Search results for " coupling"
showing 10 items of 724 documents
Growing range of correlated motion in a polymer melt on cooling towards the glass transition
1999
Many liquids cooled to low temperatures form glasses (amorphous solids) instead of crystals. As the glass transition is approached, molecules become localized and relaxation times increase by many orders of magnitude1. Many features of this ‘slowing down’ are reasonably well described2 by the mode-coupling theory of supercooled liquids3. The ideal form of this theory predicts a dynamical critical temperature T c at which the molecules become permanently trapped in the ‘cage’ formed by their neighbours, and vitrification occurs. Although there is no sharp transition, because molecules do eventually escape their cage, its signature can still be observed in real and simulated liquids. Unlike c…
Homocoupling of terminal alkynes on calcite (10.4)
2018
Abstract On-surface synthesis has been identified as highly versatile strategy to prepare molecular structures on surfaces with single-atom precision. Inspired by the classical Glaser coupling, homocoupling of terminal alkynes has attracted great attention for on-surface synthesis. This coupling is known for providing a rigid and linear linkage, which is highly interesting for the synthesis of molecular wires. For molecular wire formation, non-conductive substrates are needed for electronic decoupling. So far, however, coupling of terminal alkynes has not been performed on a bulk insulator surface. Here, we present an atomic force microscopy study, indicating that 4,4″-diethynyl-[1,1′:4′,1″…
The glass transition in polymer melts
1994
This paper presents some results of a Monte Carlo simulation for the glass transition in two- and three-dimensional polymer melts. The melt was simulated by the bond-fluctuation model on a d-dimensional cubic lattice which was combined with a two-level hamiltonian favouring long bonds in order to generate a competition between the energetic and topological constraints in the system. This competition prevents crystallization and makes the melt freeze in an amorphous structure as soon as the internal relaxation times match the observation time of the simulation set by the cooling rate. The freezing point of the melt, i.e the glass transition temperature Tg, thus depends upon the cooling rate …
Carbon nitride as photocatalyst in organic selective transformations
2020
Abstract Graphitic carbon nitride (g-C3N4) is a metal-free conjugated polymer which has become a new research hotspot in photocatalysis. It can be used for solar energy exploitation like in solar energy organic synthesis, one of the most new and appealing green applications of heterogeneous photocatalysis. This chapter resumes the state-of-the-art and progresses in the application of heterogeneous visible light photocatalysis in organic selective transformations by using C3N4 as photocatalyst.
Monte Carlo and molecular dynamics simulation of the glass transition of polymers
1998
Two coarse-grained models for polymer chains in dense glass-forming polymer melts are studied by computer simulation: the bond-fluctuation model on a simple cubic lattice, where a bond-length potential favors long bonds, is treated by dynamic Monte Carlo methods, and a bead-spring model in the continuum with a Lennard-Jones potential between the beads is treated by Molecular Dynamics. While the dynamics of both models differ for short length scales and associated time scales, on mesoscopic spatial and temporal scales both models behave similarly. In particular, the mode coupling theory of the glass transition can be used to interpret the slowing down of the undercooled polymer melt. For the…
The synergy between the CsPbBr3 nanoparticle surface and the organic ligand becomes manifest in a demanding carbon–carbon coupling reaction
2020
We demonstrate here the suitability of CsPbBr3nanoparticles as photosensitizers for a demanding photoredox catalytic homo- and cross-coupling of alkyl bromides at room temperature by merely using visible light and an electron donor, thanks to the cooperative action between the nanoparticle surface and organic capping. Fil: Rosa-Pardo, Ignacio. Instituto de Ciencia Molecular; España. Universidad de Valencia; España Fil: Casadevall, Carla. Barcelona Institute Of Science And Technology. Institut Català D'investigació Química.; España Fil: Schmidt, Luciana Carina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigac…
Glass transition of polymer melts: Test of theoretical concepts by computer simulation.
2003
Abstract Polymers are good glass formers and allow for the study of melts near the glass transition in (meta-)stable equilibrium. Theories of the glass transition imply such an equilibrium and can, hence, be tested by the study of polymer melts. After a brief summary of the basic experimental facts about the glass transition in polymers, the main theoretical concepts are reviewed: mode coupling theory (MCT), entropy theory, free-volume theory, the idea of a growing length describing the size of cooperative regions, etc. Then, two basic coarse-grained models of polymers are described, which have been developed aiming at a test of these concepts. The first model is the bond-fluctuation model …
Gold Redox Catalytic Cycles for the Oxidative Coupling of Alkynes
2011
[EN] Au(I)/Au(III) catalytic cycles are catalytically competent to perform the oxidative coupling of alkynes in the homogeneous phase at room temperature and without any protecting atmosphere. Selectfluor as oxidant, wet acetonitrile as solvent, and sodium carbonate as base are the reagents of choice. Both aromatic and alkyl alkynes can be coupled, and mechanistic studies reveal that at least two gold species having different oxidation states are implicated in the key step of the coupling.
Impact of aggregates on excitation dynamics in transparent polymer films doped by dipolar molecules
2008
Abstract Optical properties of transparent polymer films of polymethylmethacrylate doped (up to 25 wt.%) by dipolar N , N -dimethylaminobenzylidene 1,3-indandione (DMABI) molecules were studied. Formation of DMABI nanocrystallites, increasing in their density and size with dopant concentration, was revealed by optical microscopy. Transformation of the fluorescence spectrum from the molecular-like emission (for the low dopant concentration below 1 wt.%) to highly red-shifted fluorescence corresponding to self-trapped excitons in the crystallites was observed. It was shown, that due to the resonant energy transfer in the blend, the DMABI nanocrystals can serve as efficient fluorescence marker…
Intramolecular coupling of acetylenic groups of bis(alkynyl)phosphanes and silanes mediated by benzynezirconocene: a route to new mono- and tricyclic…
2004
Benzo-zirconacyclohexadiene-phospha or silacyclobutene fused ring systems are easily prepared via a benzynezirconocene intermediate by means of thermolysis of Cp 2 ZrPh 2 in the presence of bis(alkynyl)phosphanes or silanes. These polyunsaturated systems are the source of a variety of new mono- or tricyclic heterocycles incorporating either one or two heteroatoms.