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Electronic Density (electronic + density)
Selected AbstractsOptimizing the formula of rare earth-bearing materials: A computational chemistry investigationINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2007Marjorie Bertolus Abstract We present a computational investigation into the nature of bonds formed by rare earth elements (REE) in materials. This study focuses on the incorporation of neodymium in minerals called apatites, which are derived from fluorapatite: Ca10(PO4)6F2. These minerals, which allow many substitutions on all three Ca, P, and F sites, are considered as potential host phases for radioactive elements separated from nuclear waste. Nd and trivalent actinides have very similar physical and chemical properties, and Nd is not radioactive and much more easily handled. It is therefore very often used as a surrogate for actinides with oxidation degree three in experimental studies. Several formulas can be considered to substitute Nd3+ to Ca2+ and maintain charge balance of the apatite. Existing experimental and theoretical studies, however, mostly concern the Ca9Nd(PO4)5SiO4F2 formula, where the Nd incorporation is compensated by the replacement of one PO by a SiO group. Moreover, only the cation position has been studied, whereas the silicate position and its influence on stability are unknown. We present a more general investigation of possible charge compensations on the one hand, and of the various resulting configurations on the other. All possible configurations of the two formulas Ca9Nd(PO4)5 SiO4F2 and Ca8NdNa(PO4)6F2 have been considered. Calculations have been performed within the framework of density functional theory (DFT). A computation scheme that permits good accuracy in these systems within reasonable computation times is determined. The results obtained for cohesion energies, geometries, and electronic densities are discussed. As for the formulation, it is shown that the Ca8NdNa(PO4)6F2 formula is less stable than the fluorapatite, while Ca9Nd(PO4)5 SiO4F2 is more stable. For the structures, it is found that Nd substitutes preferably in the second cationic site. Moreover, the most stable structures exhibit the shortest Na,Nd or Nd,Si distances. Local charge balance therefore seems favorable. Then, it is shown that Nd forms covalent bonds both in apatite and in britholite, while Na forms ionic bonds. Finally, a first correlation between the material stability and the covalent character of the bonds formed is established. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Two-step method for precise calculation of core properties in moleculesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2005A. V. Titov Abstract Precise calculations of core properties in heavy-atom systems that are described by the operators heavily concentrated in atomic cores, such as hyperfine structure and P,T-parity nonconservation effects, require accounting for relativistic effects. Unfortunately, four-component calculation of molecules containing heavy elements is very consuming already at the stages of calculation and transformation of two-electron integrals with a basis set of four-component spinors. In turn, the relativistic effective core potential (RECP) calculations of valence (spectroscopic, chemical, etc.) properties of molecules are very popular, because the RECP method allows one to treat quite satisfactorily the correlation and relativistic effects for the valence electrons of a molecule and to reduce significantly the computational efforts. The valence molecular spinors are usually smoothed in atomic cores, and, as a result, direct calculation of electronic densities near heavy nuclei is impossible. In this paper, the methods of nonvariational and variational one-center restoration of correct shapes of four-component spinors in atomic cores after a two-component RECP calculation of a molecule are discussed. Their efficiency is illustrated in correlation calculations of hyperfine structure and parity nonconservation effects in heavy-atom molecules YbF, BaF, TlF, and PbO. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] A comparison of modern data analysis methods for X-ray and neutron specular reflectivity dataJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2007A. Van Der Lee Data analysis methods for specular X-ray or neutron reflectivity are compared. The methods that have been developed over the years can be classified into different types. The so-called classical methods are based on Parrat's or Abelčs' formalism and rely on minimization using more or less evolved Levenberg,Marquardt or simplex routines. A second class uses the same formalism, but optimization is carried out using simulated annealing or genetic algorithms. A third class uses alternative expressions for the reflectivity, such as the Born approximation or distorted Born approximation. This makes it easier to invert the specular data directly, coupled or not with classical least-squares or iterative methods using over-relaxation or charge-flipping techniques. A fourth class uses mathematical methods founded in scattering theory to determine the phase of the scattered waves, but has to be coupled in certain cases with (magnetic) reference layers. The strengths and weaknesses of a number of these methods are evaluated using simulated and experimental data. It is shown that genetic algorithms are by far superior to traditional and advanced least-squares methods, but that they fail when the layers are less well defined. In the latter case, the methods from the third or fourth class are the better choice, because they permit at least a first estimate of the density profile to be obtained that can be