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1.
Summary A kinetic study of the anaerobic oxidation of cysteine (H2 L) by iron(III) has been performed over thepH-range 2.5 to 12 by use of a stopped-flow high speed spectrophotometric method. Reaction is always preceded by complex formation. Three such reactive complex species have been characterized spectrophotometrically: FeL + (max=614 nm, =2 820 M–1cm–1); Fe(OH)L (max=503 nm; shoulder at 575 nm, =1 640 M–1cm–1); Fe(OH)L 2 2– (max=545 nm; shoulder at 445 nm, =3 175 M–1 cm–1). Formation constants have been evaluated from the kinetic data: Fe3++L 2– FeL +: logK 1 M =13.70±0.05; Fe(OH)2++L 2– Fe(OH)L: logK 1 MOH =10.75±0.02; Fe(OH)L+L 2– Fe(OH)L 2 2– ; logK 2 MOH =4.76±0.02. Furthermore the hydrolysis constant for iron(III) was also obtained: Fe(OH)2++H+ Fe aq 3+ : logK FeOH=2.82±0.02). Formation of the mono-cysteine complexes, FeL + and Fe(OH)L, is via initial reaction of Fe(OH)2+ with H2 L (k=1.14·104M–1s–1), the final product depending on thepH. FeL + (blue) formed at lowpH decomposes following protonation with a second-order rate constant of 1.08·105M–1s–1. Fe(OH)L (purple) decomposes with an apparent third order rate constant ofk=3.52·109M–2s–1 via 2 Fe(OH)L+H+ products, which implies that the actual (bimolecular) reaction involves initial dimer formation. Finally, Fe(OH)L 2 2– (purple) is remarkably stable and requires the presence of Fe(OH)L for electron transfer. A rate constant of 8.36·103M–1s–1 for the reaction between Fe(OH)L and Fe(OH)L 2 2– is evaluated.Dedicated to Prof. Dr. mult. Viktor Gutmann on the occasion of his 70th birthday  相似文献   

2.
A theoretical model is proposed to explain the trend in reactivity of cobalt(II) phthalocyanine (CoPc) and substituted cobalt(II) phthalocyanines for the oxidation of hydrazine. Our study suggests that the reaction occurs via a through bond charge transfer pathway and not via a through space charge transfer pathway as was shown in previous work for the oxidation of 2-mercaptoethanol by CoPc (G.I. Cárdenas-Jirón and D.A. Venegas-Yazigi, J. Phys. Chem. A. 106, 11398 (2002)). We propose a mechanism for the oxidation of hydrazine based on a four-step energy profile which agrees with a mechanism proposed for the electro-oxidation of hydrazine mediated by cobalt phthalocyanines confined on a graphite electrode. We show that the step in the energy profile that involves formation of a radical of hydrazine seems to be a good starting point for the study of the transfer of the first electron in the oxidation of hydrazine mediated by different substituted cobalt(II) phthalocyanines.  相似文献   

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Bichromophoric compound 3 beta-((2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17 beta-yl-[2-(N-carbazolyl)acetate] (NBD-S-CZ) was synthesized and its photochemistry was examined by fluorescence quenching, flash photolysis, and chemically induced dynamic nuclear polarization (CIDNP) methods. Fluorescence quenching measurements show that intramolecular electron transfer from the singlet excited state of the carbazole to the norbornadiene group in NBD-S-CZ occurs with an efficiency (Phi SET) of about 14 % and rate constant (kSET) of about 1.6 x 10(7) s-1. Phosphorescence and flash photolysis studies reveal that intramolecular triplet energy transfer and electron transfer from the triplet carbazole to the norbornadiene group proceed with an efficiency (TET + TT) of about 52 % and rate constant (kTET + kTT) of about 3.3 x 10(5) s-1. Upon selective excitation of the carbazole chromophore, nuclear polarization is detected for protons of the norbornadiene group (emission) and its quadricyclane isomer (enhanced absorption); this suggests that the isomerization of the norbornadiene group to the quadricyclane proceeds by a radical-ion pair recombination mechanism in addition to intramolecular triplet sensitization. The long-distance intramolecular triplet energy transfer and electron transfers starting both from the singlet and triplet excited states are proposed to proceed by a through-bond mechanism.  相似文献   

