Search results for "Isomerism"
showing 10 items of 747 documents
Heterogeneous Interactions between Gas-Phase Pyruvic Acid and Hydroxylated Silica Surfaces: A Combined Experimental and Theoretical Study
2019
The adsorption of gas-phase pyruvic acid (CH3COCOOH) on hydroxylated silica particles has been investigated at 296 K using transmission Fourier transform infrared (FTIR) spectroscopy and theoretical simulations. Under dry conditions (<1% relative humidity, RH), both the trans-cis (Tc) and trans-trans (Tt) pyruvic acid conformers are observed on the surface as well as the (hydrogen bonded) pyruvic acid dimer. The detailed surface interactions were further understood through ab initio molecular dynamics simulations. Under higher relative humidity conditions (above 10% RH), adsorbed water competes for surface adsorption sites. Adsorbed water is also observed to change the relative populations …
Exotic SiO(2)H(2) Isomers: Theory and Experiment Working in Harmony.
2016
Replacing carbon with silicon can result in dramatic and unanticipated changes in isomeric stability, as the well-studied CO2H2 and the essentially unknown SiO2H2 systems illustrate. Guided by coupled-cluster calculations, three SiO2H2 isomers have been detected and spectroscopically characterized in a molecular beam discharge source using rotational spectroscopy. The cis,trans conformer of dihydroxysilylene HOSiOH, the ground-state isomer, and the high-energy, metastable dioxasilirane c-H2SiO2 are abundantly produced in a dilute SiH4/O2 electrical discharge, enabling precise structural determinations of both by a combination of isotopic measurements and calculated vibrational corrections. …
Switching and redox isomerism in first-row transition metal complexes containing redox active Schiff base ligands.
2014
International audience; The reversible redox isomerisms in first row transition metal complexes of the type ML2 were studied. The six ML2 complexes (M = Mn(III) (), Fe(II) (), Co(III) (), Ni(II) (), Cu(II) () and Zn(II) ()) were synthesized with a redox active Schiff base ligand [2-(3,5-di-tert-butyl-2-hydroxyphenylamino)-4-chlorophenol] (H3L) presenting different oxidation states from -2 to 0 (L(2-), L(-) and L(0)). EPR spectra and magnetic susceptibility measurements indicate the presence of complexes of the type [Mn(III)(L(2-))(L(-))] () with S = 1/2, [Fe(II)(L(-))2] () with S = 2, [Co(III)(L(2-))(L(-))] () with S = 1/2, [Ni(II)(L(-))2] () with S = 1, [Cu(II)(L(-))2] () with S = 1/2 and …
The substituent effect of π-electron delocalization in N-methylamino-nitropyridine derivatives: crystal structure and DFT calculations
2020
AbstractThe crystal and molecular structures of 3-(N-methylamino)-2-nitropyridine, 5-(N-methylamino)-2-nitropyridine and 2-(N-methylamino)-5-nitropyridine have been characterized by X-ray diffraction. To perform conformational analysis, the geometries of the compounds as well as their conformers and rotamers were optimized at the B3LYP/6-311++G(3df,3pd) level. The resulting data were used to analyze the π-electron delocalization effect in relation to the methylamino group rotation in ortho-, meta- and para-substitution positions. Quantitative aromaticity indices were calculated based on which we estimated the electronic structures of the analyzed compounds. The substituent effect of the met…
Mechanism and cis/trans Selectivity of Vinylogous Nazarov-type [6π] Photocyclizations
2017
Vinylogous Nazarov-type cyclizations yield seven-membered rings from butadienyl vinyl ketones via a photochemical [6π] photocyclization followed by subsequent isomerization steps. The mechanism of this recently developed method was investigated using unrestricted DFT, SF-TDDFT, and CASSCF/NEVPT2 calculations, suggesting three different pathways that lead either to pure trans, pure cis, or mixed cis/trans configured products. Singlet biradicals or zwitterions occur as intermediates. The computational results are supported by deuterium-labeling experiments.
