension and surface widths, as well as the interplay between the surface polarization densities and the hydrogen bond network structure. We conclude that the mechanical and structural properties of the water surfaces could be tuned by both magnitude and direction of the strong external E‐fields. We also recognize that more surface properties with application value, such as dielectric permittivity tensor or surface potential, could also be regulated by the external E‐fields.     

Polarizability and complex conductivity of dilute suspensions of spherical colloidal particles with charged (polyelectrolyte) coatings

The dielectric relaxation of polyelectrolyte-coated colloidal particles is examined via "exact" numerical solutions of the governing electrokinetic equations. The charged polymer coatings are characterized by a nominal charge density, thickness, and permeability. Brush-like segment density profiles are considered here, but more sophisticated segment and charge density profiles are accommodated by the model. The role of added counterions and nonspecific adsorption is considered briefly before examining how the experimentally measured conductivity and dielectric constant increments reflect the frequency of the applied electric field, the strength of the electrolyte, and characteristics of the polymer coatings, namely the charge, charge density, and permeability. Finally, a strategy is suggested by which dielectric spectroscopy and electrophoresis can be used to characterize polymer-coated particles. This approach complements experiments where electroviscous effects such as dynamic light scattering and sedimentation are weak.     

Theoretical third-order hyperpolarizability of paratellurite from the finite field perturbation method

Density functional theory was used to estimate the third-order hypersusceptibility chi (3) of the alpha-TeO2 paratellurite (as a model structure for TeO2 glass) and the same value for alpha-SiO2 cristobalite (as a model structure for glassy silica). The attempt was made to gain a physical insight into the nature of the extraordinarily high hypersusceptibility of TeO2 glass. A finite field perturbation method implemented in the CRYSTAL code with the "sawtooth" approach was employed. The chi (3) values calculated for alpha-TeO2 were found to be of the same order as that measured for TeO2 glass and much higher than the values computed for alpha-SiO2 which, in turn, were close to that of glassy silica.     

The calculation of static polarizabilities of 1-3D periodic compounds. the implementation in the crystal code

The Coupled Perturbed Hartree-Fock (CPHF) scheme has been implemented in the CRYSTAL06 program, that uses a gaussian type basis set, for systems periodic in 1D (polymers), 2D (slabs), 3D (crystals) and, as a limiting case, 0D (molecules), which enables comparison with molecular codes. CPHF is applied to the calculation of the polarizability alpha of LiF in different aggregation states: finite and infinite chains, slabs, and cubic crystal. Correctness of the computational scheme for the various dimensionalities and its numerical efficiency are confirmed by the correct trend of alpha: alpha for a finite linear chain containing N LiF units with large N tends to the value for the infinite chain, N parallel chains give the slab value when N is sufficiently large, and N superimposed slabs tend to the bulk value. CPHF results compare well with those obtained with a saw-tooth potential approach, previously implemented in CRYSTAL. High numerical accuracy can easily be achieved at relatively low cost, with the same kind of dependence on the computational parameters as for the SCF cycle. Overall, the cost of one component of the dielectric tensor is roughly the same as for the SCF cycle, and it is dominated by the calculation of two-electron four-center integrals.     

A Self-consistent Reaction Field Model of Solvation Using Distributed Multipoles. II: Second Energy Derivatives and Application to Vibrational Spectra

The use of distributed multipoles in the formalism of the reaction field factors allowed us to develop a computational scheme adapted to quantum chemical computations on a molecule solvated by a dielectric continuum. The algorithm used to compute the second energy derivatives is developed and, after its implementation in a quantum chemical computational code, permits the computation of the vibrational frequencies of the solute. This approach has been tested successfully on two test molecules and applied to the study of the solvent effect on the structure and the vibrational spectrum of a series of parasubstituted benzonitriles.     

Density functional studies of hydrogen-bonded systems: II. Solvation of the H2O–CO complex by a nonpolar solvent

A dielectric continuum approach (SCIPCM) in the framework of density functional theory has been applied to study the structures, energetics and vibrational spectra of hydrogen-bonded H₂O–CO and H₂O–OC complexes in a non-polar solvent. The dielectric constants for Ar (1.63), Kr (1.83) and Xe (2.19) were used in order to mimic the low-temperature matrix isolation experiments. We have found that calculations which include a dielectric reaction field around the complexes are able to reproduce the experimentally observed spectral changes. The correction of the calculated interaction energy for the basis set superposition error is discussed in the framework of the self-consistent reaction field approach.     

