Charge Distribution Of Ions Research Articles

Carbon dioxide capture by aminoalkyl imidazolium-based ionic liquid: a computational investigation

Efficient technologies/processes for CO(2) capture are greatly desired, and ionic liquids are recognized as promising materials for this purpose. However, the mechanisms for selectively capturing CO(2) by ionic liquids are unclear. In this study, the interactions between CO(2) and 1-n-amino-alkyl-3-methyl-imidazolium tetrafluoroborate, an amino imidazolium ionic liquid (AIIL), in its CO(2) capturing process, are elucidated with both quantum chemistry and molecular dynamics approaches on the molecular level. The effects of the straight aminoalkyl chain length in imidazolium-based cations on CO(2) capture are explored, and thereby the factors governing CO(2) capture for this ionic liquid family, e.g., ionic liquid structure, charge distribution, intermolecular interactions, thermodynamic properties and absorption kinetics, are analyzed. Molecular dynamics simulations are used to study the diffusion of the involved compounds and liquid structures of the CO(2)-AIIL systems. The results show that the amino-alkyl chain length plays an important role in governing the absorption properties of AIILs, including the free energies of absorption, equilibrium constants, desorption temperature, absorption rate constants, diffusion coefficients, and organization of CO(2) around cations and anions. This study provides useful information about rational design of ionic liquids for efficient CO(2) capture.

Probing the structure and stability of calcium carbonate pre-nucleation clusters

Recent advances in our understanding of the emergence of biomineral phases from solution has provoked new and challenging questions. A consensus is beginning to form, attesting to the existence of pre-nucleation clusters of calcium carbonate in solution, which subsequently aggregate to initiate solid growth. The structure and stability of these clusters has not yet been fully determined; this needs to be addressed if biomineralisation mechanisms are to be exploited. Here, we present the results of an exhaustive computational study in the search for possible candidate pre-nucleation clusters that might arise in clusters up to 80 ions in size. Both anhydrous and hydrated clusters have been studied. A significant sample of the clusters were simulated, using molecular dynamics, to elucidate the metastability of these nanoclusters in water. An analogous study was conducted for hydrated clusters containing aspartate, to observe the effects of this amino acid on the structure and stability of candidate pre-nucleation clusters. Our results suggest that pre-nucleation cluster stability is a balance between ionic coordination and ion hydration. We find that clusters are generally dynamic in the lower limit of stability, forming chains to which ions frequently aggregate or dissolve. Larger calcium carbonate clusters retain a higher level of coordination in solution but swell to maximise hydration. The effect of additives on the structure and stability of clusters as a function of cluster size is intriguing, with trends in our data suggesting that aspartate can limit ion dissolution, but still allow for dynamic ordering and increasing ion hydration. Finally we find a bias in the ionic charge distributions for relatively dense clusters, indicating that these clusters sustain a negatively charged surface.

Isotopic yield measurement in the heavy mass region for239Pu thermal neutron induced fission

Despite the huge number of fission yield data available in the different evaluated nuclear data libraries, such as JEFF-3.1.1, ENDF/B-VII.0, and JENDL-4.0, more accurate data are still needed both for nuclear energy applications and for our understanding of the fission process itself. It is within the framework of this that measurements on the recoil mass spectrometer Lohengrin (at the Institut Laue-Langevin, Grenoble, France) was undertaken, to determine isotopic yields for the heavy fission products from the ${}^{239}$Pu(n${}_{th}$,f) reaction. In order to do this, a new experimental method based on $\ensuremath{\gamma}$-ray spectrometry was developed and validated by comparing our results with those performed in the light mass region with completely different setups. Hence, about 65 fission product yields were measured with an uncertainty that has been reduced on average by a factor of 2 compared to that previously available in the nuclear data libraries. In addition, for some fission products, a strongly deformed ionic charge distribution compared to a normal Gaussian shape was found, which was interpreted as being caused by the presence of a nanosecond isomeric state. Finally, a nuclear charge polarization has been observed in agreement, with the one described on other close fissioning systems.

