Terms Of Electron Density Research Articles

Electronic transport properties and CO adsorption characteristics on TiO2 molecular device – A first-principles study

The transport and CO adsorption properties on rutile TiO2 molecular device is studied using DFT method. The transport characteristics of TiO2 nanostructure are described in terms of electron density, density of states and transmission spectrum. The applied bias voltage in TiO2 device increases the peak maxima in the valence band as well as in the conduction band. The electron density in TiO2 nanostructure is found to be more along oxygen sites than in titanium sites. The transmission spectrum of TiO2 molecular device provides clear insights on transmission of electrons at various energy intervals. The adsorption properties of CO on TiO2 molecular device is studied in terms of voltage and resistance characteristics along the scattering region. The findings of the present study give a clear vision on improving the electronic transport and CO adsorption properties on TiO2 molecular device, which can be used as CO sensor.

Density functional theory computational study on solvent effect, molecular conformations, energies and intramolecular hydrogen bond strength in different possible nano-conformers of acetaminophen

The molecular modeling analysis via density functional theory has been performed on all possible conformers of acetaminophen. Computations performed at B3LYP/6-311++G(d,p) level using the Gaussian 09 program package. The polarizable continuum model method was used to investigate solvent effects on the electronic and structure properties. The optimized structures, total energies, energy gap, chemical potential, global hardness, softness, global electrophilicity and dipole moments were calculated. From 4 different obtained conformers, A1 is the most stable one with intramolecular C–H⋯O hydrogen bonding. The investigated structures contain intramolecular hydrogen bonded (HB) ring which was analyzed in detail by topological parameters in terms of electron density (ρ) and its Laplacian (∇2ρBCP) at the bond critical points and ring critical points from Bather's atom in molecular quantum theory. These parameters could be treated as measure of HB strength. Results show that the C–H⋯O and C–H⋯C bonds possess low ρ and non-covalent character (∇2ρBCP>0) whereas C–H and C–O have covalent character (∇2ρBCP<0). The energy of HB for the most stable conformer was −16.11kJ/mol. Charge density distributions and chemical reactive sites of studied conformers have been obtained by molecular electrostatic potential surface.

Bonding in molecular crystals from the local electronic pressure viewpoint

ABSTRACTThe spatial distribution of the internal pressure of an electron fluid, which spontaneously arises at the formation of a molecule or a crystal, is linked to the main features of chemical bonding in molecular crystals. The local pressure is approximately expressed in terms of the experimental electron density and its derivatives using the density functional formalism and is applied to identify the bonding features in benzene, formamide and chromium hexacarbonyl. We established how the spatial regions of compression and stretching of the electron fluid in these solids reflect the typical features of chemical bonds of different types. Thus, the internal electronic pressure can serve as a bonding descriptor, which has a clear physical meaning and reveals the specific features of the variety of the chemical bonds expressing them in terms of the electron density.

Site-Directed Mutagenesis of a Hyperthermophilic Endoglucanase Cel12B from Thermotoga maritima Based on Rational Design.

To meet the demand for the application of high activity and thermostable cellulases in the production of new-generation bioethanol from nongrain-cellulose sources, a hyperthermostable β-1,4-endoglucase Cel12B from Thermotoga maritima was selected for further modification by gene site-directed mutagenesis method in the present study, based on homology modeling and rational design. As a result, two recombinant enzymes showed significant improvement in enzyme activity by 77% and 87%, respectively, higher than the parental enzyme TmCel12B. Furthermore, the two mutants could retain 80% and 90.5% of their initial activity after incubation at 80°C for 8 h, while only 45% for 5 h to TmCel12B. The K m and V max of the two recombinant enzymes were 1.97±0.05 mM, 4.23±0.15 μmol·mg-1·min-1 of TmCel12B-E225H-K207G-D37V, and 2.97±0.12 mM, 3.15±0.21 μmol·mg-1·min-1 of TmCel12B-E225H-K207G, respectively, when using CMC-Na as the substrate. The roles of the mutation sites were also analyzed and evaluated in terms of electron density, hydrophobicity of the modeled protein structures. The recombinant enzymes may be used in the hydrolysis of cellulose at higher temperature in the future. It was concluded that the gene mutagenesis approach of a certain active residues may effectively improve the performance of cellulases for the industrial applications and contribute to the study the thermostable mechanism of thermophilic enzymes.

