Adsorptive Voltammetric Measurement of Trace Level of Iron(III) with 4–(2–Pyridylazo) Resorcin

Abstract

The polarographic behavior of the complex of iron–4– (2–pyridylazo) resorcin(PAR) was studied. In HAc– NaAc– EDTA buffer solution, the complex can be adsorped on a hanging mercury drop electrode giving a sensitive adsorptive complex reduction peak with a peak potential at -0.36V(vs. SCE). Optimum experimental conditions were found by the use of 0.08mol/L HAc, 0.06mol/L NaAc, 5.0 × 10^?3mol/L EDTA and 1.0 × 10^?5mol/L PAR. With preconcentration for 60s, the derivative peak height of the complex compound is linearly proportional to the concentration for Fe in the range from 1.0 × 10^?9mol/L to 1.0 × 10^?7mol/L. For a 2–min pre–concentration time, the detection limit found was 2.0 × 10^?10mol/L. This method has high sensitivity and selectivity. It has been applied to the determination of trace iron in food and water samples without any pre–separation step.     

Enantiospecific Synthesis of (–)-Cuspareine and (–)-Galipinine

An enantiospecific route to the synthesis of tetrahydroquinoline alkaloids (–)-cuspareine and (–)-galipinine is reported. Coupling of an iodide derivative of D-serine with aromatic dithianes and Pd-catalyzed intramolecular C–N coupling are the key steps in the synthesis.     

Theoretical study of silicon–sulfur clusters (SiS2)n (n = 1–6)

 The possible geometrical structures and relative stability of (SiS₂) _n (n=1–6) silicon–sulfur clusters are explored by means of density functional theory quantum chemical calculations. The effects of polarization functions and electron correlation are included in these calculations. The electronic structures and vibrational spectra of the most stable geometrical structures of (SiS₂) _n are analyzed by the same method. As a result, the regularity of the (SiS₂) _n cluster growth is obtained, and the calculation may used for predicting the formation mechanism of the (SiS₂) _n cluster. Received: 17 November 1999 / Accepted: 3 November 2000 / Published online: 3 May 2001     

Determination of the stability constants of Pb–(DIPSO)x–(OH)y and Pb–(AMPSO)x–(OH)y systems

In this work, complexation between lead ion and the ligands 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) and N-(1,1-dimethyl-2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid (AMPSO), which are commercial pH buffers, is presented. Both ligands form complexes with lead in their pH buffer range (between pH 6.5 and 8.5 for DIPSO and between pH 8.0 and 9.0 for AMPSO). The final models and the overall stability constants, which are reported here, were determined by direct current polarography and glass electrode potentiometry [only for the Pb–(DIPSO)_x–(OH)_y system] at 25.0 °C and 0.1 M KNO₃ ionic strength. For the Pb–(DIPSO)_x–(OH)_y system, the proposed final model contains PbL, PbL₂, PbL₂(OH), and PbL₂(OH)₂ with stability constants, as log β, of 3.4 ± 0.1, 6.35 ± 0.15, 12.8 ± 0.2, and 18.0 ± 0.3, respectively. For the Pb–(AMPSO)_x–(OH)_y system, the species observed are PbL, PbL(OH), and PbL(OH)₂ with stability constants, as log β, of 2.9 ± 0.5, 9.4 ± 0.1, and 14.5 ± 0.2, respectively. For AMPSO, the possible adsorption of the ligand at the mercury electrode surface was evaluated by alternating current polarography through calculation of the capacitance of the double layer.     

The Influence of Di-(2-Ethylhexyl)Phosphoric Acid on the Properties of Microemulsion in the Sodium Di-(2-Ethylhexyl)Phosphate–Di-(2-ethylhexyl)Phosphoric Acid–Decane–Water System

It has been shown that the presence of small amounts of di-(2-ethylhexyl)phosphoric acid (below 6 mol % in the mixture of surfactants) in a sodium di-(2-ethylhexyl)phosphate–di-(2-ethylhexyl)phosphoric acid–decane–water system widens the region of microemulsion existence with respect water and decreases the slope of the dependence of hydrodynamic droplet diameter of on water-to-sodium di-(2-ethylhexyl) phosphate molar ratio. At di-(2-ethylhexyl)phosphoric acid concentrations in its mixture with sodium di-(2-ethylhexyl)phosphate higher than 6 mol %, the fraction of water bound with ions in microemulsion droplets decreases, the region of microemulsion existence narrows, specific conductance decreases, and the slope of the dependence of the hydrodynamic droplet diameter of on the water-to-sodium di-(2-ethylhexyl) phosphate molar ration increases.     