Propensities in the solvation of M+–Benzene systems (M = Na, K, Rb) investigated by cluster dynamics

The study of size-specific interactions of alkali ions (M+) with aromatic compounds is crucial to understand the mechanisms governing the selectivity in protein channels. In particular, the investigation of the aqueous solvent effect on M+–π systems is of fundamental importance. The related processes are typically governed by several intermolecular interaction contributions as hydrogen bonds, dispersion, induction and electrostatics, which are often weak and difficult to evaluate in detail. In the present paper, the behavior of the M+–Benzene (M=Na, K, Rb, Benzene=Bz) aggregates surrounded by water molecules is analyzed performing molecular dynamics (MD) simulations. As the accuracy of such simulations depends on the reliability of the used intermolecular potential energy formulation we adopt a potential model based on a combination of electrostatic and non electrostatic components, whose reliability has been previously tested on some prototype systems by comparing predictions of the model with both accurate ab initio calculations and/or high level experimental data, has been used. The non electrostatic component has been described as sum of improved Lennard Jones (ILJ) functions, whose parameters have been derived from polarizabilities of atoms, groups of atoms and/or molecules. The electrostatic contribution has been calculated as a sum of Coulombic potentials arising from the interaction between permanent ion charge and/or permanent molecular charge distributions, which, at long range, for M+–Bz, H2O–Bz and M+–H2O reproduces the ion-quadrupole, dipole-quadrupole and the ion-dipole interactions, respectively. Energetics and structure of the clusters are found to depend on the competition between ion and benzene solvations. MD results show that while the solvation of the ion originates a diminution of the M+–Bz interaction, that of the aromatic compound enhances its interaction with the ion. Moreover, it has been found that such behavior is size-specific dependent.

Splitting of type-I (N-B, P-Al) and type-II (N-Al, N-Ga) donor-acceptor pair spectra in 3C-SiC

Discrete series of lines have been observed for many years in donor-acceptor pair (DAP) spectra in 3C-SiC. In this work, the splitting of both type-I (N-B, P-Al) and type-II (N-Al, N-Ga) DAP spectra in 3C-SiC has been systematically investigated by considering the multipole terms. For type-I spectra, in which either N or B substitutes on C sites or P and Al replace Si, the splitting energy of the substructure for a given shell is almost the same for both pairs. For type-II spectra, in which N is on the C site while Al and Ga acceptors replace Si, we find that, when compared with literature data, the splitting energy for a given shell is almost independent of the identity of the acceptor. For both type-I and type-II spectra, this splitting energy can be successfully explained by the octupole term V-3 alone with k(3)=-2x10(5) angstrom(4) meV. Comparing the experimental donor and acceptor binding energies with the values calculated by the effective-mass model, this suggests that the shallow donor (N, P) ions can be treated as point charges while the charge distribution of the acceptor ions (Al, Ga, B) is distorted in accord with the T-d point group symmetry, resulting in a considerable value for k(3). This gives a reasonable explanation for the observed splitting energies for both type-I and type-II DAP spectra.

Phonon dispersions and piezoelectricity in bulk and multilayers of hexagonal boron nitride

A unified theory of phonon dispersions and piezoelectricity in bulk and multilayers of hexagonal boron nitride (h-BN) is derived. The dynamical matrix is calculated on the basis of an empirical force constant model of intralayer valence and interlayer van der Waals interactions. Coulomb interactions are calculated by Ewald's method, adapted for the three-dimensional (3D) and the multilayer case. The deformation of the ionic charge distribution with long-wave lattice displacements is taken into account. Special attention is devoted to the nonanalytic long-range Coulomb contribution to the dynamical matrix which is different for the 3D crystal and the multilayer case. Consequently there is a splitting of the transverse optical (TO) and longitudinal optical (LO) phonon branches of ${E}_{1u}$ symmetry and a discontinuity of the ${A}_{2u}$ branch at the $\ensuremath{\Gamma}$ point in 3D h-BN. No such splitting and discontinuity at $\ensuremath{\Gamma}$ are present in multilayer crystals with a finite number $\mathcal{N}$ of layers. There a diverging bundle of $\mathcal{N}$ overbending optical phonon branches emerges from $\ensuremath{\Gamma}$. Born's long-wave theory is applied and extended for the study of piezoelectricity in layered crystals. While 3D h-BN and h-BN multilayers with an even number of layers (symmetry ${D}_{6h}$) are not piezoelectric, multilayers with an uneven number of ${\mathcal{N}}_{u}$ layers (symmetry ${D}_{3h}$) are piezoelectric; the piezoelectric coefficient ${e}_{1,11}$ is inversely proportional to ${\mathcal{N}}_{u}$.

