An asymptotic finite element method for improvement of solutions of boundary layer problems

Summary A scheme that uses singular perturbation theory to improve the performance of existing finite element methods is presented. The proposed scheme improves the error bounds of the standard Galerkin finite element scheme by a factor of O(ⁿ⁺¹) (where is the small parameter andn is the order of the asymptotic approximation). Numerical results for linear second order O.D.E.'s are given and are compared with several other schemes.     

Enhancement of a 24.77-nm line emitted by the plasma of a boron nitride capillary discharge irradiated by a high-intensity ultrashort laser pulse

Investigations of plasma produced by a boron nitride capillary discharge irradiated with a guided 20-TW Ti: sapphire laser pulse at a peak intensity of 4 x 10(18) W/cm2 are presented. The guided laser radiation in the plasma channel generated He-like ions that, subject to suitable plasma temperature, recombined into Li-like nitrogen ions. Intense radiation at a wavelength of 24.77 nm was observed, indicating possible lasing at the 3d(5/2) - 2p(3/2) transition in Li-like nitrogen.     

The structure of the bitetrahedryl molecule—A major shift due to electron correlation: Effects of carbonyl substituents,implications for the structure of coupled tricyclo[3.1.0.02,6]hexyl,and extension to cubylcubane

Ab initio molecular quantum mechanics has been applied to a number of coupled ring systems including biterahedryl and cubylcubane. Basis sets at least as large as double zeta augmented by carbon d functions (DZ +d) were used throughout. For biterahedryl, electron correlation effects qualitatively alter the molecular structure, decreasing the central C? C bond distance while increasing the adjacent C? C distance. Carbonyl substituents, contrary to some previous thinking, do not qualitatively alter the bitetrahedryl structure. This finding points to a structural problem with the monomer bicyclo[3.1.0.0^2,6]hexane that carries over to the coupled bicyclo[3.1.0.0^2,6]hexyl molecule. Finally, for the cubylcubane molecule synthesized in 1988, the only significant difference between Hartree–Fock theory and experiment occurs for the central C? C distance, which, as in the other coupled molecules, is too long. These results significantly lengthen the (still very short) list of closed-shell hydrocarbon molecules for which Hartree–Fock theory encounters structural difficulties.     

The mechanism of the H2 + CL2 reaction: Ignition behind reflected shocks

A detailed shock-tube investigation of the ignition in H₂ + Cl₂ + Ar mixtures in a shock tube is presented, and the mechanism of the reaction is discussed. Ignition delay times were determined from pressure and heat flux measurements behind reflected shock waves. The induction times measured ranged between 35 and 2100 μsec over the temperature range of 830–1260°K. The experimental results of close to seventy tests can be correlated by the relationship where the concentrations are expressed in mole/cm³. The above relationship served as a basis for a computer modeling of the ignition delay times. Ten calculations, simulating typical laboratory experiments, were run by the computer for each reaction scheme and the obtained temperature and composition dependence of the induction times were compared with the ones observed experimentally. A reaction scheme based on a simple exothermal chain propagation could not reproduce the experimental relationship. When the energy branching reaction HCl*(ν) + Cl₂ → (HCl₃) → HCl + Cl + Cl was added to the reaction scheme, a much better agreement with the experiment was obtained. It is believed that the above reaction does take place and that it is the main supplier of atoms to the system.     

Probing the anomeric effect. The diaminomethylene group: Calculations of N?C?N-containing molecular systems1

A study of systems containing the title moiety is described, with special reference to the anomeric effect. We have calculated ab initio, using Gaussian-80 with the 3-21G basis set, all basic conformations of methylene-diamine (H₂N? CH₂? NH₂) and its N-methyl derivative with full geometry optimization of energy minima and barriers. The structural data thus obtained, were then employed to parameterize Allinger's MM2-80 force field in a procedure similar to that described for oxygen derivatives, including hydrogen-bonding effects and C? N bond shortening in tertiary amines. This modification, termed MM2-AE was then used to calculate larger molecules, including N,N′-di- and N,N,N′,N′-tetramethyl-methylenediamine, various 1,3-diazane systems, and 1,4,5,8-tetraazadecalin derivatives of established (x-ray) structures. The results are discussed in light of their verificative and predictive power and appear to validate MM2-AE as a useful computational procedure.     

Modified MM2 scheme for computation of O?C?N-containing systems

A modification of Allinger's MM2 force field for the anomeric effect in O? C? N systems is presented. For optimal consistency, it was parameterized by alternate use of ab initio (3-21G level) and X-ray results to account for the energetic and structural manifestations of the effect in the gas or condensed phase, respectively. The results obtained with the modified force field are in good agreement with those from both theoretical and experimental methods. The parameterization scheme explicitly treats all structural parameters of the C? O? C? N? C moiety as well as C? N bond lengths in tertiary amines contained within an anomeric unit. In addition, it includes directional H-bond type interactions. A limited number of parameters is put forward, in accord with the general MM2 force-field approach. © 1993 John Wiley & Sons, Inc.     

New approach for the removal of metal ions from water: adsorption onto aquatic plants and microwave reaction for the fabrication of nanometals

We adsorb heavy metal ions such as Ag(+), Pb(2+), and Ru(3+) onto an aquatic plant and convert the adsorbed ions to the corresponding nanometallic particles by the polyol reaction carried out in a microwave oven.