Solubility of naphthalene in aqueous solutions of poly(ethylene glycol)–poly(propylene glycol)–poly(ethylene glycol) triblock copolymers and (2-hydroxypropyl)cyclodextrins

The solubility of naphthalene was investigated in aqueous solutions of triblock copolymers poly(ethylene glycol)–poly(propylene glycol)–poly(ethylene glycol) (PEG–PPG–PEG) and (2-hydroxypropyl)cyclodextrins. The results with solutions of the individual solubilizers were as expected: the solubility enhancement was much higher with a micelle-forming copolymer than with the non-micellizing one and with (2-hydroxypropyl)--cyclodextrin (HPBCD) than with (2-hydroxypropyl)--cyclodextrin (HPACD). Although the formation of inclusion complexes between HPACD and PEG and between HPBCD and PPG is well established, the naphthalene solubility in mixed solutions does not significantly deviate from that predicted for a mixture of independent solubilizers. Thus the interactions between HPCD and PEG–PPG–PEG copolymers are not strong enough to disrupt micelles and aggregates formed by those copolymers. In fact, slight synergetic deviations were observed with the micellizing copolymer, indicating the existence of ternary naphthalene/HPCD/copolymer interactions. For pharmaceutical applications, it is important that the solubilization efficacy of PEG–PPG–PEG copolymers and that of cyclodextrins modified by the 2-hydroxypropyl group would not be compromised if these two types of solubilizers were co-administered.     

Conductivity of Al2(WO4)3–WO3 and Al2(WO4)3–Al2O3 Composites

Russian Journal of Electrochemistry - Composites of (1 – x)Al2(WO4)3–xWO3 and (1 – x)Al2(WO4)3–xAl2O3 are synthesized and their conductivity is studied as dependent on the...     

Development of polybenzoxazine–silica–titania (PBZ–SiO2–TiO2) hybrid nanomaterials with high surface free energy

In the present work, silica and titania reinforced polybenzoxazine (PBZ–SiO₂–TiO₂) hybrid nanomaterial possessing high surface free energy have been developed using dimethylol-functional benzoxazine monomer (4HBA-BZ), tetraethoxysilane (TEOS), 3-(isocyanatopropyl)triethoxysilane (ICPTS), titaniumisopropoxide (TIPO) through an in situ sol–gel process. Data obtained from the contact angle measurement indicate that the hybrid materials are hydrophilic in nature and possess a high surface free energy. For example, hybrid PBZ obtained from 1:1:0.6:0.4 (m:m:w:w) ratio of 4HBA-BZ:ICPTS:TEOS:TIPO (PBST₄) exhibit a high surface free energy of 38.2 mJm^?2 which is higher than that of neat polybenzoxazine (29.5 mJm^?2). Further data resulted from thermal studies indicate that the hybrid PBZ possess higher values of T_g, thermal stability and char yield than those of neat PBZ.     

Study of cis–trans stereochemistry of 2-(2-chlorobenzylideneamino)phenol and 2-(2-chlorophenyl imino)methyl)phenol (Schiff bases) by GC and GC-MS spectrometry

Cis–trans stereochemistry of 2-(2-chlorobenzylideneamino)phenol (1) and 2-(2-chlorophenyl imino)methyl)phenol (2) (Schiff bases) was studied by GC-MS spectrometry, and cis–trans conversion of the two compounds in solution was investigated by GC. Thus, 2 exists as a sole trans configuration and its conversion to cis was unsuccessful in any circumstance, while compound 1 exists as a mixture of two configurations in solution, occurrence of which were dependent on the temperature, heating time, solvent, and acid catalyst. X-ray crystallography of solid 1 presented a sole trans configuration.     

Asymmetric synthesis of (–)-leiocarpin A via (–)-(S)-goniothalamin employing Julia–Kocienski olefination

A concise and enantioselective syntheses of antileukemic natural products such as (–)-(S)-goniothalamin and (–)-leiocarpin A has been accomplished in excellent yields. By employing reported conditions on suitable substrates via Julia–Kocienski olefination, intramolecular lactonization, and subsequently dehydroxylative olefination, (–)-(S)-goniothalamin was synthesized. Then Sharpless asymmetric dihydroxylation–intramolecular Michael addition on (–)-(S)-goniothalamin provided (–)-leiocarpin A.     

