Redistribution Of Electron Density Research Articles

First-principles simulation of h interacting with transition elements in molybdenum for nuclear material application

Based on first-principles simulations, we have studied systematically the interaction of transition elements (TEs) with hydrogen (H) in interstitial lattice and vacancy in molybdenum. The results indicated that the stable position of H can be remarkably affected by the TEs, which can be attributed to the valence electron density redistribution surrounding the TEs. The interactions between H and TEs display mainly the attraction, which is explained by the elastic mechanism and the Pauling electronegativity. The elastic mechanism mainly characterizes the H-TE interaction for the TE atoms with the small atomic volume and the large electronegativity in comparison with molybdenum, the opposite cases can be explained by the Pauling electronegativity viewpoint. The appearance of one H atom around the TE atom can affect negatively the binding of the second H. The positive binding energy among H, vacancy and TE can easily lead to the H-TE-vacancy cluster formation in molybdenum. The existence of the TE atom in the vicinity of vacancy can affect the interaction between H and vacancy, while the presence of the H atom around vacancy may influence the interplay between TE and vacancy. The present results give a detailed physical analysis on the interaction between H and TE as well as among H, vacancy and TE in molybdenum and could further help us design the future Mo-based material with regard to the choice of alloy composition for the consideration of the H retention.

Комбинированное квантово-классическое моделирование фотоиндуцированного перераспределения электронной плотности от биополимерных сегментов к фотохромным зондам

Комбинированное квантово-классическое моделирование фотоиндуцированного перераспределения электронной плотности от биополимерных сегментов к фотохромным зондам

Vacancy-defect-dipole amplifies the thermoacoustic conversion efficiency of carbon nanoprobes

The immense potential of carbon nanoprobes (CNPs) for using as contrast agents has propelled much recent research and development in the field of thermoacoustic (TA) molecular imaging, while the proper engineering and design of such materials with required high TA conversion efficiency is still a highly challenging task. In this work, we proposed a controllable strategy to amplify the TA conversion efficiency of the CNPs by constructing vacancy defect (VD) dipoles, and systematically demonstrated the amplification mechanism through theoretical and experimental investigations. First-principles calculation results indicate that, when a carbon atom is removed from the CNPs by chemical approach, owing to local electron density redistribution, the VDs are formed at the positions of the original carbon atoms and act as the structural origin of permanent electric dipoles with the dipole moment several orders higher than that of non-defect sites. Under pulsed microwave irradiation, the VD dipoles are polarized repeatedly and significantly contribute to the conversion efficiency from absorbed electromagnetic waves to ultrasound through enhanced dielectric relaxation losses. We experimentally synthesized graphene samples with different VD densities and VD types to demonstrate the efficiency of the proposed strategy, and results coincide well with the theoretical proposition. This work offers feasible guidance to the systematic development and rational design of new high-conversion-efficiency TA CNPs via VD engineering.

Transformation of electron density distribution induced by the cation point defects in uranium dioxide

The ab initio calculations on electronic structure of the large fragments of UO2 crystal lattice was performed using Molecular Dynamics and fully relativistic Discrete Variational method. The four types of clusters corresponding to stoichiometric crystal and to the crystals with isolated uranium vacancy and uranium interstitial impurity were considered. We investigated the transformation of valence and vacant bands and the nature of chemical bonding. The estimation of electron density redistribution among atoms of several coordination spheres around defects was performed. In contrast to previous ab initio and semi-empirical calculations, our model included the point symmetry of defects and the possibility of charge transfer between defects and all nearest and next nearest neighbors. We obtained that the deformation of crystal structure and the covalent interactions minimizes (or completely prevent) the electron density transfer between atoms in the vicinity of defects.

On the Theory of Initiating Effect of Continuous Heating on Structural Transformations and Phase-Transition Temperature in Crystals

A model and kinetic theory of structural changes in crystals upon continuous heating are proposed; this theory takes into account nonadiabatic atomic transitions between different states of the system of nuclei and electrons. The nonadiabatic transitions initiate structural transformations determined by a change in the short-range order during the electron-density redistribution. It is shown that continuous heating stimulates structural changes in a crystal and reduces the temperature of the known phase transition.