Improving approximate determination of the noninteracting electronic kinetic energy density from electron density

This work describes a new approach for approximate obtaining the positively defined electronic kinetic energy density (KED) from electron density. KED is presented as a sum of the Weizsäcker KED, which is calculated in terms of electron density exactly, and unknown Pauli KED. The latter is presented via local Pauli potential and Gritsenko–van Leeuwen–Baerends kinetic response potential, to which the second‐order gradient expansion is applied. The resulting expression for KED contains only one empirical parameter. The approach allowed to correctly reproduce all the features of KED, and electron localization descriptors as electron localization function and phase‐space defined Fisher information density for main types of bonds in molecules and molecular crystals. It is also demonstrated that the method is immediately applicable to derivation of mentioned bonding descriptors from experimental electron density. Herewith the method is significantly free from the drawback of Kirzhnits approximation, which is now commonly accepted for evaluation of the electronic kinetic energy characteristics from precise X‐ray diffraction experiment. © 2015 Wiley Periodicals, Inc.

정전 탐침법과 유체시뮬레이션을 이용한 DC플라즈마 특성 연구

Using a parallel plate DC plasma system was prepared. Using this equipment, we investigated the basic discharge characteristics of DC argon plasma in terms of electron density, temperature, voltage and current waveforms and plasma potential. The effects of the electrode gap distance, input voltage, ballast resistance and pressure were measured using electrostatic probe. Plasma simulation using fluid approximation has been performed. External circuit effects was included in the simulation. Measured and calculated current voltage characteristics show similar tendencies.

Halogen–halogen interactions in perhalogenated ethanes: An interacting quantum atoms study

In this paper, intramolecular halogen–halogen interactions in perhalogenated ethanes CX3–CX′3 (X, X′=F, Cl, Br) were investigated using Bader’s theory of Atoms in Molecules along the potential energy surface defined by the XCCX′ torsion angle. Besides examining the topology of the electron density in terms of critical points, we made use of Pendás’ interacting quantum atoms energy decomposition scheme in order to cast light on the nature of those interactions. Previous conclusions on the important role of the exchange interaction energy in such non-covalent interactions were confirmed, the presence or absence of bond critical points between halogens being rationalized in terms of the competition between primary and secondary exchange channels. Moreover, additional information was inferred from the source function that enables to quantify the role of distant atoms. Finally, we assessed the influence of the chosen computational protocol (basis set and dispersion correction) on the reported energetic decomposition.

Bonding and spectroscopic properties of complexes of SO2–O2and SO2–N2and its atmospheric consequences

van der Waals complexes of sulfur dioxide (SO2) with oxygen (O2) and nitrogen (N2) have been investigated by using MP2 and aug-cc-pVXZ (X = D, T) basis set. Two minimum structures with symmetry C1 and Cs have been located at the intermolecular potential energy surface (IPES) of the complex of SO2-O2. Stacked Cs structure of SO2-O2 is found to have greater stability than C1 structure. In the case of SO2-N2, one minimum structure with Cs symmetry has been characterized. In this study, CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ and interaction energy calculation at complete basis set (CBS) limit has been employed for better energetic description. The natural bond orbital (NBO) calculation demonstrates the bonding in terms of charge transfer from X-atom lone pair of X2 (X = O or N) to the antibonding SO orbital of SO2. The strength of various intra and inter bonds in the complexes were calculated in terms of electron density at bond critical points (BCP) using quantum theory of atoms in molecules (QTAIM). Frequency calculations for these complexes show a number of interactions induced by low frequency modes in the far IR region. Symmetry adapted calculation were also computed for the complexes and is established that the ratio of dispersion to induction effect is large for the most stable conformers. The atmospheric implications are also discussed for these complexes.

