On the convergence behavior of the restarted GMRES algorithm for solving nonsymmetric linear systems

The solution of nonsymmetric systems of linear equations continues to be a difficult problem. A main algorithm for solving nonsymmetric problems is restarted GMRES. The algorithm is based on restarting full GMRES every s iterations, for some integer s>0. This paper considers the impact of the restart frequency s on the convergence and work requirements of the method. It is shown that a good choice of this parameter can lead to reduced solution time, while an improper choice may hinder or preclude convergence. An adaptive procedure is also presented for determining automatically when to restart. The results of numerical experiments are presented.     

Caracterisation et Etude par effet Mössbauer d'une nouvelle variete haute pression de FeOOH

A new high pressure form of FeOOH, isostructural with InOOH, has been prepared by hydrothermal synthesis under very high pressure conditions. Characterization of the new phase was performed by D.T.A., T.G.A., infrared and Mössbauer spectroscopy. The compound behaves as an antiferromagnet with a Néel temperature of approximately 570° K. The hyperfine magnetic field at 0° K is about 525 kOe. Thermal decomposition occurs below the Néel temperature. Comparison with other FeOOH phases is made.     

A theoretical investigation on the interaction of a new gene vector with DNA

A new neutral gene vector, based on a lipopolythiourea N-(2-(3-[2-(2-(3-[2-(3-methyl-thioueido)-ethyl] -thioureido)-ethylamino)-ethyl]-thioureido)-ethyl)- N′, N′- ditetradecyl-succinamide (DTTU) has recently been synthetized but its behavior is difficult to study at the experimental level. Density functional theory (DFT) calculations have thus been performed to predict its interaction mode with B-DNA. Its acidic properties are first computed and suggest that DTTU should be non-charged when interacting with DNA. Different ways of DTTU/DNA associations based on hydrogen bonding–namely external and groove-binding interactions—are then investigated. Our calculations clearly point out that external interaction is preferred with respect to groove-binding, due to three bifurcated hydrogen bonds between DTTU thiourea groups and DNA phosphates. Such results can be explained by the absence of charged groups in groove-binding whereas the negative charge of DNA phosphates deeply strengthens hydrogen bonds.     

Aza-Michael Addition in Explicit Solvent: A Relative Energy Gradient–Interacting Quantum Atoms Study

Aza-Michael additions are key reactions in organic synthesis. We investigate, from a theoretical and computational point of view, several examples ranging from weak to strong electrophiles in dimethylsulfoxide treated as explicit solvent. We use the REG-IQA method, which is a quantum topological energy decomposition (Interacting Quantum Atoms, IQA) coupled to a chemical-interpretation calculator (Relative Energy Gradient, REG). We focus on the rate-limiting addition step in order to unravel the different events taking place in this step, and understand the influence of solvent on the reaction, with an eye on predicting the Mayr electrophilicity. For the first time, a link is established between an REG-IQA analysis and experimental values.     

Modular and practical synthesis of 6-substituted pyridin-3-yl C-nucleosides

A novel modular and practical methodology for preparation of 6-substituted pyridin-3-yl C-nucleosides was developed. The Heck reaction of 2-chloro-5-iodopyridine with a 3'-TBDMS-protected glycal gave a 6-chloropyridin-3-yl nucleoside analogue, which was then desilylated, selectively reduced, and reprotected to give the TBDMS-protected 6-chloropyridin-3-yl C-2'-deoxyribonucleoside as a pure beta-anomer in a total yield of 39% over four steps. This key intermediate was then subjected to a series of palladium-catalyzed cross-coupling reactions, aminations, and alkoxylations to give a series of protected 1beta-(6-alkyl-, 6-aryl-, 6-hetaryl, 6-amino-, and 6-tert-butoxypyridin-3-yl)-2'-deoxyribonucleosides. 6-Unsubstituted pyridin-3-yl C-nucleoside was prepared by catalytic hydrogenation of the chloro derivative and 6-oxopyridine C-nucleoside by treatment of the 6-tert-butoxy derivative with TFA. Deprotection of all the silylated nucleosides by Et3N.3HF gave a series of free C-nucleosides (10 examples).     

