State-specific reactions and autoionization dynamics of Ar produced by synchrotron radiation

The long-lived excited states of doubly charged rare gases can markedly affect their reactivity. In this paper we demonstrate the presence of strong state-specific effects in the charge exchange of Ar²⁺ (³P, ¹D and ¹S) with several neutral targets (He, Ne, Kr, Xe, D₂, and CH₄). State sensitive measurements have been performed by producing the different Ar²⁺ electronic states via tunable synchrotron radiation (Elettra-Trieste, Italy and SuperACO-Orsay, France). From the product ion yield data of charge transfer, state-selected total cross-sections have been deduced. Using the state-specific reactivity of Ar²⁺ towards different neutral targets, it has been possible to extract the photon-energy-dependent production branching of the three doubly charged states and to investigate the autoionization dynamics of neutral or singly charged Ar in the vicinity of the double ionization threshold.     

Relative acidities and structure analysis of cis and trans isomers of 1,5‐oxazaspiro[5.5] undecane derivatives by multinuclear magnetic resonance

The relative acidities of the cis and trans isomers of a series of 1,5‐oxazaspiro[5.5]undecane derivatives were determined by measuring ΔpK in acid‐base titrations followed by ¹ H NMR. Relative structural stabilities were determined by measuring substituent chemical shift and γ‐gauche effects in ¹³C, ¹⁵N, and ¹⁷O NMR. Crystallographic characterization of a model spiro[5.5]undecane is presented to support the basicity in solid state. Copyright © 2010 John Wiley & Sons, Ltd.     

Reversible click chemistry at the service of macromolecular materials. Part 1: Kinetics of the Diels-Alder reaction applied to furan-maleimide model compounds and linear polymerizations

The model Diels-Alder reaction of furfuryl acetate with N-methylmaleimide and the linear polymerization of the corresponding difunctional monomers were followed by both UV and ¹H NMR spectroscopy. The results obtained from this study provided clear-cut indications about the most appropriate conditions to be applied in the preparation of novel macromolecular materials based in part on renewable resources and possessing thermoreversible, mendable and recycling features, among other properties.     

Formation of out of plane oxime metallacycles in [Cu2] and [Cu4] complexes

The reaction of Cu(ClO₄)₂·6H₂O with dimethylglyoxime (H₂dmg) in a 1:1 mole ratio in aqueous methanol at room temperature affords the dinuclear complex [Cu₂(μ-Hdmg)₄] (1). Reaction of 1 with [Cu(bpy)(H₂O)₂](ClO₄)₂ (bpy = 2,2′-bipyridine) in a 1:1 mole ratio in aqueous methanol at room temperature yields the tetranuclear complex [Cu₄(μ-Hdmg)₂(μ-dmg)₂(bpy)₂(H₂O)₂](ClO₄)₂ (2). The direct reaction of Cu(ClO₄)₂·6H₂O with H₂dmg and bpy in a 2:2:1 mole ratio in aqueous methanol at room temperature also yields 2 quantitatively. The complexes 1 and 2 were structurally characterized by X-ray crystallography. Unlike the binding in Ni/Co-dmg, two different types of N?O bridging modes during the oxime based metallacycle formation and stacking of square planar units have been identified in these complexes. The neutral dinuclear complex 1 has CuN₄O coordination spheres and complex 2 consists of a dicationic [Cu₄(μ-Hdmg)₂(μ-dmg)₂(bpy)₂(H₂O)₂]²⁺ unit and two uncoordinated ClO₄^? anions having CuN₄O and CuN₂O₃ coordination spheres. The two copper(II) ions are at a distance of 3.846(8) Å in 1 for the trans out of plane link and at 3.419(10) and 3.684(10) Å in 2 for the trans out of plane and cis in plane arrangements, respectively. The average Cu–N_oxime distances are 1.953 and 1.935 Å, respectively. The average basal and apical Cu?O_oxime distances are 1.945, 2.295 and 2.429 Å. The UV–Vis spectra of 2 is similar to the spectrum of the reaction mixture of 1 and [Cu(bpy)(H₂O)₂]²⁺. Variable temperature magnetic properties measurement shows that the interaction between the paramagnetic copper centers in complex 1 is antiferromagnetic in nature. The EPR spectra of frozen solution of the complexes at 77 K consist of axially symmetric fine-structure transitions (ΔM_S = 1) and half-field signals (ΔM_S = 2) at ca. 1600 G, suggesting the presence of appreciable Cu–Cu interactions.     