Mechanism of oxygen transfer from the vanadium(V) complexes with ligands O2 2− and O3 2−

The oxygen transfer from the vanadium(V) complexes containing peroxo, oxo, and ozonide ligands to electron-donating substrates (ethylene and vinyl methyl ether) was studied in the framework of the DFT with the M06 hybrid functional. The O atom of the ozonide fragment in the O3VO(η-O2)− complex is transferred to the electron-donating substrates with lower energy expenses than the O atom of the peroxo group of the isomeric triperoxo complex, in spite of the fact that the presence of the peroxo group in the ozonide complexes stabilizes it favoring the formation of the V-Oc bond according to the data of topological analysis of the electron density in terms of the theory of atoms in molecules (AIM). The calculation results are in agreement with the experimental kinetic data indicating somewhat lower reactivity of triperoxovanadate compared to the isomeric oxygenyl complexes.

Helicon Plasma Discharge for Processing Wall Material

A helicon wave plasma (HWP) discharge with strong magnetic field was investigated. The HWP was produced with an internal Nagoya III antenna that is perpendicular to the magnetic field and driven by a 13.56 MHz radio-frequency (RF) source. HWP was characterized in terms of electron density, electron temperature and plasma potential using a single Langmuir probe in Ar gas. The result of Langmuir probe shows that the electron density increases with RF power, but saturate above 700 W at a density of 2×1019 m-3. Scanning electron microscopy (SEM) was extensively used to characterize the quality of the graphite surface. The result of SEM shows the surface of graphite that exposed to plasma processing has exhibited smoother and compacter surface topography. Meanwhile, the concentration of impurity on the graphite surface decreases with plasma processing.

Removal and transformation of pharmaceuticals in wastewater treatment plants and constructed wetlands

Abstract. Since trace organic compounds such as pharmaceuticals in surface water have been a relevant threat to drinking water supplies, in this study removal of pharmaceuticals and transformation of pharmaceuticals into metabolites were investigated in the main source of micropollutants such as WWTPs and engineered constructed wetlands. Pharmaceuticals were effectively removed by different WWTP processes and wetlands. Pharmaceutical metabolites with relatively low log D value were resulted in the low removal efficiencies compared to parent compounds with relatively high log D value, indicating the stability of metabolites. And the constructed wetlands fed with wastewater effluent were encouraged to prevent direct release of micropollutants into surface waters. Among various pharmaceuticals, different transformation pattern of ibuprofen was observed with significant formation of 1-hydroxy-ibuprofen during biological treatment in WWTP, indicating preferential biotransformation of ibuprofen. Lastly, transformation of pharmaceuticals depending on their structural position was investigated in terms of electron density, and, the electron rich C1 = C2 bond of carbamazepine was revealed as an initial transformation position.

SU-E-J-182: Validation of Two Mathematical Formalisms for Tissue Characterization in Dual Energy Computed Tomography

Purpose: While dose calculations are typically performed using a simplistic correspondence of HU to electron density (ED), recent developments in DECT for radiotherapy could provide significant improvements in characterizing human tissues for such purpose. We aim to compare and validate two DECT mathematical formalisms and evaluate their accuracy in terms of ED and effective atomic number (Z) for radiotherapy applications. Methods: Two cylindrical phantoms (Catphan 504 and Gammex 467) containing tissue substitutes are scanned with a Philips Gemini GXL CT at 90, 120 and 140 kV. Two mathematical formalisms are developed and implemented using MATLAB, allowing the extraction of ED and effective Z maps of various materials, given a pair of CT images taken at two distinctive energies. The first formalism is based on a parameterization of XCOM cross sections and uses generic photon spectra provided by the manufacturer. The second formalism is based on a stoichiometric calibration of HU and uses experimental data and the substitutes' composition. A novel definition of effective Z is developed for both formalisms. Results: With the 90–120 kV energy pair, the extraction of relative ED of the Catphan materials leads to a maximum relative error of 5% for the XCOM-based formalism and 2% for the stoichiometric-based formalism. In the instance of the Gammex materials, higher density materials, as bones, present errors up to 31% and 15% respectively. Conclusion: While the stoichiometric-based formalism demonstrates a clear advantage over the XCOM-based formalism in the analysis of CT data acquired clinically, both yield reasonable accuracy for low-Z elements materials. Conversely, the results for high-Z materials are negatively affected by discontinuities present in photoelectric effect cross sections. An advanced formalism, which would precisely parameterize this effect, is expected to yield improvements in accuracy and lead the way to a successful implantation of DECT in radiotherapy treatment planning.

