Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

The Mechanism of Thionyl Chloride Reaction with Dialkyl Alkylphosphonothionate Using 31 P NMR

Based on 31 P NMR studies of thionyl chloride reaction with dialkyl alkylphosphonothionates, a method for preparation of alkylphosphonic dichloride has been investigated. A mechanism via intermediacy of ester chloride is suggested.     

Novel nonperipheral octa-3-hydroxypropylthio substituted metallo-phthalocyanines: synthesis,characterization, and investigation of their electrochemical,photochemical and computational properties

The current study describes the synthesis, electrochemical, computational, and photochemical properties of octa (3-hydroxypropylthio) substituted cobalt (II) ( 4 ), copper (II) ( 5 ), nickel (II) ( 6 ) and zinc(II) ( 7 ) phthalocyanine derivatives. These novel compounds were characterized by elemental analysis,¹H,¹³C NMR, FT-IR, UV-Vis, and MS. The redox behaviors of these metallo-phthalocyanines were investigated by the cyclic voltammetric method. The optimized molecular structure and gauge-including atomic orbital (GIAO)¹H and¹³C NMR chemical shift values of these phthalocyanines in the ground state had been calculated by using B3LYP/6–31G(d,p) basis set. The outcomes of the optimized molecular structure were given and compared with the experimental NMR values. The photochemical properties including photodegradation and singlet oxygen generation of zinc(II) phthalocyanine were studied in DMSO solution for the determination of its photosensitizer behaviors.     

Electronic structure and properties of isoelectronic magic clusters: Al13X (X=H,Au,Li,Na,K,Rb,Cs)

The equilibrium structure, stability, and electronic properties of the Al(13)X (X=H,Au,Li,Na,K,Rb,Cs) clusters have been studied using a combination of photoelectron spectroscopy experiment and density functional theory. All these clusters constitute 40 electron systems with 39 electrons contributed by the 13 Al atoms and 1 electron contributed by each of the X (X=H,Au,Li,Na,K,Rb,Cs) atom. A systematic study allows us to investigate whether all electrons contributed by the X atoms are alike and whether the structure, stability, and properties of all the magic clusters are similar. Furthermore, quantitative agreement between the calculated and the measured electron affinities and vertical detachment energies enable us to identify the ground state geometries of these clusters both in neutral and anionic configurations.     

A small molecule discrimination map of the antibiotic resistance kinome

Kinase-mediated resistance to antibiotics is a significant clinical challenge. These enzymes share a common protein fold characteristic of Ser/Thr/Tyr protein kinases. We screened 14 antibiotic resistance kinases against 80 chemically diverse protein kinase inhibitors to map resistance kinase chemical space. The screens identified molecules with both broad and narrow inhibition profiles, proving that protein kinase inhibitors offer privileged chemical matter with the potential to block antibiotic resistance. One example is the flavonol quercetin, which inhibited a number of resistance kinases in vitro and in vivo. This activity was rationalized by determination of the crystal structure of the aminoglycoside kinase APH(2″)-IVa in complex with quercetin and its antibiotic substrate kanamycin. Our data demonstrate that protein kinase inhibitors offer chemical scaffolds that can block antibiotic resistance, providing leads for co-drug design.     

Modeling the geometric, electronic, and redox properties of iron(III)-containing amphiphiles with asymmetric [NN'O] headgroups

Two iron(III)-containing amphiphiles 1 and 2 have been synthesized with the [NN'O] ligands HL(tBu-ODA) (2-((octadecyl(pyridin-2-ylmethyl)amino)methyl)-4,6-di-tert-butylphenol) and HL(I-ODA) (2-((octadecyl(pyridin-2-ylmethyl)amino)methyl)-4,6-diiodophenol), respectively. Compound 1 is monometallic, whereas EXAFS data suggest that 2 is a mixture of mono- and bimetallic species. The archetypical [Fe(III)(L(NN'O))(2)](+) complexes 3-9 have been isolated and characterized in order to understand the geometric, electronic, and redox properties of the amphiphiles. Preference for a monometallic or bimetallic nuclearity is dependent on (i) the nature of the solvent used for synthesis and (ii) the type of the substituent in the phenol moiety. In methanol, the tert-butyl-, methoxy-, and chloro-substituted 3, 4, and 5 are monometallic species, whereas the bromo- and iodo-substituted 6 and 7 form bimetallic complexes taking advantage of stabilizing methoxo bridges generated by solvent deprotonation. In dichloromethane, the bromo- and iodo-substituted 8 and 9 are monometallic species; however, these species favor meridional coordination in opposition to the facial coordination observed for the tert-butyl- and methoxy-substituted compounds. Molecular structures for species 5, 7, 8, and 9 have been solved by X-ray diffraction. Furthermore, the electronic spectrum of the amphiphile 1 was expected to be similar to those of facial/cis archetypes with similar substituents, but close resemblance was observed with the profile for those meridional/cis species, suggesting a similar coordination mode. This trend is discussed based on DFT calculations, where preference for the meridional/cis coordination mode appears related to the presence of tertiary amine nitrogen on the ligand, as when a long alkyl chain is attached to the [NN'O] headgroup.     