Molecular Simulation of the Interaction between Imidazole Glycerol Phosphate Dehydrase and Nitrogen-Containing Heterocyclic Phosphate Inhibitors

A series of nitrogen-containing heterocyclic compounds as imidazole glycerol phosphate dehydrase(IGPD) inhibitors were successfully screened based on IGPD substrates;however,the mechanism is not clear.In this study,the B3LYP density functional theory method with the 6-31G** basis set as implemented in the Gaussian 03 program was selected to optimize the nitrogen-containing heterocyclic phosphates.These complex structures were constructed using molecular docking and optimization.The mode of interaction was discussed with regards to their electronic structures(frontier orbital energies and composition,the atomic charges,the natural bond orbital,etc.) and complex spatial structures(recognition functional domains of the inhibitor of IGPD,intermolecular hydrogen bonding,van der Waals interactions,etc.).The charge distribution of the nitrogen-containing heterocycle,the phosphate ion charge distribution,and the frontier orbital LUMO energy levels of the inhibitor were determined to be intrinsic factors that affect inhibitor activity.The conclusion of our study will provide valuable information for the screening and optimization of new herbicides targeted at IGPD.

Energy and charge distributions of fast hydrogen ions and atoms reflected from Cu in the case of grazing incidence

The energy and charge distributions of protons and hydrogen atoms reflected from the Cu surface in the case of grazing incidence angles are measured at energies of incident particles (H+ and H0) of 200 and 250 keV. The charged fractions of reflected particles are analyzed. A weak dependence of the neutral fraction of reflected particles on the scattering angle is discovered for incidence angles of 1°–2° and an energy of scattered particles of 60 keV or less. It is shown that the neutral fraction of reflected particles with an energy of 60–80 keV or more is independent of the scattering angle and is determined by the ratio of the cross sections for the electron capture and loss by ions in the material.

Nanoporous Carbon Supercapacitors in an Ionic Liquid: A Computer Simulation Study

Supercapacitors composed of carbon nanotube (CNT) micropores in the room-temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4(-)) are studied via molecular dynamics (MD)computer simulations. It is found that the distribution of RTIL ions inside the micropore varies significantly with the pore size. Internal solvation of small (6,6) and (7,7) CNTs with an electrified interior wall is effected almost exclusively via counterions. Surprisingly, these counterions, even though they all have the same charge, lead to a charge density characterized by multiple layers with alternating signs. This intriguing feature is attributed to the extended nature of RTIL ion charge distributions, which result in charge separation through preferential orientation inside the electrified nanotubes. In the case of larger (10,10) and (15,15) CNTs, counterions and coions develop multilayer solvation structures. The specific capacitance normalized to the pore surface area is found to increase as the CNT diameter decreases from (15,15) to (7,7). As the pore size further reduces from (6,6) to(5,5), however, the specific capacitance diminishes rapidly. These findings are in excellent agreement with recent experiments with carbon-based materials. A theoretical model based on multiple charge layers is proposed to understand both the MD and experimental results.

Characterization of ion emission of an extreme ultraviolet generating discharge produced Sn plasma

The ion emission of a Sn-based discharge produced extreme ultraviolet producing plasma is characterized with the combined use of different time-of-flight techniques. An electrostatic ion spectrometer is employed to measure the average charge distribution of the emitted Sn ions. A dedicated Faraday cup configuration is used to measure the total ion flux from the source for different discharge energies. High-energy Sn ions emitted by the plasma with energies up to 100 keV have been identified. The number of high-energy ions increases for higher electrical input energy into the plasma while the signal associated with the expanding plasma ions does not show such dependence. The ion energy distribution for a bulk of detected ions is calculated based on the Faraday cup measurements and compared with theoretical plasma expansion dynamics.

A molecular dynamics study of the influence of ionic charge distribution on the dynamics of a molten salt

The distribution of charge in an ion of a fused salt is known to be an important determinant of liquid dynamics. However, the details of this relationship remain poorly understood. We present the results of molecular dynamics simulations on a model molten salt system and show that changes in the distribution of ionic charge can have a profound effect on liquid dynamics. In particular, we observe complex relationships between the distribution of charge, the rate of ionic rotation, and the translational diffusion of ions in the liquid.

High-resolution spatial and temporal microscopy with intense-laser-induced rescattering electrons

Following the recent developments in intense laser techniques, ultrafast phenomena of the order of a few femtoseconds to attoseconds have been discussed. We are developing new methods for ultrafast imaging for transient atomic and molecular systems. We apply the recently developed quantitative rescattering theory together with a fitting procedure to extract effective potentials as well as elastic differential cross sections of the target ions with free electrons within the single active electron model. Using experimental photoelectron spectra for rare gas atoms of Ne, Ar, Kr, and Xe, we show that the extracted atomic potentials are in good agreement with those obtained theoretically. The current method of retrieval does not require precise knowledge of the peak laser intensities. The results show that accurate charge distribution of target ions indeed can be retrieved from experimental photoelectron spectra generated by lasers, thus paving the way for using infrared laser pulses for dynamic chemical imaging of transient states of molecules with temporal resolution of few femtoseconds.

