Negative Point Charges Research Articles

A Simple Model for the Pauli Repulsion with Possible Utility in QM, MM and Chemical Education.

The Pauli repulsion is the intermolecular force responsible for the volume and low compressibility of condensed-phase matter at normal conditions. A simple model for this force is presented, wherein per-atom electron densities are represented as spherical charge distributions that are prevented from significantly overlapping. In the example of two noble gas atoms approaching one another beyond their van der Waals radii, the distance between the centers of the electronic charge distributions becomes larger than the distance between the nuclei, giving rise to an unfavorable electrostatic interaction. For the purpose of calculating this interaction, the model is further simplified by representing the per-atom electron density as a negative point charge, loosely inspired by the classical Drude oscillator. The dispersion interaction is simplified to an R-6 term, centered on the aforementioned point charges. Despite the gross simplicity of the resulting formalism, near-quantitative agreement with high-level QM interaction energies of noble gas dimers is achieved. Accordingly, the present model is thought to have utility in force fields, in post-HF and post-DFT dispersion corrections, and in chemical education.

Effects of ligands on excitation energies of [UO2X4]2− and UO2X2 (X = F, Cl) with the equation‐of‐motion coupled‐cluster theory

AbstractExperimental excitation energies (EEs) of low‐lying states for uranyl halides are relatively independent of the type of the equatorial ligands, while many theoretical results indicate that the ligands have a significant effect on the electronic structure of uranyl. In this work, vertical and adiabatic EEs for [UO2X4]2− and UO2X2 (X = F, Cl) are compared at the same computational level, that is, the equation‐of‐motion coupled‐cluster method, to investigate effects of the number and type of ligands on EEs of uranyl. Furthermore, accuracy of a perturbative treatment of spin‐orbit coupling (SOC) on EEs of these molecules is also studied. Our results show that reliable EEs are achieved when a sufficient number of spin‐free excited states are considered in dealing with SOC perturbatively. Difference between the lowest adiabatic EEs for these molecules is calculated to be within 0.2 eV, which are in reasonable agreement with experimental results. On the other hand, difference between EEs for other higher states of [UO2X4]2− and those of UO2X2 is sizeable. In addition, reasonable EEs can still be obtained when the ligands are replaced by negative point charges. Our results indicate that the number of ligands has a more pronounced effect on EEs of these uranyl chlorides and fluorides than the type of the ligands.

A New Electric Field Mill Network to Estimate Temporal Variation of Simplified Charge Model in an Isolated Thundercloud.

The gross charge distribution in an electrified cloud has already been estimated by polarity distribution of the electrostatic field on the ground surface. While either a dipole or a tripole charge structure is commonly accepted, the increase–decrease and motion of each point charge in those models are both still unclear. This paper presents a new network of electric field mills for multipoint electrostatic measurement to evaluate the temporal variations of a simple cloud charge model with second-scale resolution. Details of our newly developed equipment are described, with an emphasis on its advantages. This network was deployed in the north Kanto area of Japan and operated during the summer season in 2020. In order to simplify the relationship between cloud charge positions and the horizontal distribution of the measured electrostatic field, an isolated thundercloud is focused on. As an initial analysis, a negative point charge model is applied to an isolated cloud observed on 27 August 2020. The quantity and height of the point charge were estimated as being approximately −20 C and 7 km, respectively. The calculated charge location is generally coincident with the C-band radar echo regions. Significant correspondence is demonstrated between the intensity distribution of the electrostatic fields measured at seven sites and that calculated with estimated point charge. This result indicates the possibility to determine the amounts and positions of cloud charges inside the dipole charge structure based on multipoint measurement of the electrostatic field.

Quantifying the Separation of Positive and Negative Areas in Electrostatic Potential for Predicting Feasibility of Ammonium Sulfate for Protein Crystallization.