Electric fields drive bond homolysis

Electric fields have been used to control and direct chemical reactions in biochemistry and enzymatic catalysis, yet directly applying external electric fields to activate reactions in bulk solution and to characterize them ex situ remains a challenge. Here we utilize the scanning tunneling microscope-based break-junction technique to investigate the electric field driven homolytic cleavage of the radical initiator 4-(methylthio)benzoic peroxyanhydride at ambient temperatures in bulk solution, without the use of co-initiators or photochemical activators. Through time-dependent ex situ quantification by high performance liquid chromatography using a UV-vis detector, we find that the electric field catalyzes the reaction. Importantly, we demonstrate that the reaction rate in a field increases linearly with the solvent dielectric constant. Using density functional theory calculations, we show that the applied electric field decreases the dissociation energy of the O–O bond and stabilizes the product relative to the reactant due to their different dipole moments.

We demonstrate that electric fields can homolytically cleave a peroxide bond in different solvents with a rate that is proportional to the solvent dielectric constant.     

A combined molecular dynamics+quantum mechanics method for investigation of dynamic effects on local surface structures

A combined molecular dynamics (MD)+quantum mechanics (QM) method for studying processes on ionic surfaces is presented. Through the combination of classical MD and ab initio embedded-cluster calculations, this method allows the modeling of surface processes involving both the structural and dynamic features of the substrate, even for large-scale systems. The embedding approach used to link the information from the MD simulation to the cluster calculation is presented, and rigorous tests have been carried out to ensure the feasibility of the method. The electrostatic potential and electron density resulting from our embedded-cluster model have been compared with periodic slab results, and confirm the satisfying quality of our embedding scheme as well as the importance of applying embedding in our combined MD+QM approach. We show that a highly accurate representation of the Madelung potential becomes a prerequisite when the embedded-cluster approach is applied to temperature-distorted surface snapshots from the MD simulation.     

Effect of Incorporating Aromatic and Chiral Groups on the Dielectric Properties of Poly(dimethyltin esters)

High‐dielectric constant materials are critical for numerous applications such as photovoltaics, photonics, transistors, and capacitors. There are numerous polymers used as dielectric layers in these applications but can suffer from having a low dielectric constant, small band gap, or ferroelectricity. Here, the structure–property relationship of various poly(dimethyltin esters) is described that look to enhance the dipolar and atomic polarization component of the dielectric constant. These polymers are also modeled using first principles calculations based on density functional theory (DFT) to predict such values as the total, electronic, and ionic dielectric constant as well as structure. A strong correlation is achieved between the theoretical and experimental values with the polymers exhibiting dielectric constants >4.5 with dissipation on the order of 10⁻³–10⁻².

     

Self-consistent reaction field calculations on the proton transfer in ammonia-formic acid systems as a model for hydrogen bonding in amino acids in solution

The reaction field (RF ) model of solvent effects, implemented within the SCF -CNDO /2 scheme of calculation, has been applied to analyze the proton transfer in the NH₃…HCOOH system in the presence of several polarizable media. The aim of such a study was to characterize the tatutomeric equilibrium between the neutral and zwiterionic forms of H-bonded amino acids in aprotic solvents. Qualitative results concerning the energetics of this equilbrium show the stabilization of two different H-bonded complexes, corresponding to two separate minima in the free energy surface. These well known double minima potentials are found to be dependent on both the intermolecular N? O distance and the strength of the reaction field. The behavior of this model is qualitatively consistent with experimental observations of nitrogen-substituted amino acids in solution: both show, for low values of the dielectric constant, tautomeric equilibria where the H-bonded complexes appear to be more stable than the corresponding monomeric forms. The charge transfer process associated with the proton migration along the H-bond is also discussed. It is found that the amount of charge transferred increases with the N? O distance and with the RF strength, In order to test the general approach and compare it with previous work, calculations on the real monomeric systems glycine, β-alanine, and γ-amino butyric acid was also performed.     

Parallelization of the integral equation formulation of the polarizable continuum model for higher-order response functions

We present a parallel implementation of the integral equation formalism of the polarizable continuum model for Hartree-Fock and density functional theory calculations of energies and linear, quadratic, and cubic response functions. The contributions to the free energy of the solute due to the polarizable continuum have been implemented using a master-slave approach with load balancing to ensure good scalability also on parallel machines with a slow interconnect. We demonstrate the good scaling behavior of the code through calculations of Hartree-Fock energies and linear, quadratic, and cubic response function for a modest-sized sample molecule. We also explore the behavior of the parallelization of the integral equation formulation of the polarizable continuum model code when used in conjunction with a recent scheme for the storage of two-electron integrals in the memory of the different slaves in order to achieve superlinear scaling in the parallel calculations.     

