Search results for "Intramolecular force"
showing 10 items of 599 documents
Synthesis and solid state structures of macromolecular cylindrical brushes with varying side chain length
2004
Abstract Cylindrical brushes with a macromolecular backbone and well-defined side chains of different length (4≤ P n sc ≤38) were synthesized by polymerization of macromonomers. The effect of side chain length on the intermolecular order has been investigated by means of X-ray scattering on the melt-extruded samples. The increase of the side chain molar mass results in an increase of the intermolecular distance d according to d ∼( M n sc ) 0.476 . The contour length of the cylindrical brush polymers per main chain monomer unit, l m , was determined to slightly vary with side chain molar mass M n sc according to l m ∼( M n sc ) 0.059 . The resulting values are much less than the maximum exte…
Two pentadehydropeptides with different configurations of the ΔPhe residues
2009
Comparison of the crystal structures of two pentadehydropeptides containing DeltaPhe residues, namely (Z,Z)-N-(tert-butoxycarbonyl)glycyl-alpha,beta-phenylalanylglycyl-alpha,beta-phenylalanylglycine (or Boc(0)-Gly(1)-Delta(Z)Phe(2)-Gly(3)-Delta(Z)Phe(4)-Gly(5)-OH) methanol solvate, C(29)H(33)N(5)O(8) x CH(4)O, (I), and (E,E)-N-(tert-butoxycarbonyl)glycyl-alpha,beta-phenylalanylglycyl-alpha,beta-phenylalanylglycine (or Boc(0)-Gly(1)-Delta(E)Phe(2)-Gly(3)-Delta(E)Phe(4)-Gly(5)-OH), C(29)H(33)N(5)O(8), (II), indicates that the Delta(Z)Phe residue is a more effective inducer of folded structures than the Delta(E)Phe residue. The values of the torsion angles phi and psi show the presence of two …
The [3,3′-Co(1,2-C2B9H11)2]– anion as a platform for new materials: synthesis of its functionalized monosubstituted derivatives incorporating synthon…
2003
[3,3′-Co(8-C5H10O-1,2-C2B9H10)(1′,2′-C2B9H11)] (2) was synthesized by reaction of the caesium salt of [Co(C2B9H11)2]− with dimethyl sulfate in the presence of sulfuric acid as catalyst and tetrahydropyrane as solvent; the zwitterionic compound (2) yielded [3,3′-Co(8-C4H4N-(CH2)5-O-1,2-C2B9H10)(1′,2′-C2B9H11)]− (6), [3,3′-Co(8-C8H6N-(CH2)5-O-1,2-C2B9H10)(1′,2′-C2B9H11)]− (7), and [3,3′-Co(8-C12H8N-(CH2)5-O-1,2-C2B9H10)(1′,2′-C2B9H11)]− (8) through the nucleophilic addition of potassium pyrrolyl, indolyl and carbazolyl, respectively. The nucleophilic addition of the same salts on [3,3′-Co(8-C4H8O2-1,2-C2B9H10)(1′,2′-C2B9H11)] (1) yielded, respectively, [3,3′-Co(8-C4H4N-(CH2)2-O-(CH2)2-O-1,2-C…
Multiplicity of Reaction Pathways in the Processes of Oxygen Transfer to Secondary Amines by Mo(VI) and W(VI) Peroxo Complexes.
1996
Oxidation of N,N-benzylmethylamine, N,N-benzylisopropylamine, and N,N-benzyl-tert-butylamine by both anionic and neutral Mo(VI) and W(VI) oxodiperoxo complexes yields the corresponding nitrones quantitatively. The oxidation reactions employing anionic oxidants were performed in CHCl(3) and follow second-order kinetics, first order with respect to the amine and to the oxidant. The data were rationalized on the basis of a rate-determining nucleophilic attack of the amine onto the peroxide oxygen of the oxidant, with a transition state in which N-O bond formation and O-O bond cleavage occur in a concerted way (electrophilic oxygen transfer mechanism). This attack yields the corresponding hydro…
Pressure effect studies on spin crossover systems
2005
Abstract In the present review article we discuss the results of investigations of the influence of hydrostatic pressure (up to 1.2 GPa) on the spin transition behaviour in coordination compounds of 3d transition metal ions. The systems under investigation are mononuclear spin crossover compounds of iron(II) and chromium(II), dinuclear complexes of iron(II) exhibiting coexistence of intramolecular anti-ferromagnetic coupling and thermal spin crossover, and 1D, 2D and 3D polynuclear spin crossover complexes of iron(II). It is demonstrated that the application of hydrostatic pressure serves as a tool for modifying the ligand field strength in a controlled manner.
