Minimally corrected partial atomic charges for non-covalent electrostatic interactions

We develop a new scheme for determining molecular partial atomic charges (PACs) with external electrostatic potential (ESP) closely mimicking that of the molecule. The PACs are the ‘minimal corrections’ to a reference set of PACs necessary for reproducing exactly the tensor components of the Cartesian zero-, first- and second- molecular electrostatic multipoles. We evaluate the quality of ESP reproduction when ‘minimally correcting’ (MC) Mulliken, Hirshfeld or iterative-Hirshfeld reference PACs. In all these cases, the MC-PACs significantly improve the ESP while preserving the reference PACs’ invariance under the molecular symmetry operations. When iterative-Hirshfeld PACs are used as reference, the MC-PACs yield ESPs of comparable quality to those of the ChElPG charge fitting method.     

A new method for deriving atomic charges and dipoles for n, -alkanes: investigation of transferability and geometry dependence

Atomic charge models are known to be unsatisfactory for representing the ab initio electrostatic potential (ESP) of n, -alkanes. A new method for deriving atomic charges and dipoles is proposed and applied to n-alkanes ranging from C₄ to C₁₀. Electrostatic parameters found by this method reproduce accurately the ab initio ESP. The issues of transferability and conformational dependence are also addressed by introducing charges and dipoles taken from a truncated distributed multipole analysis, in the same spirit as the restrained electrostatic potential method. A transferable model is proposed for larger alkanes (>C₁₀). We also estimate the error made when using a set of Boltzmann-weighted electrostatic parameters for all conformers. The reduced number of electrostatic sites considered in our model makes it suitable for computer simulation of liquid n-alkanes.     

Quantum microscopic vs. classical macroscopic calculations on the phenomenon of electrostatic influence

In order to compare microscopic and macroscopic approaches to the phenomenon of electrostatic influence, we have studied the atomic charges of an electric conductor, obtained either from macroscopic classical electrostatics, or microscopic quantum ab initio calculations. A torus was chosen as conducting material, built from valence monoelectronic atoms and influenced by an external point charge. The classical electric charges are obtained by integrating the macroscopic density over “atomic" sectors. This density is determined from a numerical integration of linearized electrostatic equations. The quantum charges are defined from Natural Orbitals in MP2/6-31G* calculations on clusters of different sizes. The overall agreement is good, with reasonable discrepancies due (i) to the continuity of the macroscopic model, which ignores the oscillations on atomic distances; and (ii) to the linearity constraint in the macroscopic equations.     

High resolution imaging of contact potential difference using a novel ultrahigh vacuum non-contact atomic force microscope technique

An ultrahigh vacuum scanning Kelvin probe force microscope (UHV SKPM) based on the gradient of electrostatic force was developed using the technique of a UHV non-contact atomic force microscope (NC-AFM) capable of atomic level imaging, and used for simultaneous observation of contact potential difference (CPD) and NC-AFM images. The CPD images with a potential resolution of less than 10 meV were observed in the UHV SKPM, demonstrating an atomic level resolution. The change of potential corresponding to the charges on the insulated surface of polypropylene have been observed in UHV SKPM. We also demonstrated a reliable method to obtain the CPD from the bias voltage dependence curves of the frequency shift in all of the scanning area. The results are consistent with comparing the barrier height images in that the work functions of adatoms are greater than the work function of corner holes.     

Charge distribution in structurally disordered systems

In this paper we calculate the charge distribution n(Q) for a structurally disordered system of identical atoms. The atoms have non-zero charges associated to them only because the spatial configuration around each atom is different. The systems considered are those for which an atomic basis set is adequate and an iterative tight binding scheme, where the matrix elements depend on the atomic charges, is used. We study the effect of including explicitly the electrostatic interaction among the charges associated to the atoms in the calculation of n(Q). We propose that the atomic positions of a totally random configuration be modified by amounts proportional to the electrostatic forces on the atoms. We call this a relaxation effect. We find that the new atomic configurations give a narrower n(Q) although they have practically the same energy and radial distribution function as the original configuration.     

