Is the structure of anisotropic pyrolytic carbon a consequence of growth by the Volmer-Weber island growth mechanism?

Pyrolytic carbon deposits, formed on flat zirconia surfaces exposed to the oxypyrolysis of CH₄ at 1673 K, were examined by SEM and found to resemble Johnson-Mehl tessellations. Carbon deposits were also observed that were isolated on the zirconia surface. These deposits were either hemispheres or resembled Johnson-Mehl tessellations with rounded edges. Examination of cross-sections and internal structures of the carbon hemispheres, produced by both FIB milling and mechanical damage, showed that the hemispheres were comprised of concentric layers of carbon. These observations are consistent with growth by the Volmer-Weber 3D island mechanism, in which isolated deposits grow and impinge upon one another, and then coalesce to eventually form a continuous film. A simple mathematical model of Volmer-Weber growth was used to demonstrate that this mechanism can produce the structures observed in this study, and more generally can provide insights into some aspects of pyrolytic carbon structure and growth. In particular, columnar structures, tessellated surfaces and growth cones, all known for anisotropic pyrolytic carbons, are consistent with growth by the Volmer-Weber 3D island mechanism.     

How does heme axial ligand deletion affect the structure and the function of cytochrome b562?

We have recently generated a new mutant of cytochrome b₅₆₂ (cytb₅₆₂)in which Met7, one of the axial heme ligands, is replaced byAla (M7A cytb₅₆₂). The M7A cytb₅₆₂ can bind heme and the UV-visibleabsorption spectrum is of a typical high-spin ferric heme. Toinvestigate the effect of the lack of Met7 ligation on the structuralintegrity of cytb₅₆₂, thermal transition analyses of M7A cytb₅₆₂were conducted. From the thermodynamic parameters obtained,it is concluded that the folding of M7A cytb₅₆₂ is comparableto the apoprotein despite the presence of heme. On the otherhand, exogenous ligands such as cyanide and azide ions are readilybound to the heme iron, indicating that the axial coordinationsite is available for substrate binding. The peroxidase activityof this mutant is thus examined to evaluate new enzymatic functionat this site and M7A cytb₅₆₂ was found to catalyze an oxidationreaction of aromatic substrates with hydrogen peroxide. Theseobservations demonstrate that the Met7/His102 bis-ligation tothe heme iron is crucial for the stable folding of cytb₅₆₂,whereas the functional conversion of cytb₅₆₂ is successfullyachieved by the loose folding together with the open coordinationsite.     

Self-propagating high-temperature synthesis of advanced ceramics in the Mo–Si–B system: Kinetics and mechanism of combustion and structure formation

The goal of this work is to investigate the combustion mechanisms of reactions in the Mo–Si–B system and to obtain ceramic materials using the SHS method. It is concluded that the following processes are defined by the SHS for Si-rich Mo–Si–B compositions: silicon melting, its spreading over the surfaces of the solid Mo and B particles, followed by B dissolution in the melt, and formation of intermediate Mo₃Si-phase film. The subsequent diffusion of silicon into molybdenum results in the formation of MoSi₂ grains and molybdenum boride phase forms due to the diffusion of molybdenum into B-rich melt. The formation of MoB phase for B-rich compositions may occur via gas-phase mass transfer of MoO₃ gaseous species to boron particles. The stages of chemical interaction in the combustion wave are also investigated. The obtained results indicate the possibility of both parallel and consecutive reactions to form molybdenum silicide and molybdenum boride phases. Thus the progression of combustion process may occur through the merging reaction fronts regime and splitting reaction fronts regime. Molybdenum silicide formation leads the combustion wave propagation during the splitting regime, while the molybdenum boride phase appears later. Finally, targets for magnetron sputtering of promising multi-phase Mo–Si–B coatings are synthesised by forced SHS compaction method.     

Hydrothermal vs electrodeposition: How does deposition method affect the electrochemical capacitor performance of manganese dioxide?

