A comprehensive modeling study of hydrogen oxidation

A detailed kinetic mechanism has been developed to simulate the combustion of H₂/O₂ mixtures, over a wide range of temperatures, pressures, and equivalence ratios. Over the series of experiments numerically investigated, the temperature ranged from 298 to 2700 K, the pressure from 0.05 to 87 atm, and the equivalence ratios from 0.2 to 6. Ignition delay times, flame speeds, and species composition data provide for a stringent test of the chemical kinetic mechanism, all of which are simulated in the current study with varying success. A sensitivity analysis was carried out to determine which reactions were dominating the H₂/O₂ system at particular conditions of pressure, temperature, and fuel/oxygen/diluent ratios. Overall, good agreement was observed between the model and the wide range of experiments simulated. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 603–622, 2004     

Radical-initiated polyolefinic cyclizations in condensed cyclopentanoid synthesis. Total synthesis of (±)-Δ9(12)-capnellene

Total synthesis of Δ⁹⁽¹²⁾-capnellene is reported. The key reaction involves construction of two rings in a single step via a tandem radical cyclization.     

On the Chaotic Behavior of a Third-Order Log-Domain Filter

Operation of log-domain filters revolves around the large-signal forward-active mode exponential current-voltage relationship of the bipolar junction transistor, which is used to map the input currents to the logarithmic domain, where the analog processing takes place, and to convert the filtered voltage waveforms back to the linear domain. Sometimes, the inherent internal nonlinearity of such circuits is the cause of unwanted externally-nonlinear effects. In this paper a differential third-order log-domain Chebyshev low-pass filter is designed by applying the method of operational simulation of doubly-terminated LC ladders. When the six grounded shunt capacitors of this filter are properly replaced with three half-sized floating capacitors, as is common practice in internally-linear fully-differential capacitively-loaded circuits, under particular conditions the resulting system exhibits limit-cycle oscillations, period-doubling bifurcations and chaotic behavior for zero-input. The presence of chaos is confirmed by extracting the spectrum of Lyapunov exponents of the system from the time series of the output voltage.     

Synthesis and characterization of N-heterocyclic carbene complexes of 1,3,2-dioxaborolane-4,5-dione (NHC-boryl oxalates)

NHC-boranes react with oxalic acid in acetonitrile slowly at room temperature and rapidly at 80?°C to form NHC-boryl oxalates. For example, heating of 1,3-dimethylimidazol-2-ylidene borane with oxalic acid in acetonitrile (80?°C, 2?h) provides the cyclic NHC-boryl oxalate derived from two sequential acid/base reactions in 59% isolated yield. The new NHC-boryl oxalates are robust white solids that are formally complexes of 1,3,2-dioxaborolane-4,5-dione, itself an unknown heterocycle.     

Discodermolide/Dictyostatin hybrids: synthesis and biological evaluation

[structure: see text] Two hybrid analogues of discodermolide and dictyostatin (3, 26) have been designed and synthesized. These are the first macrocyclic analogues of discodermolide and biological activities were evaluated and compared with linear discodermolide analogues.     

Death,taxes, and the genetic code?

The genetic code is an enduring feature of biology: only rare circumstances result in changes to translation of the code, at least in nature. Researchers are devising methods to engineer ribosome-synthesized polypeptides containing novel and potentially useful amino acids.     

Heterocyclic alpha-alkylidene cyclopentenones obtained via a Pauson-Khand reaction of amino acid derived allenynes. A scope and limitation study directed toward the preparation of a tricyclic pyrrole library

The synthesis of a novel class of tricyclic pyrroles has been accomplished by using a Pauson-Khand/Stetter/Paal-Knorr reaction sequence. Full details of the Pauson-Khand reaction of amino acid tethered allenynes 4a-e and 9a-d are disclosed. The study of this reaction led to the discovery of an unprecedented substituent effect on the diastereoselectivity of the Mo(CO)6 mediated allenic Pauson-Khand reaction. It was found that amino acid tethered allenynes with aromatic side chains afford alpha-alkylidene cyclopentenones with the opposite diastereoselectivity compared to those with aliphatic side chains. This effect has been attributed to complexation of the metal mediator to the aromatic ring in the substrate. Furthermore, an isomerization of one of the diastereomers of the alpha-alkylidene cyclopentenones was encountered, leading to eventual decomposition. The stable diastereomers were found to react well in the Stetter reaction leading to 1,4-diketones that were converted to pyrroles. The observation that the first generation of 2-alkyl-substituted pyrroles was unstable led to a second generation of 2-carboxamide pyrroles with sufficient stability for biological tests which are in progress.