Shape‐similarity relations based on topological resolution

Shape‐similarities of electron density clouds of molecules provide important clues concerning chemical and physical properties, including information about their reactivities in biochemical systems. The concept of topological resolution is used for quantifying molecular similarities: within a hierarchy of finer and cruder topologies, the crudest topology that already provides discrimination between two objects (such as two fuzzy electron density clouds) is used to define a measure of their similarities. The finer this topology, the more similar the two objects. This approach, the method of topological resolution‐based similarity measures (TRBSM), can be combined with a geometrically motivated resolution‐based similarity measure (RBSM) within a metric space. Some of the relations between these two approaches are discussed in this contribution, with special emphasis on applications to electron densities. This revised version was published online in July 2006 with corrections to the Cover Date.     

Optimized Atomic Partial Charges and Radii Defined by Radical Voronoi Tessellation of Bulk Phase Simulations

We present a novel method for the computation of well-defined optimized atomic partial charges and radii from the total electron density. Our method is based on a two-step radical Voronoi tessellation of the (possibly periodic) system and subsequent integration of the total electron density within each Voronoi cell. First, the total electron density is partitioned into the contributions of each molecule, and subsequently the electron density within each molecule is assigned to the individual atoms using a second set of atomic radii for the radical Voronoi tessellation. The radii are optimized on-the-fly to minimize the fluctuation (variance) of molecular and atomic charges. Therefore, our method is completely free of empirical parameters. As a by-product, two sets of optimized atomic radii are produced in each run, which take into account many specific properties of the system investigated. The application of an on-the-fly interpolation scheme reduces discretization noise in the Voronoi integration. The approach is particularly well suited for the calculation of partial charges in periodic bulk phase systems. We apply the method to five exemplary liquid phase simulations and show how the optimized charges can help to understand the interactions in the systems. Well-known effects such as reduced ion charges below unity in ionic liquid systems are correctly predicted without any tuning, empiricism, or rescaling. We show that the basis set dependence of our method is very small. Only the total electron density is evaluated, and thus, the approach can be combined with any electronic structure method that provides volumetric total electron densities—it is not limited to Hartree–Fock or density functional theory (DFT). We have implemented the method into our open-source software tool TRAVIS.     

Evaluation of integrals over STOs on different centers and the complementary convergence characteristics of ellipsoidal‐coordinate and zeta‐function expansions

One‐electron integrals over three centers and two‐electron integrals over two centers, involving Slater‐type orbitals (STOs), can be evaluated using either an infinite expansion for 1/r₁₂ within an ellipsoidal‐coordinate system or by employing a one‐center expansion in spherical‐harmonic and zeta‐function products. It is shown that the convergence characteristics of both methods are complimentary and that they must both be used if STOs are to be used as basis functions in ab initio calculations. To date, reports dealing with STO integration strategies have dealt exclusively with one method or the other. While the ellipsoidal method is faster, it does not always converge to a satisfactory degree of precision. The zeta‐function method, however, offers reliability at the expense of speed. Both procedures are described and the results of some sample calculation presented. Possible applications for the procedures are also discussed. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 1–13, 1999     

Localized electron pair theory for the calculation of ground state energies of large molecules

The quantum mechanical energy is examined in which groups of one, two, three, and four localized electron pairs found within a molecule are separately computed. From these results, the interaction energies of the electron pairs taken one, two, three, and four at a time form the terms of a convergent molecular mechanics like expansion of the molecular ground state energy. This procedure can be used with any size consistent quantum mechanical method. The computational time for large molecules depends chiefly upon the order needed in the energy expansion to obtain sufficient convergence and not on the particular quantum mechanical method used. Preliminary results within the framework of a semiempirical CNDO/2 model Hamiltonian show at the Hartree–Fock and Møller–Plesset perturbation levels that relative energies converge to within a few tenths of a kcal/mol of the exact values at the four body level for molecules that have little delocalization. In strained ring and aromatic systems, convergence is however not nearly as rapid. Results can be improved somewhat by using larger interacting fragments containing two or more electron pairs over three or more atomic centers. © 1992 by John Wiley & Sons, Inc.     

