- Sustained and Damped pH Oscillation in the Periodate-Thiosulfate Reaction in a Continuous-Flow Stirred Tank Reactor
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The reaction between IO4- and S2O32- exhibits a variable stoichiometry in acetate buffer.With a large excess of IO4-, the products are IO3- and SO42-.In excess thiosulfate, IO4- is reduced
- Rabai, Gyula,Beck, Mihaly T.,Kustin, Kenneth,Epstein, Irving R.
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- The reduction of I2 by 1-hydroxyalkyl radicals in aqueous solution. A pulse radiolysis study
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The one electron reduction of iodine by various 1-hydroyalkyl radicals in aqueous solution is described, which leads first to the formation of I2-, and to I- at longer times. Rate constants of 5×109, 6.5×109, or 8×109dm3 mol-1 s-1 were obtained for the reaction between iodine and the radical of methanol, ethanol or 2-propanol, respectively.
- Ershov,Janata
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- STUDIES ON THE μ-(NN')-ETHYLENEDIAMINETETRA-ACETATO-DI-μ-OXO-BIS-, 2-, COMPLEX IN AQUEOUS SOLUTIONS. FORMATION OF AN AQUO-ION AND REDOX PROPERTIES
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The aquation of 2- (edta=ethylenediaminetetra-acetate) to W2O42+ (or a closely related species) proceeds to completion in HCl>=2 mol dm3-.First-order kinetics have been observed, with the rate constant (1.16*10-4 s-1 at 25 deg C) in 2 mol dm-3 HCl some 60 times less than for the corresponding reaction of 2-.With HClO4>=2 mol dm-3 aquation is accompained by oxidation to WVI.No aquation (or oxidation) has been observed with HClO4-3, under which condition redox reactions of the edta complex could be investigated.At 25 deg C, I=0.50 mol dm-3 (H-LiClO4) rate constants for the first stage of the 1:2 oxidations of 2- with 2- and 3+ (phen=1,10-phenantroline) are 6.3*105 dm3 mol-1 s-1 and >107 dm3 mol-1 s-1 respectively.For the 2- reaction activation parameters are ΔH+=5.4 kcal mol-1 s-1 and ΔS+=-13.8 cal K-1 mol-1 respectively.Rate constants (dm3 mol-1 s-1) for other oxidants, I=0.10 mol dm-3, are 3- (0.058), 3+ (2.0) (bipy=2,2'-bipyridine), and I3- (ca. 20).The complex 2- is a much stronger reducing agent than 2-.
- Soares, Allan B.,Taylor, Craig R.,Sykes, Geoffrey A.
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- General pathway of sulfur-chain breakage of polythionates by iodine confirmed by the kinetics and mechanism of the pentathionate-iodine reaction
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The pentathionate-iodine reaction has been studied spectrophotometrically at T = 25.0 ± 0.1 °C and at an ionic strength of 0.5 M in both the absence and presence of an initially added iodide ion at the pH range of 3.95-5.15. It was found that the pH does not affect the rate of the reaction; however, the iodide ion produced by the reaction strongly inhibits the oxidation. Therefore, it acts as an autoinhibitor. The kinetic curves also support the fact that iodide inhibition cannot be explained by the formation of the unreactive triiodide ion, and S5O6I- along with the iodide ion has to be involved in the initiating rapid equilibrium being shifted far to the left. Further reactions of S5O6I -, including its hydrolysis and reaction with the iodide ion, lead to the overall stoichiometry represented by the following equation: S 5O62- + 10I2 + 14H2O → 5SO42- + 20I- + 28H+. A nine-step kinetic model with two fitted parameters is proposed and discussed, from which a rate equation has also been derived. A brief discussion about the general pathway of sulfur-chain breakage of polythionates supported by theoretical calculations has also been included.
- Xu, Li,Csek?, Gy?rgy,Kégl, Tamás,Horváth, Attila K.
