Search results for "stereo"
showing 10 items of 6147 documents
Complexes of Tungsten(VI) with Mucic Acid: a Spectrophotometric and Polarimetric Study in Aqueous Solution
1986
Abstract Spectrophotometric work on mucic acid-W(VI) system shows the formation of three different oxoanion complexes in aqueous solution; their stability is dependent upon pH. One of the complexes is monomeric tungstodimucate and the other two are 2/2 species. An anomalous cryoscopic behaviour, similar to that of W(VI) tartaric system, has been observed for the dimeric complex formed at higher pH. The stoichiometries and conditional dissociation constants have been polarimetrically determined by means of competitive reactions between the mucic and tartaric ligands. 1 H and 13 C NMR spectra have been interpreted for the similar complex species of both mucic and tartaric acids.
Photocatalytic activity of transition-metal-loaded titanium(IV) oxide powders suspended in aqueous solutions: Correlation with electron–hole recombin…
2001
Photocatalytic reactions by transition-metal (V, Cr, Fe, Co, Cu, Mo, or W) loaded TiO2 (M-TiO2) powders suspended in aqueous solutions of methanol, (S)-lysine (Lys), or acetic acid were investigated. The photoactivities of various samples were compared with the rate constant (kr) of recombination of photoexcited electrons and positive holes determined by femtosecond pump–probe diffuse reflection spectroscopy (PP-DRS). As a general trend, increased loading decreased the rate of formation of the main products (H2 , pipecolinic acid (PCA), and CO2) under UV (>300 nm) irradiation, and the effect became more intense on increasing the loading. In PP-DRS, these M-TiO2 gave similar decays of absorp…
The interaction of organotins with native DNA
1992
The compounds R2SnCl2 and R3SnCl (RMe, Et, nBu, nOct, Ph, in ethanol solution) as well as the aqueous species [Me2Sn(OH2)n]2+ and [Me3Sn(OH2)2]+, react with aqueous native DNA, yielding solid phases. According to the pointcharge model treatment of the 119Sn Mossbauer parameter nuclear quadrupole splitting, trans-octahedral R2Sn(O2PXY)2, and trigonalbipyramidal R3Sn(O2PXY), (RMe, Et, nBu), would occur in the pellets, the tin atoms being coordinated by phosphodiester groups of the nucleic acid. The precipitates from Ph2SnIV would consist of the DNA complex as well as of the Ph2SnIV distannoxane obtained by hydrolysis of the reactant, whilst nOct2SnCl2, nOct3SnCl and Ph3SnCl would mainly yield…
Induced-Fit Molecular Recognition with Water-Soluble Cavitands
2000
Synthesis of novel water-soluble cavitands 1 and 2 and their complexes—the caviplexes—is described. The solubility in water derives from four primary ammonium groups on the lower rim and eight secondary amide groups on the upper rim. Cavitands 1 and 2 exist as D2d velcraplex dimers in aqueous solution but the addition of lipophilic guests 15–24 induces conformational changes to the vase-like structures. The internal cavity dimensions are 8×10 A, and the exchange rates of guests in the caviplexes are slow on the NMR timescale (room temperature and 600 MHz). The direct observation of bound species and the stoichiometry of the complexes is reported. The association constants (Ka) between 0.4×1…
Violurato complexes of nickel(II). Formation equilibria. Deprotonation equilibria of the coordinated ligands and related stereochemical changes
1986
The formation equilibria of nickel(II) violurato-complexes in water and dimethylsulphoxide-water (80∶20) as well as deprotonation of the coordinated ligands and related stereochemical changes are reported. The stability constants of the Ni2+-H2V− complexes logβ1=5.06, Iogβ2=9.38, logβ3=12.98 as well as the acidity constants of the [Ni(H2V)3]− complex, logβj1= 8.37, logβj2=15.76, logβj3=22.37 are determined at 25 °C and 0.1M NaClO4. A new violurato-complex of Ni2+, Na4[Ni(HV)3]5H2O, is isolated and characterized.