EXPERIMENTAL INVESTIGATIONS OF ION CHARGE DISTRIBUTIONS, EFFECTIVE ELECTRON DENSITIES, AND ELECTRON-ION CLOUD OVERLAP IN ELECTRON BEAM ION TRAP PLASMA USING EXTREME-ULTRAVIOLET SPECTROSCOPY

Spectra in the extreme ultraviolet range from 107 to 353 A emitted from Fe ions in various ionization stages have been observed at the Heidelberg electron beam ion trap (EBIT) with a flat-field grating spectrometer. A series of transition lines and their intensities have been analyzed and compared with collisional-radiative simulations. The present collisional-radiative model reproduces well the relative line intensities and facilitates line identification of ions produced in the EBIT. The polarization effect on the line intensities resulting from nonthermal unidirectional electron impact was explored and found to be significant (up to 24%) for a few transition lines. Based upon the observed line intensities, relative charge state distributions (CSD) of ions were determined, which peaked at Fe23+ tailing toward lower charge states. Another simulation on ion charge distributions including the ionization and electron capture processes generated CSDs which are in general agreement with the measurements. By observing intensity ratios of specific lines from levels collisionally populated directly from the ground state and those starting from the metastable levels of Fe XXI, Fe X and other ionic states, the effective electron densities were extracted and found to depend on the ionic charge. Furthermore, it was found that the overlap of the ion cloud with the electron beam estimated from the effective electron densities strongly depends on the charge state of the ion considered, i.e. under the same EBIT conditions, higher charge ions show less expansion in the radial direction.

Pendular proteins in gases and new avenues for characterization of macromolecules by ion mobility spectrometry

Polar molecules align in electric fields when the dipole energy (proportional to field intensity E x dipole moment p) exceeds the thermal rotational energy. Small molecules have low p and align only at inordinately high E or upon extreme cooling. Many biomacromolecules and ions are strong permanent dipoles that align at E achievable in gases and room temperature. The collision cross-sections of aligned ions with gas molecules generally differ from orientationally averaged quantities, affecting ion mobilities measured in ion mobility spectrometry (IMS). Field asymmetric waveform IMS (FAIMS) separates ions by the difference between mobilities at high and low E and hence can resolve and identify macroion conformers based on the mobility difference between pendular and free rotor states. The exceptional sensitivity of that difference to ion geometry and charge distribution holds the potential for a powerful method for separation and characterization of macromolecular species. Theory predicts that the pendular alignment of ions in gases at any E requires a minimum p that depends on the ion mobility, gas pressure, and temperature. At ambient conditions used in current FAIMS systems, p for realistic ions must exceed approximately 300-400 Debye. The dipole moments of proteins statistically increase with increasing mass, and such values are typical above approximately 30 kDa. As expected for the dipole-aligned regime, FAIMS analyses of protein ions and complexes of approximately 30-130 kDa show an order-of-magnitude expansion of separation space compared with smaller proteins and other ions.

Disparate Behavior of Carbonyl and Thiocarbonyl Compounds: Acyl Chlorides vs Thiocarbonyl Chlorides and Isocyanates vs Isothiocyanates

The reaction of benzoyl chloride with methanol catalyzed by pyridine is 9 times more rapid than is the same reaction with thiobenzoyl chloride. The difference in reactivity, as well as the dealkylation reactions that occur when the reaction of thiobenzoyl chloride is catalyzed by bases such as Et(3)N, can be understood in terms of the charge distributions in the intermediate acylammonium ions. The reaction of PhNCO with ethanol occurs at a much higher rate (4.8 x 10(4)) than that of PhNCS, corresponding to a difference in activation free energies for the additions of 6 kcal/mol. Transition states for each of these reactions were located, and each involves two alcohol molecules in a hydrogen bonded six-membered ring arrangement. Information concerning differences in reactivity was derived from analysis of Hirshfeld atomic charge distributions and calculated hydrogenolysis reaction energies.

