Electrostatic potential and field approximation for aluminosilicates in cation-substituted forms

Water molecules were removed from the X-ray structure models of zeolites with comparatively small-sized unit cells (BIK, BRE, EDI, GIS, GOO, LAU, and PHI), and cations were replaced by cations of only one type (Mg or Zn). The dehydrated zeolite models obtained were optimized with the Bush and Catlow force fields. The dependences of atomic multipole moments up to the fourth order along the so-called cumulative coordinate were calculated for these models by the density functional method with periodic boundary conditions. These dependences were used to estimate the electrostatic potential and electric field in the cross section of a zeolite that was not used to calculate them. The data obtained with atomic charges borrowed directly from calculation results were quite satisfactory.     

Electrostatic potential topography for exploring electronic reorganizations in 1,3 dipolar cycloadditions

Topographical analysis of the molecular electrostatic potential (MESP) along a reaction path is employed for bringing out sequential electronic reorganizations for 1,3-dipolar cycloadditions of ethyne to fulminic acid as well as diazomethane. A simple and consistent set of rules for portraying electronic mechanisms of chemical reactions using the MESP topography is applied for this purpose. The MESP topography at each point on the concerted reaction path is associated with a classical electronic structure yielding a clear picture of the electronic reorganization along the reaction path.     

Electrostatic potential around charged finite rodlike macromolecules: nonlinear Poisson-Boltzmann theory

In this paper, we show that the far field electrostatic potential created by a highly charged finite size cylinder within the nonlinear Poisson-Boltzmann (PB) theory, is remarkably close to the potential created within the linearized PB approximation by the same object at a well-chosen fixed potential. Comparing the nonlinear electrostatic potential with its linear counterpart associated to a fixed potential boundary condition (called the effective surface potential), we deduce the effective charge of the highly charged cylinder. Values of the effective surface potential are provided as a function of the bare surface charge and Debye length of the ionic solution. This allows to compute the anisotropic electrostatic interaction energy of two distant finite rods.     

Electrostatic potential mapping of polycyclic aromatic hydrocarbon diol epoxides using a dipole

Electrostatic potential maps of benz(a)anthracene diol epoxide, benzphenanthrene diol epoxide, chrysene diol epoxide, and tryphenylene diol epoxide, which are ultimate carcinogens derived from the corresponding polycyclic aromatic hydrocarbons, have been studied using a dipole of length 1 Å and strength 1 Debye. The net charge distributions in the molecules were obtained using the MNDO molecular orbital method. The maps were drawn for two closest distances of approach between the charged ends of the dipole and atoms of the molecules. Using the electric field directions and values of electrostatic potentials, reactive sites and relative reactivities of these molecules have been examined. Existence of a bay region appears to be important for the carcinogenic activity of a diol epoxide in this class of carcinogens.     

Electrostatic potential minimum of the aromatic ring as a measure of substituent constant

The molecular electrostatic potential minimum (Vmin) observed for the arene pi system of a substituted benzene derivatives is found to correlate linearly with the substituent constant sigma(p) degrees . The use of Vmin as a measure of substituent effect is further confirmed by obtaining a linear correlation between Vmin and a thermodynamic measure of the substituent effect obtained from an isodesmic reaction scheme involving benzene derivatives. Vmin and the recently proposed electrostatic potential value at the nucleus of the para carbon atom (Vc) show a nearly identical trend toward quantification of substituent effects. Both quantities have been compared at three different density functional theory methods, viz. B3LYP/6-311+G(2d,2p), BPW91/6-311G(d,p), and B3LYP/aug-cc-pvtz, as well as the at the MP2/6-31+G(d,p) level of theory, showing remarkable consistency among them.     

Electrostatic potential profile and nonlinear current in an interacting one-dimensional molecular wire

We consider an interacting one-dimensional molecular wire attached to two metal electrodes on either side of it. The electrostatic potential profile across the wire-electrode interface has been deduced solving the Schrodinger and Poisson equations self-consistently. Since the Poisson distribution crucially depends on charge densities, we have considered different Hamiltonian parameters to model the nano-scale wire. We find that for very weak electron correlations, the potential gradient is almost zero in the middle of the wire but are large near the chain ends. However, for strong correlations, the potential is essentially a ramp function. The nonlinear current, obtained from the scattering formalism, is found to be less with the ramp potential than for weak correlations. Some of the interesting features in current-voltage characteristics have been explained using one-electron formalism and instabilities in the system. Dedicated to Professor C N R Rao on his 70th birthday     

Electrostatic effect of streaming potential on ion-exchange phenomena in resins for ion chromatography

Ion-exchange phenomena in the inner phase of resins for ion chromatography were studied by measuring streaming potentials in a flow system. On the basis of the values obtained, the electrostatic behaviour in connection with anion-exchange reactions in the resin phase was evaluated. The electrostatic potentials in the resin phase varied with the exchanging ion species in the solution phase under various salt conditions. On comparing retention times in ion chromatography, it was found that the electrostatic behaviour influences the selectivity coefficient of the resin.     

