FeBr_3-catalyzed dibromination of alkenes and alkynes

The dibromination of alkenes and alkynes with bromosuccinimide and sodium bromide catalyzed by FeBr₃ under mild conditions has been developed.The trans-dibromo compounds were exclusively obtained with excellent yields.     

Dissociation Channel Dependence on Peptide Size Observed in Electron Capture Dissociation of Tryptic Peptides

Electron capture dissociation (ECD) of a series of five residue peptides led to the observation that these small peptides did not lead to the formation of the usual c/z ECD fragments, but to a, b, y, and w fragments. In order to determine how general this behavior is for small sized peptides, the effect of peptide size on ECD fragments using a complete set of ECD spectra from the SwedECD spectra database was examined. Analysis of the database shows that b and w fragments are favored for small peptide sizes and that average fragment size shows a linear relationship to parent peptide size for most fragment types. From these data, it appears that most of the w fragments are not secondary fragments of the major z ions, in sharp contrast with the proposed mechanism leading to these ions. These data also show that c fragment distributions depend strongly on the nature of C-terminal residue basic site: arginine leads to loss of short neutral fragments, whereas lysine leads to loss of longer neutral fragments. It also appears that b ions might be produced by two different mechanisms depending on the parent peptide size. A model for the fragmentation pathways in competition is proposed. These relationships between average fragment size and parent peptide size could be further exploited also for CID fragment spectra and could be included in fragmentation prediction algorithms.     

Conformational analysis of 18-azacrown-6 and its bonding with late first transition series divalent metals: insight from DFT combined with NPA and QTAIM analyses

Density functional theory calculations, together with quantum theory of atoms in molecules (QTAIM) analyses, have been performed to investigate 18-azacrown-6 complexes of the high-spin late first transition series divalent metal ions in the gas phase and, in some cases, in aqueous solution simulated by a polarizable continuum model. Six intramolecular H-H bonding interactions in the meso-complexes are found to arise from folding of the ligand upon its electrostatic interaction with the metal ions, which are largely absent in the lowest-energy C(2h) conformer of the free ligand. The ligand-to-metal charge transfer obtained from QTAIM analysis, among other things, is found to be an important factor that controls the stability of these complexes. The inter-relationship between the ligand preorganization energy, the zero-point corrected formation energy of the metal complexes, and the H-H bonding pair distances, as well as the dependence of the electron density and the total energy density at the H-H bond critical points on the H-H bonding pair distances, provides a physical basis for understanding and explaining the stabilizing nature of these closed-shell interactions, which are often viewed as steric clashes that lead to complex destabilization.     

Translocation versus cyclisation in radicals derived from N-3-alkenyl trichloroacetamides

Under radical reaction conditions, two different and competitive reaction pathways were observed for N-(α-methylbenzyl)trichloroacetamides with a N-3-cyclohexenyl substituent: 1,4-hydrogen translocation and radical addition to a double bond. However, for radicals with an acyclic alkenyl side chain, the direct cyclisation process was exclusively observed. The dichotomy between translocation and direct radical cyclisation in these substrates has been theoretically studied using density functional theory (DFT) methods at the B3LYP/6-31G** computational level.     

Using local search to speed up filtering algorithms for?some NP-hard constraints

This paper proposes to use local search inside filtering algorithms of combinatorial structures for which achieving a desired level of consistency is too computationally expensive. Local search quickly provides supports for many variable-value pairs, thus reducing the effort required to check and potentially filter the rest of them. The idea is demonstrated on the SomeDifferent constraint, a graph coloring substructure. An experimental evaluation confirms its significant computational gain in many cases.     

A physico-chemical investigation of poly(ethylene oxide)-block-poly(L-lysine) copolymer adsorption onto silica nanoparticles

The adsorption behavior of poly(ethylene oxide)-b-poly(L-lysine) (PEO(113)-b-PLL(10)) copolymer onto silica nanoparticles was investigated in phosphate buffer at pH 7.4 by means of dynamic light scattering, zeta potential, adsorption isotherms and microcalorimetry measurements. Both blocks have an affinity for the silica surface through hydrogen bonding (PEO and PLL) or electrostatic interactions (PLL). Competitive adsorption experiments from a mixture of PEO and PLL homopolymers evidenced greater interactions of PLL with silica while displacement experiments even revealed that free PLL chains could desorb PEO chains from the particle surface. This allowed us to better understand the adsorption mechanism of PEO-b-PLL copolymer at the silica surface. At low surface coverage, both blocks adsorbed in flat conformation leading to the flocculation of the particles as neither steric nor electrostatic forces could take place at the silica surface. The addition of a large excess of copolymer favoured the dispersion of flocs according to a presumed mechanism where PLL blocks of incoming copolymer chains preferentially adsorbed to the surface by displacing already adsorbed PEO blocks. The gradual addition of silica particles to an excess of PEO-b-PLL copolymer solution was the preferred method for particle coating as it favoured equilibrium conditions where the copolymer formed an anchor-buoy (PLL-PEO) structure with stabilizing properties at the silica-water interface.     

