Rearrangement Of Chemical Bonds Research Articles

Elastic properties and chemical bonding in fluoritelike Be2B, AlBeB, MgBeB, and NaBeB

Calculations of lattice constants and elastic moduli were carried out by means of FP LMTO-GGA for fluoritelike beryllium semiboride Be2B and its ternary phases on its basis: AlBeB, MgBeB, and NaBeB. Changes in the indicated parameters are discussed from the viewpoint of chemical bond rearrangement in ternary boride phases.

Electron density and electrostatic potential of KMnF3: a phase-transition study.

Three accurate X-ray diffraction experiments (Mo Kalpha, T = 190, 240 and 298 K) were carried out to track the temperature dependence of the electron density in the cubic perovskite potassium manganese trifluoride, KMnF3, from room temperature to just above that of the phase transition to the tetragonal structure (186 K), and to correlate the parameters of the critical points with the phase-transition mechanism. The data obtained were approximated by the Hansen-Coppens multipole model expanded up to hexadecupoles; the anharmonicity of the atomic displacements up to the fourth level was considered. Topological analysis shows only two types of chemical bond at room temperature, Mn-F and K-F. However, at low temperature the K-F bonds blocking the rotation of the MnF6 octahedra are weakened and new Mn-K bonds are formed to keep the crystal structure from disintegrating. The Mn-K bonds become stronger as the temperature approaches 186 K. This rearrangement of chemical bonds can be regarded as a precursor effect, which starts 50-60 degrees above the phase-transition temperature. The effective one-particle potential of the F atom has a single minimum at 298 K and four well separated minima (with a shift of 0.2 A from the equilibrium position towards the structural holes) at 190 K. Parameters of the critical points of the electron density indicate closed-shell type interactions between K-F and Mn-K pairs, whereas the Mn-F bond can be considered as an intermediate type. The topology of the electrostatic potentials is discussed as well.

Kinetic and Thermodynamic Control by Chemical Bond Rearrangement on a Si(001) Surface

Bleiben oder gehen? Die Produktverteilung für an die Si(001)-Oberfläche adsorbiertes Aceton kann durch Erhöhung der Oberflächentemperatur von kinetisch kontrolliert (Vierring-Spezies; siehe Schema) nach thermodynamisch kontrolliert (Dissoziations-Spezies) verschoben werden. Damit ist gezeigt, dass organische Moleküle an die Si(001)-Oberfläche reversibel binden.

Kinetic and thermodynamic control by chemical bond rearrangement on a Si(001) surface.

Kinetic and thermodynamic control by chemical bond rearrangement on a Si(001) surface.

The role of hydrogen bonding in the enzymatic reaction catalyzed by HIV-1 protease.

The hydrogen-bond network in various stages of the enzymatic reaction catalyzed by HIV-1 protease was studied through quantum-classical molecular dynamics simulations. The approximate valence bond method was applied to the active site atoms participating directly in the rearrangement of chemical bonds. The rest of the protein with explicit solvent was treated with a classical molecular mechanics model. Two possible mechanisms were studied, general-acid/general-base (GA/GB) with Asp 25 protonated at the inner oxygen, and a direct nucleophilic attack by Asp 25. Strong hydrogen bonds leading to spontaneous proton transfers were observed in both reaction paths. A single-well hydrogen bond was formed between the peptide nitrogen and outer oxygen of Asp 125. The proton was diffusely distributed with an average central position and transferred back and forth on a picosecond scale. In both mechanisms, this interaction helped change the peptide-bond hybridization, increased the partial charge on peptidyl carbon, and in the GA/GB mechanism, helped deprotonate the water molecule. The inner oxygens of the aspartic dyad formed a low-barrier, but asymmetric hydrogen bond; the proton was not positioned midway and made a slightly elongated covalent bond, transferring from one to the other aspartate. In the GA/GB mechanism both aspartates may help deprotonate the water molecule. We observed the breakage of the peptide bond and found that the protonation of the peptidyl amine group was essential for the peptide-bond cleavage. In studies of the direct nucleophilic mechanism, the peptide carbon of the substrate and oxygen of Asp 25 approached as close as 2.3 A.