Novel heterometal-organic complexes as first single source precursors for up-converting NaY(Ln)F4 (Ln = Yb, Er, Tm) nanomaterials

First heterometal-organic single source precursors for NaYF(4) nanomaterials as a host matrix for up-conversion emission are reported. These novel heterobimetallic derivatives NaY(TFA)(4)(diglyme) (1), [Na(triglyme)(2)][Y(2)(TFA)(7)(THF)(2)] (2) and Na(2)Y(TFA)(5)(tetraglyme) (3) (TFA = trifluoroacetate), which were fully characterized by elemental analysis, FT-IR and (1)H NMR spectroscopy, TG-DTA data as well as single crystal X-ray structures, are advantageous in terms of being anhydrous and having lower decomposition temperatures in comparison to the homometallic precursor Y(TFA)(3)(H(2)O)(3). In addition, they also contain chelating glyme ligands, which act as capping reagents during decomposition to control the NaYF(4) particle size and render them monodisperse in organic solvents. On decomposition in 1-octadecene, the molecular derivatives 1 and 3 are converted, in the absence of any surfactant or capping reagent, to cubic NaYF(4) nanocrystals at significantly lower temperatures (below 250 °C). At higher temperature, a mixture of the cubic and hexagonal phases was obtained, the relative ratio of the two phases depending on the reaction temperature. A pure hexagonal phase, which is many folds more efficient for UC emission than the cubic phase, was obtained by calcining nanocrystals of mixed phase at 400 °C. In order to co-dope this host matrix with up-converting lanthanide cations, analogous complexes NaLn(TFA)(4)(diglyme) [Ln = Er (4), Tm (5), Yb (6)] and Na(2)Ln(TFA)(5)(tetraglyme) [Ln = Er (7), Yb (8)] were also prepared and characterized. The decomposition in 1-octadecene of suitable combinations and appropriate molar ratios of these yttrium, ytterbium and erbium/thulium derivatives gave cubic and/or hexagonal NaYF(4): Yb(3+), Er(3+)/Tm(3+) nanocrystals (NCs) capped by diglyme or tetraglyme ligands, which were characterized by IR, TG-DTA data, EDX analysis and TEM studies. Surface modification of these NCs by ligand exchange reactions with poly acrylic acid (PAA) and polyethyleneglycol (PEG) diacid 600 was also carried out to render them water soluble. The THF solutions of suitable combinations of the diglyme derivatives were also used to elaborate the thin films of NaYF(4):Yb(3+), Er(3+)/Tm(3+) on a glass or Si wafer substrate by spin coating. The multicolour up-conversion fluorescence was successfully realized in the Yb(3+)/Er(3+) (green/red) and Yb(3+)/Tm(3+) (blue/violet) co-doped NaYF(4) nanoparticles and thin films, which demonstrates that they are promising UC nanophosphors of immense practical interest. The up-conversion excitation pathways for the Er(3+)/Yb(3+) and Tm(3+)/Yb(3+) co-doped materials are discussed.     

Semiclassical line broadening calculations,using an ab initio potential energy surface,in Q‐branch and S‐branch of N2 perturbed by H2

In order to improve the accuracy of Raman combustion diagnostics, the broadening coefficients of N₂ perturbed by H₂ for Q‐branch and S‐branch have been calculated by the semiclassical Robert–Bonamy model using an ab initio potential energy surface. The calculations have been performed for a large range of temperatures (from 77 to 800 K) and J rotational quantum numbers (from 0 to 60). This paper shows that our results and their temperature dependence are in good agreement with earlier published experimental and theoretical data. Moreover, our results improve the semiclassical calculations made earlier with an adapted analytical potential. The results, obtained at high temperatures and for a large range of rotational quantum numbers, are presented in order to be implemented for optical diagnostics in combustion media. Copyright © 2012 John Wiley & Sons, Ltd.     

Isolation of isomangiferin from honeybush (Cyclopia subternata) using high-speed counter-current chromatography and high-performance liquid chromatography

Isomangiferin was isolated from Cyclopia subternata using a multi-step process including extraction, liquid–liquid partitioning, high-speed counter-current chromatography (HSCCC) and semi-preparative reversed-phase high-performance liquid chromatography (HPLC). Enrichment of phenolic compounds in a methanol extract of C. subternata leaves was conducted using liquid–liquid partitioning with ethyl acetate–methanol–water (1:1:2, v/v). The enriched fraction was further fractionated using HSCCC with a ternary solvent system consisting of tert-butyl methyl ether–n-butanol–acetonitrile–water (3:1:1:5, v/v). Isomangiferin was isolated by semi-preparative reversed-phase HPLC from a fraction containing mostly mangiferin and isomangiferin. The chemical structure of isomangiferin was confirmed by LC–high-resolution electrospray ionization MS, as well as one- and two-dimensional NMR spectroscopy.     

A study of some parameters relevant to the response of harwell PMMA dosimeters to gamma and electron irradiation

The effects on the radiation response of Harwell polymethylmethacrylate (PMMA) dosimeters of dose-rate, radiation type, temperature during irradiation and post-irradiation storage, and post-irradiation stability, are of importance to the operators of commercial irradiation facilities.

This paper describes recent studies of the effects of some of these parameters on the radiation response of Harwell Red 4034, Amber 3042, and Gammachrome YR Perspex dosimeters, and provides data on batch to batch variation and shelf-life.