refined using the classical methods of the first class. It is also shown that different analysis programs may calculate different reflectivities for a similar chemical system. One reason for this is that the representation of the layers is either described by chemical composition or by scattering length or electronic densities, between which the conversion of the absorptive part is not straightforward. A second important reason is that routines that describe the convolution with the instrumental resolution function are not identical. [source] The relaxations of temporal bond polarizabilities of methylviologen adsorbed on the Ag electrode by 514.5 nm excitation: a Raman intensity studyJOURNAL OF RAMAN SPECTROSCOPY, Issue 3 2009Chao Fang Abstract An algorithm to elucidate the temporal bond polarizabilities from the surface enhanced Raman (SERS) intensities was employed to the case of methylviologen (MV) adsorbed on the Ag electrode. This enables us to obtain the properties of its SERS mechanisms and the effect of its adsorption. The analysis shows that the charge transfer and electromagnetic mechanisms involving in this MV SERS system possess different relaxation times for its various temporal bond polarizabilities. The physics is that the process involved in the charge transfer mechanism will take longer time than that involved in the electromagnetic mechanism since it needs more time to redistribute the charges during relaxation. The time division between these two mechanisms is figured out to be around 3 ps for this system. Adsorption also enhances the relaxation of the temporal bond polarizabilities, in general. The adsorption effect is indicated by the temporal bond polarizabilities close to the final stage of relaxation. They are, in fact, the quantities parallel to the bond electronic densities in the molecular orbital (MO) concept. For comparison, the case of MV solid was also analyzed. Copyright © 2008 John Wiley & Sons, Ltd. [source] Understanding quantum dots: overheating of the LO-phonon modesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2007K. Král Abstract Longitudinal optical phonons have been used recently to explain the electronic energy relaxation in quantum dots. In this theory they served as a reservoir, on which the electron executes multiple scattering acts. Rather naturally such a phonon subsystem is expected to be passive, namely, in a long-time limit of development the whole system should be able to achieve such a stationary state in which statistical distributions of both subsystems, electron and phonons, do not change in time. Here we remind briefly that the recent approach to the relaxation theory in quantum dots displays a non-passivity of such a reservoir. We remind briefly the method of a partial elimination of this phonon overheating effect by using the Lang-Firsov transformation. We apply the modified relaxation theory to the electronic relaxation at low electronic densities in quantum dot systems and come to conclusions about the role of e-LO scattering mechanism in these situations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Origin of Anomalous Pt,Pt Distances in the Pt/Alumina Catalytic SystemCHEMPHYSCHEM, Issue 12 2004Karl Sohlberg Prof. Intriguing Pt3structures observed on the Pt/,-alumina catalytic system by atomic-resolution Z -contrast scanning transmission electron microscopy (STEM) are investigated with first-principles calculations (see electronic densities in the Pt3 plane with and without the capping OH group). [source] Synthesis of 3-(Tosylalkyl)indazoles and their Desulfonylation Reactions , A New Entry to 3-Substituted Indazoles by an Unprecedented Friedel,Crafts ProcessEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 19 2009Silvia Campetella Abstract Reaction of indazoles with aldehydes in the presence of p -toluenesulfinic acid affords the corresponding sulfonyl indazoles in satisfactory yields. The reported Friedel,Crafts process is rather unusual on indazoles because of the reduced electronic density of the heterocycle. The obtained sulfonyl indazoles can be desulfonylated under reductive conditions, finally leading to 3-alkylated indazoles.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Effect of residual vanadyl ions on the spectroscopic analysis of humic acids: a multivariate approachEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 3 2008E. H. Novotny Summary In a study of the vanadyl (VO2+)-humic acids system, the residual vanadyl ion suppressed fluorescence and specific electron paramagnetic resonance (EPR) and NMR signals. In the case of NMR, the proton rotating frame relaxation times (T1,H) indicate that this suppression is due to an inefficient H-C cross polarization, which is a consequence of a shortening of T1,H. Principal components analysis (PCA) facilitated the isolation of the effect of the VO2+ ion and indicated that the organic free radical signal was due to at least two paramagnetic centres and that the VO2+ ion preferentially suppressed the species whose electronic density is delocalized over O atoms (greater g -factor). Additionally, the newly obtained variables (principal components , PC) indicated that, as the result of the more intense tillage a relative increase occurred in the accumulation of: (i) recalcitrant structures; (ii) lignin and long-chain alkyl structures; and (iii) organic free radicals with smaller g -factors. [source] Grid-based density functional calculations of many-electron systemsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2008Amlan K. RoyArticle first published online: 10 DEC 200 Abstract Exploratory variational pseudopotential density functional calculations are performed for the electronic properties of many-electron