Preparation and Characterization of a Sol-Gel Prepared Ferroelectric Sandwich Structure

A novel silicon-based PbTiO₃/Pb(Zr,Ti)O₃/PbTiO₃ (PT/PZT/PT) sandwich structure has been prepared using a sol-gel method. The annealing temperature is greatly reduced compared with those structures without PT layers. Capacitance-voltage (C-V), leakage current-voltage (I-V), polarization-field (P-E), dielectric-frequency response and polarization fatigue of the sandwich structure are examined. The relative dielectric constant, the coercive field and the remanent polarization of the PZT films are measured to be about 900, 18 kV/cm and 16 C/cm² respectively. The current density is less than 5 × 10^–9 A/cm² below 200 kV/cm. The dielectric constant of the structure remains constant at low frequency, and decreases to some degree at high frequency. The retained polarization does not change significantly after 8 × 10⁹ read/write cycles. The PZT films are proved to have very good dielectric and ferroelectric properties. The new PT/PZT/PT sandwich structure can be valuable for memory devices and other applications.     

Thermodynamical study of the thermoelectric effect for magnesium silicide

The thermoelectric effect of magnesium silicide is studied by using a thermodynamical method in the presence of an electric field. The thermoelectric potential is evaluated from the partial derivative of free energy with respect to charge in which the free energy is calculated at the B3LYP/6-31G(d,p) level of density functional theory. This free energy is also utilized to determine the average dipole moment from which the polarizability, alpha; molar polarization, Psi; and dielectric constant can be computed. The present calculation for the dielectric constant (approximately 24-20) is in very good agreement with the experimental value (20). This accurate dielectric constant can be used to derive the relation of the thermoelectric potential with respect to temperature, from which the thermoelectric power or the Seebeck coefficients are calculated. The present result shows good agreement with experiment measurement for the Seebeck coefficients. In comparison, that calculation from the energy band structure theory is far off from the experimental values.     

Force Field Benchmark of Organic Liquids: Density, Enthalpy of Vaporization, Heat Capacities, Surface Tension, Isothermal Compressibility, Volumetric Expansion Coefficient, and Dielectric Constant

The chemical composition of small organic molecules is often very similar to amino acid side chains or the bases in nucleic acids, and hence there is no a priori reason why a molecular mechanics force field could not describe both organic liquids and biomolecules with a single parameter set. Here, we devise a benchmark for force fields in order to test the ability of existing force fields to reproduce some key properties of organic liquids, namely, the density, enthalpy of vaporization, the surface tension, the heat capacity at constant volume and pressure, the isothermal compressibility, the volumetric expansion coefficient, and the static dielectric constant. Well over 1200 experimental measurements were used for comparison to the simulations of 146 organic liquids. Novel polynomial interpolations of the dielectric constant (32 molecules), heat capacity at constant pressure (three molecules), and the isothermal compressibility (53 molecules) as a function of the temperature have been made, based on experimental data, in order to be able to compare simulation results to them. To compute the heat capacities, we applied the two phase thermodynamics method (Lin et al. J. Chem. Phys.2003, 119, 11792), which allows one to compute thermodynamic properties on the basis of the density of states as derived from the velocity autocorrelation function. The method is implemented in a new utility within the GROMACS molecular simulation package, named g_dos, and a detailed exposé of the underlying equations is presented. The purpose of this work is to establish the state of the art of two popular force fields, OPLS/AA (all-atom optimized potential for liquid simulation) and GAFF (generalized Amber force field), to find common bottlenecks, i.e., particularly difficult molecules, and to serve as a reference point for future force field development. To make for a fair playing field, all molecules were evaluated with the same parameter settings, such as thermostats and barostats, treatment of electrostatic interactions, and system size (1000 molecules). The densities and enthalpy of vaporization from an independent data set based on simulations using the CHARMM General Force Field (CGenFF) presented by Vanommeslaeghe et al. (J. Comput. Chem.2010, 31, 671) are included for comparison. We find that, overall, the OPLS/AA force field performs somewhat better than GAFF, but there are significant issues with reproduction of the surface tension and dielectric constants for both force fields.     