One and two body densities for excited states of the helium confined atom

A study of the first excited states of the helium atom confined under impenetrable spherical walls is carried out. Both single particle and two body, intracule and extracule, densities are constructed. Crossing levels and Hund's rule are analyzed in terms of the contribution to the total energy from kinetic, electron–nucleus, and electron–electron energies. A study about the behavior of the single particle and two body densities is carried out. The Multiconfiguration Parameterized Optimized Effective Potential method is employed with a cut-off factor to account for Dirichlet boundary conditions. Single particle density is analytically constructed whereas the Monte Carlo algorithm is used to calculate two body densities.     

Electron correlation via frozen Gaussian dynamics

We investigate the accuracy and efficiency of the semiclassical frozen Gaussian method in describing electron dynamics in real time. Model systems of two soft-Coulomb-interacting electrons are used to study correlated dynamics under non-perturbative electric fields, as well as the excitation spectrum. The results show that a recently proposed method that combines exact-exchange with semiclassical correlation to propagate the one-body density-matrix holds promise for electron dynamics in many situations that either wavefunction or density-functional methods have difficulty describing. The results also however point out challenges in such a method that need to be addressed before it can become widely applicable.     

Simple voltammetric approach for characterization of two-step surface electrode mechanism in protein-film voltammetry

Many enzymes embedding multivalent metal ions or quinone moieties as redox-active centres undergo electrochemical transformation via two successive electron transfer steps. If electrochemical features of such redox enzymes are analyzed with “protein-film voltammetry”, one frequently meets a challenging reaction scenario where the two electron transfers take place at the same formal potential. Under such conditions, one observes voltammogram with a single oxidation-reduction pattern hiding voltammetric features of both redox reactions. By exploring some aspects of the two-step surface EECrev mechanism one can develop simple methodology under conditions of square-wave voltammetry to enable recognizing and characterizing each electron transfer step. The method relies on the voltammetric features of the second electron transfer, which is coupled to a follow-up chemical reaction. The response of the second electron transfer step shifts to more positive potentials by increasing the rate of the chemical reaction. The proposed methodology can be experimentally applied by modifying the concentration of an electrochemically inactive substrate, which affects the rate of the follow-up chemical reaction. The final voltammetric output is represented by two well-separated square-wave voltammetric peaks that can be further exploited for complete thermodynamic and kinetic analysis of the EECrev mechanism.

     

关于嵌段聚醚氨酯形态结构的研究

本文主要研究了在溶液聚合中,用两种加料方式对嵌段聚醚氨酯(SPEU)本体形态结构的影响。由透射电镜观察的结果发现,在预聚反应过程中采用两种加料方式,其最终聚合产物(SPEU)都具有明显的两相结构;加料方式不同主要影响SPEU中分散相的形态和其内的聚集态结构:(1)采用将端羟基的聚四次甲基醚(PTMEG)溶液滴加到4,4′-二苯基甲烷二异氰酸酯(MDI)的溶液中,俟反应完成后再滴加入扩链剂(Extender)的加料方式所制得的SPEU其分散相在形态上呈现为小棒条状,这种分散微区的宽度一般在10—60nm范围内,从选区电子衍射结果证明这种微区的结构是结晶的。(2)而采用将MDI的溶液滴加到PTMEG溶液中,俟反应完成后再滴加Extender的加料方式所制备的SPEU其分散相在形态上呈现为球形,它们的直径多在100至300nm范围内,由电子衍射证明这种分散微区的结构是无定形的。     

Absolute partial cross sections and kinetic energy analysis for the electron impact ionization of ethylene

We present absolute partial electron impact ionization cross sections for ethylene in the electron energy range between threshold and 1000 eV measured with a two sector field double focusing mass spectrometer. Ion kinetic energy distribution functions have been measured at all electron energies by applying a deflection field method. Multiplication of the measured relative cross sections by the appropriately determined discrimination factors lead to accurate relative partial cross sections. Normalization of the sum of the relative partial cross sections to an absolute total cross section gives absolute partial cross section values. The initial kinetic energy distributions of several fragment ions show the presence of two or more contributions that exhibit different electron energy dependencies. Differential cross sections with respect to the initial kinetic energy of the ions are provided and are related to specific ion production channels. The electron threshold energies for the direct and numerous other dissociative ionization channels are determined by quantum chemical calculation and these allow the determination of the total kinetic energy release and the electron energy loss for the most prominent dissociative ionization channels.     

