Search results for "Hydrogen"

Calix[4]arenes bridged at the lower rim

1990

Calix[41 arenes, in which two opposite p-positions are linked by an aliphatic chain, and their derivatives show transport or complexation properties strongly dependent on the length of this chain. Bicyclo-calix[4]arenes, a special case of 1,3-bridged calix[41 arenes, and annelated calix[41 arenes, a special case of 1 ,&bridged calix [4] arenes, are also described. Although the preferred conformation of calix[4]arenes is the coneconformation due to its stabilization by a cyclic array of hydrogen bonds between the phenolic hydroxyl goups, these molecules are flexible, with a cone to cone inversion rate of about 150 8-1 for a calix[glarene at 47OC (ref. 1). The cone (or any other) conformation…

REGIONAL, COMPARATIVE AND ONTOGENETIC STUDIES ON LACTATE DEHYDROGENASE OF NERVOUS TISSUE

1962

Phospholylmethano P-chirogenic-phosphine-borane as P-(η2-BH3)-chelating ligands of rhodium (I): Synthesis, characterization and asymmetric hydrogenat…

2021

International audience; The stereoselective synthesis of new phospholylmethano P-chirogenic-phosphine-borane was achieved by P-C bond formation of the bridge, using electrophilic or nucleophilic P*-building blocks. These P1CH2P2*.BH3 ligands behaved as chelating (κ1-P1)-(η2-BH3) entities towards the cationic rhodium(I) centre. The resulting chiral rhodium complexes were tested in asymmetric rhodium catalyzed hydrogenation of methyl 2-(acetamido)acrylate.

Titanium dioxide in dental enamel as a trace element and its variation with bleaching

2017

Background Titanium is a less studied trace element in dental enamel. Literature relates an increased Titanium concentration with a decreased enamel crystal domain size, which in turn is related to a higher color value. The aim of our study was to analyze the effect of tooth bleaching agents on its concentration in dental enamel by means of confocal Raman spectroscopy. Material and methods Human teeth were randomly distributed in six experimental groups (n=10) and submitted to different bleaching protocols according to the manufacturer´s instructions. Confocal Raman spectroscopy was carried out in order to identify and quantify the presence of titanium dioxide molecules in enamel prior to a…

Applications and stereoselective syntheses of P-chirogenic phosphorus compounds.

2016

International audience; Phosphorus compounds bearing chirality on the P-center are usually qualified as P-chirogenic or P-stereogenic. This chemical class concerns natural products, agrochemistry, molecular materials, biology and pharmacy, although it is certainly in coordination chemistry and in asymmetric catalysis using chiral transition metal complexes that P-chirogenic phosphorus compounds are the most used. The chiral phosphine ligands and their uses in asymmetric metal-catalyzed reactions have been widely reviewed in literature. However, an overview covering the applications as well as the stereoselective syntheses of all classes of phosphorus compounds has not yet been provided. Thi…

Dependence of the distortion of the square pyramids in N,N-dimethylethylenediammonium pentachloroantimonate(III) on the geometry of hydrogen bonds

2001

Abstract N ,N-Dimethylethylenediammonium pentachloroantimonate(III) crystallizes in the monoclinic system, in space group P21/c (a = 12.460(2), b = 10.252(2), c = 10.330(2) Å, β = 97.75(3)°, V = 1307.5(4) Å3, Z = 4, dc = 1.997, dm = 1.99(2) g/cm3). The crystal structure of [(CH3)2NH(CH2)2NH3][SbCl5] consists of isolated [SbCl5]2- anions and [(CH3)2NH(CH2)2NH3]2+ cations. The [SbCl5]2- anion has a distorted square pyramidal geometry, presenting one short axial and four long equatorial Sb-Cl bonds. The square pyramids are characteristically stacked one close to the other, parallel to the c axis. The voids between the anionic sublattice are filled by [(CH3)2NH(CH2)2NH3]2+ cations. The five non…

Iron (II) isothiocyanate complexes with substituted pyrazines: Experimental and theoretical views on their electronic structure

2015

Abstract Synthesis, structural, magnetic, Mossbauer and thermal studies of isothiocyanate iron (II) complexes with substituted pyrazines (iodo-, bromo- and amino-derivatives) are discussed here. Complexes with iodo- and bromo-derivatives were found to have the composition [Fe(Ipz)2(SCN)2(H2O)2]·2Ipz (1) and [Fe(Brpz)2(SCN)2(H2O)2]·2Brpz (2), whereas in the case of amino-functionalized pyrazine the formation of [Fe(NH2pz)4(SCN)2] (3) was observed. 3D organization of the molecular complexes is stabilized within different hydrogen, halogen and lone pair–π interactions. Spin state of iron (II) ions in 1–3 was determined as high spin by Mossbauer and magnetic measurements. DFT calculations for t…

Photoreforming of organics in aqueous suspension of Nb2O5/C3N4 composites to obtain H2

2023

Photocatalysis based on the use of semiconductors has gained, during the recent years, increased attention as promising sustainable process for H2 and fine chemicals production, given the possibility of activating such photocatalysts by visible light radiation. The photo-reforming activity in this work has been measured under both UV-LED irradiation and natural sunlight illumination. Moreover, the presence of metallic Pt as co-catalysts has been studied. Carbon nitride (C3N4) based materials are good candidates for solar energy conversions. The choice of C3N4 and Nb2O5 arises from the fact that their heterojunction can lead to lower recombination between the photo-generated electrons and ho…

Role of Weak Hydrogen Bonds and Halogen Bonds in 5-Halo-1,3-dimethyluracils and Their Cocrystals—A Combined Experimental and Computational Study

2016

Seven single crystals containing either N,N-dimethyluracil (DMHU) or one of its 5-halogenated derivatives (DMXU; X = F, Cl, Br, I) were prepared using N,N-dimethylformamide as the crystallization solvent. Single crystal X-ray diffraction and quantum chemical calculations carried out at the spin component scaled local MP2 level of theory were then used to study the intramolecular halogen and nonconventional hydrogen bonds present in the structures. The results were compared to and contrasted with the previously reported data for uracil and its halogenated derivatives. In particular, the intermolecular interactions in DMIU were compared to the halogen and hydrogen bonds in 5-iodouracil that, …

Chiroptical Phenomena in Reverse Micelles: The Case of (1R,2S)- Dodecyl (2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium Bromide (DMEB)

2014

(1R,2S)-Dodecyl(2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium bromide (DMEB) aggregates dispersed in carbon tetrachloride have been investigated by Fourier transform infrared (FT-IR), vibrational circular dichroism (VCD) and 1H nuclear magnetic resonance (NMR) spectroscopy at various surfactant concentration and water-to-surfactant molar ratio. Experimental data indicate that, even at the lowest investigated concentration and in absence of added water, DMEBmolecules associate in supramolecular assemblies. At higherDMEBconcentration the aggregates can confine watermolecules,making it plausible to think thatDMEB form reverse micelles and that watermolecules are quite uniformly distributed…