Conformational preferences for some 2-substituted N-methoxy-N-methylacetamides through spectroscopic and theoretical studies

The analysis of the IR carbonyl band of the 2-substituted N-methoxy-N-methylacetamides Y-CH(2)C(O)-N(OMe)Me (Y = F1, OMe 2, OPh 3, Cl 4), supported by B3LYP/6-311++G(3df, 3pd) calculations along with the NBO analysis for 1-4, indicated the existence of cis-gauche conformers i.e. (c) and (g) for 1 and 3, (c(1), c(2)) and (g(1), g(2)) for 2, and (c) and (g(1), g(2)) for 4. In the gas phase, the g conformer population prevails over the c one, for 1 and 3, the (c(1) + c(2)) population prevails over the (g(1) + g(2)) one for 2, and the (g(1) + g(2)) conformer population is more abundant than (c) one for 4. In n-hexane solution, the cis conformer is more abundant for 1-3. The occurrence of Fermi resonance in the nu(CO) region, in n-hexane, precludes the estimative of relative populations of the (c, g(1), g(2)) conformers for 4. The SCI-PCM calculations agree with the solvent effect on the nu(CO) band component relative intensities for 1-3. NBO analysis showed that the n(N) -> pi.(CO), orbital interaction is the main factor which stabilizes the gauche (g, g(1), g(2)) conformers for 1-4 into a larger extent relative to the cis (c, c(1), c(2)) ones. The n(y) -> pi(.)(Co,) sigma(C-Y) -> pi.(CO,) pi(CO) -> sigma(C-Y) and 7co orbital interactions still contribute, but into a minor extent for the stabilization of the gauche conformers relative to the cis ones. The existence of some pyramidalization at the nitrogen atom of the Weinreb amides 1-4 is responsible for the occurrence of Y(delta)-(4)center dot center dot center dot O(delta)-(9) and Y(delta)-(4)center dot center dot center dot N(delta)-(7) short contacts in the gauche (g, g(1), g(2)) conformers, which originates strong repulsive Coulombic interactions, acting in opposition to the large orbital stabilization of the gauche conformer with respect to the cis one. Therefore, a delicate balance of the Coulombic and orbital interactions seems to be responsible for the observed stabilization of the gauche (g, g(1), g(2)) and cis (c, c(1), c(2)) conformers, both in the gas phase and in the solution for 1-4. However, the cis conformer predominance, in non polar solvents, for the 2-substituted N-methoxy-N-methyl acetamides 1-3, bearing in a first raw (fluorine and oxygen) atoms, is in the opposite direction to the gauche conformer preference for the corresponding 2-substituted N,N-dialkyl-acetamides. (C) 2010 Elsevier B.V. All rights reserved.

Temperature Dependence of the Intergranular Critical Current Density in Uniaxially Pressed Bi(1.65)Pb(0.35)Sr(2)Ca(2)Cu(3)O(10+delta) Samples

We have studied the normal and superconducting transport properties of Bi(1.65)Pb(0.35)Sr(2)Ca(2)Cu(3)O(10+delta) (Bi-2223) ceramic samples. Four samples, from the same batch, were prepared by the solid-state reaction method and pressed uniaxially at different compacting pressures, ranging from 90 to 250 MPa before the last heat treatment. From the temperature dependence of the electrical resistivity, combined with current conduction models for cuprates, we were able to separate contributions arising from both the grain misalignment and microstructural defects. The behavior of the critical current density as a function of temperature at zero applied magnetic field, J (c) (T), was fitted to the relationship J (c) (T)ae(1-T/T (c) ) (n) , with na parts per thousand 2 in all samples. We have also investigated the behavior of the product J (c) rho (sr) , where rho (sr) is the specific resistance of the grain-boundary. The results were interpreted by considering the relation between these parameters and the grain-boundary angle, theta, with increasing the uniaxial compacting pressure. We have found that the above type of mechanical deformation improves the alignment of the grains. Consequently the samples exhibit an enhance in the intergranular properties, resulting in a decrease of the specific resistance of the grain-boundary and an increase in the critical current density.