Friction behavior of monolayer molybdenum diselenide nanosheet under normal electric field

The friction behavior of monolayer molybdenum diselenide (MoSe₂) under normal electric field was studied by the atomic force microscope. The friction coefficients of MoSe₂ are increasing with bias voltage applied on the Si substrate. The results show that the adhesion and electrostatic forces increase with bias and approximately follow a parabolic law. The friction force and surface potential are of the same tendency with bias application time, and the contribution of charges accumulation to friction is considerable. The mechanisms of the friction behavior under external normal electric field were explained with electrostatic force and adsorption. This study reveals a possibility of electronically controlling friction in two-dimensional MoSe₂ system, with potential applications in solid lubricant and moving parts for MEMS devices.     

The change of the vibrational frequencies of a chemisorbed diatomic caused by the electrostatic interactions with the metal surface

Results of density functional calculations are used to discuss the change of the vibrational frequency of an adsorbed molecule caused by the electrostatic interaction between a chemisorbed molecule — simulated by two effective charges — and a metallic surface.     

Non-empirical versus empirical choices for overlapping-sphere radii ratios in SCF-Xα-SW calculations on ClO4 - and SO2

Two non-empirical schemes for establishing ratios of atomic sphere radii for Xα-SW calculations are proposed and compared with known methods of partitioning molecules into atomic fragments along paths of maximum change in electrostatic potential or charge density. In the initial Xα-SW molecular charge distribution, the radius of a sphere around each atom containing the atomic number of electrons is shown to be constant within a few per cent no matter what atomic charges are assumed in constructing the charge distribution. This contrasts with the radius of a sphere containing the number of electrons put in for the atom in calculating its atomic charge density, which shows a significant inverse variation with the assumed charge. Ionization energies for ClO₄ ^- and SO₂ are calculated using ratios of sphere radii established both from the atomic number sphere sizes and from Slater's empirical atomic radii. The results are compared with experiment and with HF-LCAO calculations. The Xα-SW calculations using atomic number radii ratios are in the best overall agreement with experiment.     

外电场下4-氯-2-三氟甲基噻吩并[3,2-D]嘧啶分子的结构和光谱

4-氯-2-三氟甲基噻吩并[3,2-D]嘧啶(4-chloro-2-(trifluoromethyl)thieno[3,2-d] pyrimidine, CTTP)分子具有很高的生物和医学活性,是重要的医药中间体,因此了解外电场下对其结构和光谱的影响具有重要意义。采用密度泛函理论B3LYP方法和def2-tzvp基组,优化计算了CTTP分子在不同外电场作用下的稳定结构;然后从外电场对其结构、原子电荷分布、前线轨道、静电势、红外和拉曼光谱等方面的影响进行了分析。结果显示：随着外电场强度增加,分子结构和原子电荷分布发生改变,分子极性增强,能隙降低,化学反应活性增强,静电势分布极值点发生了明显改变,红外和拉曼在不同区域内发生振动斯塔克效应,既有蓝移又有红移,峰的强度也出现了复杂的变化。     

利用高分辨同步辐射衍射研究氨苄青霉素三水合物的电荷密度分布:密度泛函理论和多极模型之间的相关性

实验研究了氨苄青霉素三水合物的电荷密度分布,并与用密度泛函理论的量子计算结果进行比较.计算了电荷导出性质,Mulliken原子电荷,偶极矩和分子静电势.另外用多极分析对实验总体参数的进行细化.用多极处理获得的结构因子构建了傅立叶图.同时讨论电荷分布的拓扑性质,分析了（3,-1）临界点的特性.     

