Etude cristallographique de membranes obtenues a partir d'alcools polyvinyliques alcoyles

Low and wide angle X-ray diffraction was used to study the structure of paraffin grafted polyvinylalcohol membranes as a function of water content. The structure was found to be lamellar resulting in the pile-up of crystalline paraffin double layers, covered with polymer chains. Such an organization would be impossible if grafting of paraffin chains upon polyvinylalcohol were random. The membranes must be considered as two phase systems, consisting of amorphous and crystalline regions.     

Gradient reversed-phase liquid chromatographic-electrospray ionization mass spectrometric method for the comparison of smokeless powders

A gradient reversed-phase liquid chromatographic-electrospray ionization mass spectrometric (LC-ESIMS) method was developed to determine compositional variation in the organic additives of smokeless powders. The method was optimized for the separation and detection of selected powder constituents, including diphenylamine, along with isomers of its nitroso and nitro derivatives, centralite I and II, in addition to dialkylphthalate acid esters. A series of commercially available smokeless powders was prepared by organic liquid extraction and characterized using the LC-ESIMS method. The results demonstrate the differentiation of smokeless powders by their additive profile.     

Luminescent Eu(III) and Gd(III) trisbipyridine cryptates: experimental and theoretical study of the substituent effects.

Synthesis, absorption spectra and luminescebce properties of a series of lanthanide trisbipyridine cryptates Ln within R-Bpy x R-Bpy x R-Bpy, where Ln = Eu, Gd and R = H, COOH, COOCH3, CONH(CH2)2NH2 are described. Comparison of the unsubstituted parent compound with the substituted compounds shows that bipyridine substitution doesn't alter significantly the photophysical properties of the lanthanide cryptate. The absorption maximum is slightly red-shifted when three bipyridines are substituted, whereas substituting one bipyridines has a negligible effect on the absorption spectra. The experimental triplet state energy is between 21600 and 22 100 cm(-1) for the series of compounds and the luminescence lifetimes at 77 K are between 0.5 and 0.8 ms in HO2 and equal to 1.7 ms in D2O. The experimental characterizations are completed by DFT and TD-DFT calculations to assess the ability of these approaches to predict absorption maxima, triplet state energies and structural parameters of lanthanide cryptates and to characterize the electronic structure of the excited states. The calculations on the unsubstituted parent and substituted compounds show that absorption maxima and lowest 3pipi* triplet state energies can be accurately determined from density functional theory (DFT) and time-dependent (TD) DFT calculations.     

Anionic polymerization with alkaline-earth counterion. II. Polymerization of styrene initiated by dicarbanionic oligostyrylbarium in THF

The propagation kinetics of anionic polymerization of styrene initiated by dicarbanionic oligostyrylbarium (PS⁼Ba⁺⁺) in THF are described. The apparent propagation rate constant k_p increases drastically with the degree of polymerization (DP) of living chains and tends at 20°C, for the highest molecular weight (DP ? 5000), to the value determined for monocarbanionic polystyrylbarium(PS^?)₂Ba⁺⁺. At given DP, the propagation step follows usual first-order kinetics with respect to monomer, and k_p is inversely proportional to carbanion concentration; as observed for (PS^?)₂Ba⁺⁺. Similar behavior is observed in the temperature range from ?60 to +20°C. The activation energy of the propagation is 4–5 kcal/mole (16.7–21 kJ/mole). It is shown that k_p may be considered as directly proportional to the dissociation constant K_d of ion pairs (～S^?Ba^++?S～ is considered as an ion pair ～(SBa)⁺S^?～). The striking variation of k_p with the DP living chains is interpreted in terms of cyclic living chains, in which both carbanionic ends are bound to the same cation. Values of the intramolecular dissociation constant K_d of ion pairs included in such a model are computed as a function of DP, and their variation is found to fit rather well with experimental data.     

Bipyridyl- and biphenyl-DNA: a recognition motif based on interstrand aromatic stacking

The synthesis and incorporation into oligonucleotides of C-nucleosides containing the two aromatic, non-hydrogen-bonding nucleobase substitutes biphenyl (I) and bipyridyl (Y) are described. Their homo- and hetero-recognition properties in different sequential arrangements were then investigated via UV-melting curve analysis, gel mobility assays, CD- and NMR spectroscopy. An NMR analysis of a dodecamer duplex containing one biphenyl pair in the center, as well as CD data on duplexes with multiple insertions provide further evidence for the zipper-like interstrand stacking motif that we proposed earlier based on molecular modeling. UV-thermal melting experiments with duplexes containing one to up to seven I- or Y base pairs revealed a constant increase in T(m) in the case of I and a constant decrease for Y. Mixed I/Y base pairs lead to stabilities in between the homoseries. Insertion of alternating I/abasic site- or Y/abasic site pairs strongly decreases the thermal stability of duplexes. Asymmetric distribution of I- or Y residues on either strand of the duplex were also investigated in this context. Duplexes with three natural base pairs at both ends and 50 % of I pairs in the center are still readily formed, while duplexes with blunt ended I pairs tend to aggregate unspecifically. Duplexes with one natural overhang at the end of a I-I base pair tract can both aggregate or form ordered duplexes, depending on the nature of the natural bases in the overhang.     

The analysis of high explosives by liquid chromatography/electrospray ionization mass spectrometry: multiplexed detection of negative ion adducts

The negative ion electrospray ionization mass spectrometric (ESI-MS) detection of adducts of high explosives with chloride, formate, acetate, and nitrate was used to demonstrate the gas-phase interaction of neutral explosives with these anions. The relative intensities of the adduct species were determined to compare the competitive formation of the selected high explosives and anions. The relative stability of the adduct species varies, yielding preferential formation of certain anionic adducts with different high explosives. To exploit this effect, an isocratic high-performance liquid chromatography (HPLC)/ESI-MS method was developed and used for the simultaneous analysis of high explosives using two different techniques for the addition of the anionic additives; pre- and post-column. The results show that the pre-column approach provides similar results with improved selectivity for specific explosives. By detecting characteristic adduct species for each explosive, this method provides a qualitative and quantitative approach for the analysis and identification of high explosives.     

Synthesis of the first truly reactive polymer-bound NADH models

Improved reduction of benzaldehydes has been obtained with reactive polymer-bound NADH models