Hydrothermal and electrodeposition are widely utilized methods in the preparation of MnO₂ nanomaterials for electrochemical capacitor application. In this paper, these two approaches were applied to synthesize nanoscale MnO_2, and their electrochemical properties were fully investigated in different electrolyte environments. Because of different mechanisms, the hydrothermal method gave an ultrathin layer of MnO₂ nanosheets with a large accessible electrochemical surface area, while the electrodeposition method resulted in the formation of MnO₂ nanorods with a rather limited opening area. The electrochemical performance of hydrothermally prepared MnO₂ was much better in specific capacitance and overall resistance, except the cycling durability, as a result of its unique structure.     

The mechanism of the ketene-imine (staudinger) reaction in its centennial: still an unsolved problem?

[Reaction: see text]. Although Staudinger reported the reaction between ketenes and imines 100 years ago (1907), this process is still the most general and useful method for the synthesis of beta-lactams and their derivatives. This reaction is a [2 + 2] thermal cycloaddition in which two chiral centers may be generated in one preparative step. Staudinger reactions involving alpha,beta-unsaturated imines or ketenes have issues concerning the [2 + 2] or [4 + 2] periselectivity of the reaction. This Account discusses how the main factors that determine the regiochemical and stereochemical outcomes of this reaction were elucidated with computational and experimental data. This fruitful interplay between theory and experiment has revealed that the [2 + 2] cycloaddition is actually a two-step process. The first step is a nucleophilic addition of the nitrogen atom of the imine on the sp-hybridized carbon atom of the ketene. This attack forms a zwitterionic intermediate that evolves toward the final beta-lactam cycloadduct. The second step can be viewed as a four-electron conrotatory electrocyclization that is subject to torquoelectronic effects. When alpha,beta-unsaturated imines are used, the zwitterionic intermediates yield either the corresponding 4-vinyl-beta-lactams or the alternative 3,4-dihydropyridin-2(1 H)-ones. In this latter case, the cyclization step consists of a thermal disrotatory electrocyclization. In the context of stereoselectivity, it is usually assumed that the first step takes place through the less hindered side of the ketene. The cis-trans selectivity of the reaction depends on the geometry of the imine. As the general rule, ( E)-imines form cis-beta-lactams whereas ( Z)-imines yield trans-beta-lactams. Most of the experimental results point to the two-step model. The asymmetric torquoselectivity of the conrotatory ring closure of the second step accounts for the stereochemical discrimination in the reaction of chiral ketenes or chiral imines. Nevertheless, recent studies have revealed that isomerization paths in the imine or in the zwitterion may determine the stereochemistry of the reaction. Thus, if the rotation about the N1-C4 bond of the zwitterion intermediate is faster than the cyclization, the formation of trans-beta-lactams from ( E)-imines is biased. Alternatively, in some cases, the ( E)-( Z) isomerization of the starting imines prior to the cycloaddition steps also results in the formation of trans-cycloadducts. Although the main variables that govern the outcome of the reaction have been elucidated, there are still several aspects of the reaction yet to be disclosed. Finally, the discovery of the catalytic version of the reaction is a new and formidable mechanistic challenge and will be a nice playground for forthcoming theoretical-experimental discussions.     

How does a chain-extended polylactide behave?: a comprehensive analysis of the material,structural and mechanical properties

The presented contribution deals with the material properties of chain-extended polylactide. In the course of this study, two different functional additives were used, namely an epoxidized and a maleated styrene-acrylic copolymer. Both additives were compounded together with polylactide using a conventional twin-screw extruder, and were then injection moulded to standardized testing specimens. The main focus of the investigation is on structural changes as well as the mechanical performance, e.g. crack propagation and arresting mechanisms that are affected by reactive chain extension. The first section of the experimental part consists of results regarding modifications achieved on the macromolecular level. Based on size exclusion chromatography and DSC-OIT experiments, different structural changes and their influence on the material behaviour are presented. Subsequently, a comprehensive analysis of the quasi-static tests and the impact strength of notched and unnotched specimens was performed, and correlated with the following fatigue experiments. A discussion concerning morphological aspects and finally a correlation to the fracture surface topography after fatigue test completes the experimental part.     