Transition-Metal-Free C(sp2)–C(sp2) Cross-Coupling of Diazo Quinones with Catechol Boronic Esters

A transition-metal-free C(sp²)−C(sp²) bond formation reaction by the cross-coupling of diazo quinones with catechol boronic esters was developed. With this protocol, a variety of biaryls and alkenyl phenols were obtained in good to high yields under mild conditions. The reaction tolerates various functionalities and is applicable to the derivatization of pharmaceuticals and natural products. The synthetic utility of the method was demonstrated by the short synthesis of multi-substituted triphenylenes and three bioactive natural products, honokiol, moracin M, and stemofuran A. Mechanistic studies and density functional theory (DFT) calculations revealed that the reaction involves attack of the boronic ester by a singlet quinone carbene followed by a 1,2-rearrangement through a stepwise mechanism.     

Synthesis of N-benzothiazol-2-yl-amides by Pd-catalyzed C(sp~2)–H functionalization

A catalytic synthesis of N-benzothiazol-2-yl-amides from 1-acyl-3-(phenyl)thioureas was achieved in the presence of a palladium catalyst through the C(sp2)–H functionalization/C–S bond formation. This synthetic methodology can produce various N-benzothiazol-2-yl-amides in high yields with good functional group tolerance.     

Counter-current extraction and separation of U(VI) from a mixture of U(VI)–Th(IV)–Y(III) using tris-2-ethyl hexyl phosphate (TEHP)

Evaluation of tris-2-ethyl hexyl phosphate (TEHP) for counter-current extraction and separation of U(VI) from a mixture of U(VI)–Th(IV)–Y(III) from nitric acid medium was carried out under wide experimental conditions. Batch extraction studies were carried out to investigate the effect of nitric acid concentration in feed solution, U(VI)/Th(IV) ratio and extractant concentration and the results were compared with established solvent such as tri-n-butyl phosphate (TBP) for separation of U(VI) from nitric acid medium. McCabe–Thiele diagrams for extraction as well as stripping of U(VI) were constructed under simulated conditions. Based on batch experiments, six stage counter-current extraction studies were conducted under various TEHP concentration and it was observed that 0.1 M TEHP/n-paraffin was most suitable for selective recovery of U(VI) from a mixture of U(VI)–Th(IV). An optimized condition, 0.1 M TEHP/n-paraffin, 2 M HNO₃ in feed and six number of stages was evaluated for selective extraction and stripping of U(VI) from a solution containing mixture of U(VI)–Th(IV)–Y(III) in nitric acid medium. The U(VI) in strip solution was precipitated using 30 % H₂O₂ at pH ~3. Average particle size of the final precipitate was found to be ~33 μm.     

Pyrolysis kinetics of ethylene–propylene (EPM) and ethylene–propylene–diene (EPDM)

The thermal degradation kinetics of several ethylene–propylene copolymers (EPM) and ethylene–propylene–diene terpolymers (EPDM), with different chemical compositions, have been studied by means of the combined kinetic analysis. Until now, attempts to establish the kinetic model for the process have been unsuccessful and previous reports suggest that a model other than a conventional nth order might be responsible. Here, a random scission kinetic model, based on the breakage and evaporation of cleavaged fragments, is found to describe the degradation of all compositions studied. The suitability of the kinetic parameters resulting from the analysis has been asserted by successfully reconstructing the experimental curves. Additionally, it has been shown that the activation energy for the pyrolysis of the EPM copolymers decreases by increasing the propylene content. An explanation for this behavior is given. A low dependence of the EPDM chemical composition on the activation energy for the pyrolysis has been reported, although the thermal stability is influenced by the composition of the diene used.     

Improved Synthesis of (–)-Agelastatin A

Optimization of key steps in the synthesis of the architecturally unique tetracyclic antitumor alkaloid (–)-agelastatin A (1) improved the overall yield of the 11-step process (eight operations) from 9% to 23%. Changing the solvent and using a more efficient N-benzyl deprotecting-group procedure enhanced the yields of the C-ring and D-ring intermediates, (–)-4 and (–)-7, respectively. Bromination of (–)-7 with 1,3-dibromo-5,5-dimethylhydantoin, rather than N-bromosuccinimide (NBS), increased the yield of (–)-1 from 69% to more than 94% yield.     

