A class of deformational flow test cases for linear transport problems on the sphere

A class of new benchmark deformational flow test cases for the two-dimensional horizontal linear transport problems on the sphere is proposed. The scalar field follows complex trajectories and undergoes severe deformation during the simulation; however, the flow reverses its course at half-time and the scalar field returns to its initial position and shape. This process makes the exact solution available at the end of the simulation, and facilitates assessment of the accuracy of the underlying transport scheme. A procedure to eliminate possible cancellations of errors when the flow reverses is proposed.     

Syntheses of β- and γ-fluorophenyl cis- and trans-α-methylene-γ-butyrolactones

Preparation of a series of cis-γ-fluorophenyl-β-phenyl-α-methylene-γ-butyrolactones is reported via ‘allylboration’ of fluorobenzaldehydes with (E)-methyl 3-phenyl-2-((4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)methyl)acrylate. The corresponding trans-γ-fluorophenyl lactones were prepared either (i) via ‘allylboration’ using the (Z)-reagents or (ii) via an indium triflate-mediated isomerization of the cis-products. The difficulty in isomerizing difluorinated cis-products confirms the probable intermediacy of carbocations. Finally, the synthesis of cis-β-fluorophenyl-γ-phenyl-α-methylene-γ-butyrolactones was achieved via an indium-catalyzed allylation–lactonization of aldehydes with (Z)-2-(bromomethyl)-3-(fluorophenyl)acrylates.     

Studies on the oxidation of α-amino acids by N-Bromo oxidants: Kinetics of the reaction of bromide ion with N-Bromoacetamide

The rate of oxidation of amino acids (AA) by N-Bromoacetamide (NBA) was studied in aqueous buffered medium at 35°C. The rate of disappearance of [NBA] is catalyzed by the Br⁻ produced from the reduction of NBA. Analysis of the autocatalyzed reaction gives the kinetic data for the oxidation of bromide ion by NBA. The results suggest that the protonated NBA reacts with Br⁻ to form Br₂ which rapidly oxidizes amino acids. The rate constant for the reaction between protonated NBA and Br⁻ at 35°C is estimated. © 1996 John Wiley & Sons, Inc.     

Selective reductions. 56. Exploration of the B-haloisopinocampheylboranes for asymmetric reduction of ketones

A series of diisopinocampheylhaloboranes and monoisopinocampheyldihaloboranes were synthesized by the reaction of the corresponding boranes with the respective HX or X₂ (X = halogen) or by the hydroboration of α-pinene with the corresponding haloboranes. Stabilities of these haloboranes in various solvents were studied. Most of these haloboranes proved capable of reducing prochiral ketones to the alcohols with significant optical induction. When tested against a representative aromatic and aliphatic prochiral ketone, acetophenone and 3-methyl-2-butanone, respectively, α-phenethyl alcohol in 65–98% ee and 3-methyl-2-butanol in 28–59% ee were obtained. Some of them exhibited anomalous behavior.     

Recent Developments in Carbon-Based Nanocomposites for Fuel Cell Applications: A Review

Carbon-based nanocomposites have developed as the most promising and emerging materials in nanoscience and technology during the last several years. They are microscopic materials that range in size from 1 to 100 nanometers. They may be distinguished from bulk materials by their size, shape, increased surface-to-volume ratio, and unique physical and chemical characteristics. Carbon nanocomposite matrixes are often created by combining more than two distinct solid phase types. The nanocomposites that were constructed exhibit unique properties, such as significantly enhanced toughness, mechanical strength, and thermal/electrochemical conductivity. As a result of these advantages, nanocomposites have been used in a variety of applications, including catalysts, electrochemical sensors, biosensors, and energy storage devices, among others. This study focuses on the usage of several forms of carbon nanomaterials, such as carbon aerogels, carbon nanofibers, graphene, carbon nanotubes, and fullerenes, in the development of hydrogen fuel cells. These fuel cells have been successfully employed in numerous commercial sectors in recent years, notably in the car industry, due to their cost-effectiveness, eco-friendliness, and long-cyclic durability. Further; we discuss the principles, reaction mechanisms, and cyclic stability of the fuel cells and also new strategies and future challenges related to the development of viable fuel cells.     

Orientational information from TEDOR spectral sidebands.

In a dipolar-coupled spin-1/2 network of the type 15N1-(13)C-15N2, an assessment of the sensitivity of the N --> C and C --> N TEDOR sideband intensities to the Euler angles defining the orientation of the two heteronuclear dipolar vectors in the 13C and 15N chemical shift (CS) tensor principal axes system has been carried out via numerical calculations. The results clearly indicate the potential of TEDOR MAS NMR spectroscopy for the characterization of the CS tensor orientation in the molecular frame. The efficacy of the method has been experimentally illustrated by TEDOR studies on a polycrystalline sample of [1, 3-(15)N2, 2-(13)C]uracil, which is one of the four bases in RNA.     

Use of a graded gain random amplifier as an optical diode

The spectral characteristics of liquid amplifying media have been used to design and experimentally realize an optical device that prevents the propagation of a band of wavelengths in one direction and permits it in the opposite direction, thus acting as an optical diode. The addition of random scattering centers is shown to narrow the width of the forbidden band. A model is proposed to explain the observations and is verified by Monte Carlo simulations.     

Fluorescence from doubly driven four-level atoms

The unusually narrow features in the fluorescence from ⁸⁵Rb driven by two laser fields L1 and L2, reported in [1], are explained on the basis of a four-level density matrix calculation. The L2 laser enables atom transfer to the fluorescing levels connected by the strong L1 laser. In turn the L1 laser causes the Autler-Townes splitting of the upper levels connected by L2 laser. These two effects are shown to maximise fluorescence within a narrow spectral range of the scanned L2 laser due to velocity selection of atoms from co-propagating and counter propagating L1 and L2 lasers. The analysis reveals the existence of narrow spectral features from a collection of atoms at room temperature even in the absence of induced coherences between the levels.Received: 2 July 2004, Published online: 21 September 2004PACS: 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift - 32.80.Bx Level crossing and optical pumping     

Decay instability in a plasma channel

Near the critical layer, a large-amplitude laser beam creates a transverse plasma channel due to the ponderomotive force. The axis of the channel is normal to the ambient density gradient. Inside the channel, the laser beam decays into counter-propagating ion acoustic and Langmuir waves. A nonlocal theory of the process is developed for a parabolic density profile. The growth rate of the instability is significantly reduced due to the nonlocal effects