Intergranular properties of uniaxially pressed YBa(2)Cu(3)O(7-delta) ceramic samples

We performed measurements of electrical resistivity as a function of temperature, rho(T), in polycrystalline samples of YBa(2)Cu(3)O(7-delta) (Y-123) subjected to different uniaxial compacting pressures. We observed by using X-ray diffractometry that samples have a very similar composition. Most of the identified peaks are related to the superconducting Y-123 phase. Also, from the X-ray diffraction patterns performed, in powder and pellet samples, we estimated the Lotgering factor along the (00l) direction, F((00l)). The results indicate that F((00l)) increases from 0.13 to 0.16. From electrical resistivity measurements as a function of temperature, we were able to separate contributions arising from both the grain misalignment and microstructural defects. We found appreciable degradation in the normal-state transport properties of samples with an increase in uniaxial compacting pressure. It seems that this type of behavior is associated with an increase in the influence of microstructural defects at the intergranular level. The experimental results are analyzed in the framework of a current conduction model of granular samples.

Evidence for magnetic phase separation in La(0.86)Sr(0.14)Mn(1-x)Cu(x)O(3+delta) manganites from NMR and magnetic measurements

Polycrystalline La(0.86)Sr(0.14)Mn(1-x)Cu(x)O(3+delta) (x = 0, 0.05, 0.10, 0.15, 0.20) manganites were investigated by means of magnetic measurements and zero-field (139)La and (55)Mn nuclear magnetic resonance (NMR) spectroscopy. Magnetization versus temperature measurements revealed a paramagnetic to ferromagnetic transition in most samples, with lower Curie temperatures and broader transitions for samples with higher Cu contents. The details of the magnetization measurements suggested a phase-separated scenario, with ferromagnetic clusters embedded in an antiferromagnetic matrix, especially for the samples with large Cu contents (x = 0.15 and 0.20). Zero-field (139)La NMR measurements confirmed this finding, since the spectral features remained almost unchanged for all Cu-doped samples, whereas the bulk magnetization was drastically reduced with increasing Cu content. (55)Mn NMR spectra were again typical of ferromagnetic regions, with a broadening of the resonance line caused by the disorder introduced by the Cu doping. The results indicate a coexistence of different magnetic phases in the manganites studied, with the addition of Cu contributing to the weakening of the double-exchange interaction in most parts of the material.

Cholesterol Hydroperoxides Generate Singlet Molecular Oxygen [O(2) ((1)Delta(g))]: Near-IR Emission, (18)O-Labeled Hydroperoxides, and Mass Spectrometry

In mammalian membranes, cholesterol is concentrated in lipid rafts. The generation of cholesterol hydroperoxides (ChOOHs) and their decomposition products induces various types of cell damage. The decomposition of some organic hydroperoxides into peroxyl radicals is known to be a potential source of singlet molecular oxygen [O(2) ((1)Delta(g))] in biological systems. We report herein on evidence of the generation of O(2) ((1)Delta(g)) from ChOOH isomers in solution or in liposomes containing ChOOHs, which involves a cyclic mechanism from a linear tetraoxide intermediate originally proposed by Russell. Characteristic light emission at 1270 nm, corresponding to O(2) ((1)Delta(g)) monomolecular decay, was observed for each ChOOH isomer or in liposomes containing ChOOHs. Moreover, the presence of O(2) ((1)Delta(g)) was unequivocally demonstrated using the direct spectral characterization of near-infrared light emission. Using (18)O-labeled cholesterol hydroperoxide (Ch(18)O(18)OH), we observed the formation of (18)O-labeled O(2) ((1)Delta(g)) [(18)O(2) ((1)Delta(g))] by the chemical trapping of (18)O(2) ((1)Delta(g)) with 9,10-diphenylanthracene (DPA) and detected the corresponding (18)O-labeled DPA endoperoxide (DPA(18)O(18)O) and the (18)O-labeled products of the Russell mechanism using high-performance liquid chromatography coupled to tandem mass spectrometry. Photoemission properties and chemical trapping clearly demonstrate that the decomposition of Ch(18)O(18)OH generates (18)O(2) ((1)Delta(g)), which is consistent with the Russell mechanism and points to the involvement of O(2) ((1)Delta(g)) in cholesterol hydroperoxide-mediated cytotoxicity.