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- Kinetics and Mechanisms of the Oxidation of Hydrazine by Aqueous Iodine
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Kinetics for the reactions of I2 with excess N2H5+/N2H4 and I- are measured by the loss of I3- over a wide range of acidity from pH 0.35 to 8.0 at 25.0 °C, μ = 0.50 M. Pseudo-first-order rate constants increase by factors of more than 107 with increase of pH, hydrazine, and buffer concentrations. Below pH 1, I2 reacts directly with N2H5+ which has a relative reactivity that is 2.4 × 107 times smaller than N2H4 (the dominant reactant at pH ≥ 1). Kinetic evidence for IN2H4+ as a steady-state species is found below pH 3. From pH 3.5 to 6.3, rate constants are measured by stopped-flow methods and at higher pH by pulsed-accelerated-flow methods. A multistep mechanism is proposed where I2 reacts rapidly with N2H4 to form an I2N2H4 adduct (KA = 2.0 × 104 M-1) that is present in appreciable concentrations above pH 6. The adduct undergoes general base-assisted deprotonation accompanied by loss of I- in the rate-determining step. Subsequent intermediates react rapidly with another I2 to form N2 as a final product. At high pH, hydrazine acts as a general base as well as the initial nucleophile. Rate constants for various bases (H2O, CH3COO-, HPO42-, N2H4, and OH-) fit a Br?nsted β value of 0.46. Values for the second-order rate constants (M-1 s-1) for I2N2H4 reactions with CH3COO-, HPO42-, N2H4 and OH- are 7.5 × 103, 2.0 × 105, 8.5 × 105, and 4.8 × 108, respectively.
- Liu, Rong Ming,Margerum, Dale W.
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- In situ growth of mirror-like platinum as highly-efficient counter electrode with light harvesting function for dye-sensitized solar cells
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In this work, we prepared a continuous nanostructured Pt-mirror film with metallic lustre and good adhesion to the F-doped tin oxide conducting glass (FTO) substrate through a simple in situ growth method, which retains a good catalytic activity and more importantly, exhibits significant light reflection for light-harvesting. The dye-sensitized solar cells (DSCs) fabricated with Pt-M CE exhibited superior photovoltaic performance compared with the conventional Pt CE. The enhancements of the short-circuit current density and energy conversion efficiency are 15.3% and 18.5%, respectively. Such significant enhancement of the short-circuit current density was found to be related to the excellent light reflection and high catalytic activity of the Pt-M CE. This has been proved by ultraviolet and visible reflection spectra (UV/Vis), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV).
- Zhang, Nan Nan,Zhang, Bo,Li, Yu Hang,Hou, Yu,Yang, Shuang,Zhong, Ju Hua,Yang, Hua Gui
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- Electron transfer. 144. Reductions with germanium(II)
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Solutions 0.2-0.4 M in Ge(II) and 6 M in HCl, generated by reaction of Ge(IV) with H3PO2, are stable for more than 3 weeks and can be diluted 200-fold with dilute HCl to give GeCl3- preparations to be used in redox studies. Kinetic profiles for the reduction of Fe(III) by Ge(II), as catalyzed by Cu(II), implicate the odd-electron intermediate, Ge(III), which is formed from Cu(II) and Ge(II) (k = 30 M-1 s-1 in 0.5 M HCl at 24 °C) and which is consumed by reaction with Fe(III) (k = 6 x 102 M-1 s-1). A slower direct reaction between Ge(II) and Fe(III) (k = 0.66 M-1 s-1) can be detected in 1.0 M HCl. The reaction of Ge(II) with I3- in 0.01-0.50 M iodide is zero order in oxidant and appears to proceed via a rate-determining heterolysis of a Ge(II)-OH2 species (k = 0.045 s-1) which is subject to H+-catalysis. Reductions of IrCl62- and PtCl62- by Ge(II) are strongly Cl--catalyzed. The Ir(IV) reaction proceeds through a pair of 1e- changes, of which the initial conversion to Ge(III) is rate-determining, whereas the Pt(IV) oxidant probably utilizes (at least in part) an inner-sphere Pt(IV)-Cl-Ge(II) bridge in which chlorine is transferred (as Cl+) from oxidant to reductant. The 2e- reagent, Ge(II), like its 5s2 counterpart, In(I), can partake in 1e- transactions, but requires more severe constraints: the coreagent must be more powerfully oxidizing and the reaction medium more halide-rich.