Imaging and tracking an electrostatic charge micro-domain by Kelvin force microscopy as evidence of water adsorption on mica surface

Muscovite mica is a widely accepted substrate for scanning probe microscopy (SPM) investigations. However, mica has intrinsic properties that alter samples and obstruct their analysis due to free charges build-up, ionic exchange and water adsorption taking place at the surface. In addition to interfacial phenomena, there is a growing interest in electrostatic charges on insulators as they are crucial in diverse applications. Despite the high demand for studies of this nature, experimental set-ups capable of resolving charge build-up at the micro-scale are still scarce and technically limited. Here, we report the imaging of surface charge dissipation on freshly cleaved mica by Kelvin-probe Force Microscopy (KPFM). A local electrostatic charge micro-domain was generated by friction between an atomic force microscope (AFM) tip and mica, and its decay was tracked by two-dimensional mapping using KPFM. We found time-dependent charge dissipation, which is attributed to the adsorption of water molecules on mica surface.     

Performance characteristics between horizontally and vertically oriented electrodes EHD ESP for collection of low-resistive diesel particulates

The novel electrohydrodynamically-assisted electrostatic precipitator (EHD ESP) was developed to suppress particle reentrainment for collection of low resistive diesel particulates. The collection efficiency was compared between vertically and horizontally oriented electrodes of the EHD ESP using 400 cc diesel engine. The particle size dependent collection efficiency was evaluated for the particle size ranging in 20 to 5000 nm using a scanning mobility particle sizer (SMPS) and a particle counter (PC). Both horizontally and vertically oriented EHD ESP showed an excellent suppression of particle reentrainment. However, the horizontally oriented electrode EHD ESP showed significantly improved for the particle size of 300–500 nm in comparison with vertically oriented electrode EHD ESP, resulting in more than 90% collection efficiency for all particle size range. The EHD ESP has high potential especially for highly concentrated marine diesel engine emission control.     

Molecular structure,spectroscopic properties (FT-IR,Micro-Raman,and UV-vis), and DFT calculations of minaprine

Molecular geometry, experimental vibrational wavenumbers, electronic properties, and quantum chemical calculations of minaprine (C₁₇H₂₂N₄O · 2HCl), (with synonym, dihydrochloride salt of N-(4-methyl-6-phenyl-3-pyridazinyl)-4-morpholineethamine) which is widely used as a psychotropic drug at medicinal treatment, in the ground state by using density functional theory (DFT/B3LYP) method with 6–31++G(d,p) basis set have been presented for the first time. The comparison of the observed fundamental vibrational frequencies were in a very good agreement with the experimental data. Furthermore, UV-vis TD-DFT calculations, the highest occupied molecular orbitals (HOMO-1, HOMO), lowest unoccupied molecular orbitals (LUMO, LUMO + 1), molecular electrostatic potential (MEP) surface, atomic charges and thermodynamic properties of minaprine molecule have been theoretically calculated and simulated at the mentioned level.     

Electrostatic Potential Anomaly in 2D Janus Transition Metal Dichalcogenides

Symmetry breaking induced exotic physical properties is an eternal topic in scientific community. Due to lack of mirror symmetry, 2D Janus transition metal dichalcogenides (TMDs) exhibit many bizarre features; however, the physical mechanisms of most of these intrinsic properties are still unclear. Herein, a generalized and effective approach is developed to disclose the physical mechanism of electrostatic potential anomaly in 2D Janus TMDs, based on fast Fourier transform and Moore–Penrose generalized inverse matrix for separating Hartree potential and ionic potential from electrostatic potential, and conversely, calculating charge density distribution through Hartree potential. Through extensive numerical simulations and theoretical analyses, the electrostatic potential anomaly is expounded successfully, which is a pending issue in 2D Janus TMDs: the electrostatic potential energy at Se atomic layer is larger than that at S and Te atomic layers, which breaks the periodic law. Such an anomaly could be attributed to the competition between Hartree potential energy and ionic potential energy that emerges as a result of asymmetric charge transfer, atomic layer distance, and atomic species. This approach possesses universality, and is proved to be a robust method in dealing with the issues related to electrostatic potential.     