5.
Hydroxyl radicals, generated in aqueous solution from Fe2+ and H2O2, react with the formato, glycolato, lactato and mandelato complexes of (NH3)5CoIII, extracting H·, releasing CO2 and inducing the internal reduction of CoIII to Co2+; decomposition of peroxynitrous acid (O=N—OOH) in the presence of these complexes also yields Co2+, indicating partial utilization (15% at 22°C and pH 1) of a path involving OH·.  相似文献   

6.
Three new rigid bridging ligands for metal complexation (7=bmb, 8=bqb and 11=btb) were prepared from a rigid triptycene spacer connected to two bipyridine ligands using a Horner–Emmons type reaction. The triptycene spacer is substituted by methoxy groups in the case of bmb and in the case of bqb by a benzoquinone substituent. The corresponding metal complexes (ruthenium and/or osmium) were synthesised and the different luminescence behaviour was tested. They show great potential for the investigation of intramolecular electron and energy transfer reactions. The dinuclear metal complex Ru---bqb---Os is an interesting system in which the bridging ligand bqb acts as a redox switch, able to tune the conductivity for energy or electrons across the bridge.  相似文献   

7.
[reaction: see text] Photoexcitation of a zinc imidazoporphyrin-fullerene dyad with a short linkage results in formation of the charge-separated state by photoinduced electron transfer. The charge-separated state has a lifetime of 310 micros in benzonitrile at 278 K, which is the longest lifetime in solution ever reported for electron donor-acceptor-linked dyads.  相似文献   

8.
Structure, photoabsorption and excited states of two representative conformations obtained from molecular dynamics (MD) simulations of a doubly-linked porphyrin-fullerene dyad DHD6ee are studied by using both DFT and wavefunction based methods. Charge transfer from the donor (porphyrin) to the acceptor (fullerene) and the relaxation of the excited state are of special interest. The results obtained with LDA, GGA, and hybrid functionals (SVWN, PBE, and B3LYP, respectively) are analyzed with emphasis on the performance of used functionals as well as from the point of view of their comparison with wavefunction based methods (CCS, CIS(D), and CC2). Characteristics of the MD structures are retained in DFT optimization. The relative orientation of porphyrin and fullerene is significantly influencing the MO energies, the charge transfer (CT) in the ground state of the dyad and the excitation of ground state CT complex (g-CTC). At the same time, the excitation to the locally excited state of porphyrin is only little influenced by the orientation or cc distance. TD-DFT underestimates the excitation energy of the CT state, however for some cases (with relatively short donor-acceptor separations), the use of a hybrid functional like B3LYP alleviates the problem. Wavefunction based methods and CC2 in particular appear to overestimate the CT excitation energies but the inclusion of proper solvation models can significantly improve the results.  相似文献   

9.
An axially substituted titanium(IV) phthalocyanine-fullerene donor-acceptor supramolecular dyad has been prepared by two different approaches, one of them representing a convenient convergent strategy. The dyad system exhibits photoinduced electron transfer upon irradiation with visible light to produce a microsecond lived charge separated state.  相似文献   

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The rate of electron transfer from organic sulfides to [CrV(ehba)2] (ehba-2-ethyl-2-hydroxy butyric acid) decreases with a decrease in the polarity of the medium. The anionic surfactant, SDS and the cationic surfactant, CTAB have different effects on the kinetics of this reaction. The micellar inhibition observed in the presence of SDS is probably due to the decrease in the polarity and the electrostatic repulsion faced by the anionic oxidant from the anionic micelle and the partition of the hydrophobic substrate between the aqueous and micellar phases. The micellar catalysis in the presence of CTAB is attributed to the increase in the concentration of both reactants in the micellar phase. This micellar catalysis is observed to offset the retarding effects of the less polar micellar medium and the unfavorable charge-charge interaction between the + charge developed on S center in the transition state and the cationic micelle. This catalysis is contrary to the enormous micellar inhibition observed with IO4, HSO5 and HCO4 oxidation of organic sulfides.  相似文献   

12.
Organic molecule neutral red (NR), as electron transfer mediator, was introduced in the anodic electrocatalyst system for methanol oxidation and the resulting electrode was investigated by cyclic voltammetry, polarization method, and electrochemical impedance spectroscopy. For the same loading mass of platinum catalyst, 1.25 times larger exchange current density, 1.83 times higher specific activity, and better long-term cycle stability can be obtained at Pt/NR/graphite electrode, as compared to the electrode without NR. These results indicate that neutral red plays an important role on the enhanced electrocatalytic activity of platinum catalyst for methanol oxidation.  相似文献   