Synthesis and cyclopolymerization of diallylammoniomethanesulfonate

N,N‐Diallylammoniomethanesulfonate, a new zwitterionic monomer having a sulfobetaine (SB) motif, has been synthesized by reacting diallylamine with formaldehyde and sodium bisulfite. While the monomer underwent cyclopolymerization to give the unstable polysulfobetaine (PSB) in low yields, copolymerization with sulfur dioxide gave the stable PSB/SO2 copolymer in excellent yields. The polymers, as a consequence of having unquenched nitrogen valency in the repeating units, are pH‐responsive. The PSB/SO2 copolymer, upon treatment with sodium hydroxide, was converted into a water‐soluble anionic polyelectrolyte (APE). The strong zwitterionic interactions inherent in the PSB/SO2 made it insoluble in salt‐free water; the presence of large concentration (&gt;3M) of low molecular weight salts (e.g., NaCl, KI etc,) was required to neutralize the intragroup zwitterionic interactions and promote dissolution. The PSB/SO2 represents the first example of a sulfobetaine polymer having a single methylene spacer separating the charge centers. The stability of the copolymer is explained in terms of electron density on the nitrogens. POLYM. ENG. SCI., 53:2378–2388, 2013. © 2013 Society of Plastics Engineers

QTAIMC study of the X–H/H⋯O bond order indices (X = O, N, C) in molecular systems

The O–H/H⋯O bond order indices are evaluated at the B3LYP level using the Quantum Theory of Atoms in Molecules and Crystals (QTAIMC). The parameters of bond critical points, such as electron density, principal electron-density curvatures, kinetic and potential energy densities, are calculated for 32 molecular systems comprising of 40 covalent O–H bonds and 69 hydrogen H⋯O bonds of different strength. Different models for the bond order index are tested using the Cioslowski–Mixon approach as a standard. A uniform description of the bond order indices for the covalent and H-bonds in the O–H⋯O fragment, ranging from 0.97 to 2.91 Å, is achieved using the electron density and electronic kinetic energy density features. The model describing the H-bond indices of the X–H⋯O fragment (X = O, N and C) in terms of electron density and potential energy density is also suggested.

Stagnation layers at the collision front between two laser-induced plasmas: A study using time-resolved imaging and spectroscopy

When two dense laser-induced plasmas collide a stagnation layer may form at the interface region. The degree of stagnation depends on the so-called collisionality parameter given by the ratio of the geometric scale length of the experiment (in effect the initial plasma–plasma separation) to the ion–ion mean free path. Our experiments confirm that both target geometry and elemental composition strongly influence the collision process and degree of stagnation. In particular, we show that moving from flat to wedge-shaped targets and from aluminium to calcium results in a decrease in the value of the collisionality parameter accompanied by a concomitant reduction in the degree of stagnation. Time-resolved emission imaging was employed to track the size and shape of the stagnation layer which acts as a signature of hard versus soft stagnation. Characterization of the stagnation layer in terms of electron density and temperature has been performed using optical emission spectroscopy.

Incommensurate magnetic order and phase separation in the two-dimensional Hubbard model with nearest- and next-nearest-neighbor hopping

We consider the ground-state magnetic phase diagram of the two-dimensional Hubbard model with nearest- and next-nearest-neighbor hopping in terms of electronic density and interaction strength. We treat commensurate ferromagnetic and antiferromagnetic as well as incommensurate (spiral) magnetic phases. The first-order magnetic transitions with changing chemical potential, resulting in a phase separation (PS) in terms of density, are found between ferromagnetic, antiferromagnetic, and spiral magnetic phases. We argue that PS has a dramatic influence on the phase diagram in the vicinity of half-filling. The results imply possible interpretation of the unusual behavior of magnetic properties of single-layer cuprates in terms of PS between collinear and spiral magnetic phases. The relevance of our results to the magnetic properties of the ruthenates is also discussed.