Ammonium sulfate (AS) and poly(ethylene glycol) (PEG) are the most popular precipitants in protein crystallization. Some proteins are preferably crystallized by AS, while some are by PEG. The electrostatic potential is related to the preference of the precipitant agents. The iso-surfaces of the electrostatic potentials for the AS-crystallized proteins display a common shape and a distinct separation between the positive and negative areas. In contrast, the PEG-crystallized proteins show unclear positive and negative separation. In this work, we propose schemes to quantitatively evaluate the separation for predicting which precipitant is favorable for crystal growth between AS or PEG. Three methods were attempted to quantify the amplitude of the separation, separation distance, dipole moment, and shape regularity. The positive and negative areas are approximated to the spherical potentials caused by point charges. The first method is a measurement of the distance between the positive and negative point charges. The second one is an assessment including the quantity of electric charge into the distance. The last one is an approach monitoring the clarity of the positive and negative separation. The average value for 25 kinds of AS-preferring proteins was higher than that for the PEG-preferring ones in all three methods. Therefore, every method can distinguish the proteins preferring AS for crystal growth from those preferring PEG. These methods require an iso-surface of the electrostatic potential depicted at a certain contouring value. The shape of the iso-surface depends on the contouring value. The dependency on contour was examined by depicting the iso-surfaces of electrostatic potential with three values at ±0.8, ±0.5, and ±0.2 kT/e. While reducing the contouring value leads to the increase in separation distance and the decrease in shape regularity, dipole moment is independent of the alteration of contouring value. While the AS-preferring proteins are distinguishable from the PEG-preferring ones in any contouring values, the iso-surface at ±0.5 kT/e seems adequate for regular use. The dipole moment assessment is feasible for the choice of potent precipitants for crystal growth in experiments.

Періодична кулонівська динаміка двох рівних від’ємних зарядів у полі фіксованих чотирьох рівних додатних зарядів

УДК 517.9 Знайдено періодичні розв'язки -вимірних рівнянь руху Кулона двох від'ємних точкових однакових зарядів у полі чотирьох однакових додатних зарядів, зафіксованих у вершинах прямокутника.Ці системи мають рівноважний стан.Періодичні розв'язки отримано за допомогою центральної теореми Ляпунова.

Periodic Coulomb Dynamics of Three Equal Negative Charges in the Field of Equal Positive Charges Fixed in Octagon Vertices

Periodic solutions of the Coulomb equation motion for three equal negative point charges in the field of six equal positive point charges fixed in vertices of a convex octagon are found. The system possesses an equilibrium configuration. The Lyapunov center theorem is applied.

Probing the impact of the N3-substituted alkyl chain on the electronic environment of the cation and the anion for 1,3-dialkylimidazolium ionic liquids.

In this study, X-ray photoelectron spectroscopy is used to probe the impact of the N3-substituted alkyl group on the electronic environment of the cation and the anion by comparing two types of imidazolium cations, 1-alkyl-3-butylimidazolium and 1-alkyl-3-methylimidazolium. Due to the more intense inductive effect changing from methyl to butyl, the electronic environment of the cationic nitrogen can be significantly affected, which is reflected in a shift of N 1s binding energy. The magnitude of the binding energy shift is found to be more pronounced in the case of the less basic anion and inversely proportional to the basicity of the anion. The increase of the N3-substituted alkyl chain length can also influence the charge-transfer effect from the anion to the cation. This gives rise to a change in the electronic environment of the anion. Such an impact is found to be concentrated on the anion-based component bearing more negative point charges.

Theoretical study of aerobic oxidation of alcohols over Au38 nanocluster by a two-step-modeling approach

Abstract Au nanoclusters, stabilized by PVP, catalyze the aerobic oxidation of p-HBA. In this study, its principal reaction route was analyzed using Au38. First, the arrangements of substrates, including ligands, were estimated. Next, these ligands were replaced with negative point charge of −1, and each system was re-optimized. The results obtained from these models exhibited the same tendency for the two reaction pathways. By estimating the electron density transfers between the substrates and oxidants, it was suggested that the electron affinity of the active oxygen species and Au NC is a good index for determining the main reaction pathways.

Tunable model promoters in DFT simulations of catalysts.

Promoter atoms can tune a catalyst's activity and selectivity by transferring charge to and from the active site. Rational design of promoted catalysts, using density functional theory calculations, is today limited by the need to simulate many catalyst and promoter configurations. We present a simple approximation that rapidly captures some trends in promoter effects, at a cost of complexity comparable with simulating unpromoted catalysts. Negative (positive) noninteger point charges introduced into the catalyst simulate how electropositive (electronegative) promoters might affect each predicted intermediate. Calculations return Sabatier plots, relating promoters' predicted efficacy to readily measured properties such as catalyst work functions. We illustrate our approach for two reactions associated with the Fischer-Tropsch process, hydrogen-deuterium scrambling, and carbon monoxide dissociation over ruthenium. Consistent with experiment, electropositive promoters are predicted to accelerate hydrogen scrambling and unassisted CO dissociation. Simulations also provide a new prediction: electronegative promoters accelerate hydrogen-assisted CO dissociation over hydrogen-precovered surfaces by stabilizing the initial CO adsorption. © 2019 Wiley Periodicals, Inc.

