Application of integrated computer simulation approach to solid surfaces and interfaces

The atomistic understanding of the structure, reactivity, and electronic properties of solid surfaces and interfaces are essential for the design of novel catalysts and electronics/photonics devices which have high-performance and unexplored properties. Computational chemistry is expected not only to rationalize the experimental results but also to predict new features. We have applied integrated computer simulation methods including quantum chemistry, periodic density functional theory, molecular dynamics, embedded atom method, and atomic force microscopy simulation to various topics related to solid surfaces and interfaces. In the present paper, we reviewed our recent activities on supported metal catalysts, metal clusters, atomic force microscopy simulation, high-temperature superconductors, tribology, Si semiconductor and V₂O₅ catalysts. Our activities also involve the generation of a lot of new computer simulation codes. We emphasize that the integrated computer simulation system provides not only methods for scientific studies but also a key technology for industrial innovations in research and development.     

Mechanism of formation of YBa2Cu4O8 superconductor in the sol-gel synthesis

It was shown that a single phase YBa₂Cu₄O₈ (124-phase) could be formed from gels at 1 atm oxygen pressure, and the mechanism of its formation was elucidated. It was found that there are two key routes for the sol-gel formation of the 124-phase, one involving the tetragonal YBa₂Cu₃O _y with a low concentration of oxygen defects (tetra-I phase) and the other involving the Ba₂Cu₃O_5.9 as important intermediates of the 124-phase. The rapid formation of these intermediate compounds from the gel was attributed to the small particle size of the oxides and carbonates precipitating at the initial stage of heating. It was thought that the small particles characteristic of sol-gel processing lead to the rapid formation of the intermediate compounds and subsequent precipitation of the 124-phase.     

The contribution of frictional contacts to the shear strength of coarse glass bead powders and slurries

This paper investigates the use of the shear vane technique as a means of determining frictional and cohesive interparticle force contributions to the shear strength of coarse glass bead powders and slurries. To this end, the shear strength of 203-μm glass beads in air and slurried in water and kaolinite suspensions was determined as a function of vane immersion depth, vane geometry, and container size. Both vane immersion depth and container diameter are found to significantly impact the shear strength measured using the vane technique. An equation describing interparticle frictional and cohesive contributions to shear vane measurements was derived in an effort to describe the experimental results. A Janssen stress distribution model for granular materials forms the basis for this equation and appears to explain the behavior of shear strength measurements at varying immersion depths. The presence of the Janssen stress distribution can affect the interpretation of shear vane results. Rather than shear strength being a material property, as is the case with flocculated colloid slurries and polymer solutions, shear strength becomes a process property where vane depth, container size, and container material can result in significant measurement variations. Such parameters should be considered before using the shear vane results on applications involving granular material components.US Department of Energy Environmental Management Science Program project no. 90162.     

Fréchet differentiability of solution mappings for semilinear second order evolution equations

In this paper we establish the strong Fréchet differentiability of maps from the set of initial values and forcing terms into the set of solutions for semilinear second order evolution equations. Also, under the weaker conditions of nonlinear terms we establish the strong Gâteaux differentiability of the solution maps. An application of results to semilinear strongly damped wave equations is given.     

Resonant neutral-particle emission after collisions of electrons with base-stacked oligonucleotide cations in a storage ring

Electron-cation collisions have been studied for various protonated deoxyoligonucleotide monocations, using an electrostatic storage ring equipped with a merged-electron-beam device. Resonant neutral-particle emissions have been observed at a collision energy of about 4.5 eV for some of them. The resonance parameters depend on DNA base composition and sequence, and the resonances occur only in oligonucleotide monocations with base stacking. The resonance widths increase with oligonucleotide length, but saturate at trimers. Quantum-chemical estimations performed here help explain these results.     

Theoretical investigation of ethylene/1-butene copolymerization process using constrained geometry catalyst (CpSiH2NH)-Ti-Cl2

The ethylene/1-butene copolymerization using constrained geometry catalyst CpSiH₂-NH-TiCl₂ (CGC) was investigated by the density functional theory and molecular dynamics. Structures and energetics of reactants, π-complexes, transition states, and products during insertion of ethylene and 1-butene monomers into the catalytic reactive site of the CGC were investigated by the density functional theory (DFT) using the software Dmol³, while dynamics of atoms during copolymerization process was investigated by classical molecular dynamics (MD) using the New-Ryudo-CR program. The calculated results were compared with the available experimental and theoretical ones. It was found that the ethylene insertion into Ti-Me active species is energetically more favorable than the butene one and the 2,1-butene insertion is more favorable than 1,2-butene one. Once the initial ethylene insertion has taken place, the further ethylene insertion occurring with a less energy barrier, in good agreement with experimental findings.     

Bonding in some covalent derivatives of the 1,2,3,4-thiatriazole-5-thiolate anion. A topological study

The study of pseudohalides has found a deep interest to nonmetal chemists for many years. Due to the similarities between the halides and the pseudohalides, the pseudohalide family of compounds is of fundamental chemical interest. The pseudohalide concept was introduced in 1925 and since its introduction, the pseudohalide principle has been used extensively in nonmetal chemistry to predict the structure and stability of many molecular species.

The so-called ‘azidodithiocarbonate’ anion, more properly referred to as the 1,2,3,4-thiatriazole-5-thiolate anion, CS2N3-, is of particular interest. In a short communication we have recently reported the topological study of some CS2N3-containing species reported by Crawford et al. Previous reports on these compounds showed that in covalent derivatives, not only does the ring remain intact but also the site of attachment of the R group is most likely at the exocyclic sulfur atom in contrast to the previously suggested N-R connectivity. Therefore, the structure and bonding of derivatives of the CS2N3- moiety is clearly an important question.

With that in our mind, we undertook a topological analysis, based on the AIM theory, to gain more insight into the bonding in covalent derivatives of CS2N3- moiety, trying to find an explanation to the origin of the NH and S-H connectivities. The question is: which is the reason that makes all the covalent derivatives prefer the S-R connectivity while the hydracid has an N-H one?     

Enhancement of the oxygen transfer in a circulating three-phase fluidized bed bioreactor

Applied Biochemistry and Biotechnology - The addition of α-alumina to the aqueous solution of sodium alginate for imobilization of viable cells allows the production of denser particles than...