Dilute solution properties of polybutadiene modified by 4-phenyl-1,2,4-triazoline-3,5-dione
1996
The dilute solution properties of 4-phenyl-1,2,4-triazoline-3,5-dione-modified polybutadienes were studied in tetrahydrofuran by static and dynamic light scattering and compared with those from unmodified polybutadiene chains. For the unmodified polymer, the scaling laws were established and the coil interpenetration parameter k f0 was determined in the thermodynamically good solvent tetrahydrofuran ; the chains exhibited common flexible chain behavior. For the modified polybutadiene, the results indicate at infinite dilution a weak extent of intermolecular crosslinking via the urazole groups, coupled with a very high extent of intramolecular ring formation. In order to evaluate the fractio…
Recent Developments in Monte Carlo Simulations of Lattice Models for Polymer Systems
2008
A brief review is given of methodological advances made during the past decade with the Monte Carlo sampling of equilibrium properties of simple lattice models of polymer systems, and representative applications of these new algorithms are summarized. These algorithms include Wang−Landau (WL) sampling, the pruned-enriched Rosenbluth method (PERM), and topology violating dynamic Monte Carlo algorithms such as combinations of local moves, slithering snake moves, and “double bridging” moves for the bond fluctuation model. The applications mentioned concern phase-transition-like phenomena of single chains (collapse and crystallization in bad solvents; interplay of collapse and adsorption; escap…
A coarse-graining procedure for polymer melts applied to 1,4-polybutadiene
2009
We present a coarse-graining procedure for homopolymer melts mapping intra- as well as inter-molecular interactions from a chemically realistic united atom description to a bead-spring type molecular model. On the coarse-grained level the repeat units interact through bond-length and bond angle potentials and a non-bonded Lennard-Jones type interaction. The latter one is of the 7,4 form and softer than the typically employed 12,6 interactions. The coarse-graining of the intramolecular interactions follows well developed procedures, however, we point out in which way the non-bonded intramolecular interactions in the chemically realistic model should be treated. The parameters of the non-bond…
Diastereodivergent Synthesis of Fluorinated Cyclic β(3)-Amino Acid Derivatives.
2015
The ability of 2-p-tolylbenzyl carbanions to behave as a source of chiral benzylic nucleophiles has been shown in its reaction with fluorinated imines. The process takes place with high levels of stereocontrol, rendering the corresponding amines as single diastereoisomers. Subsequent cross-metathesis followed by intramolecular aza-Michael reaction makes the synthesis of fluorinated homoproline derivatives bearing three stereogenic centers possible. Furthermore, the selectivity of the cyclization process can easily be tuned up in a diastereodivergent manner simply by changing the reaction conditions.
An ELF analysis of the C–C bond formation step in the N-heterocyclic carbene-catalyzed hydroacylation of unactivated C–C double bonds
2012
The changes in electron-density along the C–C bond-formation step in the N-heterocyclic carbene-catalyzed hydroacylation of unactivated double bonds has been studied by an electron localization function (ELF) analysis at the B3LYP/6-31G** level in order to characterize the reaction mechanism. Analysis of DFT reactivity indices and the natural bond orbital and ELF analysis at the most relevant points of the intrinsic reaction coordinate indicate that the reaction path takes place through a two-stage one-step mechanism with non-polar character. In the first stage a hydrogen atom is transferred from the hydroxyl group of Breslow intermediate 12 to the terminal olefinic carbon atom, to yield a …