Application of work function measurements to the characterization of thin films and solid surfaces

Two different methods of electron work function measurements, the diode and the onset method, which can easily be incorporated in existing analytical equipment are described. For the diode method the sample is used as the anode of a diode arrangement, and the work function changes of the specimen are obtained from the shift of the break points of characteristic retarding field lines. The onset method uses the shift of the onset of the secondary electron energy distribution due to work function changes.Both methods were applied to Ta₂O₅ layers of 500 Å thickness which were produced by electrochemical oxidation of polycrystalline tantalum substrates. In particular, bombardment induced work function changes during sputter removal of these layers have been investigated.The onset method was also applied to differently oriented grains of a polycrystalline Ta specimen. Work function differences of 0.1 eV between two different grain surfaces were safely detectable.In addition, a work function difference of 0.5 eV between Au and Mo was determined which agrees well with literature values.     

Attractive electron–electron interactions within robust local fitting approximations

An analysis of Dunlap's robust fitting approach reveals that the resulting two‐electron integral matrix is not manifestly positive semidefinite when local fitting domains or non‐Coulomb fitting metrics are used. We present a highly local approximate method for evaluating four‐center two‐electron integrals based on the resolution‐of‐the‐identity (RI) approximation and apply it to the construction of the Coulomb and exchange contributions to the Fock matrix. In this pair‐atomic resolution‐of‐the‐identity (PARI) approach, atomic‐orbital (AO) products are expanded in auxiliary functions centered on the two atoms associated with each product. Numerical tests indicate that in 1% or less of all Hartree–Fock and Kohn–Sham calculations, the indefinite integral matrix causes nonconvergence in the self‐consistent‐field iterations. In these cases, the two‐electron contribution to the total energy becomes negative, meaning that the electronic interaction is effectively attractive, and the total energy is dramatically lower than that obtained with exact integrals. In the vast majority of our test cases, however, the indefiniteness does not interfere with convergence. The total energy accuracy is comparable to that of the standard Coulomb‐metric RI method. The speed‐up compared with conventional algorithms is similar to the RI method for Coulomb contributions; exchange contributions are accelerated by a factor of up to eight with a triple‐zeta quality basis set. A positive semidefinite integral matrix is recovered within PARI by introducing local auxiliary basis functions spanning the full AO product space, as may be achieved by using Cholesky‐decomposition techniques. Local completion, however, slows down the algorithm to a level comparable with or below conventional calculations. © 2013 Wiley Periodicals, Inc.     

Mechanisms of double ionization in strong laser field from simulation with Coupled Coherent States: Beyond reduced dimensionality models

The recently developed Coupled Coherent States method for solution of the time-dependent Schrödinger equation is used to simulate the strong laser field double ionization of a helium atom in six spatial dimensions. The calculated double ionization yield, as a function of laser intensity, reproduces the “shoulder”, attributable to electron recollisions. The method, which employs a Coherent State representation guided by classical trajectories also provides physical insight into the ionization mechanism. The appearance of the ‘shoulder’, is seen by analysis of the guiding trajectories to arise predominantly from sequential excitation of one electron into a highly excited orbit, followed by an electron–electron collision leading to correlated escape of the two electrons. A second little known mechanism involves recollisions resulting in ionization of the secondary electron, leaving the first in a highly excited state, from which it is later ionized by the laser field.     