Electrostatic potential for some non-spherical organic molecules: An oblate-spheroidal dipolar approximation

A spheroidal dipolar expansion is used for the determination of the electrostatic potential due to three non-spherical organic molecules: to-luene, phenol and cyclohexanone. The agreement of experimental facts with the prediction of the most probable reactive sites on each molecule deduced from calculated equipotential maps is discussed.     

Electrostatic potential around halogenated anesthetics. Acidic hydrogen; chlorine vs. fluorine

Maps of electrostatic potential around a number of halocarbon anethetics have been obtained through Gaussian ab initio calculations. Information is obtained on the mode of association of these compounds with possible receptors. The positive potential found along the extension of the C? H bonds substantiates the role of the “acidic hydrogen.” The presence of fluorines on the carbon atom to which it is bound restricts the access to this positive site through the repulsive action of the fluorine lone pairs, an observation that explains the very different behavior of chloroform and fluoroform, for example, with respect to their anesthetic potencies and hydrogen bonding abilities. In the absence of an acidic hydrogen the chlorine atoms can serve as associative sites.     

Ion-dipole S(N)2 reaction in acetone-water mixtures. Electrostatic and specific solute-solvent interactions

The rate constants of the S(N)2 reaction of sodium 4-nitrophenoxide (1) and iodomethane were determined by UV-visible spectrophotometry in acetone-water mixtures at 25, 30, and 35 degrees C. The rate-Xwater (mole fraction of water) profile shows that the reaction depends strongly on the medium. The fastest rate constant was obtained in pure acetone, and a minimum occurred at Xwater= 0.4, whereas the observed second-order rate constants increases again in the water-rich region. In pure acetone, in the presence of dicyclohexano-[18]-crown-6, increases linearly with the concentration of the crown ether as a result of the complexation of the sodium ion (KS = 104.8 M) of the ion-pair and the increase in the effective concentration of free 4-nitrophenoxide ion, which was assumed to be the only reactive species. Ion-pairing was also detected at Xwater= 0.65 with a dissociation constant Kd = 7.82 x 10(-4) M(-1). The solvatochromic behaviors of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenoxide (2), 4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one (3), and 1-methyl-8-oxy-quinolinium betaine (4) were investigated in the entire range of acetone-water mixtures. The dyes presented an increasing order of hydrophilicity compatible with their chemical structure, i.e., 2 < 3 < 4. Kinetic parameters for the methylation of 1 and the ET values of the dyes show a linear correlation of the polarity in the region of Xwater = 1.0-0.40 for 3 and 4, and it was observed that the more hydrophilic the dye the better the correlation coefficient, because of the structural similarity with 1. The activation parameter-Xwater profile shows extrema at Xwater < 0.4, reflecting an important change in the structure of the solvent that is responsible for the changes in the solvation of the reactive species including ion-pairs. These results suggest that the addition of water to acetone reduces abruptly the rate of substitution due to the preferential solvation (PS) of the phenoxide ion by the hydrogen-bonding donor (HBD) solvent. Nevertheless, the real second-order rate constant is "masked" by the association involving Na+ and 4-nitrophenoxide that extends even to water-rich mixtures. A model, based on the assumption that the free-energy terms involved in the second-order rate constant and the dissociation constant of the ion-pair have two components, is invoked to explain the kinetic data. One of the components depends on electrostatic interactions for which the main variable is the dielectric constant of the solvent mixture, and the other depends on the specific solute-solvent interactions, expressed by the activity coefficients of transfer of the species involved. The model indicates that in the range of Xwater = 1.0-0.40 the interactions are exclusively electrostatic, while for the rest of the acetone-rich region they are specific with a large contribution of the 4-nitrophenoxide ion.     

Workman-Reynolds freezing potential measurements between ice and dilute salt solutions for single ice crystal faces

Workman-Reynolds freezing potentials have been measured for the first time across the interface between single crystals of ice 1h and dilute electrolyte solutions. The measured electric potential is a strictly nonequilibrium phenomenon and a function of the concentration of salt, freezing rate, orientation of the ice crystal, and time. When all these factors are controlled, the voltage is reproducible to the extent expected with ice growth experiments. Zero voltage is obtained with no growth or melting. For rapidly grown ice 1h basal plane in contact with a solution of 10 (-4) M NaCl the maximum voltage exceeds 30 V and decreases to zero at both high and low salt concentrations. These single-crystal experiments explain much of the data captured on this remarkable phenomenon since 1948.     