- Babich, Olga A.,Gould, Edwin S.
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- Kinetics and Mechanism of Oxidation of Hydrazine by Tri-iodide Ion in Aqueous Acidic Media
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The kinetics of oxidation of hydrazine (L) by I3- follows a pseudo-first-order rate law in aqueous acid-sulphuric acid media in the range +> 2.0-1.58 x 10-3 mol dm-3.The pseudo-first-order rate constants (kobs.) exhibit: (1) at +>=0.9 mol dm-3 a linear dependence on 0 at low 0, with a tendency to a limiting value at high relative 0; (2) a decreasing and complex trend in -> and in +>; (3) a decreasing trend with decreasing dielectric constant of the medium; and (4) negligible dependence on the ionic strength of the medium, on added Cu2+(aq), and on added ethylenediaminetetra-acetate.The results are interpreted in terms of a mechanism which envisage (a) negligible reactivities of I3- and of HL+ predominant in the medium, (b) an inner-complex mechanism of electron transfer (i.e. a 1:1 ?-charge-transfer co-ordination prior to the electron transfer) involving I2 and L, and (c) an encounter reaction between IOH and L.Mechanistic ambiguities of some earlier reports of the same reaction are explained.
- Radhakrishnamurti, Patapati Subbu,Rath, Nabeen Kumar,Panda, Rama Krushna
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- Complex kinetics of a landolt-type reaction: The later phase of the thiosulfate-iodate reaction
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The thiosulfate-iodate reaction has been studied spectrophotometrically in slightly acidic medium at 25.0 ± 0.1 °C in acetate/acetic acid buffer by monitoring the absorbance at 468 nm at the isosbestic point of iodine-triiodide ion system. The formation of iodine after the Landolt time follows a rather complex kinetic behavior depending on the pH and on the concentration of the reactants as well. It is shown that the key intermediate of the reaction is I2O2, its equilibrium formation from the well-known Dushman reaction along with their further reactions followed by subsequent reactions of HOI, HIO2, S2O3OH -, and S2O3I- adequately accounts for all the experimentally measured characteristics of the kinetic curves. A 19-step kinetic model is proposed and discussed with 13 fitted and 7 fixed parameters in detail.
- Horvath, Attila K.,Varga, Denes,Nagypal, Istvan
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- The Oscillatory Landolt Reaction. Empirical Rate Law Model and Detailed Mechanism.
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The iodate oxidation of sulfite and ferrocyanide when carried out in a continuous flow stirred tank reactor exhibits large-amplitude oscillations in pH accompanied by an almost constant concentration of iodide.A description of the reaction in terms of component processes and associated empirical rate laws is used to model the dynamical behavior.Limitations and potential refinements of the empirical rate law model are discussed.A detailed mechanism consistent with the component process description is presented.