Sequestration of alkyltin(IV) cations by complexation with amino-polycarboxylic chelating agents
2013
Abstract The binding capacity of four amino-polycarboxylic ligands (APCs) [nitrilotriacetate (NTA), ethylenediamine- N , N , N′ , N′ -tetraacetate (EDTA), (S,S)-ethylenediamine- N , N ′-disuccinic acid (S,S-EDDS) and diethylenetriamine- N , N , N′ , N″ , N″ -pentaacetate (DTPA)] towards mono-, di- and tri-alkyltin(IV) cations [(CH 3 )Sn 3 + , (CH 3 ) 2 Sn 2 + , (C 2 H 5 ) 2 Sn 2 + , (CH 3 ) 3 Sn + or (C 2 H 5 ) 3 Sn + ] was studied, in aqueous solutions, by ISE-H + potentiometry, at I = 0.1 mol L − 1 (NaCl) and at T = 298.15 K. In all the systems R x Sn (4 − x)+ − APC (R = CH 3 or C 2 H 5 ) a strong 1:1 species is formed together with protonated, hydroxo and dinuclear complexes. The valu…
Ionic partition diagram of tetraphenylporphyrin at the water|1,2-dichloroethane interface
2011
diagram of 5,10,15,20-tetraphenyl-21H,23H-porphine (H2TPP) at the water|1,2-dichloroethane interface using a simple Born solvation model. This zone diagram shows under which form this porphyrin is present, i.e. neutral, monoprotonated or diprotonated, and in which phase i.e. either in the aqueous or the organic phase as a function of the aqueous pH and the interface polarisation that can be controlled externally or by the distribution of supporting electrolytes. This diagram explains why the monoprotonated form has been difficult to observe when doing biphasic pH titrations
[2]Catenanes and inclusion complexes derived from self-assembled rectangular PdII and PtII metallocycles
2012
New inclusion complexes and [2]catenanes were self-assembled from a fluorescent diazapyrenium based ligand, a PdII or PtII complex, and cyclic or acyclic electron rich aromatic guests in aqueous and organic media. The molecular rectangles display a π-deficient cavity suitable to incorporate π-donor aromatic systems. The inclusion complexes between the metallocycles and phenylenic (2a,b) and naphthalenic (3a,b–5a,b) derivatives were studied by NMR, UV-vis and fluorescence spectroscopy. The crystal structure of (3b) ⊂ 1a·6PF(6) confirmed the insertion of the guest into the cavity of the metallocycle. Following the same self-assembly strategy, the use of polyethers 6,7 as π-donors resulted in …
Desulfurization processes of thiosemicarbazonecopper(ii) derivatives in acidic and basic aqueous media
2013
This work provides structural evidence for different desulfurization processes in aqueous solutions of [CuL]+ derivatives (HL = pyridine-2-carbaldehyde thiosemicarbazone). Structural resolution has been achieved for the [{CuL(SH)}2] (1), [CuLCl]2[Cu(pic)2] (pic− = picolinato, pyridine-2-carboxilato) (2) and [Cu(HL)(NCS)](NO3) (4) compounds, together with a derivative of 4 with formula [Cu(HL)(NCS)][Cu(HL)(NCS)0.72(NO3)0.28](NO3)2 (5), whose HS−, pic− and NCS− ligands come from thiosemicarbazone breakage. The behaviour of the [CuL]+ species in basic aqueous solutions or water under reflux has been compared with that exhibited by iron(III), cobalt(III), zinc(II) and lead(II) derivatives of th…
Microheterogeneous electrocatalytic chiral recognition at monoclinic vanadium-doped zirconias: enantioselective detection of glucose.
2007
Synthetic tetragonal and monoclinic vanadium-doped zirconias (t- and m-VxZr1-xO2, 0.005x0.150) exert an effective catalytic effect toward the electrochemical oxidation of glucose in aqueous alkaline media. The catalytic effect of monoclinic specimens attached to carbon and fluorine-doped tin oxide electrodes exhibits a remarkable enantioselectivity, so that catalytic currents for the oxidation of L-glucose at +0.92 V vs AgCl/Ag are considerably larger than those obtained for the oxidation of D-glucose. This enantioselectivity can be associated with the existence of a noncentrosymmetric coordination of vanadium centers in the monoclinic crystalline form of zirconia. From the electrochemical …