Electrostatic energy in the effective fragment potential method: theory and application to benzene dimer

Evaluation of the electrostatic energy within the effective fragment potential (EFP) method is presented. The performance of two variants of the distributed multipole analysis (DMA) together with two different models for estimating the charge penetration energies was studied using six homonuclear dimers. The importance of damping the higher order multipole terms, i.e. charge dipole, was also investigated. Damping corrections recover more than 70% of the charge penetration energy in all dimers, whereas higher order damping introduces only minor improvement. Electrostatic energies calculated by the numerical DMA are less accurate than those calculated by the analytic DMA. Analysis of bonding in the benzene dimer shows that EFP with inclusion of the electrostatic damping term performs very well compared to the high-level coupled cluster singles, doubles, and perturbative triples method. The largest error of 0.4 kcal/mol occurs for the sandwich dimer configuration. This error is about half the size of the corresponding error in second order perturbation theory. Thus, EFP in the current implementation is an accurate and computationally inexpensive method for calculating interaction energies in weakly bonded molecular complexes.     

Atomic multipoles: Electrostatic potential fit,local reference axis systems,and conformational dependence

Currently, all standard force fields for biomolecular simulations use point charges to model intermolecular electrostatic interactions. This is a fast and simple approach but has deficiencies when the electrostatic potential (ESP) is compared to that from ab initio methods. Here, we show how atomic multipoles can be rigorously implemented into common biomolecular force fields. For this, a comprehensive set of local reference axis systems is introduced, which represents a universal solution for treating atom‐centered multipoles for all small organic molecules and proteins. Furthermore, we introduce a new method for fitting atomic multipole moments to the quantum mechanically derived ESP. This methods yields a 50–90% error reduction compared to both point charges fit to the ESP and multipoles directly calculated from the ab initio electron density. It is shown that it is necessary to directly fit the multipole moments of conformational ensembles to the ESP. Ignoring the conformational dependence or averaging over parameters from different conformations dramatically deteriorates the results obtained with atomic multipole moments, rendering multipoles worse than partial charges. © 2012 Wiley Periodicals, Inc.     

The potential of inorganic mass spectrometry in mineral and trace element nutrition research

Over the past two decades, new applications of inorganic mass spectrometry have been made possible by the use of stable isotopes as tracers in studies of mineral and trace element metabolism in man. Stable isotope techniques and radioisotope methods are the only reliable tools available for determination of the absorption, retention, or utilization of a nutrient by the human body. Recent developments in inorganic mass spectrometry might open new perspectives as progress in this field of research depends mainly on improving existing stable isotope techniques and on developing novel concepts. By improving precision in isotope analysis, isotope doses in experiments on man can be reduced to physiologically more meaningful levels. This will also enable reduction of the (often substantial) costs of isotopically labeling a nutrient in a test meal. Improvements in the mass spectrometric sensitivity will enable the development of new tracer techniques that have the potential to provide the information required by: 1. governmental institutions for designing food fortification programs; 2. the food industry for developing nutrient-fortified food products; and 3. public health authorities for establishing reliable dietary recommendations for intake of inorganic nutrients. In this context the current scope and limitations of thermal ionization mass spectrometry, inductively coupled mass spectrometry, accelerator mass spectrometry, and resonance ionization mass spectrometry are evaluated. Iron isotopic variations in the human body are discussed as a possible source of bias that might be a future biological limit to stable isotope-dose reduction in experiments on iron metabolism in man. Received: 9 February 2001 / Revised: 21 March 2001 / Accepted: 23 March 2001     

The potential of inorganic mass spectrometry in mineral and trace element nutrition research

Over the past two decades, new applications of inorganic mass spectrometry have been made possible by the use of stable isotopes as tracers in studies of mineral and trace element metabolism in man. Stable isotope techniques and radioisotope methods are the only reliable tools available for determination of the absorption, retention, or utilization of a nutrient by the human body. Recent developments in inorganic mass spectrometry might open new perspectives as progress in this field of research depends mainly on improving existing stable isotope techniques and on developing novel concepts. By improving precision in isotope analysis, isotope doses in experiments on man can be reduced to physiologically more meaningful levels. This will also enable reduction of the (often substantial) costs of isotopically labeling a nutrient in a test meal. Improvements in the mass spectrometric sensitivity will enable the development of new tracer techniques that have the potential to provide the information required by: 1. governmental institutions for designing food fortification programs; 2. the food industry for developing nutrient-fortified food products; and 3. public health authorities for establishing reliable dietary recommendations for intake of inorganic nutrients. In this context the current scope and limitations of thermal ionization mass spectrometry, inductively coupled mass spectrometry, accelerator mass spectrometry, and resonance ionization mass spectrometry are evaluated. Iron isotopic variations in the human body are discussed as a possible source of bias that might be a future biological limit to stable isotope-dose reduction in experiments on iron metabolism in man.