Reverse Fountain and the Nighttime Enhancement in the Ionospheric Electron Density Over the Equatorial Region: A Case Study

AbstractThis paper focuses on the linkage between reverse fountain effect and anomalous enhancement in the nighttime electron density over the equatorial ionosphere often referred as the ionospheric nighttime enhancement (INE). The Global Positioning System (GPS)‐derived total electron content (TEC) measurements over two Indian equatorial stations and ionosonde measurements are used to investigate the occurrence characteristics and mechanism of the premidnight and postmidnight INE events. The high‐resolution spatiotemporal two‐dimensional (2D) TEC maps generated by using satellite‐based augmentation system are used to visualize the reverse fountain effect. These TEC maps are found to be effective in envisaging the motion of equatorial ionization anomaly (EIA) in nighttime ionosphere and its impact on the distribution of electron density over the equatorial region. Salient features of this case study illustrate that, accompanying enhancements in the nighttime TEC, there exist decrease in the F2 layer height, prevalence of downward vertical plasma drift, and dramatic increase in the F2 region electron density. The 2D‐TEC maps reveal the persistence of well‐developed EIA until late evening hours and its gradual shift toward equator during the events of postmidnight INEs. It is found out that the premidnight INE is not invariably linked to the prereversal enhancement of the zonal electric field and could be associated with the redistribution of electron density caused by the reverse fountain effect. The present observations indicate that although the westward electric field serves as a prime driver, the INEs over the equatorial regions are associated with plasma motions caused by the retreating anomaly.

Observation of the molecular response to light upon photoexcitation

When a molecule interacts with light, its electrons can absorb energy from the electromagnetic field by rapidly rearranging their positions. This constitutes the first step of photochemical and photophysical processes that include primary events in human vision and photosynthesis. Here, we report the direct measurement of the initial redistribution of electron density when the molecule 1,3-cyclohexadiene (CHD) is optically excited. Our experiments exploit the intense, ultrashort hard x-ray pulses of the Linac Coherent Light Source (LCLS) to map the change in electron density using ultrafast x-ray scattering. The nature of the excited electronic state is identified with excellent spatial resolution and in good agreement with theoretical predictions. The excited state electron density distributions are thus amenable to direct experimental observation.

The effect of contact resistance on electrical conductivity in filled elastomer materials

This paper is focused on studying the charge transport between the particles in filled elastomer composites, and suggests a new modeling approach that would allow predicting the electrical conductivity of this material. Our approach is based on a combination of non-equilibrium Green's function (DFTB-NEGF) analysis for atomic-level electron quantum transport, and effective medium theory for calculating the said macroscopic transport properties of the material. The model can be used for predicting macroscopic material conductivity based on the information of material microstructure received by electron microscopy; explain changes of conductivity in deformed material and with temperature variation, and finally it can assist in finding the optimal material parameters such as particle shape, size distribution, and volume fraction to maximize the performance. The results of our modeling re-attest the known phenomenon that the electrical conductivity in percolated network is always dominated by direct particle-to-particle charge transport. We studied the electrical conductivity in a medium of high volume fraction of metallic particles dispersed in polymer matrix where thin polymer matrix layer covalently attached to the metallic particle contacts, and the thin polymer layer undergoes high local shear strain when hard particles slide along each other during deformation. These conditions allow the redistribution of electron density from metal to the polymer, and the conductance through normally insulating matrix can become comparable with the pinpoint conductance through the metal. These conditions can only occur in the place of direct particle contacts, therefore all criteria for percolated conductive network hold valid in this case. The integral conductance of thin “sandwiched” polymer layer strongly, but predictably depends on local contact geometry, mostly on particle surface curvature at place of contact. This allows predicting composite conductivity from the properties of each phase, loading, and particle shape and size distributions. Further, the conductance of polymer chain can significantly increase with the fluctuations of molecular shape caused by temperature and/or deformation. We infer that this effect is responsible for the major part of strain and temperature dependence of the conductivity that has been reported in the literature, although particle orientation and stacking at larger deformations may also contribute to the strain dependence.