Determination of Antioxidant Activity of Hibiscus Flowers by Flow Injection Analysis with Chemiluminescence Detection

This study reports a simple and rapid method for the determination of the antioxidant activity of the phenolics present in plants of the Hibiscus genus. The protocol is based on the ability of the phenolic compounds to inhibit luminol chemiluminescence through the inactivation of free radicals in a flow injection system. Using the optimized operational conditions, the antioxidant activities of several phenolics were measured. The results were explained on the basis of structure–reactivity relationships. The antioxidant activities of dried flowers of Hibiscus rosa-sinensis and Hibiscus sabdariffa were evaluated. The protocol was validated by the 2,2-diphenyl-1-picrylhydrazyl-hydrate free radical assay. The differences in the antiradical activities were explained based on the concentrations of total polyphenol and flavonoid concentrations determined by spectrophotometric methods. It was concluded that the chemiluminescence method was complementary for the evaluation of hydrophilic antioxidants in plants.     

New phosphorylating reagents for deoxyribonucleosides and oligonucleotides

New phosphorylating reagents 1 and 2 were prepared in three steps from 4-hydroxybenzaldehyde. They showed good efficiency in the solid phase synthesis of 5′-phosphate monoester nucleosides. End-phosphate DNA sequence synthesis demonstrated the efficiency of the new reagents (1 and 2) according to the general procedure of automated DNA synthesis. The oxidation of P(III) to P(V) and the removal of benzyl protecting groups were achieved in a single step by treatment with a 0.02 M I₂/pyridine/H₂O solution. Due to this one-pot treatment, it is possible to use the phosphorylating reagents (1 and 2) for the synthesis of base-sensitive ODNs. The reagents 1 and 2 are unique among phosphorylating reagents.     

Epoxy lactones by photooxidative rearrangement of 6β-acetoxyvouacapane

The first study of photooxidation reaction of 6β-acetoxyvouacapane isolated from Caesalpinia platyloba is reported. The reaction yielded four new epoxy lactones, 6β-acetoxy-15,16α-epoxy-13-spirocassa-12,16-olide, 6β-acetoxy-15,16β-epoxy-13-spirocassa-12,16-olide, 6β-acetoxy-12,13β-epoxycassa-16,12-olide and 6β-acetoxy-12,13α-epoxycassa-16,12-olide. All the structures were supported by 1D and 2D NMR spectroscopy as well as mass spectrometry. The stereochemistry was established on the base of single crystal X-ray diffraction.     

Coordination and Dehydrogenation of Amine–Boranes at Metal Centers

There have been a number of approaches developed for the catalyzed dehydrogenation of amine–boranes as potential dihydrogen sources for hydrogen storage applications in recent years. Key advances in this area have been recently made thanks to catalytic and stoichiometric studies. In this Minireview, the fate of amine–boranes upon coordination to a metal center is discussed with a particular emphasis on B? H activation pathways. We focus on the few cases in which coordination of the resulting dehydrogenated product could be achieved, which includes the coordination of aminoborane, the simplest unit resulting from dihydrogen release of ammonia–borane.     

Evolution of first-order sidebands from multiple FWM processes in HiBi optical fibers

The generation of two idler waves inside a high birefringent (HiBi) optical fiber through three four-wave mixing (FWM) processes is studied theoretically. The coupled-equations for the field amplitudes are derived and analytically solved, in the co-polarized and orthogonal polarization schemes. The obtained solutions take into account the delayed Raman response of the medium. The polarization sensitivity of the generation of the idler waves is analyzed. Results show that the stimulated Raman scattering does not change the efficiency of the idler wave generation in the co-polarized scheme, whereas in the orthogonal polarization scheme that nonlinear process decreases the efficiency of the four-wave mixing processes. Results also show that this set of multiple four-wave mixing processes is physically quite different from the typical single or dual pump four-wave mixing configurations. Findings show that the power transfer from the pumps to the idler fields can lead to a monotonous growth, or a periodic evolution of the sidebands along the fiber. Results show that the process efficiency varies greatly with the angle between the two pump polarizations.