Metalloporphyrins in Oxidative Catalysis. Oxygen Transfer Reactions of Oxochromium Porphyrins

The oxidation of chloro-5-10,15,20-tetramesitylporphyrinotoiron(III) with peroxyacids affords a reactive oxoiron(IV)-porphyrin cation radical species 2. The characterization of 2 and its oxochromium analogs 3, 4 and 5 are reviewed. The nature of reactive oxochromium species derived from chromyl reagents is also reviewed. The oxidation of triphenylphosphine by CrOTPP (11), CrOTTP (13) and CrOTMP (14) is described. Variations in the rate constants indicate that steric factors affect the rate of oxygen atom transfer. Activation parameters for the oxidation of triphenylphosphine by 14 are ΔH^≠ = 6.96 kcal/mol and ΔS^≠ = −39 eu. The oxidation of t-butylphenylcarbinol (18) by CrOTPP gave predominantly benzaldehyde via carbon—carbon bond cleavage while the chromium(III) porphyrin-catalyzed oxidation of 18 by iodosylbenzene afforded t-butylphenylketone.     

New materials for diode laser pumping of solid-state lasers

The authors review recent progress in the development of new materials for III-V semiconductor diode lasers useful for pumping solid-state lasers. All of the diode lasers discussed are grown on GaAs substrates. Particular emphasis is placed on the performance and reliability of high-CW-power strained-layer InGaAs-AlGaAs diode lasers emitting in the wavelength range between 0.87 and 1.1 μm, improved resistance to degradation of 0.78 to 0.87 μm diode lasers afforded by the strained-layer AlInGaAs-AlGaAs and lattice-matched GaInAsP-GaInP materials systems, and improved performance of visible diode lasers utilizing the materials system GaInP-AlGaInP     

Location of Aluminum in Substituted Calcium Silicate Hydrate (C-S-H) Gels as Determined by 29Si and 27Al NMR and EELS

Solid-state ²⁷Al and ²⁹Si magic angle spinning NMR spectroscopy has been combined with electron energy loss spectroscopy carried out in the transmission electron microscope to determine the location of Al substituting in a semicrystalline C-S-H gel present in a hydrated synthetic slag glass. The gel is found to contain mainly pentameric silicate chains in which the central silicon is substituted by aluminum.     

Mobility of trace elements from selected Australian fly ashes and its potential impact on aquatic ecosystems

Batch leaching tests have been performed on fly ashes collected from four Australian power stations fuelled by chemically different coals. Two acidic and two alkaline fly ashes were subjected to long-term (144 h) leaching tests, and the behaviour of As, B, Mo and Se was investigated to obtain data on their potential for mobilisation during fly ash-water interactions. All four elements are mobile under different conditions and over different leaching times. The concentrations of these elements released in leaching solutions with initial pH values of 4, 7 and 10 were used to assess the influence of pH conditions on element mobility from the acidic and alkaline fly ashes. The most mobile of the four elements leached were Mo from alkaline fly ashes and B from acidic fly ashes. Arsenic concentration increased with time in leachate solutions from acidic and alkaline fly ashes; however, in solutions in contact with alkaline fly ashes the As concentration, after reaching a maximum, later decreased with time. Selenium mobility shows a similar pattern to that of As, with similar leaching concentrations. Boron has the highest relative mobility of all four elements. A process possibly responsible for the decrease in concentration of B, As and Se in alkaline leaching solutions is the formation of ettringite. Equilibrium between the solid phase (ash) and the leaching solution was not reached in any of the leaching experiments. The pH of the leaching solution is the key factor affecting the mobility of these trace elements in these fly ashes.     

