1,720,974 research outputs found
Atoms in Valence Bond – AiVB : synopsis and test results
No full textThe Atoms in Valence Bond (AiVB) method, a new approach in the Valence Bond, is introduced. This approach combines the ideas behind the early Atoms in Molecules (AIM) developments, e.g. by Moffit [21], to understand a molecular wave function in terms of proper atomic wave functions, with the available framework of the VBSCF [17,18] as implemented in TURTLE [19]. The fundamental theoretical tools, to explain the AiVB concept, are shown and the initial test results are presented
Generation of Kekule valence structures and the corresponding valence bond wave function
No full textA new scheme, called “list of nonredundant bonds”, is presented to record the number of bonds and their positions for the atoms involved in Kekulé valence structures of (poly)cyclic conjugated systems. Based on this scheme, a recursive algorithm for generating Kekulé valence structures has been developed and implemented. The method is general and applicable for all kinds of (poly)cyclic conjugated systems including fullerenes. The application of the algorithm in generating Valence Bond (VB) wave functions, in terms of Kekulé valence structures, is discussed and illustrated in actual VB calculations. Two types of VBSCF calculations, one involving Kekulé valence structures only and the second one involving all covalent VB structures, were performed for benzene, pentalene, benzocyclobutadiene, and naphthalene. Both strictly local and delocalised p-orbitals were used in these calculations. Our results show that, when the orbitals are restricted to their own atoms, other VB structures (Dewar structures) also have a significant contribution in the VB wave function. When removing this restriction, the other VB structures (Dewar and also the ionic structures) are accommodated in the Kekulé valence structures, automatically. Therefore, at VBSCF delocal level, the ground states of these systems can be described almost quantitatively by considering Kekulé valence structures only at a considerable saving of time
Delocalization in valence bond-hyperconjugation
No full textWe consider delocalization in small molecules. The valence bond technology allows an arbitrary division of the 1-electron space in strictly separate, but nonorthogonal, spaces. Allowing or obstructing orbital mixing between subspaces during the SCF procedure is associated with incorporating or eliminating the hyperconjugation effect. We show an example for tert-butyl and trimethylsilyl cations and radicals. For H2, for which really extensive basis sets are feasible, we extend the basis so far that the delocalization or hyperconjugation effect has nearly disappeare
Atoms in Valence Bond. Method, implementation and application
No full textThe Atoms in Valence Bond (AiVB) approach is presented. The main goal was to develop a new and innovative approach, within the existing Valence Bond framework, to build and analyze the molecular VB wave function in terms of atoms and their atomic states, in a very user-friendly environment. The necessary theoretical tools, detailed implementation in TURTLE, an ab initio VB/VBSCF program and application are described. The procedure to build the molecular Atoms in ValenceBond (AiVB) wave function is not arbitrary, and is carried out by specifying the atomic states of the atoms in the different atomic AiVB structures. Presently the AiVB wave function is restricted to atomic orbitals from atomic Hartree-Fock calculations, but this is not a fundamental limitation. All possible projections of the atomic state, for each of the atoms, have to be included as separate VB structures. The AiVB approach extends the standard concept of VB structure, which is limited to a single spin-function, in order to realistically describe the atoms in their atomic states. The AiVB structure assumes a more general form where it is expressed as a sum of N-electron Slater determinants with a fixed expansion coefficients in front of each Slater determinant, which is not optimized variationally. The AiVB structures have a good quantum numbers S and Ms of the molecule considered. The Slater determinants will have different orbitals however the spin function may or may not be the same
VBSCF Methods: Classical Chemical Concepts and Beyond
No full textThe aim of this research has been to extend the ab initio Valence Bond Self-Consistent Field (VBSCF) methodology and to apply this method to the electronic structure of molecules. The valence bond method directly deals with the chemical structure of molecules in a pictorial language, which chemists are familiar with. One of the problems in this case is the manual generation of the structures where the spin-coupling patterns of the electrons correspond to the classical (Kekulé valence) structures for polycyclic conjugated systems, which is cumbersome because the number of such structures grows very rapidly with the size of the system. A procedure has been developed which can generate these structures automatically using the geometry of the molecules. Further, it has also been shown that, for cyclic conjugated systems, the consideration of only Kekulé valence structures in the VB wave functions already gives an excellent description of their electronic structure. Another methodological improvement that has been made in this work is on improving the convergence of the VBSCF wave function. A second-order converging VBSCF method has been developed in this thesis based on a Newton-Raphson scheme. The new method shows excellent convergence when the singly occupied orbitals do not mix with each other. When good starting orbitals are available, the same convergence behaviour has been found in the full optimisation. The convergence efficiency of the method has been compared with the Super-CI method. Finally, a combination of Super-CI and Newton-Raphson methods has been shown to be computationally more efficient than either the Super-CI or the Newton-Raphson method alone. In the combined method the first few iterations are performed with Super-CI until reasonably small orbital gradients or correction vectors, and then the final iterations are performed using the Newton-Raphson method. Finally, the VBSCF method has been applied to calculate the resonance energies of cyclic conjugated systems. The resonance energy of cyclic conjugated systems is considered as an important measure of their aromaticity. The effect of the choice of the orbitals on the calculated resonance energies has been explored. It has been shown that resonance energies calculated with the VB-delocal method are more reliable than those obtained from the VB-local method. Furthermore, the results for phenanthrene and anthracene show that the extra stability of the kinked benzenoid systems over their linear counterparts is a result of the larger resonance energies in the bent benzenoids. At last, the empirical parameters used in other semi-empirical VB methods and conjugated circuit theory have been quantified with the results of the VBSCF calculations on cyclic conjugated molecules