Methane aromatization on Zn-modified zeolite in the presence of a co-reactant higher alkane: How does it occur?

By using ¹³C solid-state NMR and GC–MS, the analysis of the ¹³C-label transfer from methane-¹³C into the products of methane and propane co-aromatization on Zn/H-BEA zeolite at 823–873 K has been performed. A high degree involvement of ¹³C-carbon atoms of methane into aromatic products (benzene, toluene, xylenes) has been demonstrated. The main pathway of methane conversion into aromatics has been determined to consist in the methylation of aromatics, which is produced exclusively from propane, by methane. The methoxy species formed by the dissociative adsorption of methane on ZnO species of the zeolite is responsible for the methylation.     

Surface composition and possible rearrangement of disperse Pt and Rh catalysts: does the presence of carbon and oxygen contribute to different catalytic properties?

The surface composition of Rh and Pt blacks (as determined by XPS) shows carbon and oxygen impurities in the untreated state. Oxygen on Pt is present as adsorbed O as well as OH/H₂O groups and oxidized carbon. Rh was partly oxidized to Rh₂O₃, in agreement with UPS showing hardly any Fermi‐edge intensity in untreated Rh as opposed to untreated Pt. High Fermi‐edge intensities indicated a predominant metallic surface after an in situ treatment with H₂ at 483 K, increasing the purity (XPS) to ∼90%. This treatment reduced Rh to metal and removed its C impurity. Pt, in turn, retained much carbon after H₂ treatment, present mainly as graphitic carbon. A minor amount of CO was also detected, some of the O 1s peak belonging to it. The two metals were tested in methylcyclopentane reactions. Considering the necessity of carbon for nondegradative reactions and oxygen enhancing fragmentation, a correlation is suggested between the typical impurities of Pt and Rh and their respective catalytic propensities: the high fragmentation activity of Rh and the predominant nondegradative reactions to C₆ “ring opening products” on Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Phase equilibria in the Nd2O3–BaO–Fe2O3 system: Crystal structure,oxygen content,and properties of intermediate oxides

The phase equilibria in the ½Nd₂O₃–BaO–½Fe₂O₃ system were systematically studied at 1373 K in air and presented in the form of a phase diagram. By X-ray diffraction (XRD) analysis of quenched samples, the homogeneity ranges and crystal structure were determined for the following intermediate oxides: Nd_1-xBa_xFeO_3-δ with 0.0 ≤ x ≤ 0.05 (space group (SG) Pnma) and with 0.6 ≤ x ≤ 0.7 (SG Pm-3m), Ba_6+yNd_2-yFe₄O_15-δ with 0.0 ≤ y ≤ 0.4 (SG P6₃mc), and Ba_1.1Nd_1.9Fe₂O_7-δ (SG P4₂/mnm). The structural parameters of single-phase oxides were refined by the Rietveld method. The changes in oxygen content for Nd_1-xBa_xFeO_3-δ (0.6 ≤ x ≤ 0.7), Ba₆Nd₂Fe₄O_15-δ, and Ba_1.1Nd_1.9Fe₂O_7-δ versus temperature in air were determined by thermogravimetric analysis. Gradual substitution of neodymium by barium in the Nd_1-xBa_xFeO_3-δ (0.6 ≤ x ≤ 0.7) oxides leads to a decrease of oxygen content. Partial ordering via formation of separate domains with a_p × a_p × 5a_p superstructure in Nd_0.4Ba_0.6FeO_3-δ, which cannot be detected by XRD, noticeably influence its thermal expansion and electrical conductivity.     