Paramagnetic 1H-NMR relaxation probes of stereoselectivity in metalloporphyrin catalyzed olefin epoxidation

Enantioselective catalytic epoxidation of olefins is an important problem from both practical and mechanistic points of view. The origins of chiral induction by asymmetric porphyrin and salen complexes were investigated by FT-NMR T1 relaxation techniques. A new chiral vaulted porphyrin (1) that carries (S)-binaphthyl-L-alanine straps across both faces of the porphyrin macrocycle was synthesized and characterized. (R)-styrene oxide was obtained in > 90% ee in the initial stages of styrene epoxidation with F5PhIO catalyzed by 1-Fe(III)Cl. The transition state for olefin epoxidation with high-valent metal-oxo species was modeled by coordinating epoxides to paramagnetic copper complexes of the corresponding ligands. The epoxide enantiomer that better fit the chiral cavity of the catalyst, as revealed by T1 relaxation measurements, was also the major product of catalytic olefin epoxidation. These results are consistent with the "lock-and-key" mechanism of asymmetric catalysis by metalloporphyrins. The copper complex of a chiral salen ligand showed no differentiation in terms of T1 relaxation rates between the enantiomers of cis-beta-methylstyrene oxide in contrast to the high enantioselectivity observed for catalytic epoxidation.     

Microstructure and microanalysis of hardened ordinary Portland cement pastes

Hardened ordinary Portland cement pastes of various ages have been examined by analytical transmission electron microscopy (TEM) and electron microprobe analysis (EMPA). The stability of the various hydrate phases in the electron microscope is discussed. Although all are subject to damage in varying degrees, even the least stable phase, AFt, can be recognized in relict form in the TEM. The basic framework of the microstructure and the differentiation into inner and outer hydration product are well-established at 24 h hydration. Although the dominant inner product formed within the boundaries of the original anhydrous grains is C-S-H gel, particles of AFt, AFm, Ca(OH)₂, a magnesium-rich phase and an iron-rich phase are occasionally observed within the inner product. The CaSi ratio of the C-S-H gel determined by TEM shows significant variation from one region to another in a given paste. There is no relationship between the average CaSi ratio of the C-S-H and the maturity of the paste, although young pastes appear to show a bimodal distribution. Microanalysis by EMPA gives CaSi ratios in substantial agreement with those found by TEM but it is essentially impossible to obtain by EMPA analyses of outer product C-S-H without admixture of other phases, particularly sulphoaluminate phases. Despite the presence of small amounts of embedded phases as revealed by TEM, single-phase inner product C-S-H can be analysed by EMPA. The compositions of AFt and AFm phases have been obtained by TEM and the results do not require the substitution of silicon in the formulae.     

Accurate modeling of monolithic inductors using conformal meshing for reduced computation

Accurate component modeling is a key factor to successful wireline and wireless circuit design in Si/SiGe BiCMOS and RF CMOS. This article presents the application of two planar electromagnetic simulation methods for reducing the memory and computation time requirement for accurate simulation of inductors fabricated with thick analog metal layers. First, a conformal subsectioning technique is briefly discussed in the context of reducing the numerical complexity of octagonal and circular spiral inductor analysis. Second, this article discusses a method for determining if more than a two-sheet model of thick metals is needed for accurate inductor simulation. Finally, the conformal mesh is applied to a 3.3-nH inductor fabricated using the IBM 0.13-/spl mu/m RF CMOS process technology. The simulated and measured results are compared.     

Verifying Concurrent Data Structures by Simulation

We describe an approach to verifying concurrent data structures based on simulation between two Input/Output Automata (IOAs), modelling the specification and the implementation. We explain how we used this approach in mechanically verifying a simple lock-free stack implementation using forward simulation, and briefly discuss our experience in verifying three other lock-free algorithms which all required the use of backward simulation.     

Study of the fracture toughness and fracture morphology of polybenzimidazole

Polybenzimidazole (PBI) is a relatively new polymeric material exhibiting unusual properties that are attributable to its aromatic-heterocyclic monomer structure. Owing to its high strength, stiffness and excellent stability in hostile chemical and thermal environments, PBI is being used increasingly in critical applications. As a result, understanding the failure mechanisms of the material is vital. This paper presents the results of a study of the fracture toughness and fracture morphology of polybenzimidazole. The standard compact tension specimen was used as the basic experimental specimen in this study. The fracture tests were performed in an Instron tensile testing machine. The effects of varying the loading rate, and the ratio of the initial crack length,a, to the ligament length,W, were investigated. The fracture surface morphology was examined using optical and scanning electron microscopy. The results of this study indicate that the precracking technique significantly affects the measured fracture toughness. Also, an increase in the loading rate causes a significant decrease in fracture toughness. Examination of the fracture morphology reveals that PBI fracture surfaces exhibit many of the characteristics expected of a tough engineering plastic.