Arsenite oxidation and arsenate determination by the molybdene blue method

Based on the similarity in properties of arsenate and phosphate, the colorimetric method using the molybdene blue complex was tested in order to determine low As(V) concentration in waters. The influence of complex formation time, daylight, temperature and competitive anions (silicate and sulphate) upon complex formation was determined. Optimal complex formation was reached in 1 h at 20±1 °C and was slightly favoured when developed in daylight. The formation rate declined with decreasing reaction temperature and no influence of any of the competitive anions tested (at concentrations usually found in natural waters of granitic areas) was noted. The detection limit of this method was 20 μg As(V) l⁻¹. This simple, fast and sensitive arsenic determination method is suitable for field analysis, especially for waters containing low levels of phosphate and organic matter. Through arsenate determination, this colorimetric method allowed the arsenite oxidation efficiency of five common industrial oxidants to be compared. H₂O₂ and MnO_2(s) were not considered as effective oxidants as a high excess was necessary to ensure As(III) oxidation. NaOCl and KMnO₄ were promising oxidants as they allowed complete arsenite oxidation with a small excess for NaOCl or even less than the electron stoichiometric ratio in the case of KMnO₄. FeCl₃ was the most effective oxidant among the reagents tested here.     

Noncovalent functionalization of multi-walled carbon nanotubes by pyrene containing polymers

Multi-walled carbon nanotubes (MWNTs) have been solubilized in water and in various organic solvents by noncovalent side-wall functionalization by pyrene containing polymers.     

Epoxides,cyclic sulfites,and sulfate from natural pentacyclic triterpenoids: theoretical calculations and chemical transformations

Several triterpenic derivatives, with the A-ring functionalized, were semisynthesized from oleanolic and maslinic acids. The reactivities of sulfites, sulfate, and epoxides in these triterpene compounds were investigated under different reaction conditions. Moreover, contracted A-ring triterpenes (five-membered rings) were obtained, by different treatments of the sulfate 7. From the epoxide 8, deoxygenated and halohydrin derivatives were semisynthesized with several nucleophiles. Ozonolysis and Beckmann reactions were used to yield 4-aza compounds, from five-membered ring olanediene triterpenes. The X-ray structure of sulfate 7 is given and compared with density functional theory geometries. Theoretical (13)C and (1)H chemical shifts (gauge-invariant atomic orbital method at the B3LYP/6-31G*//B3LYP/6-31G* level) and (3)J(H,H) coupling constants were calculated for compounds 5-9 and 34-36, identifying the (R)- or (S)-sulfur and alpha- or beta-epoxide configurations together with 4-aza or 3-aza structures.     

Beitrag zur Kenntnis der Pt(II)-Verbindungen mit Acetoxim

Zusammenfassung Es wurde die thermische Zersetzung dercis-undtrans-Isomeren des [PtAox ₂Cl₂] sowie des [PtAox ₃Cl]Cl mit Hilfe derTG-undTD-Kurven im Falle des reinen Komplexes sowie im Falle einer Mischung mit Al₂O₃ (1:3) geklärt. Dietrans-Form ist stabiler als diecis-Form, die Zersetzung wird durch Al₂O₃ nicht beeinflu\t. [PtAox ₃Cl]Cl wird infolge der Erhitzung in die stabileretrans-Form [PtAox ₂Cl₂] umgewandelt und verhält sich auch weiterhin wie diese.

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A contribution to the study of the compounds of Pt(II) with acetoxime

The thermal decomposition ofcis- andtrans-[PtAox ₂Cl₂] and of [PtAox ₃Cl]Cl was studied by means ofTGA andTDA on pure substances as well as on mixtures with Al₂O₃ (1:3). Thetrans form is more stable then thecis form, the presence of Al₂O₃ has no influence on its decomposition. The [PtAox ₃Cl]Cl is transformed by heating into the more stabletrans-isomer and its further behaviour is identical with that of thetrans isomer.

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Mit 4 Abbildung     

Infrared Study of Solid Dispersions of β-Cyclodextrin with Naphthalene Derivitaves

The inclusion complexes of β-cyclodextrin (β-CD) with naphthalene and some of its derivatives in the solid state have been studied by infrared spectroscopy. Digital subtraction, deconvolution and curve fitting have been used to investigate the interactions between the naphthalene derivatives andβ-CD. Several preparation methods for the solid dispersions have been tested, using FTIR as an effective tool to evaluate the interactions at the molecular level. The effects of temperature and humidity on the spectra have been also analyzed. A carbonyl moiety in the guest molecule can increase the stability of the complex by establishing specific interactions with the hydroxyl groups of the CD cavity rims. The stability of the complexes is higher for 2-naphthylacetate than that for 1-naphthyl acetate, andso is the degree of association of its carbonyl groups. On the other hand, 2-acetylnaphthaleneforms very stable inclusioncomplexes although its carbonyl groups appear to be significantly less associated.     