Vibronic Spectral Behaviour of Molecules: Theoretical study on the vibrational structure of 9-phenyl anthracene S1?S0 absorption and fluorescence

Underlying the completely-optimized S₀ and S₁ molecular geometries of 9-phenylanthracene, the vibronic ‘line’ spectra of the S₀ S₁ absorption and fluorescence have been calculated within the Condon approximation. On this basis the corresponding theoretical spectrum contours as obtained by superposition of individual Gaussian line width functions are presented. For the spectroscopic peaks an analysis of the vibronic levels is given. The theory/experiment comparison of the respective vibrationally structured spectra of 9-phenyl-anthracene turns out to be satisfactory. It confirms likewise the geometric structure data calculated for both electronic states.     

A new ternary phase in the system CaO–Al2O3–Cr2O3: Crystal structure and thermal expansion of CaAl2Cr2O7

Polycrystalline material of a novel phase in the system CaO–Al₂O₃–Cr₂O₃ has been obtained by solid-state reactions. Chemical analysis indicated the composition CaAl₂Cr₂O₇. Single-crystal growth of the new compound using borax as a mineralizer was successful. Diffraction experiments at ambient conditions on a crystal with composition CaAl_2.13Cr_1.87O₇ yielded the following basic crystallographic data: space group P 3, a = 7.7690(5) Å, c = 7.6463(5) Å, V = 399.68(6) ų, Z = 3. Structure determination and subsequent least-squares refinements resulted in a residual of R(|F|) = 2.3% for 1440 independent observed reflections and 113 parameters. To the best of our knowledge, the structure of CaAl_2.13Cr_1.87O₇ or CaAl₂Cr₂O₇ represents a new structure type. It belongs to the group of double layer structures where individual double layers contain octahedrally and tetrahedrally coordinated cation positions. Linkage between neighboring sheet packages is provided by additional calcium cations. Furthermore, thermal expansion has been studied in the interval between 29 and 790°C using in situ high-temperature single-crystal diffraction. No indications for a structural phase transition were observed. From the evolution of the lattice parameters the thermal expansion tensor has been obtained. A pronounced anisotropy is evident. The response of structural building units to variable temperature has been discussed.     

New insights into the mechanistic details of the carbonic anhydrase cycle as derived from the model system [(NH(3))(3)Zn(OH)](+)/CO(2): how does the H(2)O/HCO(3)(-) replacement step occur?

The full reaction path for the conversion of carbon dioxide to hydrogencarbonate has been computed at the B3LYP/6-311+G** level, employing a [(NH(3))(3)Zn(OH)](+) model catalyst to mimic the active center of the enzyme. We paid special attention to the question of how the catalytic cycle might be closed by retrieval of the catalyst. The nucleophilic attack of the catalyst on CO(2) has a barrier of 5.7 kcal mol(-1) with inclusion of thermodynamic corrections and solvent effects and is probably the rate-determining step. This barrier corresponds well with prior experiments. The intermediate result is a Lindskog-type structure that prefers to stabilize itself via a rotation-like transition state to give a Lipscomb-type product, which is a monodentate hydrogencarbonate complex. By addition of a water molecule, a pentacoordinated adduct with pseudo-trigonal-bipyramidal geometry is formed. The water molecule occupies an equatorial position, whereas the hydrogencarbonate ion is axial. In this complex, proton transfer from the Zn-bound water molecule to the hydrogencarbonate ion is extremely facile (barrier 0.8 kcal mol(-1)), and yields the trans,trans-conformer of carbonic acid rather than hydrogencarbonate as the leaving group. The carbonic acid molecule is bound by a short O...H-O hydrogen bond to the catalyst [(NH(3))(3)Zn(OH)](+), in which the OH group is already replaced by that of an entering water molecule. After deprotonation of the carbonic acid through a proton relay to histidine 64, modeled here by ammonia, hydrogencarbonate might undergo an ion pair return to the catalyst prior to its final dissociation from the complex into the surrounding medium.