Mn promotion effects in Co/TiO2 Fischer-Tropsch catalysts as investigated by XPS and STEM-EELS
No full textThe addition of small amounts of Mn to a Co/TiO2 catalyst affects the catalytic performance in the Fischer-Tropsch synthesis (FTS) by increasing the activity and suppressing the CH4 yield. These variations in the catalyst selectivity are due to Mn promotion effects that influence the final catalyst active site distribution, playing a role under reaction conditions. The use of STEM-EELS and XPS has provided more insight into the location of Mn as a function of the catalyst surface composition observed after the main preparation steps, that is, calcination and reduction. XPS shows that after calcination the catalysts contain mainly a Co3O4 phase, and that after 4 h of reduction in H-2 flow at 350 degrees C, a fair fraction of the Co3O4 is fully reduced to Coo. The STEM-EELS measurements reveal the existence of a clear Co-Mn association in the calcined catalyst, in which the Mn apparently forms a mixed oxidic phase with the Co particles and thus causes a decrease in the Co reducibility with respect to a Mn-free Co/TiO2 catalyst. The Mn tends to segregate over the TiO2 support after reduction, as indicated by the increase in the Mn/Co and Mn/Ti atomic ratios obtained from XPS. Hence, the Mn compounds migrate from the Co particles and are enriched at the surface of the TiO2 support during Co3O4 reduction to Coo. However, the STEM-EELS images obtained for the reduced catalyst reveal a remaining Co-Mn interaction, which presumably causes the promotion effect in FTS. (c) 2004 Elsevier Inc. All rights reserved
Strobes: An Oscillatory Combustion
No full textStrobe compositions belong to the class of solid combustions. They are mixtures of powdered ingredients. When ignited, the combustion front evolves in an oscillatory fashion, and flashes of light are produced by intermittence. They have fascinated many scientists since their discovery at the beginning of the 20th century. However, the chemical and physical processes involved in this curious oscillatory combustion remain unknown. Several theories have been proposed: One claims that two different reactions occur: one during the slow dark phase and another during the fast flash phase. The alternation between the phases is ascribed to heat variations. Other theories suggest that the formation of intermediate species during the dark phase and the change of phase are caused by variations in their concentration. A ternary strobe composition with ammonium perchlorate, magnalium, and barium sulfate is analyzed. The role of barium sulfate is studied by replacing it by other metal sulfates that have different physical properties (melting points), and the burning of the compositions is recorded with a high-speed camera and a spectrometer coupled with a charge-coupled device (CCD) camera. Experimental results show noticeable differences in the physical and chemical processes involved in the strobe reaction
Strobes: Pyrotechnic Compositions That Show a Curious Oscillatory Combustion
No full textStrobes are pyrotechnic compositions which show an oscillatory combustion; a dark phase and a flash phase alternate periodically. The strobe effect has applications in various fields, most notably in the fireworks industry and in the military area. All strobe compositions mentioned in the literature were discovered by trial and error methods and the mechanisms involved remain unclear. Many oscillatory systems such as Belousov–Zhabotinsky reactions, cool flames, self-propagating high-temperature synthesis have been observed and theories developed to elucidate their unstable behavior based on chemical interactions or based on physical processes. These systems are compared to experimental observations made on strobe mixtures
Assignment of phantom bands in the solid-state infrared and Raman spectra of coronene: the importance of a minute out-of-plane distortion
No full textThe molecular geometry and the normal modes properties of coronene are investigated by means of DFT B3LYP and restricted/Hartree–Fock calculations utilizing basis sets of triple zeta +polarization quality. The interpretation of the infrared and Raman spectra of coronene, especially in solid state, is critically revised. The phantom bands in the solid state, previously not understood, are readily assigned after considering a minute out-of-plane molecular distortion from D6h to C2h
Growth Process Conditions of Tungsten Oxide Thin Films Using Hot-Wire Chemical Vapor Deposition
No full textWe report the growth conditions of nanostructured tungsten oxide (WO3−x) thin films using hot-wire chemical vapor deposition (HWCVD). Two tungsten filaments were resistively heated to various temperatures and exposed to an air flow at various subatmospheric pressures. The oxygen partial pressure was varied from 6.0×10−6 to 1.0 mbar and the current through the filaments was varied from 4.0 to 9.0 A, which constitutes a filament temperature of 1390–2340 ◦C in vacuum. It is observed that the deposition rate of the films is predominantly determined by the oxygen partial pressure; it changes from about 1 to about 36,000nmmin−1 in the investigated range. Regardless of the oxygen partial pressure and filament temperature used, thin films with a nanogranular morphology are obtained, provided that the depositions last for 30 min or shorter. The films consist either of amorphous or partially crystallized WO3−x with high averaged transparencies of over 70% and an indirect optical band gap of 3.3±0.1 eV. A prolonged deposition time entails an extended exposure of the films to thermal radiation from the filaments, which causes crystallization to monoclinic WO3 with diffraction maxima due to the (0 0 2), (2 0 0) and (0 2 0) crystallographic planes, furthermore the nanograins sinter and the films exhibit a cone-shaped growth. By simultaneously influencing the surface mobility, by heating the substrates to Tsurface = 700±100 ◦C, and the deposition rate, a very good control of the morphology of the deposited films is obtained. Nanowire films, nanocrystallite films and closed crystallite films were thus deposited. These differently nanostructured c-WO3 films all possess an indirect optical band gap of 2.5±0.1 eV
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