Synthesis, magnetic properties, and magnetic structure of a natrochalcite structural variant, KM(II)2D3O2(MoO4)2 (M = Mn, Fe, or Co)

We report the syntheses, crystal structures, and magnetic properties of KMn(2)(H(3)O(2))(MoO(4))(2) (MnH), KMn(2)(D(3)O(2))(MoO(4))(2) (MnD), KFe(2)(H(3)O(2))(MoO(4))(2) (FeH), KFe(2)(D(3)O(2))(MoO(4))(2) (FeD), KCo(2)(H(3)O(2))(MoO(4))(2) (CoH), and KCo(2)(D(3)O(2))(MoO(4))(2) (CoD), and the magnetic structures of MnD and FeD. They belong to the structural variant (space group I2/m) of the mineral natrochalcite NaCu(2)(H(3)O(2))(SO(4))(2) (space group C2/m) where the diagonal within the ac-plane of the latter become one axis of the former. The structure of MnD, obtained from Rietveld refinement of a high-resolution neutron pattern taken at 300 K, consists of chains of edge-sharing octahedra bridged by MoO(4) and D(3)O(2) to form layers, which are connected to K through the oxygen atoms to form the three-dimensional (3D)-network. The X-ray powder diffraction patterns of the other two compounds were found to belong to the same space group with similar parameters. The magnetic susceptibilities of MnH and FeH exhibit long-range ordering of the moments at a Ne?el temperature of 8 and 11 K, respectively, which are accompanied by additional strong Bragg reflections in the neutron diffraction in the ordered state, consistent with antiferromagnetism. Analyses of the neutron data for MnD and FeD reveal the presence of both long- and short-range orderings and commensurate magnetic structures with a propagation vector of (?, 0, ?). The moments are antiferromagnetically ordered within the chains with alternation between chains to generate four nonequivalent nuclear unit cells. For MnD the moments are perpendicular to the chain axis (b-axis) while for FeD they are parallel to the b-axis. The overall total is a fully compensated magnetic structure with zero moment in each case. Surprisingly, for KCo(2)(D(3)O(2))(MoO(4))(2) neither additional peaks nor increase of the nuclear peaks' intensities were observed in the neutron diffraction patterns below the magnetic anomaly at 12 K which was identified to originate from a small quantity of a ferromagnetic compound, Co(2)(OH)(2)MoO(4).     

Computational study of FOX‐7 synthesis in a solvated reaction system

Ethene and two kinds of nitrating reagents (HNO₃ and N₂O₅) in a variety of solvents were included in respective molecular systems, and each underwent a two‐stage electrophilic and free radical nitro‐substitution reaction to obtain the corresponding nitroethene compounds. Subsequent halogenation (using Cl₂ and Br₂) and amination (using NH₃) were then performed in solvents, also by electrophilic and radical substitution, to produce the desired 1,1‐diamino‐2,2‐dinitroethene (FOX‐7) derivatives. The reaction energy barrier in the nitration stage for obtaining each kind of mononitro ethene exhibited a stepwise decreasing trend when the reaction was carried out in H₂O‐solvated and CH₃OH‐solvated systems, no matter what nitrating agent was used. Related energy barrier data showed that the nitration reaction is more feasible in an H₂O‐solvated than a CH₃OH‐solvated system. The modeling results suggested that N₂O₅ is the better agent for nitration to proceed in water, bromine is suitable for halogenation, and the bromine derivatives are convenient for further amination in an H₂O‐solvated system. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Can a dipole-bound electron form a pseudo-atom? An atoms-in-molecules study of the hydrated electron

Non-nuclear local maxima, or attractors, of electron density are a rare but very interesting feature of the electron density distribution in molecules and solids. Recently, non-nuclear attractors (NNAs) and the corresponding pseudoatoms of electron density have been identified with the quantum theory of atoms in molecules for some anionic clusters formed by several polar solvent molecules and an excess electron bound in either a solvated-electron or dipole-bound fashion. This contribution reports a detailed study of the topology of the electron density for a series of dipole-bound water cluster anions, as calculated with Hartree-Fock, M?ller-Plesset perturbation theory, and coupled-cluster methods together with basis sets augmented with extra diffuse basis functions to accommodate the excess electron. For dipole-bound clusters, electron densities obtained with insufficient inclusion of electron correlation effects and tight basis sets feature a well-pronounced pseudoatom due to the excess electron, which ultimately disappears when a higher level of electronic structure theory and a more diffuse basis set are used. On the other hand, for solvated-electron clusters, where the excess electron is surrounded by solvent molecules, the existence of NNAs does not seem to be an artifact of the method employed, but rather a genuine feature of the electron density distribution. Pseudoatoms of electron density thus appear to be an exclusive feature of confined environments and are unlikely to be found on the tip of a cluster dipole or on solid surfaces.