systems in the 3D cartesian coordinate grid (CCG). The atom-centered localized gaussian basis set, electronic density, and the two-body potentials are set up in the 3D cubic box. The classical Hartree potential is calculated accurately and efficiently through a Fourier convolution technique. As a first step, simple local density functionals of homogeneous electron gas are used for the exchange-correlation potential, while Hay-Wadt-type effective core potentials are employed to eliminate the core electrons. No auxiliary basis set is invoked. Preliminary illustrative calculations on total energies, individual energy components, eigenvalues, potential energy curves, ionization energies, and atomization energies of a set of 12 molecules show excellent agreement with the corresponding reference values of atom-centered grid as well as the grid-free calculation. Results for three atoms are also given. Combination of CCG and the convolution procedure used for classical Coulomb potential can provide reasonably accurate and reliable results for many-electron systems. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Substituent effects on ion complexation of para - tert -butylcalix[4]arene esters,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2006Márcio Lazzarotto Abstract Phenoxy-carboxy-methoxy- p-tert -butylcalix[4]arene esters were synthesized in order to evaluate the role of electronic parameters on the complexation of alkaline metal cations. Extraction constants of metal picrates to organic phase were determined. Plots of log (KR/KH) against Hammett , and , gave good linear correlations. The best correlations with , were obtained for K+ and Rb+, while the best correlations with , were obtained for Li+ and Na+. All Hammett plots gave a straight descending line, which is consistent with a dependence of the electronic density on the CO. Treatment of data using the Yukawa,Tsuno equation revealed a variation in the contribution of resonance in the complexation of alkaline metal ions, which is maximum for Na+ and minimum for Rb+. Electronic parameters were calculated for a related acyclic model structure and only the HOMO energy showed a good correlation with log (KR/KH). Copyright © 2007 John Wiley & Sons, Ltd. [source] Spatio-temporal inhomogeneities of laser induced plasma on surface solutionsLASER PHYSICS LETTERS, Issue 11 2004J. Ben Ahmed Abstract A transient plasma created by focusing a nanosecond laser pulse on the surface of ionic solutions of Ca++ and Mg++ is studied spectroscopically. This kind of plasma has a properties which vary significantly with time and over its extended volume. The emissions of ionic and atomic line of calcium and magnesium are analyzed to get information on the electronic temperature, the electronic density and on the kinetics of the population of low atomic and ionic states: 4P1 of CaI, 5S1/2 of CaII, 3P1 of MgII and 3P1/2 of MgII. The temperature is maximal at the plasma center and it drops at the edges. The study of the term as a function of time, where r is the radial position, shows a continue decreasing with time, which implies that the inhomogeneity of the temperature develops with time. On the another side, the electronic density deceases exponentially with time and does not vary significantly with space. These measurements allow us to follow the kinetics of the transformation of calcium (magnesium) from Ca++Mg++ to Ca+(Mg+) to give at the end Ca (Mg) and to understand the behavior of self-absorption phenomena which is observed in the resonance line of CaII at t , 2000 ns to increase with time. The experimental results are simulated by a kinetics model of LTE laser induced plasma in its late relaxation period. (© 2004 by ASTRO, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Simulation of electronic density of states and optical properties of PbB4O7 by first-principles DFT methodPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2009Hui Wang Abstract First-principles calculations based on the density functional theory and the generalized gradient approximation were carried out to systematically investigate the electronic structure and linear optical properties of lead tetraborate (PbB4O7; PTB) single crystals. An indirect band gap of 4.34 eV is obtained. The valence band top is at the X-point and the conduction band is at the ,-point. The calculated total and partial densities of states indicate that the top valence band is constructed from O 2p states and the low conduction band mainly consists of Pb 6p states. The optical absorption spectra show several prominent structures but with very small directional anisotropy. PTB crystals have a higher refractive index but a small birefringence. The calculated frequency-dependent refractive index is in good agreement with experimental data. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Application of entropy calculations to the determination of transition temperature in zirconiumPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 8 2005Pornjuk Srepusharawoot Abstract The Helmholtz free energy is determined in order to calculate the bcc,hcp phase transition temperature in zirconium. The molecular dynamics method is employed to calculate the vibrational entropy which comprises both the harmonic and anharmonic vibrational entropy. Electronic effects are estimated and taken into account. The anharmonic vibrational entropy is determined from the anharmonic free energy and the electronic entropy is calculated via the electronic density of states. By considering the Helmholtz free energy, the bcc,hcp transition temperature from our simulations is 1047 ± 210 K compared to 1135 K from experimental results. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||