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81.
82.
Correia I Costa Pessoa J Duarte MT Henriques RT Piedade MF Veiros LF Jakusch T Kiss T Dörnyei A Castro MM Geraldes CF Avecilla F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(9):2301-2317
The Schiff base N,N'-ethylenebis(pyridoxylideneiminato) (H(2)pyr(2)en, 1) was synthesized by reaction of pyridoxal with ethylenediamine; reduction of H(2)pyr(2)en with NaBH(4) yielded the reduced Schiff base N,N'-ethylenebis(pyridoxylaminato) (H(2)Rpyr(2)en, 2); their crystal structures were determined by X-ray diffraction. The totally protonated forms of 1 and 2 correspond to H(6)L(4+), and all protonation constants were determined by pH-potentiometric and (1)H NMR titrations. Several vanadium(IV) and vanadium(V) complexes of these and other related ligands were prepared and characterized in solution and in the solid state. The X-ray crystal structure of [V(V)O(2)(HRpyr(2)en)] shows the metal in a distorted octahedral geometry, with the ligand coordinated through the N-amine and O-phenolato moieties, with one of the pyridine-N atoms protonated. Crystals of [(V(V)O(2))(2)(pyren)(2)].2 H(2)O were obtained from solutions containing H(2)pyr(2)en and oxovanadium(IV), where Hpyren is the "half" Schiff base of pyridoxal and ethylenediamine. The complexation of V(IV)O(2+) and V(V)O(2) (+) with H(2)pyr(2)en, H(2)Rpyr(2)en and pyridoxamine in aqueous solution were studied by pH-potentiometry, UV/Vis absorption spectrophotometry, as well as by EPR spectroscopy for the V(IV)O systems and (1)H and (51)V NMR spectroscopy for the V(V)O(2) systems. Very significant differences in the metal-binding abilities of the ligands were found. Both 1 and 2 act as tetradentate ligands. H(2)Rpyr(2)en is stable to hydrolysis and several isomers form in solution, namely cis-trans type complexes with V(IV)O, and alpha-cis- and beta-cis-type complexes with V(V)O(2). The pyridinium-N atoms of the pyridoxal rings do not take part in the coordination but are involved in acid-base reactions that affect the number, type, and relative amount of the isomers of the V(IV)O-H(2)Rpyr(2)en and V(V)O(2)-H(2)Rpyr(2)en complexes present in solution. DFT calculations were carried out and support the formation and identification of the isomers detected by EPR or NMR spectroscopy, and the strong equatorial and axial binding of the O-phenolato in V(IV)O and V(V)O(2) complexes. Moreover, the DFT calculations done for the [V(IV)O(H(2)Rpyr(2)en)] system indicate that for almost all complexes the presence of a sixth equatorial or axial H(2)O ligand leads to much more stable compounds. 相似文献
83.
The reaction of aluminium(III) with Hydroxynaphtol Blue (HNB) in aqueous media at apparent pH 5.5 results in a red complex that is stable for at least 4 hr. Beer's Law is obeyed up to 1.6 microg/ml of aluminium(III) with an apparent molar absorptivity of 1.66 x 10(4) l.mol(-1). cm(-1) at 569 nm. This paper proposes procedures for aluminium(III) determination by ordinary and first-derivative spectrophotometry. The results demonstrated that the linear dynamic range is 0.03-1.60 microg/ml for ordinary spectrophotometry and 11.8-320.0 ng/ml for first derivative spectrophotometry. The HNB is not selectivity for aluminium, but the addition of EDTA allows the aluminium determination in the presence of accepted amounts of Ca(II), Mg(II), Mn(II), Ba(II), Sr(II), Cd(II), Pb(II), La(III), In(III), Bi(III) and Zn(II). The interference of Cu(II) and Hg(II) can be masked by thiosulphate. Ions such as UO(2)(II), Mo(VI), Co(II), Ti(IV) and PO(4)(III) do interfere seriously. This method was applied for aluminium determination in copper-base alloy, zinc-base alloy, magnesium-base alloy, iron ore, manganese ore, cement, dolomite, feldspar and limestone. The results indicated high accuracy and precision. 相似文献
84.
Ballester P Capó M Costa A Deyà PM Frontera A Gomila R 《Molecules (Basel, Switzerland)》2004,9(5):278-286
A dinuclear self-assembled cationic macrocycle based on Pt(II)-N(pyridine) coordinative bonds and having competitive triflate anions, as metal counterions, is used in the construction of [2]rotaxane and [2]pseudorotaxane architectures assisted by hydrogen bonding. The kinetic lability of the Pt(II)-N(pyridine) coordinative bond controls the dynamics of the [2]rotaxane. 相似文献
85.