(Invited) Defect Induced Oxygen Redox Chemistry

Lithium- and sodium-rich layered transition-metal oxides have recently been attracting significant interest because of their large capacity achieved by additional oxygen-redox reactions. However, layered transition-metal oxides exhibit structural degradation such as layer exfoliation or cracks upon deep charge, which is a major obstacle to achieve higher energy-density batteries. Defect are usually considered as a negative factor to realize stable operation of cathode materials. Here we show, by applying optimal design strategies, defect can not only effectively generate redox-active “orphaned” nonbonding oxygen 2p orbitals to “activate” additional oxygen-redox reactions, but also contribute to “stabilizing” reversible phase transformation with its effective negative point charge.

The σ-Hole Coulombic Interpretation of Trihalide Anion Formation.

It is shown that the interactions of dihalogen molecules XY with halide anions Z- to form trihalide anions (XYZ)- can be satisfactorily described as Coulombic, involving the σ-holes on the atoms Y, but only if polarization is taken into account. We have approximated the polarizing effect of the halide anion Z- by means of a unit negative point charge. The CCSD/aug-cc-pVTZ computed interaction energies ΔE correlate well with the most positive electrostatic potentials associated with the induced σ-holes over a ΔE range of -12 to -63 kcal mol-1 . The (XYZ)- anions are more stable when the central atom is the largest, as has been observed, because the central atom is then the most polarizable, making the electrostatic potential associated with its σ-hole more positive.

The Electric Field as a "Smart" Ligand in Controlling the Thermal Activation of Methane and Molecular Hydrogen.

While pristine [TaO2 ]+ is rather inert in its thermal reactions with both methane and molecular hydrogen, the cluster oxide becomes reactive upon ligation with the closed-shell ligand CO2 . While the presence of CO2 increases the intrinsic barriers of the C-H and H-H bond activation steps, the efficiency of the overall reaction to generate the insertion product [Ta(R)(OH)O]+ (R=CH3 , H) is greatly enhanced by the superb leaving-group properties of CO2 . The ligand effect of CO2 has been well mimicked by the electric field of a negative point charge, and mechanistic aspects are addressed by high-level quantum chemical calculations.

Control of Product Distribution and Mechanism by Ligation and Electric Field in the Thermal Activation of Methane.

An unexpected mechanistic switch as well as a change of the product distribution in the thermal gas-phase activation of methane have been identified when diatomic [ZnO].+ is ligated with acetonitrile. Theoretical studies suggest that a strong metal-carbon attraction in the pristine [ZnO].+ species plays an important role in the rebound of the incipient CH3. radical to the metal center, thus permitting the competitive generation of CH3. , OH. , and CH3 OH. This interaction is drastically weakened by a single CH3 CN ligand. As a result, upon ligation the proton-coupled single electron transfer that prevails for [ZnO].+ /CH4 switches to the classical hydrogen-atom-transfer process, thus giving rise to the exclusive expulsion of CH3. . This ligand effect can be modeled quite well by an oriented external electric field of a negative point charge.

Two New Theories for the Current Charge Relativity and the Electric Origin of the Magnetic Force Between Two Filamentary Current Elements

This paper presents two new theories and a new current representation to explain the magnetic force between two filamentary current elements as a result of electric force interactions between current charges. The first theory states that a current has an electric charge relative to its moving observer. The second theory states that the magnetic force is an electric force in origin. The new current representation characterizes a current as equal amounts of positive and negative point charges moving in opposite directions at the speed of light. Previous work regarded electricity and magnetism as different aspects of the same subject. One effort was made by Johnson to unify the origin of electricity and magnetism, but this effort yielded a formula that is unequal to the well-known magnetic force law. The explanation provided for the magnetic force depends on three factors: 1) representing the electric current as charges moving at the speed of light, 2) considering the relative velocity between moving charges, and 3) analyzing the electric field spreading in the space due to the movement of charges inside current elements. The electric origin of the magnetic force is proved by deriving the magnetic force law and Biot-Savart law using the electric force law. This paper is helpful for unifying the concepts of magnetism and electricity.

Periodic and Bounded Solutions of the Coulomb Equation of Motion of Two and Three Point Charges with Equilibrium in the Line

Periodic and bounded solutions of the Coulomb equation of motion in the line are obtained for two and three identical negative point charges in the fields of two and three symmetrically located fixed point charges. The systems possess equilibrium configurations. The Lyapunov, Siegel, Moser, and Weinstein theorems are applied.

