Single-molecule magnet behavior for an antiferromagnetically superexchange-coupled dinuclear dysprosium(III) complex

A family of five dinuclear lanthanide complexes has been synthesized with general formula [Ln(III)(2)(valdien)(2)(NO(3))(2)] where (H(2)valdien = N1,N3-bis(3-methoxysalicylidene)diethylenetriamine) and Ln(III) = Eu(III)1, Gd(III)2, Tb(III)3, Dy(III)4, and Ho(III)5. The magnetic investigations reveal that 4 exhibits single-molecule magnet (SMM) behavior with an anisotropic barrier U(eff) = 76 K. The step-like features in the hysteresis loops observed for 4 reveal an antiferromagnetic exchange coupling between the two dysprosium ions. Ab initio calculations confirm the weak antiferromagnetic interaction with an exchange constant J(Dy-Dy) = -0.21 cm(-1). The observed steps in the hysteresis loops correspond to a weakly coupled system similar to exchange-biased SMMs. The Dy(2) complex is an ideal candidate for the elucidation of slow relaxation of the magnetization mechanism seen in lanthanide systems.     

Formation and reactivity of gem-difluoro-substituted pyridinium ylides: Experimental and DFT investigation

According to DFT B3LYP/6-31G* calculations the reaction of difluorocarbene with pyridines proceeds reversibly with the formation of thermodynamically unstable intermediates, difluoro-substituted pyridinium ylides, which dissociate to carbene and pyridine with low activation barrier. The equilibrium constant of the reaction increases with increasing electron-withdrawing ability of substituents in the pyridine ring. Difluoroylides were generated from 4-cyano, 4-benzoyl- and 4-ethoxycarbonyl-substituted pyridines under difluorocarbene generation conditions (CF₂Br₂/Pb/Bu₄NBr/CH₂Cl₂/ultrasound) and trapped with dimethyl maleate or fumaronitrile. 3-Fluoroindolizines were isolated as final products of the reaction which involves dehydrofluorination of the primary cycloadducts followed by dehydrogenation by active MnO₂.     

Further studies of intramolecular Michael reactions of nitrosoalkenes for construction of functionalized bridged ring systems

A wide variety of stabilized carbanions have been found to participate as nucleophiles in intramolecular Michael-type conjugate additions to in situ generated nitrosoalkenes to form bridged carbocyclic systems. The vinylnitroso platforms for these cyclizations have been prepared via two key steps involving ring-closing metathesis of vinyl chlorides and regioselective conversion of vinyl chlorides to α-chloroketones with sodium hypochlorite in glacial acetic acid/acetone. An alternative approach to preparation of some cyclization substrates has involved use of more reactive enol ethers as precursors to the requisite α-chloroketones. A sulfonamide anion has also been found to be an effective nucleophile in this type of reaction, leading to formation of a 6-azabicyclo[3.2.1]octane.     

Chemoenzymatic synthesis of HIV-1 V3 glycopeptides carrying two N-glycans and effects of glycosylation on the peptide domain

[structure: see text] A highly efficient chemoenzymatic synthesis of HIV-1 V3 domain glycopeptides carrying two N-linked core tri- and pentasaccharides was achieved. The synthesis consisted of two key steps: a solid-phase synthesis of the cyclic, 47-mer V3 domain peptide containing two GlcNAc residues and a novel endoglycosidase-catalyzed transglycosylation that simultaneously added two N-glycan moieties to the peptide precursor from the oligosaccharide oxazoline donor substrates. The availability of the synthetic glycopeptides allowed the probing of the effects of glycosylation on the HIV-1 V3 domain. It was demonstrated that glycosylation influenced the global conformations of the V3 domain and provided protection of the V3 domain against protease digestion.     

A New Look at the Burnside-Schur Theorem

The famous Burnside–Schur theorem states that every primitivefinite permutation group containing a regular cyclic subgroupis either 2-transitive or isomorphic to a subgroup of a 1-dimensionalaffine group of prime degree. It is known that this theoremcan be expressed as a statement on Schur rings over a finitecyclic group. Generalizing the latter, Schur rings are introducedover a finite commutative ring, and an analogue of this statementis proved for them. Also, the finite local commutative ringsare characterized in permutation group terms. 2000 MathematicsSubject Classification 20B10, 20B15, 05E99.     

Spatially resolved ultra-trace analysis of elements combining resonance ionization with a MALDI-TOF spectrometer

A combined setup for spatially resolved mass analysis of trace amounts of elements and macromolecules is presented. Using a MALDI-TOF mass spectrometer, a laser spectroscopic setup for resonant ionization of neutral atoms has been implemented. This allows for an efficient and selective detection of trace elements by means of resonance ionization mass spectrometry (RIMS). The instrumental scheme is described, and methodological developments are presented. In a first application pure, laser desorption/ionization with TOF-MS was used to measure mass distributions of cosmic nanodiamonds. For further applications regarding the spatially resolved ultra-trace analysis of elements in solid samples, an implanted target was used to characterize both laser desorption/ionization and laser desorption/resonance ionization for the detection of trace elements within. A perspective of the setup is given and future investigations are outlined.