Hartl F Mahabiersing T Chardon-Noblat S Da Costa P Deronzier A 《Inorganic chemistry》2004,43(22):7250-7258
IR, UV-vis, and EPR spectroelectrochemistry at variable temperatures and in different solvents were applied to investigate in situ the formation of electroactive molecular chains with a nonbridged Os-Os backbone, in particular, the polymer [Os(0)(bpy)(CO)(2)](n) (bpy = 2,2'-bipyridine), from a mononuclear Os(II) carbonyl precursor, [Os(II)(bpy)(CO)(2)Cl(2)]. The one-electron-reduced form, [Os(II)(bpy(.)(-))(CO)(2)Cl(2)](-), has been characterized spectroscopically at low temperatures. This radical anion is the key intermediate in the electrochemical propagation process responsible for the metal-metal bond formation. Unambiguous spectroscopic evidence has been gained also for the formation of [[Os(0)(bpy(*)(-))(CO)(2)](-)](n), the electron-rich electrocatalyst of CO(2) reduction. The polymer species are fairly well soluble in butyronitrile, which is important for their potential utilization in nanoscience, for example, as conducting molecular wires. We have also shown that complete solubility is accomplished for the monocarbonyl-acetonitrile derivative of the polymer, [Os(0)(bpy)(CO)(MeCN)(2)Cl](n). 相似文献
86.
In this work a laboratory polymerization scale process was studied for the production of polybutadiene with high content of cis-1,4 repeating units. A Ziegler-Natta catalytic system based on neodymium versatate (catalyst), diisobutylaluminium hydride (cocatalyst) and tert-butyl chloride (chlorinating agent) was used. The influence of solvent nature (pure grade) and possible contaminants (electron donors) in a recovered solvent from a butadiene-styrene anionic polymerization industrial plant on the stereoselectivity and catalytic activity, molecular weight and molecular weight distribution of the resultant polybutadienes was studied. The polymers were characterized by infrared spectroscopy and size exclusion chromatography. Polybutadienes with cis-1,4 units content in the range of 99-98% were produced. The polymers weight-average molecular weight, , varied from 2.23 × 105 to 4.47 × 105 and the molecular weight distribution, MWD, from 3.1 to 5.1. 相似文献
87.
88.
We expose and discussPenrose's thesis: Nature produces harnessable noncomputable processes, but none at the classical level. We then suggest a partial counterexample to it, based on aGedanken experiment about an undecidable family of integrable Hamiltonian systems that could lead to a sort of idealized solution to the Halting problem for Turing machines.1. Partially supported by a Fulbright/CNPq-Brazil grant. Permanent address after November 5, 1990: Center for the Study of Mathematical Theories of Communication, IDEA/School of Communications, Federal University of Rio de Janeiro, Av. Pasteur 250, 22290 Rio de Janeiro, RJ Brazil. 相似文献
89.
O. Costa de Beauregard 《Foundations of Physics》1982,12(9):861-871
We show that particle-antiparticle exchange and covariant motion reversal are two physically different aspects of the same mathematical transformation, either in the prequantal relativistic equation of motion of a charged point particle, in the general scheme of second quantization, or in the spinning wave equations of Dirac and of Petiau-Duffin-Kemmer. While, classically, charge reversal and rest mass reversal are equivalent operations, in the wave mechanical case mass reversal must be supplemented by exchange of the two adjoint equations, implying
.Denoting by M the rest mass reversal, P the parity reversal, T the Racah time reversal, and Z the
exchange, the connection with the usual scheme of charge conjugation, parity reversal, and Wigner motion reversal, is with, of course, 相似文献
90.
Sérgio L. C. Ferreira Marcus L. S. F. Bandeira Valfredo A. Lemos Hilda C. dos Santos A. C. Spinola Costa Djane S. de Jesus 《Analytical and bioanalytical chemistry》1997,357(8):1174-1178
A simple and very sensitive method has been developed for the determination of ascorbic acid based on the oxidation of ascorbic
acid to dehydroascorbic acid by iron(III), followed by a complexation of iron(II) with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol(Br-PADAP).
The iron(II) complex is formed immediately, with absorption maxima at 560 and 748 nm and a molar absorptivity of 1.31 × 105 l mole–1cm–1 and 5.69 × 104 l mole–1cm–1, respectively. The ascorbic acid determination is possible with a linear range up to 2.4 μg ml–1, a calibration sensitivity of 0.744 ml μg–1 at 560 nm and 0.323 ml μg–1 at 748 nm, and a detection limit of 15 ng ml–1 and 44 ng ml–1, respectively. The procedure was used for the ascorbic acid determination in several fruit juices and pharmaceutical formulations.
The results demonstrated a good precision (R.S.D. < 1%) and are in agreement with those obtained with others methods. The
Br-PADAP method proposed is six times more sensitive than the method using the iron(II)-1,10-phenanthroline system.
Received: 7 May 1996 / Revised: 1 July 1996 / Accepted: 8 August 1996 相似文献