A conceptual DFT analysis of the plausible mechanism of some pericyclic reactions

Out of several pericyclic reactions, Diels-Alder (DA) reaction is one of the most widely used synthetic processes. In the present work, several models and methodologies have been used to determine and to analyze the plausible mechanism of some representative DA cycloaddition reactions. A comparison between the dual descriptor and the bond reactivity indices corresponding to the natural bond orbital of the reagents is included, which provides a complete description of the plausible reaction mechanism. In the next step, two very recent models are used to determine the local electronic density transfer and redistribution between the reactants involved. The description of the local electronic density transfer has been made in two stages; first, the variation in the net charge on the atoms is obtained, and then, the electronic density transfer between the natural bond orbitals is calculated. The values obtained using the two models are correlated with the experimental rate constants of the reactions. Finally, the natural bond orbitals are obtained at several steps along the reaction path and the variation in their partial occupation is compared with the corresponding electron density transfer among these orbitals. Furthermore, frontier molecular orbital (FMO) approach has been employed to understand the more feasible way of interaction between the DA pair. Relative electrophilicity descriptors like net electrophilicity (∆ω±), net reactivity index (NRI, Δ $$ {\omega}_R^{\pm } $$ ), and electrophilicity difference (∆ω) between DA pairs have also been employed to describe the studied reaction mechanisms especially whether they follow non-polar-concerted/polar-stepwise pathway along with their classification in terms of normal or inverse electron demand. Furthermore, adaptive natural density partitioning method (AdNDP) and energy decomposition analyses (EDA) in conjunction with natural orbital for chemical valence (NOCV) have been made use of in order to analyze the actual bonding situation in the transition state (TS).

Creating Competitive Active Sites on CNTs Walls by N‐Doping and Sublayer Co4N Encapsulating for Efficient Hydrogen Evolution Reaction

AbstractAn excellent catalyst can be effectively designed by adjusting its electronic structure. In this paper, an efficient hydrogen evolution reaction (HER) catalyst, carbon nanotubes with N doping and sublayer Co4N encapsulating (N−CNTs@Co4N) supported on Ni foam (NF), was designed and prepared through a complementary electronic structure principle. The as‐prepared N−CNTs@Co4N@NF catalyst shows outstanding hydrogen evolution activity in 1.0 M KOH solution. DFT calculation indicated that carbon nanotubes wall of N−CNTs@Co4N@NF catalyst provided more competitive catalytic active sites for hydrogen evolution reaction than N doped CNTs and Co4N singly. The greatly improved catalytic ability was owing to the redistribution of electron density for N−CNTs after Co4N embeds in, leading to a moderate absorption energy of H. This study demonstrated the successful rational design and experimental verification on developing a novel non‐noble metal catalyst.

Solitonic-like excitations in cations of linear conjugated systems

A quantum-chemical study of the atomic charges and bond orders in the cations of the linear conjugated systems was performed. It is shown that total charge in the collective system of the π-electrons generates the soliton-like wave of the alternated partial charges along the conjugated chain not only in ground state but also in the excited state. The excitation is accompanied by the change of the soliton phase and the wave dimension. Additionally, it is established that the electron density redistribution at the atoms and bonds also forms the soliton-like wave. In paper, the dependence of the solitonic wave shape on the dimension and section of the polymethine is studied; established regularities in the charge distribution in excited state could be used for the molecular design of organic semiconducting materials.

SMALL-ANGLE SCATTERING OF NEUTRONS ON Sr<sub>2</sub>FeMoO<sub>6–δ</sub> SAMPLES WITH DIFFERENT-DEGREE SUPERSTRUCTURAL ORDERING OF Fe/Mo CATIONS

Single-phase Sr2FeMoO6-δ samples with different-degreesuperstructiral ordering Fe/Mo cations superstructural ordering (P, 76, 86 and 93 %) were obtained by the solid-phase technique. Based on the results of measuring the magnetic characteristics in the samples, we found that an increase in magnetization (26.41, 32.36 and 42.66 A·m2·kg–1), magnetic moment (1.33, 3.07 and 3.58 μB/f.u.) and Curie temperatures (422, 428 and 437 K) withparameter P (76, 86 and 93 %) can be explained by the presence of antistructural defects, as well as antiferromagnetic inclusions. This determines the redistribution of electron density, which is accompanied by the change in electronic configuration of a part of Fe/Mo cations. Based on the temperature dependences of the magnetic moment of the samples measured in ZFC and FC modes, and on small-angle polarized neutron scattering (SANS), we found that the samples are in a magnetically inhomogeneous state. An important result to mention is that we detected the difference between the slope of the SANS curves of samples with different oxygen content, which demonstrates a different microstructure of inhomogeneities. The main inhomogeneities are magnetic inclusions with the dimensions depending on the superstructural ordering of Fe/Mo cations. According to the Porod law, it was shown that the Sr2FeMoO6-δ samples with wave vector values 0.1 > q > 0.002 Å–1 contain polydisperse grains with a smooth surface. For q > 0.1 Å–1 a deviation from the Porod law is observed, confirming the presence of magnetic inhomogeneities with a diameter < 6 nm in the grains.