Correlation effects, image charge effects and finite size in the macro-ion-electrolyte system: A field-theoretic approach

We consider a model of a macro-ion surrounded by small ions of an electrolyte solution. The finite size of ionic charge distributions of ions, and image charge effects are considered. From such a model it is possible to construct a statistical field theory with a single fluctuating field and derive physical interpretations for both the mean field and two-point correlation function. For point-like charges, at the level of a Gaussian (or saddle point) approximation, we recover the standard Poisson-Boltzmann equation. However, to include ionic correlation effects, as well as image charge effects of individual ions, we must go beyond this. From the field theory considered, it is possible to construct self-consistent approximations. We consider the simplest of these, namely the Hartree approximation. The Hartree equations take the form of two coupled equations. One is a modified Poisson-Boltzmann equation; the other describes both image charge effects on the individual ions, as well as correlations. Such equations are difficult to solve numerically, so we develop an (a WKB-like) approximation for obtaining approximate solutions. This, we apply to a uniformly charged rod in univalent electrolyte solution, for point like ions, as well as for extended spherically symmetric distributions of ionic charge on electrolyte ions. The solutions show how correlation effects and image charge effects modify the Poisson-Boltzmann result. Finite-size charge distributions of the ions reduce both the effects of correlations and image charge effects. For point charges, we test the WKB approximation by calculating a leading-order correction from the exact Hartree result, showing that the WKB-like approximation works reasonably well in describing the full solution to the Hartree equations. From these solutions, we also calculate an effective charge compensation parameter in an analytical formula for the interaction of two charged cylinders.

A Numerical Model for Ion Charge Distribution of Plasmas in Collisional Radiative Steady State

A numerical model for the charge state distribution of plasmas in a collisional radiative steady state (CRSS) is established by averaging over the atomic process rate coefficients in universal kinetic equations. It is used to calculate the mean ion charge and ion population for a given temperature and density of the plasmas, ranging from low Z to high Z elements. The comparisons of the calculated results with those of other non-local thermodynamic equilibrium kinetics codes show that this model possesses acceptable precision. Furthermore, the NLTE effects are investigated by virtue of the model, and the differences between CRSS and LTE models for low density plasmas are quite evident.

Charge exchange of medium energy H and He ions emerging from solid surfaces

Charge exchange of medium energy H and He ions emerging from clean solid surfaces is studied extensively using a toroidal electrostatic analyzer with an excellent energy resolution. The charge distributions of He ions scattered from sub-monolayers near a surface are non-equilibrated, resulting in a surface peak even for poly-crystal solids. By solving simultaneous rate equations numerically, we derive electron capture and loss cross sections for Ni and Au surfaces. Based on a free electron gas model, non-equilibrated He+ fractions dependent on emerging angle reveals uniform electronic surfaces for metals and corrugated surfaces for Si and graphite with covalent bonds. It is also found that equilibrium charge fractions of H+ are independent of surface materials (Z2) and in contrast equilibrium He+ fractions depend pronouncedly on Z2. The data obtained are compared with semi-empirical formulas.

FIRST-PRINCIPLES STUDY OF THE EFFECTS OF GOLD ADSORPTION ON THE Al(001) SURFACE PROPERTIES

In this paper, we have studied theoretically the effects of gold adsorption on the Al (001) surface, using ab initio pseudo-potential method in the framework of the density functional theory. Having found the hollow sites at the Al (001) surface as the most preferred adsorption sites, we have investigated the effects of the Au adsorption with different coverages (Θ =0.11, 0.25, 0.50, 0.75, 1.00 ML) on the geometry, adsorption energy, surface dipole moment, and the work function of the Al (001) surface. The results show that even though the work function of the Au substrate increases with the Au coverage, the surface dipole moment decreases with the changes in coverage from Θ =0.11 to 0.25 ML. We have explained this behavior by analyzing the electronic and ionic charge distributions. Furthermore, by studying the diffusion of Au atoms into the substrate, we have shown that at room temperature the diffusion rate of Au atoms into the substrate is negligible but increasing the temperature to about 200°C the Au atoms significantly diffuse into the substrate, in agreement with the experiment.

Monte Carlo particle trajectory simulation for classification of C 3+ cluster ion structure utilizing Coulomb explosion imaging

A numerical estimation of the spatial structures of impinging clusters from the charge distribution of fragment ions measured in our experimental system has been performed using a simple Monte Carlo particle trajectory simulation. A calculation using the numerical results summarized in a 2D symmetric frame for the C 3 fragment ions provides a technique for evaluating the accuracy and condition of classification of cluster structures, i.e., triangle or linear chain, utilized in our measurement system.