Lead(II) thiocyanate complexes with bibracchial lariat ethers: an X-ray and DFT study

Compounds of formula [Pb(L2)(NCS)2] (1) and [Pb(L4)(SCN)2] (2) (where L2 is the lariat crown ether N,N'-bis(3-aminobenzyl)-4,13-diaza-18-crown-6 and L4 is the Schiff-base lariat crown ether N,N'-bis(3-(salicylaldimino)benzyl)-4,13-diaza-18-crown-6) were isolated and structurally characterized by X-ray diffraction analyses. The X-ray crystal structures of both compounds show the metal ion coordinated to the six donor atoms of the crown moiety, leaving the corresponding pendant arms uncoordinated. The coordination sphere of lead(II) is completed by two thiocyanate groups that coordinate either through their nitrogen (1) or sulfur (2) atoms. The organic receptor adopts a syn conformation in 1, while in 2 it shows an anti conformation. To rationalize these unexpected different conformations of the L2 and L4 receptors in compounds 1 and 2, as well as the different binding modes found for the thiocyanate ligands, we have carried out theoretical calculations at the DFT (B3LYP) level. These calculations predict the syn conformation being the most stable in both 1 and 2 complexes. So, the anti conformation found for 2 in the solid state is tentatively attributed to the presence of intermolecular pi-pi interactions between phenol rings, for which the dihedral angle between the least-squares planes of both rings amounts to 2.6 degrees and the distance between the center of both rings is 3.766 A. On the other hand, the analysis of the electronic structure has revealed that the Pb-ligand bonds present highly ionic character in this family of compounds. They also suggest a greater transfer of electron density from the NCS- ligands when they coordinate through the less electronegative S atom. The Pb-SCN covalent bond formation mainly occurs due to an effective overlap of the occupied 3p z orbitals of the S atoms and the unoccupied 6p z AO of the Pb atom, while the Pb-NCS bonding interaction is primarily due to the overlap of the 6s and 7s AO of Pb with sp(1.10) hybrids of the N donor atoms. Our electronic structure calculations can rationalize the different coordination of the thiocyanate groups in compounds 1 and 2: the simultaneous formation of two Pb-SCN bonds is more favorable for S-Pb-S angles close to 180 degrees , for which the overlap between the occupied 3p z orbitals of the S atoms and the unoccupied 6 pz AO of the Pb atom is maximized.     

An effective enantioselective approach to the securinega alkaloids: total synthesis of (-)-norsecurinine

[reaction: see text] A highly versatile approach to the enantioselective synthesis of securinega alkaloids is presented. Crucial steps are a palladium-catalyzed enantioselective imide alkylation, a vinylogous Mannich reaction, and a ring-closing metathesis process. Through this strategy, the synthesis of (-)-norsecurinine has been accomplished in nine steps and 11% overall yield.     

Functional polythiiranes 7: Surface activity of polythiiranes with PEO side chains

 Some comb-like polythiiranes with PEO side chains were prepared from the corresponding macromonomers. These new materials are amphiphiles and act as surfactants. Their surface tension and interfacial tension are measured and studied in this paper on account of their structures. The lowering of surface tension measured in polymers bearing methyl terminal group in PEO side chains, are in the same range as these observed with polymers of identical structures but different main chains. An increase of the hydrophobic units in the main chain, obtained in copolymers with methylthiirane does not significantly modified the surface tensions. Better lowering is afforded with structures bearing large alkyl groups as terminal group of PEO side chains. On the contrary, some of these macro-molecules with an optimized EO content largely lower the water/xylene surface tension. The main interest of these new materials is their very low cmc and the stabilization of L₁-type microemulsions. Received: 20 May 1997 Accepted: 25 November 1997     

Trifluoromethoxy substituted anilines: metalation as the key step for structural elaboration

Trifluoromethoxy-substituted anilines undergo hydrogen/lithium permutation ("metalation") with optional site selectivity depending on the N-protective group employed. N-tert-Butoxycarbonyl-2- and -4-(trifluoromethoxy)aniline react with tert-butyllithium at the nitrogen-adjacent 6- and 2-position affording, after electrophilic trapping, products 1-6. In contrast, deprotonation of the para isomer occurs at the oxygen-neighboring 3-position, giving rise to the acid 12, when the amino group is carrying two trimethylsilyl groups. sec-Butyllithium attacks 3-trifluoromethoxy-N-mono(trimethylsilyl)aniline at the 2-position, but 3-trifluoromethoxy-N,N-bis(trimethylsilyl)aniline at the 4-position to provide respectively the acids 10 and 11 after carboxylation. The synthesis of two new benzodiazepines illustrates (19 and 22) the preparative potential of the aniline functionalization mediated by organometallic reagents.     

ECE reaction pathways in the electrochemical reduction of dicyanocobalamin: Kinetics of ligand substitution in vitamin B12r cyanocob(II)alamin

The reduction of dicyanocob(III)alamin leads in a first stage to monocyanocob(II)alamin which can be partially converted into the base-off and base-on Co(II) complexes (B12r). The latter species are easier to reduce than the starting Co(III) complex leading to a single two-electron wave at low cyanide concentrations and/or low diffusion rates. Upon raising one of these two parameters two successive one-electron waves tend to be obtained corresponding to the Co(III)/Co(II) and Co(II)/Co(I) conversion respectively. The kinetics of the reduction process is investigated using potential-dependent potentiostatic chronoamperometry which allows a simpler analysis than cyclic voltammetry for systems involving a slow initial charge-transfer step. It is seen that the second electron, at the level of the first wave, comes from the electrode and not from the cyano-Co(II) complex in the solution. The reduction thus follows an ECE rather than a DISP-type mechanism in conditions where they can be distinguished by the usual electrochemical kinetic techniques. This contrasts with that which occurs in organic electrochemistry where the electron transfers are generally fast, while in the present case they are slow. The analysis of the reduction kinetics as a function of cyanide concentration gives some insight into the mechanism of the ligand substitution reaction at the Co(II). The kinetic data are discussed in terms of SN1-, SN2- and SNAr-like mechanisms.