Directional Change of Interfacial Electric Field by Carbon Insertion in Heterojunction System TiO2/WO3.

Z-scheme transfer is an ideal photocatalytic system with stronger redox ability, but its design and construction still lack understanding. Herein, the work function difference and the band bending are found to be the determining factors for the construction of the Z-scheme transfer mechanism of photoexcited charges in TiO2/WO3. The control of work function and band bending achieved by carbon insertion results from the hybridization of orbitals and redistribution of electron density, as demonstrated by ultraviolet photoelectron spectroscopy and photocatalytic analysis. The heterojunction system, TiO2/WO3, with controlled work function and band bending, shows 2 times faster •OH radical formation rate (0.011 μmol min-1) compared to the undisturbed system. First-principles calculation reveals that the changes in work function and band bending result in an interfacial electric field, which shifts the charge transfer mechanism from type II to Z-scheme. This work proves that the design of work function and band bending allows reconstructing charge transfer mechanism by forming the interfacial electric field in heterojunction systems.

Excited Electronic States in Total Isotropic Scattering from Molecules

Ultrafast X-ray scattering experiments are routinely analyzed in terms of the isotropic scattering component. Here, we present an analytical method for calculating total isotropic scattering for ground and excited electronic states directly from ab initio two-electron densities. The method is generalized to calculate the isotropic elastic, inelastic, and coherent mixed scattering. The presented computational results focus on the potential for differentiating between electronic states and the decomposition of the total scattering in terms of elastic and inelastic scattering. For the specific example of the umbrella motion in the first excited state of ammonia, we show that the redistribution of electron density along this coordinate leaves a comparably constant fingerprint in the total scattering that is similar in magnitude to the effect of changes in molecular geometry.

Ultrathin graphene encapsulated Cu nanoparticles: A highly stable and efficient catalyst for photocatalytic H2 evolution and degradation of isopropanol

Cu is a promising candidate for developing efficient and cost-effective catalysts for metal-induced photocatalysis. A highly desirable yet great challenge is the preparation of stable Cu-based photocatalysts, and its application in photocatalytic H2 evolution and photodegradation are desirable to explore. In this study, ultrathin graphene-coated metal Cu nanoparticles (NPs) are fabricated through a two-step pyrolysis process using MOFs as precursors and show remarkable stability even in an oxidative atmosphere. Benefiting from the nanostructure, the optimized Cu@C/SrTiO3 shows a photocatalytic H2 evolution rate of 255.3 µmol·g−1·h−1, and the apparent quantum efficiency (AQE) reaches 3.8% at 420 nm. The acetone evolution rate of Cu@C/SrTiO3 is approximately 21-fold higher than that of Cu/STO in photocatalytic isopropyl alcohol (IPA) degradation under visible light. Moreover, the graphene layer demonstrated a quasi-cocatalyst effect. Time-resolved photoluminescence and theoretical calculations reveal that the ultrathin graphene layer promotes charge separation and induces electron density redistribution over Cu@C, which distinctly boosts H2 evolution. This work provides an effective method to protect Cu NPs and a broader prospect for exploring non-precious Cu-based nanocomposites in photocatalytic conversion.

К теории инициирующего действия непрерывного нагрева на структурные перестройки и температуру фазового перехода в кристаллах

A model and kinetic theory of structural changes in crystals upon continuous heating are proposed. Nonadiabatic transitions of atoms between different states of a system of nuclei and electrons are taken into account. Non-adiabatic transitions initiate structural rearrangements determined by a change in the short-range order upon the redistribution of electron density. It was shown that continuous heating stimulates structural changes in the crystal and lowers the temperature of a known phase transition.

Complexation Features of N-Vinylsuccinimide in Monomer–Solvent and Monomer–Monomer Systems

The results of investigations of the complexation of N-vinylsuccinimide (VSI) in monomer–solvent and monomer–monomer systems and the effect of these processes on the kinetics of copolymerization of VSI with monometers of differing reactivity are summarized. Conditions for the synthesis of compositionally homogeneous copolymers were established. Taking into account the electronic structure of VSI and co-components, a mechanism of complexation in VSI–dimethylsulfoxide, VSI–acetic anhydride, and VSI–maleic anhydride systems is proposed. The electron density redistribution in the VSI molecule upon complexation has been quantitatively assessed.