FTIR and FT‐Raman spectra and density functional computations of the vibrational spectra,molecular geometry and atomic charges of the biomolecule: 5‐bromouracil

FTIR and FT‐Raman spectra of 5‐bromouracil in the powder form were recorded in the region 400–4000 cm⁻¹ and 50–4000 cm⁻¹, respectively. The observed wavenumbers were analysed and assigned to different normal modes of vibration of the molecule. Quantum chemical calculations were performed to support the assignments of the observed wavenumbers. The performance of the B3LYP hybrid density functional (DFT) method was compared with other methods. With the 6–31 G** and 6–311 + G(2d,p) basis sets, the calculated geometry, dipole moments and harmonic vibrations were determined. A comparison with the uracil molecule was made, and specific scale factors were deduced and employed in the predicted wavenumbers of 5‐bromouracil. The total atomic charges and thermodynamic parameters were calculated, and are discussed briefly. Structure and harmonic vibrations of 5‐bromouracil were also calculated in the presence of water within a simple model with one molecule. It is observed that the bromine atom at position 5 exhibits smaller inductive effects than the fluorine atom, producing a small distortion of the electrostatic potential around the ring and a reduction of the molecular dipole moment. Copyright © 2007 John Wiley & Sons, Ltd.     

Electrostatic potential of point charges inside dielectric oblate spheroids

As a sequel to a previous paper on electrostatic potential of point charges inside dielectric prolate spheroids [1], this note further presents the exact solution to the electrostatic problem of finding the electric potential of point charges inside a dielectric oblate spheroid that is embedded in a dissimilar dielectric medium. Numerical experiments have demonstrated the convergence of the proposed series solutions.     

Interaction of two macroparticles in a nonequilibrium plasma

The interaction of two macroparticles in a nonequilibrium plasma at elevated pressures has been investigated. An asymptotic theory of screening, which leads to a two-exponential dependence of the macroparticle potential on distance with different screening constants, is used to determine the electrostatic energy of the system of charges associated with the two macroparticles. The dependence of the electrostatic energy on interparticle distance has been found to have a minimum, as in an equilibrium plasma. The interaction force between the macroparticles has been determined; it turned out to be asymmetric—for different charges, the forces acting on the first and second macroparticles are not equal. This is the result of an asymmetric charge separation near macroparticles with differing charges and indicates that the interaction force in a nonequilibrium plasma is nonpotential. The forces are equal for identical macroparticles or in an equilibrium plasma and the potential energy of the interaction between the macroparticles has been determined for these cases. Attraction between likely charged particles with different (in magnitude) charges has been found to be possible when they come very close together. Relations to determine the modified coupling parameter for an interaction potential that consists of two exponential terms with different screening constants have been derived.     

Experimental investigation on charging characteristics and penetration efficiency of PM2.5 emitted from coal combustion enhanced by positive corona pulsed ESP

The electrostatic precipitator (ESP) has been extensively used for collecting aerosol particles emitted from coal combustion, but its collection efficiency of PM2.5 (Particulate matter whose aerodynamic diameter is less than 2.5 μm) is relatively low due to insufficient particle charging. The positive pulsed ESP is considered to enhance particle charging and improve collection efficiency. A laboratory-scale pulsed ESP with wire-plate electrode configuration was established to investigate the particle charging and penetration efficiency under controlled operating conditions of different applied impulse peak voltages, impulse frequencies, dust loadings and residence times. The results show that most particles larger than 0.2 μm are negatively charged, while most particles smaller than 0.2 μm are positively charged. For a given operating condition, the particle penetration efficiency curve has the highest penetration efficiency for particles with a diameter near 0.2 μm, and there is always a negative correlation between the particle penetration efficiency and the average number of charges per particle. Under the same operating conditions, the particle penetration efficiency decreases with increasing impulse peak voltage and impulse frequency, but increases as the dust loading increases. The results imply that residence time of 4 s is optimum for particle charging and collection. PM2.5 number reduction exceeding 90% was achieved in our pulsed ESP.