Search results for "bond"
showing 10 items of 3527 documents
Rational design of covalently bridged [FeIII2MIIO] clusters
2010
We are reporting the first supramolecular dimeric units of basic carboxylates. The neutral [FeIII 2MIIO] motif for different 3d M metals is covalently bound through 2,2′-bipyrimidine. We have structurally characterized the hexanuclear clusters and the related trinuclear building blocks. Their magnetic properties have been fully analyzed and DFT calculations have been performed as a supplementary tool. All results evidence a weak antiferromagnetic interaction through the bpym bridge between isolated spin ground states (in some examples) arising from intra-Fe 2MO core exchange couplings. Fil: Alborés, Pablo. Johannes Gutenberg Universitat Mainz; Alemania. Consejo Nacional de Investigaciones C…
Conductance in isodielectric mixtures. III.i-butyronitrile with chlorobenzene,o-dichlorobenzene,p-dichlorobenzene, 1,2-dichloroethane, andn-pentanol
1975
The conductance of tetrabutylammonium tetraphenylboride, picrate, perchlorate, and nitrate has been measured at 25°C in mixtures ofi-butyronitrile with chlorobenzene,o-dichlorobenzene,p-dicholorobenzene, 1,2-dichloroethane, andn-pentanol covering the range of dielectric constants 10≤D≤20. In these mixtures of polar solvents, both association constants and ionic mobilities depend on ion-solvent interaction energies and on free volume in a manner that is specific for each system. This failure of the primitive model (rigid charged spheres in a continuum) is shown to be the consequence of short-range spatial and energetic interactions between ions and adjacent solvent molecules.
Second harmonic generation in SiO2 sol-gel films functionalized with Ethyl-[4-(4-nitro-phenylazo)-phenyl]-(2-oxiranylmethoxy-ethyl)-amine (ENPMA) mol…
2010
Abstract High concentrations of Ethyl-[4-(4-nitro-phenylazo)-phenyl]-(2-oxiranylmethoxy-ethyl)-amine (ENPMA) chromophores were linked covalently to two different kinds of SiO 2 matrices prepared by the sol–gel method. The matrices differed in their reticulation level, determined by the organometallic precursor used during the synthesis. The materials were deposited as films by spin-coating. The chromophores in the films were oriented in a non-centrosymmetric way by a modified Corona poling set-up, and the nonresonant second order non-linear optical coefficient d 33 (0) of the films was determined by means of the Maker fringe technique. It was found that there is an optimum concentration of …
Preparation and characterization of organofunctionalized bentonite clay bearing aminophosphonic groups in heavy metal uptake
2019
Abstract Bentonite (Bnt) with covalently immobilized N-propylaminodi(methylenephosphonic) groups (Bnt-ADPA) was obtained through a two-step synthesis including aminosilanization and phosphorylation of resulting aminobentonite through Moedritzer-Irani reaction. The grafting of an aminosilane layer in the interplanar space of the bentonite was investigated by XRD, and the presence of N and P-containing groups was confirmed by FTIR, 31P and 29Si MAS NMR. The immobilization of the organic functional groups increased the interplanar distance in bentonite matrix at 52% that essentially improved filtration properties compared to raw bentonite. The number of surface acid groups was evaluated to 0.8…
Effects of temperature on electron paramagnetic resonance of dangling oxygen bonds in amorphous silicon dioxide
2011
The properties of electron paramagnetic resonance (EPR) signal of oxygen dangling bonds in amorphous SiO2 ("non-bridging oxygen hole centers", NBOHC) in excimer laser-irradiated amorphous SiO2 were studied in the temperature range 20K to 295K. NBOHCs strongly affect optical and chemical properties of amorphous SiO2 -based (nano) structures and their surfaces. The behaviour of their EPR signal is complicated due to a nearly degenerate electronic ground state. It was found that EPR signal has a non-Curie (~1/T) T-dependence down to 40K, indicating that EPR-based concentration estimates routinely obtained at T = 77K underestimate the center concentrations at least by a factor of 1.7. The estim…
Diffusion and reactions of interstitial oxygen species in amorphous SiO2: A review
2008
This article briefly summarizes the diffusion and reactions of interstitial oxygen species in amorphous SiO 2 (a-SiO 2 ). The most common form of interstitial oxygen species is oxygen molecule (O 2 ), which is sensitively detectable via its characteristic infrared photoluminescence (PL) at 1272 nm. The PL observation of interstitial 0 2 provides key data to verify various processes related to interstitial oxygen species: the dominant role of interstitial O 2 in long-range oxygen transport in a-SiO 2 ; formation of the Frenkel defect pair (Si-Si bond and interstitial oxygen atom, 0°) by dense electronic excitation; efficient photolysis of interstitial O 2 into O° with F 2 laser light (λ= 157…
A Wavy Two-Dimensional Covalent Organic Framework from Core- Twisted Polycyclic Aromatic Hydrocarbons
2019
A high degree of crystallinity is an essential aspect in two-dimensional covalent organic frameworks, as many properties depend strongly on the structural arrangement of the different layers and their constituents. We introduce herein a new design strategy based on core-twisted polycyclic aromatic hydrocarbon as rigid nodes that give rise to a two-dimensional covalent organic framework with a wavy honeycomb (chairlike) lattice. The concave–convex self-complementarity of the wavy two-dimensional lattice guides the stacking of framework layers into a highly stable and ordered covalent organic framework that allows a full 3D analysis by transmission electron microscopy revealing its chairlike …
C–S bond cleavage by cobalt: synthesis, characterization and crystal structure determination of 1,2-di-(o-salicylaldiminophenylthio)ethane and its Co…
2002
1,2-Di-(o-salicylaldiminophenylthio)ethane reacts with Co(II) salts to form a complex with oxidative cleavage of the C–S bond, to result in the formation of a Co(III) complex of the cleaved ligands.
Novel chiral three-dimensional iron(III) compound exhibiting magnetic ordering at T(c) = 40 K.
2002
The preparation and crystal structure determination of the iron(III) compound of formula [(NH(4))(2)[Fe(2)O(ox)(2)Cl(2)].2H(2)O](n) (1) (ox = oxalate dianion) are reported here. Complex 1 crystallizes in the orthorhombic system, space group Fdd2, with a = 14.956(7) A, b = 23.671(9) A, c = 9.026(4) A, and Z = 8. The structure of complex 1 consists of the chiral anionic three-dimensional network [Fe(2)O(ox)(2)Cl(2)](2-) where the iron(III) ions are connected by single oxo and bisbidentate oxalato groups. The metal-metal separations through these bridging ligands are 3.384(2) and 5.496(2) A, respectively. Ammonium cations and crystallization water molecules are located in the helical pseudohex…
Unravelling the mechanism of the ketene-imine Staudinger reaction. An ELF quantum topological analysis
2015
The mechanism of the ketene-imine Staudinger (KI-S) reaction between t-butyl-cyano ketene and N-phenyl phenylimine has been studied using DFT methods at the MPWB1K/6-311G(d) computational level. The reaction takes place through a two-step mechanism: (i) the first step is the nucleophilic attack of the imine nitrogen lone pair on the central carbon of the ketene yielding a zwitterionic (ZW) intermediate; (ii) the second step, which is the rate- and stereoselectivity-determining step, is a ring-closure process achieved by a nucleophilic attack of the terminal carbon atom of the ketene on the imine carbon atom. Due to the unfeasibility of a cis/trans and an E/Z stereoisomerisation at the ZW in…