On-orbit and ground testing of space environment interactions with materials

During the past 12 years, a significant number of different materials have been returned to Earth after extended exposure to space in low Earth orbits. The process steps required to conduct on-orbit exposure experiments and a brief historical outline of materials experiments to date will be described. Examples of material degradation, with emphasis on coatings, will be presented using on-orbit and post-flight photographs of spacecraft hardware and flight test specimens. Results of post-flight testing will be compared with test results from ground-based systems. Oxidation of coatings, darkening of surfaces due to UV-induced chemical bond rearrangement, and displacement of oxygen atoms from pigments due to proton and electron impact, mechanical failure due to thermal cycling, damage due to microscopic particle impact, and multiple bond ruptures due to high-energy charged particles are observed on hardware returned from orbit. A current materials flight experiment that includes a significant number of coating specimens, along with many other materials, is the Materials International Space Station Experiment (MISSE). MISSE is flying today on-board the International Space Station (ISS) and will be discussed as an example of the current infrastructure available for on-orbit testing.

Cell attachment of polypropylene surface-modified by COOH + ion implantation

Carboxy ion (COOH +) implantation was performed at the energy of 50 keV with fluences ranging from 1×10 14 to 1×10 15 ions/cm 2 at room temperature for polypropylene (PP). The effects of ion implantation on cells (immune macrophages, 3T3 mouse fibroblasts and human endothelial cells) were studied in vitro. Tests of cell attachment gave interesting results that the 3T3 mouse fibroblasts and human endothelial cells cultured on the surface of the implanted PP showed much better attachment and proliferation than that on pristine PP. At the same time, the COOH + ion implantation also induced low macrophage attachment with normal cellular morphology. Results of X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) analysis showed that COOH + ion implantation caused the rearrangement of chemical bonds and the formation of some new O-containing groups, which was responsible for the enhancement of the biocompatibility of PP.

In situ detection of F 2 laser-induced oxidation on hydrogen-terminated Si(111) and Si(110) surfaces by Fourier transform infrared transmission spectroscopy

Although oxidation of silicon has been intensively investigated experimentally and theoretically, the composition and structure of the interface region between the bulk oxide and silicon are not very well known. In this work, we report on the vacuum UV laser-induced oxidation of well-defined atomically flat Si(111)-(1×1):H and Si(110)-(1×1):H surfaces, prepared by wet-chemical processing, using an F 2 laser (157 nm). The silicon samples were irradiated under ultra high vacuum conditions in water and oxygen atmospheres. The photo-induced partial oxidation of the first monolayer, controlled by the oxidant pressure, was monitored by in situ Fourier transform infrared transmission spectroscopy with different photon detectors. The chemical reactions occurring at the surface were monitored by the shift and decreasing strength of the stretching vibration of SiH surface bonds. The IR absorption spectra recorded during surface processing yield detailed information on the rearrangement of chemical bonds with a sensitivity reaching monolayer resolution.

Selective bond dissociation and rearrangement with optimally tailored, strong-field laser pulses.

We used strong-field laser pulses that were tailored with closed-loop optimal control to govern specified chemical dissociation and reactivity channels in a series of organic molecules. Selective cleavage and rearrangement of chemical bonds having dissociation energies up to approximately 100 kilocalories per mole (about 4 electron volts) are reported for polyatomic molecules, including (CH3)2CO (acetone), CH3COCF3 (trifluoroacetone), and C6H5COCH3 (acetophenone). Control over the formation of CH(3)CO from (CH3)2CO, CF3 (or CH3) from CH3COCF3, and C6H5CH3 (toluene) from C6H5COCH3 was observed with high selectivity. Strong-field control appears to have generic applicability for manipulating molecular reactivity because the tailored intense laser fields (about 10(13) watts per square centimeter) can dynamically Stark shift many excited states into resonance, and consequently, the method is not confined by resonant spectral restrictions found in the perturbative (weak-field) regime.

Effects on synchronized motion in elementary chemical processes

The influence of the synchronous motion of various atoms on the probability of elementary chemical reactions in a condensed medium is theoretically investigated. The factor of synchronization is defined as the ratio of the probability of the multibond transition W to the probability W 0 of the transition along one degree of freedom calculated in the Condon approximation at fixed equilibrium positions of the other atoms. It is shown that the synchronous rearrangement of several chemical bonds involves in general three types of synchronization electronic, electron-configurational and energetic synchronization. The factor of electron synchronization is smaller than or equal to unity. The factor of electron-configurational synchronization is usually greater than unity. The factor of energy synchronization K E S is the most important for adiabatic reactions. It represents the product of an exponential ( K F S) and an entropy ( K s S) factor. The entropy factor K s S is usually not very restrictive for multibond rearrangements, and the path of the transition is determined mainly by the exponential factor K F S.

