β-N-Heterocyclic Cyclobutane Carboximides: Synthesis through a Tandem Base-Catalyzed Amidation/aza-Michael Addition Protocol and Facile Transformations

A stereoselective one-pot double derivatization of cyclobutene-1-carboxylic acid via a mild organic base catalyzed amidation/aza-Michael addition of benzo[d]oxazol-2(3H)-ones has been developed. This unprecedented tandem reaction provides access to novel β-N-heterocyclic cyclobutane carboximide derivatives with a trans geometry. The carboximide moiety reacts smoothly with nucleophiles, allowing access to diverse derivatives of trans-β-N-heterocyclic cyclobutanecarboxylic acid, including peptidomimetic structures.     

Bacterial iron transport: coordination properties of azotobactin, the highly fluorescent siderophore of Azotobacter vinelandii

Azotobacter vinelandii, a nitrogen-fixing soil bacterium, secretes in iron deficiency azotobactin delta, a highly fluorescent pyoverdin-like chromopeptidic hexadentate siderophore. The chromophore, derived from 2,3-diamino-6,7 dihydroxyquinoline, is bound to a peptide chain of 10 amino acids: (L)-Asp-(D)-Ser-(L)-Hse-Gly-(D)-beta-threo-HOAsp-(L)-Ser-(D)-Cit-(L)-Hse-(L)-Hse lactone-(D)-N(delta)-Acetyl, N(delta)-HOOrn. Azotobactin delta has three different iron(III) binding sites which are one hydroxamate group at the C-terminal end of the peptidic chain (N(delta)-Acetyl, N(delta)-HOOrn), one alpha-hydroxycarboxylic function in the middle of the chain (beta-threo-hydroxyaspartic acid), and one catechol group on the chromophore. The coordination properties of its iron(III) and iron(II) complexes were measured by spectrophotometry, potentiometry, and voltammetry after the determination of the acid-base functions of the uncomplexed free siderophore. Strongly negatively charged ferric species were observed at neutral p[H]'s corresponding to a predominant absolute configuration Lambda of the ferric complex in solution as deduced from CD measurements. The presence of an alpha-hydroxycarboxylic chelating group does not decrease the stability of the iron(III) complex when compared to the main trishydroxamate siderophores or to pyoverdins. The value of the redox potential of ferric azotobactin is highly consistent with a reductive step by physiological reductants for the iron release. Formation and dissociation kinetics of the azotobactin delta ferric complex point out that both ends of this long siderophore chain get coordinated to Fe(III) before the middle. The most striking result provided by fluorescence measurements is the lasting quenching of the fluorophore in the course of the protonation of the ferric azotobactin delta complex. Despite the release of the hydroxyacid and of the catechol, the fluorescence remains indeed quenched, when iron(III) is bound only to the hydroxamic acid, suggesting a folded conformation at this stage, around the metal ion, in contrast to the unfolded species observed for other siderophores such as ferrioxamine or pyoverdin PaA.     

Stability of Semi-Discrete Shock Profiles by Means of an Evans Function in Infinite Dimensions

Semi-discrete shock profiles are traveling wave solutions of hyperbolic systems of conservation laws under discretization in space. The existence of semi-discrete shocks has been investigated in earlier papers. Here the spectral stability of those nonlinear waves is addressed, and formulated in terms of a variational delay differential operator. Constructing a generalized Evans function, in infinite dimensions, it is shown how to derive stability criteria. Some examples are given when the criterion is fully explicit, e.g., for extreme Lax shocks. Additionally, connection is made with the alternative approach proposed by Chow, Mallet-Paret, and Shen (Journal of Differential Equations 1998), regarding the stability of traveling waves in general Lattice Dynamical Systems.     

Investigation of nanoparticle generation during femtosecond laser ablation of metals

The production of nanoparticles via femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume.     

Lead zirconate titanate (PZT) films deposited by a hydrothermal method

We present a hydrothermal method to deposit PZT films. It uses a single-step hydrothermal process and user-friendly precursors. Pb(Zr_0.52,Ti_0.48)O₃ polycrystalline films, 5–10 μm thick, were produced in an aqueous basic solution (4M KOH) on a titanium substrate in a closed autoclave at 180 °C. A time-depending study to see the evolution of the films' formation is described. Some actuation testing is also presented.     

Copolyimides containing alicyclic and fluorinated groups: Characterization of the film microstructure

The microstructures of a series of copolyimide films were characterized with different experimental methods such as density measurements, X‐ray diffraction, ultraviolet‐visible spectrophotometry, positron annihilation spectroscopy, and dynamic mechanical analysis. The experimental data were linked to the chemical structures of the polymers and especially the alicyclic and fluorinated monomers. Some analysis responses were directly dependent on the fluorine atoms and, therefore, did not provide clear information about the microstructures. The chain organization in the amorphous films appeared to be significantly dependent on the effect of the casting solvent. The influence of the alicyclic group content was quite significant for a nonsubstituted diamine but was strongly attenuated with a fluorinated diamine. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2998–3010, 2003     

MixingC rmaps of the interval without maximal measure

We construct aC ^rtransformation of the interval (or the torus) which is topologically mixing but has no invariant measure of maximal entropy. Whereas the assumption ofC ^∞ ensures existence of maximal measures for an interval map, it shows we cannot weaken the smoothness assumption. We also compute the local entropy of the example.     

Action de l'acide fluorhydrique sur les azirines : synthèse d'α-fluorocétones et de difluoroamines - étude de l'orientation de la réaction

Addition of Olah's reagent in benzene or methylene chloride to azirines leads to the formation of difluoroamines or α-fluoroketones. This reaction has been applied to the synthesis of 17 α-fluoropregnenolone. The formation of fluoroketone proceeds via a cationic intermediate. Yield is improved when using a more nucleophilic fluorinating reagent obtained by addition of triethylamine to Olah's reagent.     

Photochemical reactivity of trifluoromethyl aromatic amines: the example of 3,5-diamino-trifluoromethyl-benzene (3,5-DABTF)

This work presents the application of an on-line photoreactor to the detection of 3,5-diamino-trifluoromethyl-benzene (3,5-DABTF) in aqueous solutions. When irradiated at 310 nm, this compound is defluorinated to 3,5-diaminobenzoic acid by a nucleophilic substitution of the fluoride by water. Concomitantly, defluorination intermediates polymerize through amide bonds to give dark-colored compounds. We take advantage of the photocatalyzed defluorination and the subsequent decrease in pH to develop an original and specific photoreactor. Continuous recording of pH and temperature in the outlet of the reactor by a dual electrode gives us an opportunity to optimize the system. In the photoreactor, 3,5-DABTF is immediately and totally transformed as attested by the rapid drop of the flowing solution pH from 6.2 to 3.2 and the chromatographic analysis of the outgoing solutions. The detection remains effective from 1 to 1000 parts per million.     

Theoretical description of the electronic structure of the alkali hydride cation NaH+

A theoretical determination of the electronic structure of NaH+ is presented. Potential energy curves and dipole moments have been computed for 48(2)Lambda(+) electronic states (i.e., correlated adiabatically up to Na(6s) + H+) through a model-potential-type method over a wide range of R. Equilibrium distances, transition energies, depths of wells, and/or heights of humps predicted at short and large interatomic separations are reported and compared with available experimental and theoretical values. Variations of the static dipole polarizabilities versus internuclear distance have been determined for the two lowest states.