Fluorothiolate–dithioacid complexes of ruthenium(III) and osmium(III): crystal structure of [Os(SC6F5)2(S2CNEt2)(PMe2Ph)2]

Treatment of the coordinative unsaturated complexes [M(SR_F)₃(PMe₂Ph)₂] (M = Os or Ru; R_F = C₆F₅ or C₆F₄H-4) with MS₂Z (M = Na, S₂Z = S₂CNEt₂; M = K, S₂Z = S₂COEt) and [Os(SR_F)₃(PMe₂Ph)₂] (R_F = C₆F₅ or C₆F₄H-4) with MS₂Z [M = Na; S₂Z = S₂P(OEt)₂] in Me₂CO solution, gave the paramagnetic Os^III and Ru^III derivatives, [M(SR_F)₂(S₂Z)(PMe₂Ph)₂]. X-ray crystallography shows that [Os(SC₆F₅)₂(S₂CNEt₂)(PMe₂Ph)₂] has an octahedral geometry with trans-fluorothiolates, cis-phosphines and a chelating N,N-diethyldithiocarbamate ligand.     

The Mechanism of Acid-Catalyzed Nucleophilic Substitution in Decahydro-closo-Decaborate(2–) Anions

The reactions of salts formed by the B₁₀H^2– ₁₀anion with carboxylic acids were studied. From the model systems Cat₂B₁₀H₁₀+ HCOOH (Cat = Et₄N⁺, Bu₄N⁺, Ph₄P⁺, Ph₄As⁺), several intermediates were isolated and characterized. A mechanism was proposed for the replacement of the exo-polyhedral hydrogen atoms in B₁₀H^2– ₁₀by carboxylate groups in the reactions of Cat₂B₁₀H₁₀with carboxylic acids.     

Intramolecular Rearrangements of >Si(O–C·=O)(CH2–CH3) and >Si(CH2–CH·–CH3)(CH2–CH3) Radicals

Using ESR and IR spectroscopy, the structures of >Si(O–C^·=O)(CH₂–CH₃) (1) and >Si(CH₂–CH^·–CH₃)(CH₂–CH₃) (2) radicals were deciphered. The directions and kinetic parameters of reactions of intramolecular rearrangements in these radicals were determined. The reactions of hydrogen atom abstraction in radical (1) from the CH₂ and CH₃ groups were studied. It was found that the endothermic reaction of hydrogen atom abstraction from the methyl group occurs at a higher rate than the exothermic reaction with the methylene group. The differences are determined by changes in the size of a cyclic transition state. Based on the experimental data, the strengths of separate C–H bonds in surface fragments are compared. The rearrangement >Si(CH₂–CH^·–CH₃)(CH₂–CH₃) >Si(C^·(CH₃)₂)(CH₂–CH₃) was discovered and its mechanism was determined. One of its steps is the skeletal isomerization Si- (2)- _. (1)Si- (1)- _. (2). Experimental data are analyzed using the results of quantum-chemical calculations of model systems.     

Fabrication of hollow SiO2 and Au (core)–SiO2 (shell) nanostructures of different shapes by CdS template dissolution

Core–shell silica (SiO₂) coated CdS nanorods (NR) and nanospheres (NS) were prepared (SiO₂@CdS) by deposition of a Si–O–Si amorphous layer over the CdS surface through the hydrolysis of 3-mercaptopropyltrimethoxysilane and tetraethylorthosilicate. Nanoporous SiO₂ matrix (NPSM), hollow SiO₂ nanotubes (HSNT) and nanospheres (HSNS) useful for efficient adsorption and catalytic processes were prepared by chemical dissolution of CdS–NS (size: 9–10 nm) and CdS–NR (length: 116–128 nm and width: 6–11 nm) template from SiO₂@CdS with 2 M HNO₃. These SiO₂ nanostructures were characterized by optical absorption, TEM, EDX, SAED and BET surface area analysis. TEM images revealed the fabrication of slightly distorted HSNS (size: 9–12 nm) and closed HSNT (length: 30–45 nm and diameter: 9–14 nm) of shorter dimensions than the CdS–NR template used. The BET surface area (112–134 m² g^?1) of NPSM and HSNS is found to be larger than the surface area (29–51 m² g^?1) of SiO₂@CdS composites indicating hollow SiO₂ morphology. Silica coated Au (SiO₂@Au) composites formed by CdS dissolution from Au (2 wt%) deposited CdS–NR core-encapsulated into SiO₂ shell (SiO₂@Au–CdS–NR) exhibited a surface plasmon band at 550 nm and displayed high catalytic activity for 4-nitrophenol reduction by Au nanoparticle.