Cis- and trans molybdenum oxo complexes of a prochiral tetradentate aminophenolate ligand : Synthesis, characterization and oxotransfer activity
2020
Abstract Reaction of [MoO2Cl2(dmso)2] with the tetradentate O2N2 donor ligand papy [H2papy = N-(2-hydroxybenzyl)-N-(2-picolyl)glycine] leads to formation of the dioxomolybdenum(VI) complex [MoO2(papy)] (1) as a mixture of cis and trans isomers. Recrystallization from methanol furnishes solid cis-1, whereas the use of a dichloromethane-hexane mixture allows for the isolation of the trans-1 isomer. Both isomers have been structurally characterized by X-ray crystallography and the energy difference between the isomeric pair has been investigated by electronic structure calculations. Optimization of two configurational isomers in the gas phase predicts the trans isomer to lie 2.5 kcal/mol lower…
The separation of cis- and trans-1,3-cyclohexanediol isomers by copper complexation. Crystal structures of cis-1,3-cyclohexanediol and copper(II) chl…
2002
Abstract A 1:1 complex between copper(II) chloride and cis-1,3-cyclohexanediol has been synthesized and its crystal structure determined. The structure is composed of two different dinuclear [CuCl2(c-13chd)]2 units in which the copper(II) ions are surrounded by the two oxygen atoms of the diol and the two halide ions in a distorted cis-square planar arrangement in both cases. A fifth coordination site is occupied by a halide ion in one dinuclear unit for both copper ions and an O atom belonging to the adjacent CuCl2(c-13chd) moiety in the other dinuclear unit for both copper ions, thus forming a distorted square pyramidal geometry for the metals in all cases. The ability of CuCl2 to prefere…
Highly Stable Neutral and Positively Charged Dicarbollide Sandwich Complexes
2005
Novel sandwich metallacarboranes commo-[3,3'-Ni(8-SMe2-1,2-C2B9H10)2] (1), commo-[3,3'-Co(8-SMe2-1,2-C2B9H10)2]+ (2+), commo-[3,3'-Ru(8-SMe2-1,2-C2B9H10)2] (4) and commo-[3,3'-Fe(8-SMe2-1,2-C2B9H10)2] (5) have been prepared by reaction of [10-SMe2-7,8-nido-C2B9H10]- with NiCl2 x 6 H2O, CoCl2, [RuCl2(dmso)4] and [FeCl2(dppe)], respectively. Reduction of 2+ with metallic Zn leads to the neutral and isolable complex commo-[3,3'-Co(8-SMe2-1,2-C2B9H10)2] (3). Theoretical calculations using the ZINDO/1 semiempirical method show three energy minima for complexes 1-3 and 5 that agree with the presence of three different rotamers in solution at low temperature, while four relative energy minima have…
Artemisia arborescens essential oil composition, enantiomeric distribution and antimicrobial activity from different wild populations from the Medite…
2016
International audience; Aerial parts of Artemisiaarborescens were collected from different sites of the Mediterranean area (southwestern Algeria and southern Italy) and the chemical composition of their essential oil (EO) extracted by hydrodistillation was studied by both gas chromatography (GC) equipped with an enantioselective capillary column and GC/mass spectrometry (GC/MS). The EOs obtained were tested against several Listeriamonocytogenes strains. Using GC and GC/MS, 41 compounds were identified, accounting for 96.0-98.8% of the total EO. All EOs showed a similar terpene profile, which was rich in chamazulene, -thujone, and camphor. However, the concentration of such compounds varied …
Purification and characterization of geranyl diphosphate synthase from Vitis vinifera L. cv Muscat de Frontignant cell cultures
1993
A geranyl diphosphate synthase (EC 2.5.1.1), which catalyzes the formation of geranyl diphosphate from dimethylallyl diphosphate and isopentenyl diphosphate, was isolated from Vitis vinifera L. cv Muscat de Frontignan cell cultures. Purification of the enzyme was achieved successively by ammonium sulfate precipitation and chromatography on DEAE-Sephacel, hydroxylapatite, Mono Q, Phenyl Superose, Superose 12, and preparative nondenaturing polyacrylamide gels. The enzyme formed only geranyl diphosphate as a product. In all cases, neither neryl diphosphate, the cis isomer, nor farnesyl diphosphate was detected. The enzyme showed a native molecular mass of 68 [plus or minus] 5 kD as determined …