The Chemistry of Flames

Combustion scientists are primarily concerned with the fuels most often burned as energy sources (coal, petroleum products, and natural gas), with the goal of learning to burn them as efficiently, intensely, and cleanly as possible. Discovering those slight rearrangements of chemical bonds that together account for the net chemical transformation is the key to understanding how combustion proceeds. Once these reactions have been defined, the chemist can determine the rate coefficient of each reaction as a function of temperature and assemble the information into flame models. The computer programs that use these models to predict experimental results combine two sets of equations describing (1) the diffusive and reactive rates of change in concentration of all the molecules in the flame and (2) the flow of the reacting gases. Although the details of hydrocarbon-flame models are still disputed, many of their general features are clear and the basic reactions well-known; computer models for methane combustion already include over 100 elementary reactions. Experimental data on NO/sub x/ formation have led to ingenious methods for purifying postcombustion gases. Currently, the two most actively studied processes in combustion chemistry are the formation and oxidation of soot. The synthetic fuels most likely to replace fossilmore » ones will probably contain less hydrogen, making them more prone to produce soot. How to understand and overcome this problem will be the next challenge for combustion chemists and engineers.« less

Enskog theory for chemically reacting fluids

Presented here is a theory of nonequlibrium processes for a dense fluid composed of chemically reactive, rigid-sphere molecules. Explicit consideration is given to the relocations of the molecular centers of mass that result from the occurrence of reaction and the accompanying rearrangements of chemical bonds. The collision operators are separated into source and flux parts in order to produce equations of motion which faithfully mimic those of continuum mechanics. As a consequence of this we are led naturally to the discovery of ’’collisional transfer’’ contributions to the mass fluxes of the reactive species. To transform these concepts into a computationally tractable theory we invoke the approximate Enskog functional relationship between the singlet and pair-space distribution functions. This is followed by the development of a procedure for estimating the nonequilibrium corrections to the reaction rates as well as the coefficients of viscosity and diffusion, with special attention being devoted to the collisional transfer contributions to the species’ mass fluxes.

Energy Utilization through Coupled Systems

Living systems are characterized by a continuous of substance and energy. Both chemical and physical occur. Matter may move from one point to another by diffusion or active transport. Alternatively, it may flow through a sequence of chemically distinct compounds by enzymatically catalyzed reactions. Physically-distant compounds may react with each other through the of electrons along biochemical conductors. Changes in energy are associated with all such movements or conversions. Consider any biochemical system in which a net physical or chemical change occurs without inflow or outflow of matter or energy. A net change can occur only when there is a displacement from equilibrium; therefore, such change must always be accompanied by some negative free-energy change. This negative free-energy change is the difference in free energies of formation of the products as compared with the reactants. It is expressed as AF = AH - TAS, where AH is the heat of the reaction and TAS is the absolute temperature times the changes in entropy. Entropy changes result from rearrangement of chemical bonds, changes in concentration, movements through gradients, etc. AH changes are primarily the result of making and breaking chemical bonds, ionization, electronic excitation, formation of resonance hybrids, etc. Whereas the entropy increase may occasionally be so great that AH is positive (heat absorbed from the surroundings), the more common situation is for AH to be negative. This results in a of heat away from the system (which is commonly isothermal). This energy, together with that associated with the entropy change, is irrevocably lost. In the interests of economy of energy, how might the free-energy loss be diminished? For a net conversion of material to occur in a chemical reaction, the rate of the forward reaction, f, obviously must exceed the rate of the back reaction, b. The ratio of these rates is given by AF = - RT In f/b. If the net rate is to be large and at the same time the energy loss to be small, the reaction must be highly reversible; that is, b must be large. Examples of such reactions are to be found in eperimization of sugar phosphates and in transketolase reactions. We have estimated the free-energy changes of these reactions in the carbon-reduction cycle in