Electronic and optical behaviour of lanthanum doped CaTiO3 perovskite

To improve the efficiency of perovskite based solar cells, doping of heavier elements in Perovskite materials (ABX3) can modulate its electronic and optical properties significantly. Thus it is important to understand the possible microscopic origin of the band gap modification and optical enhancement after heavier element doping using first-principles studies. Here we investigate the effect of La doping, while substituting the Ca atom, on the electronic and optical properties in CaTiO3 perovskite material using generalized gradient approximation within density functional theory. We observe a decrease in lattice constants and bond lengths in LaxCa1−xTiO3, mainly due to re-distribution of electronic charge density between La and Oxygen, as confirmed by charge density contour. We further notice a widening of electronic band gap and an upward shift of Fermi level into the conduction band, thus characterizing LaxCa1−xTiO3 as an n-type material. DOS diagram attributes this shift mainly due to the appearance of La p-DOS and d-DOS and their repulsion with N p-DOS, when La enters into the host lattice at Ca site. Investigation of optical properties upon La Doping in CaTiO3 exhibits further shifting of polarization and refractive index to lower values as compared to its pure counterpart, due to dominating semiconducting behavior and hence one observes a blue shift in absorption and reflection spectrum accordingly. Energy loss function is found to be consistent with absorption and extinction coefficient measured in case of LaxCa1−xTiO3. All these results are found to be consistent with the existing experimental and first-principles studies.

Charge Redistribution Effects in Hexacyanometallates Evaluated from XPS Data

The charge redistribution effects determine the physical and functional properties of transition metal hexacyanometallates. The π‐back bonding interaction between the CN ligand and the metal (M) linked at its C end and the σ donation from the N end to a second transition metal (T), when a coordination polymer is formed, are relevant features of such effects. The π‐back donation interaction contributes to the overlapping of the electron clouds of M and T metals in the ···–T–N≡C–M–C≡N–T‐··· chain. In this contribution, we demonstrate that XPS spectroscopy is a useful tool to sense the change in the electron density redistribution in hexacyanometallates related to these two mechanisms. The results herein discussed contribute to understand the physical and chemical properties of these materials. Initially, a series of potassium salts, Kn[M6–n(CN)6] (M = CrIII, FeII, RuII, OsII, CoIII, RhIII, IrIII, PtIV) is considered in order to evaluate the role of the π‐back donation on the electron density found at the N atom, which determines the strength for the T‐NC coordination bond and of the mentioned electron clouds overlapping. Then, a representative set of transition metal (T) hexacyanometallates, T2(1‐y)K2y[FeII(CN)6], is studied in order to shed light on the electron density redistribution when a coordination bond is formed at the N end. Within such coordination polymers, the copper (2+) salts are particularly attractive by the unique interaction between the copper atom and the hexacyanometallate ion, and the potential applications of these copper (2+) complexes in heterogeneous catalysis and other areas. From these reasons, the recorded XPS spectra for Cu (2+) hexacyanometallates were evaluated in detail. In accordance with a previous study of these copper (2+) coordination polymers using other techniques, XPS data is sensing an atypical strong bonding interaction with the CN ligand. The experimental XPS data herein discussed are relevant for the coordination chemistry of transition metal hexacyanometallates and for the research community involved in their applications.

O-doped graphdiyne as metal-free catalysts for nitrogen reduction reaction

Electrocatalytic nitrogen reduction reaction (NRR) is a sustainable and promising strategy for the conversion of N2 into NH3. However, electrocatalytic NRR is vitally dependent on metal-based catalysts, and it remains a huge challenge in achieving effective NRR on metal-free catalysts. Here we report that O-doped graphdiyne (GDY) as a metal-free NRR electrocatalyst in aqueous solution. O doping in the GDY results in the redistribution of electron density, where the sp-C atom nearest to the O atom provides enhanced capture to N2 molecules. The influence of surface strain on the progression of NRR on O-doped GDY surface has been investigated using the density functional theory (DFT) calculation method. The DFT calculations predict that the potential-limiting steps (PLS) do not vary with the strain, which is *NHNH2→*NH2NH2 for alternating mechanism and *NH2→*NH3 for distal mechanism under the strain range of -1%∼ +5%; the limiting potentials of alternating mechanism is higher than that of distal mechanism; the limiting potentials of two mechanisms decrease with the tensile strain. It can be inferred that the NRR activity increases with a tensile strain of 0%∼ +5%. Our study not only gets deep insights into the catalytic activity of O-doped GDY but also highlights the significance of strain engineering for catalyst design.