Quantitative estimates of transferability of the QTAIM descriptors. Case study of the substituted hydropyrimidines

A theoretical study of the quantum-topological properties of three substituted in the positions 4, 5 and 6 hydropyrimidine molecule derivatives in terms of electron density and electronic energy characteristics has been carried out. The uncertainty in the intramolecular QTAIM descriptors obtained within the Kohn–Sham method with B3LYP, BLYP and BHHLYP functionals, as well as the Møller–Plesset second-order perturbation theory using the same 6-311++G** basis set has been considered and the quantitative estimates of transferability of the QTAIM descriptors have been derived. These values might be used as the threshold in the quantitative evaluation of the transferability of the bond or atomic characteristics at the above-mentioned level of approximation. It was found that electron density at the bond critical points, which is the most transferable characteristic, does not allow making a decision concerning the transferability of submolecular moieties in the substituted hydropyrimidines possessing a noticeable flexibility of the heterocyclic ring and side chains. The examinations of the chemical origins of the physiological properties of the substituted hydropyrimidines demands a consideration of maximal set of the QTAIM descriptors. Our results show that changes occurring due to conformational transitions in compounds under investigation regard the bonds and atoms not only of the side chains but also the heterocyclic ring. This observation is useful in the search for new biologically-active substituted hydropyrimidines.

Stochastic heating in asymmetric capacitively coupled RF discharges

Electron dynamics in a strongly asymmetric capacitively coupled radio-frequency (RF) discharge at low pressures is investigated by a combination of various diagnostics, analytical models and simulations. Electric fields in the sheath are measured phase and space resolved using fluorescence dip spectroscopy in krypton. The results are compared with a fluid sheath model. Experimentally obtained input parameters are used for the model. The excitation caused by beam-like highly energetic electrons is measured by phase resolved optical emission spectroscopy (PROES) and compared with the results of a hybrid Monte Carlo model based on the electric field resulting from the sheath model. The plasma itself is characterized by Langmuir probe measurements in terms of electron density, electron mean energy and electron energy distribution function (EEDF). The RF voltage and the current to the chamber wall are measured in parallel. At low pressures the plasma series resonance (PSR) effect is observed. It leads to high frequency oscillations of the current (non-sinusoidal RF current waveforms) and, consequently, to a faster sheath expansion. The measured current is compared with an analytical PSR model. Another analytical model using experimentally obtained input parameters determines the influence of beams of highly energetic electrons on the time averaged isotropic EEDF as measured by Langmuir probes. The main result is the observation of beams of highly energetic electrons during the sheath expansion phase, that are enhanced by the PSR effect. The paper shows that the nature of stochastic heating is closely related to electron beams and the PSR effect.

Development of a simple 2.45 GHz microwave plasma with a repulsive double hexapole configuration

A simple and inexpensive 2.45 GHz microwave plasma source with a repulsive double hexapole configuration is described and characterized. In this work, the operation of the source is shown to be flexible in terms of electron density, electron temperature, and plasma uniformity even at low-pressures (approximately millitorr). It allows for easy control of the electron temperature (2-3.8 eV) and density ( approximately 10(9)-10(10) cm(-3)) by removing either of the two hexapoles or by varying the separation distance between the two hexapoles. Characterization was done via information gathered from the usual Langmuir probe measurements for electron temperature and density. The source makes a resonant surface with its repulsive double hexapole magnetic configuration providing an additional longitudinal confinement near the walls midway between the two hexapoles. Magnetic field maps are presented for varying double hexapole distances. Power delivery for various settings is also presented.

Beyond Point Charges: Dynamic Polarization from Neural Net Predicted Multipole Moments.

Intramolecular polarization is the change to the electron density of a given atom upon variation in the positions of the neighboring atoms. We express the electron density in terms of multipole moments. Using glycine and N-methylacetamide (NMA) as pilot systems, we show that neural networks can capture the change in electron density due to polarization. After training, modestly sized neural networks successfully predict the atomic multipole moments from the nuclear positions of all atoms in the molecule. Accurate electrostatic energies between two atoms can be then obtained via a multipole expansion, inclusive of polarization effects. As a result polarization is successfully modeled at short-range and without an explicit polarizability tensor. This approach puts charge transfer and multipolar polarization on a common footing. The polarization procedure is formulated within the context of quantum chemical topology (QCT). Nonbonded atom-atom interactions in glycine cover an energy range of 948 kJ mol(-1), with an average energy difference between true and predicted energy of 0.2 kJ mol(-1), the largest difference being just under 1 kJ mol(-1). Very similar energy differences are found for NMA, which spans a range of 281 kJ mol(-1). The current proof-of-concept enables the construction of a new protein force field that incorporates electron density fragments that dynamically respond to their fluctuating environment.