Constriction of a microwave-induced plasma by a magnetic pinch

The microwave-induced plasma has been widely studied as a spectrochemical excitation source. The plasma is generally maintained in a quartz tube, and energy coupled via a cavity or antenna. Previous work has shown the importance of the electron density; plasma performance is improved as the instrumental parameters are adjusted to increase the electron density. Another method to increase the electron density is to constrict the plasma. Additionally, energy losses via wall-collision would be decreased if the plasma were moved away from the walls. This communication presents such a study, in which the plasma is ‘pinched’ by the application of an external magnetic field. Preliminary results show an increase in intensity by a factor of two. The constriction also tends to improve the atomization processes. Measurements performed on plasmas which contain carbon monoxide show a greater increase in the carbon atomic emission than the carbon monoxide molecular emission.     

Computation of screened two‐electron matrix elements

Computational studies are presented for atoms in screening environments. Numerical solution of atomic Hartree–Fock equations in the Slater type orbitals basis are presented for screened Debye electron–electron as well as electron–nucleus interaction. Slater integrals are evaluated using a Gauss–Laguerre quadrature. Detailed numerical results are presented for various atoms and Debye lengths establishing the method as well as showing the opposing effects of electron–nucleus and electron–electron screening that may induce ambiguity in physical properties extracted from experimental plasma data. A detailed discussion of convergence properties is given. The relevant outcome of the present study revealed the fact that the present expansion method and the previously published Legendre expansion method (Winkler, Int. J. Quantum Chem. 2010, 110, 3129) have their best convergence properties in different regions of the Debye screening length. This is important for applications. In addition, the new expansion method can be implemented for other screening potentials, where other approaches fail. © 2013 Wiley Periodicals, Inc.     

Preparation of Graphene‐ZnS Nanocomposites via Hydrothermal Method Using Two Sulfide Sources

Graphene‐ZnS nanocomposites were prepared by hydrothermal method using functionalized graphene sheets (FGS) as matrix with Na₂S and thioacetamide (TAA) as sulfide sources, respectively. The X‐ray diffraction (XRD) patterns reveal that face‐centered cubic ZnS was obtained. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results demonstrate different morphological characteristics of the two samples. The optical behaviors of both samples were examined by means of photoluminescence (PL) spectroscopy. The different effects of the two sulfide sources on the formation of the nanocomposites were also discussed to explain the reasons for the differences of morphological characteristics and optical behaviors between two samples.     

Polymer colloids and silver nanoparticles hybrid materials

The present study presents two different methods to obtain hybrid material formed by the poly [styrene (ST)–poly(ethylene glycol) methyl ether methacrylate (PEGMA) 1100] and silver (Ag⁰). The aim has been to cover the polymeric particles with Ag⁰ shell. The first method consisted of mixing Ag⁰ nanoparticles dispersion with poly (ST-PEGMA 1100) dispersion, while in the second method, the Ag⁰ nanoparticles have been generated in situ. The hybrid materials have been characterized by MO, dynamic light scattering, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray energy dispersive spectrometry, and ultraviolet–visible spectrophotometry. The results confirm the obtaining of two types of morphologies. In the first case, the nanoparticles have been arranged in the interspatial zones of the polymer particles, while in the second method, the Ag⁰ nanoparticles have covered the polymer particles. Thus, the film obtained using the second method is more suitable for the practical application, as a separation membrane, using the antiseptic properties of Ag⁰.     

Structural and electronic properties of La‐doped CaTiO3 crystal

There is experimental and computational evidence that some important properties such as electrical conductivity and ferroelectricity in the CaTiO₃ crystal change according to the dopant states. Using an INDO quantum‐chemical computational method modified for crystal calculations we explore the stability of the La‐doped CaTiO₃ crystal in both phases, cubic and orthorhombic. The calculations are carried out by means of the supercell model based on the LUC (large unit cell) approach as it is implemented into the CLUSTERD computer code. The equilibrium geometry for impurity is found together with the crystalline lattice distortions. Atomic displacements and relaxation energies are analyzed in a comparative manner for the two crystallographic phases. A new effect of electron transfer from the local one‐electron energy level within the band‐gap to the conduction band is observed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002