Tetradentate vs pentadentate coordination in copper(II) complexes of pyridylbis(aminophenol) ligands depends on nucleophilicity of phenol donors

The ligand binding preferences of a series of potentially pentadentate pyridylbis(aminophenol) ligands were explored. In addition to the previously reported ligands 2,2'-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H(2)L(1)) and 6,6'-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)bis(2,4-di-tert-butylphenol) (H(2)L(1-tBu)), four new ligands were synthesized: 6,6'-(2-methyl-2(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)bis(2,4-dibromophenol) (H(2)L(1-Br)), 6,6'-(2-methyl-2(pyridine-2-yl)propane-1,3diyl)bis(azanediyl)bis(methylene)bis(2-methoxyphenol) (H(2)L(1-MeO)), 2,2'-(2-methyl-2(pyridine-2-yl)propane-1,3diyl)bis(azanediyl)bis(methylene)bis(4-nitrophenol) (H(2)L(1-NO2)), and 2,2'-(2-phenylpropane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H(2)L(2)). These ligands, when combined with copper(II) salts and base, form either tricopper(II) species or monocopper(II) species depending on the nucleophilicity of the phenol groups in the ligands. All copper complexes were characterized by X-ray crystallography, cyclic voltammetry, and spectroscopic methods in solution. The ligands in trimeric complexes [{CuL(1)(CH(3)CN)}(2)Cu](ClO(4))(2) (1), [{CuL(1)Cl}(2)Cu] (1a), and [{CuL(2)(CH(3)CN)}(2)Cu](ClO(4))(2) (1b) and monomeric complex [CuL(1-tBu)(CH(3)OH)] (2) coordinate in a tetradentate mode via the amine N atoms and the phenolato O atoms. The pyridyl groups in 1, 1a, and 2 do not coordinate, but instead are involved in hydrogen bonding. Monomeric complexes [CuL(1-Br)] (3a), [CuL(1-NO2)] (3b), and [CuL(1-MeO)Na(CH(3)OH)(2)]ClO(4) (3c) have their ligands coordinated in a pentadentate mode via the amine N atoms, the phenolato O atoms, and the pyridyl N atom. The differences in tetradentate vs pentadentate coordination preferences of the ligands correlate to the nucleophilicity of the phenolate donor groups, and coincide with the electrochemical trends for these complexes.     

Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

La_0.67Ba_0.33MnO₃ (LBMO) thin film is deposited on a 36.7°C SrTiO₃ bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.     

Efficient and selective enzymatic acylation reaction: separation of furanosyl and pyranosyl nucleosides

Candida antarctica lipase-B (CAL-B) immobilized on lewatite selectively acylated the primary hydroxyl group of the furanosyl nucleoside in a mixture of 1-(alpha-D-arabinofuranosyl)thymine and 1-(alpha-D-arabinopyranosyl)thymine. This selective biocatalytic acylation of furanosyl nucleoside has enabled us an easy separation of arabinofuranosyl thymine from an inseparable mixture with arabinopyranosyl thymine. The primary hydroxyl selective acylation methodology of arabinonucleoside has also been successfully used for the separation of 1-(beta-D-xylofuranosyl)thymine and 1-(beta-D-xylopyranosyl)thymine from a mixture of the two, which demonstrate the generality of the enzymatic methodology for separation of furanosyl and pyranosyl nucleosides.