Polaron Structure and Transport in Fullerene Materials: Insights from First-Principles Calculations

Organic semiconductors offer a low-cost alternative to inorganic semiconductors. However, their usefulness is limited by a relatively low mobility of polaron charge carriers. Past research indicates a positive correlation between charge density and charge mobility in organic semiconductors. This relationship is usually attributed to the phenomenon of excess charges filling traps. Here, we explore whether charge density may also affect mobility via influence on intermolecular couplings. Density functional theory (DFT) with a long-range corrected (LC-BLYP) functional is used to calculate charge densities and electronic couplings of negative charges on C70 fullerenes in the presence of nearby negative point charges, which provides an upper limit calculation of the influence of nearby polarons. We find that in C70 systems with relatively low couplings, the presence of additional charges has an effect of maximizing intermolecular couplings and hence transport. This effect drops off quickly with distance, suggesting that it is relevant only at extremely high charge densities that are an unlikely event in current C70 devices. The effect of charge density on couplings may be useful in understanding transport in very limited regions of C70 materials where the local charge density is high; however, it is unlikely to affect overall device performance.

A new perturbation theory for electrolyte solutions.

Developing physically based equations of state for electrolyte solutions is demanding due to the long range behaviour of the Coulombic interaction potentials. In this work, we present a new perturbation approach for nonprimitive model electrolyte solutions consisting of hard spheres with a positive or negative point charge or with point dipoles. We overcome the problem of diverging correlation integrals by separating the interaction potentials into short ranged parts and a long ranged contribution. For the point charges, the division is done like in most implementations of the Ewald sum. The perturbation expansion to 3rd order is formulated using the short ranged part of the potentials only, which results in converging correlation integrals for which we provide simple analytical expressions. The long range contribution to the Helmholtz energy is taken into account by a analytical term that has recently been presented by Rodgers and Weeks [J. M. Rodgers and J. D. Weeks, J. Chem. Phys. 131, 244108 (2009)]. In order to assess the proposed theory, we present molecular simulation data for Helmholtz energies of the same model electrolyte solutions. Predictions for the Helmholtz energy from the new theory are found to be in very good agreement with results from the molecular simulations for all state points we regarded.

Riesz External Field Problems on the Hypersphere and Optimal Point Separation

We consider the minimal energy problem on the unit sphere $\mathbb{S}^d$ in the Euclidean space $\mathbb{R}^{d+1}$ in the presence of an external field $Q$, where the energy arises from the Riesz potential $1/r^s$ (where $r$ is the Euclidean distance and $s$ is the Riesz parameter) or the logarithmic potential $\log(1/r)$. Characterization theorems of Frostman-type for the associated extremal measure, previously obtained by the last two authors, are extended to the range $d-2 \leq s < d - 1.$ The proof uses a maximum principle for measures supported on $\mathbb{S}^d$. When $Q$ is the Riesz $s$-potential of a signed measure and $d-2 \leq s <d$, our results lead to explicit point-separation estimates for $(Q,s)$-Fekete points, which are $n$-point configurations minimizing the Riesz $s$-energy on $\mathbb{S}^d$ with external field $Q$. In the hyper-singular case $s > d$, the short-range pair-interaction enforces well-separation even in the presence of more general external fields. As a further application, we determine the extremal and signed equilibria when the external field is due to a negative point charge outside a positively charged isolated sphere. Moreover, we provide a rigorous analysis of the three point external field problem and numerical results for the four point problem.

FAAF](-) (A = O, S, Se, Te) or How Electrostatic Interactions Influence the Nature of the Chemical Bond.

The present contribution analyzes the special bonding situation of the [FAAF](-) anions (A = O, S, Se, Te), where the addition on an extra charge turns the covalent A-A bond of the neutral into a charge-shift (CS) bond. By means of theoretical calculations, we demonstrate that electrostatic interactions produced by negative point charges can transform the covalent A-A bond in the A2(+) cation into a CS one. Consequently, the electric field created by the fluorine atoms can be behind the unusual bonding situation of [FAAF](-), showing how electrostatic interactions can influence and alter the nature of the chemical bond.

Atomic Charges and the Electrostatic Potential Are Ill-Defined in Degenerate Ground States.

A system in a spatially degenerate ground state responds in a qualitatively different way to positive and negative point charges. This means that the molecular electrostatic potential is ill-defined for degenerate ground states due to the ill-defined nature of the electron density. It also means that it is impossible, in practice, to define fixed atomic charges for molecular mechanics simulations of molecules with (quasi-)degenerate ground states. Atomic-polarizability-based models and electronegativity-equalization-type models for molecular polarization also fail to capture this effect. We demonstrate the ambiguity in the electrostatic potential using several molecules of different degree of degeneracy, quasi-degeneracy, and symmetry.