13.
A new binaphthalene (R)-1 with two substituted TTF and trichloroquinone units is reported. Both absorption and ESR spectral studies show that electron transfer occurs between TTF and trichloroquinone units of (R)-1 in the presence of metal ions (Pb2+, Sc3+, Zn2+, and Ca2+). We also propose a possible mechanism for this electron transfer process. But, the CD spectral change of (R)-1 in the presence of metal ions is rather small.  相似文献   

14.
Treatment of α,β-unsaturated ketones with Mg metal in the presence of trimethylsilyl choride (TMSCl) brought about facile and regioselective reductive dimerization to give the corresponding bis(silyl enol ethers), 1,6-bis(trimethylsilyloxy)-1,5-dienes. Similar Mg-promoted reductive dimerization of 1,3-cyclic diketones in the presence of TMSCl followed by acid-catalyzed hydrolysis led to selective formation of the corresponding 1,6-diketo-2,4-dienes in moderate to good yields.  相似文献   

15.
Photostimulated vectorial electron transfer through the lecithin bilayer membrane was studied in a system based on lipid vesicles with CdS as a photosensitizer located either in the vesicle inner cavity or outside the vesicles. 1,4-Bis(1,2,6-triphenyl-4-pyridyl)benzene (benzoviologen) was used as an effective lipophilic highly reversible electron carrier incorporated into the bilayer membrane. A peculiarity of this electron carrier is its ability to be reversibly reduced on one and two electrons. The interface electron transfer across the border “vesicular cavity-membrane” was studied by stationary and pulse photolysis. The primary photoreduced form of benzoviologen appears to be that reduced by one electron; however, on stationary photolysis, most benzoviologen molecules appear to be affected by two-electron reduction, which can result from the low mutual mobility of CdS nanoparticles and benzoviologen molecules. The CdS photostimulated transmembrane electron transfer from the internal sacrificial donor to the external acceptor (as regards the vesicular cavity) was performed for [CoEDTA] as the final electron acceptor. The rate constants of electron transfer from the membrane-embedded two-electron reduced form of benzoviologen have been determined in the case of [CoEDTA] and O2 as acceptors.  相似文献   

16.
Contemporary continuum-based models of solvation in polar media are surveyed and assessed, with special focus on non-equilibrium solvation. A new hybrid approach combining molecular-level treatment of inertial solvent response, and inclusion of inertialess solvent response at the continuum level, is presented and illustrated in terms of calculated equilibrium solvation free energies for small molecular ions and reorganization free energies for model dumbbell systems.  相似文献   

17.
The aim of this work is to evaluate the efficiency of the determination of As(III) by anodic stripping voltammetry (ASV) using a lateral gold electrode and to study the modifications of the electrode surface during use. Potential waveforms (differential pulse and square wave), potential scan parameters, deposition time, deposition potential and surface cleaning procedure were examined for they effect on arsenic peak intensity and shape. The best responses were obtained with differential pulse potential wave form and diluted 0.25 M HCl as supporting electrolyte. The repeatability, linearity, accuracy and detection limit of the procedure and the interferences of cations and anions in solution were evaluated. The applicability of the procedure for As(III) determination in drinking waters was tested. Cyclic voltammetry (CV) was used to study the electrochemical behaviour of As(III) and for the daily monitoring of electrode surface. Also scanning electron microscopy (SEM) analysis was used to control the electron surface. Finally we evaluated the possibility to apply the equations valid for flow systems also to a stirred system, in order to calculate the number of electrons transferred per molecule during the stripping step.  相似文献   

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In order to investigate the coupling of molecular vibrations and electron distribution, dynamic electron transfers in B2H6 and Cu(PH3)2(BH4) are lated by using a new variational method. In both molecules, the dynamic electron density near bridging hydrogen atoms decreases to form the density valley by exciting specific vibrational modes. On the other hand, in both sides of the valley density hills grow up. For these molecules, similar contour maps are given by the modes with different symmetry which have large contribution of the bridging ligands. While the dynamic electron transfer of B2H6 arises in symmetric form, the vibrational modes of the Cu complex gives the asymmetric redistribution of the dynamic electron density. This is attributed to the difference of the symmetry between the two molecules.  相似文献   

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