Reduction of the contribution of electrode polarization effects in the radiowave dielectric measurements of highly conductive biological cell suspensions

Electrode polarization effects in dielectric spectra of highly conductive biological cell suspensions cause a severe difficulty in the estimation of dielectric parameters of cells under physiological conditions. This problem becomes particularly serious with the increase of the electrical conductivity of the sample, preventing the use of low frequencies in the characterization of biological systems, especially aqueous biological systems.Although a variety of methods to correct the electrode polarization have been proposed in the past, no simple technique for its correction has been available so far. Since the magnitude of the polarization effect can be time-dependent owing to changes in the conductance of the suspending medium or to possible alteration in the electrode surface structure, it is clear that correction procedure should be based on a kind of "self-correction" method, avoiding the so-called "comparison methods" which, on the contrary, require time-independent effects.This note is aimed to address this problem considering an electrode polarization modelled by a constant phase angle (CPA) element in series with the sample admittance. A scaling-law frequency dependence has found to describe the a.c. response of the interface between the electrode and the bulk electrolyte solution. Although this approach has been extensively proposed in the past in the analysis of dielectric spectra of biological suspensions, we have somewhat modified the way it has been previously applied and have re-examined in detail its effectiveness in typical systems of biological interest. The results give support to the proposed analysis, allowing the complete low-frequency dielectric spectra characterization at frequencies of the order of 1 kHz for samples with a bulk ionic conductivity as large as that of the order of 1 mho/m. Typical examples with different dielectric behaviours are extensively discussed in order to show the applicability of the proposed method to biological samples.     

Some calculations of the solvent dependences of phosphorus nuclear shielding

Some CNDO/S parameterized calculations of phosphorus shielding, as a function of the dielectric constant of the medium, are presented. Pople's SOS perturbation approach together with the solvaton model are employed. For trivalent phosphorus compounds a shielding decrease by up to 15 ppm is predicted as ε increases from 1 to 80. For pentavalent phosphorus a much smaller shielding variation, in the opposite direction, is predicted. In general, a consideration of changes in phosphorus charge density is insufficient to account for the variation in nuclear shielding.     

MOLCAS 7: The Next Generation

Some of the new unique features of the MOLCAS quantum chemistry package version 7 are presented in this report. In particular, the Cholesky decomposition method applied to some quantum chemical methods is described. This approach is used both in the context of a straight forward approximation of the two‐electron integrals and in the generation of so‐called auxiliary basis sets. The article describes how the method is implemented for most known wave functions models: self‐consistent field, density functional theory, 2nd order perturbation theory, complete‐active space self‐consistent field multiconfigurational reference 2nd order perturbation theory, and coupled‐cluster methods. The report further elaborates on the implementation of a restricted‐active space self‐consistent field reference function in conjunction with 2nd order perturbation theory. The average atomic natural orbital basis for relativistic calculations, covering the whole periodic table, are described and associated unique properties are demonstrated. Furthermore, the use of the arbitrary order Douglas‐Kroll‐Hess transformation for one‐component relativistic calculations and its implementation are discussed. This section especially focuses on the implementation of the so‐called picture‐change‐free atomic orbital property integrals. Moreover, the ElectroStatic Potential Fitted scheme, a version of a quantum mechanics/molecular mechanics hybrid method implemented in MOLCAS, is described and discussed. Finally, the report discusses the use of the MOLCAS package for advanced studies of photo chemical phenomena and the usefulness of the algorithms for constrained geometry optimization in MOLCAS in association with such studies. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Moments dipolaires et conformations de l'hexaméthyl-phosphotriamide (HMPT) et des dérivés aziridinylés correspondants

Some physical properties (density, refractive index, dielectric constant, dipole moment) have been determined for the phosphoramides

Variations of dielectric constant and dipole moment as a function of the structure show a minimum for n = 2. In order to explain this minimum, the dipole moments are evaluated for various conformations, using both a rigid dipoles summation and quantum calculations (CNDO/2). The values adopted for the bond angles and for the interatomic distances are discussed. The conformation in which the three nitrogen lone pairs are essentially parallel to the PO axis (one in the same direction, two in the opposite direction) seems to be applicable to all the phosphoramides studied. In the case of HMPT (n = 0), however, the arrangement in which the nitrogen lone pair is parallel to the PO axis in the opposite direction, and the two others in a plane perpendicular to PO, seems to be more reasonable. These structures are compared with those for some related compounds. Relationships between the basicity (deduced from proton NMR chemical shifts of hcci₃) and the orientation of dipole moment are discussed.     

Molecular theory on dielectric constant at interfaces: a molecular dynamics study of the water/vapor interface

Though the local dielectric constant at interfaces is an important phenomenological parameter in the analysis of surface spectroscopy, its microscopic definition has been uncertain. Here, we present a full molecular theory on the local field at interfaces with the help of molecular dynamics simulation, and thereby provide microscopic basis for the local dielectric constant so as to be consistent to the phenomenological three-layer model of interface systems. To demonstrate its performance, we applied the theory to the water/vapor interface, and obtained the local field properties near the interface where the simple dielectric model breaks down. Some computational issues pertinent to Ewald calculations of the dielectric properties are also discussed.