- Gaspar, Vilmos,Showalter, Kenneth
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- Kinetics and Mechanism of the Autoinhibitory Iodide-Thiocyanate Reaction
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The kinetics and mechanism of the reaction between iodine and thiocyanate have been investigated in the pH range 1-9.Two limiting stoichiometries are found: at pH > 4, 4 I2 + SCN- + 4 H2O -> SO42- + ICN + 7 I- + 8 H+
- Simoyi, Reuben H.,Epstein, Irving R.,Kustin, Kenneth
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- Electrocatalytic behavior of mixed-valent RuORu(CN) 6 4- SiMo12 O40 4- Hybrid film modified electrodes toward oxidation of neurotransmitters and iodate reduction
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Mixed-valent ruthenium oxide/hexacyanoruthenate/silicomolybdate (RuORu(CN) 6 4- SiMo12 O40 4-) hybrid film was grown on glassy carbon electrode by repetitive cyclic voltammetric scanning in H2 SO4 solution (pH 1.5) containing Ru3+, Ru(CN) 6 4- and SiMo12 O40 4- ions. Simultaneous cyclic voltammetry and electrochemical quartz crystal microbalance measurements demonstrate the steady growth of hybrid film. In pure supporting electrolyte, the modified electrode exhibits five obvious redox couples. The electrochemical responses of modified electrode resemble that of a surface immobilized redox couple. The hybrid film electrodes showed electrocatalytic activity toward oxidation of neurotransmitters, ascorbic acid and reduction of IO3-. The neurotransmitters oxidized on modified electrode at less positive potentials compared to bare electrode. The feasibility of using our modified electrodes for analytical application was also investigated.
- Chen, Shen-Ming,Song, Jun-Liang,Thangamuthu
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- Revised explanation of the pH oscillations in the iodate-thiosulfate- sulfite system
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A revised four-step model based on the reinvestigation of the kinetics of the subsystems is suggested for the description of the dynamics of the iodate-sulfite-thiosulfate reaction. It is shown that the rate equation of the iodate-sulfite reaction contains an undiscovered term that depends on the square of the concentration of the hydrogen ion as an autocatalyst making the reaction supercatalytic . In contrast to earlier studies but already supposed by a more recent one, the parallel oxidation of thiosulfate by iodate into tetrathionate and sulfite has also been established experimentally as well as the rate coefficients of the parallel pathways. Combining these kinetic studies yields a new model that does not include the cross-catalytic effect of thiosulfate and the inhibitory effect of sulfite supposed hypothetically by Rabai and Beck to describe the dynamics of the combined system. In fact, the supercatalytic effect of the hydrogen ion on the sulfite-iodate reaction and the H+ dependence of the thiosulfate-iodate reaction along with the parallel formation of sulfite and tetrathionate are sufficient for perfect reflection of all the most important characteristics of the pH-time curves of the composite system. Nevertheless, the revised model significantly decreases the number of the kinetic parameters necessary to simulate the experimental results.
- Horvath, Attila K.
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- Pulse radiolysis study of I- oxidation with radical anions Cl2.- in an aqueous solution
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Radiation-chemical transformations of chloride solutions in the presence of iodide additives were studied by pulse radiolysis. Radical anion Cl 2.- oxidize I- ion, while in the secondary reactions Cl2 reacts with I- to form a mixed trihalide ion ICl2-. A reaction model that satisfactorily describes the experimental data was proposed.
- Ershov,Janata,Gordeev
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- The reduction of I2 by H2O2 in aqueous solution
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The reduction of I2 by hydrogen peroxide, a primary water radiolysis product, has been identified as a key reaction that would influence iodine volatility in nuclear reactor accident conditions (1-3). Although there have been a number of studies of the reduction of I2, there exists a great degree of controversy regarding the intermediates involved, the effect of buffers, and the general rate law (1-9). Because the rates and the mechanism of this reaction are important in predicting the pH dependence of iodine behaviour in reactor containment building after a postulated reactor accident, we have undertaken a kinetic study of I2 reduction by H2O2 in aqueous solution over a pH range of 6-9. The experiments were performed using stopped-flow instrumentation and monitoring the decay of I3- spectrophotometrically. The effects of buffer catalysis have been examined by comparison of kinetic data obtained in sodium barbital (5,5-diethylbarbituric acid), disodium citrate, and disodium hydrogen phosphate buffers. The effect of buffers, combined with the complex acid dependence of the rate law, explains many of the discrepancies reported in earlier literature.
- Ball,Hnatiw
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- Iodide modified electrodes for the electrochemical detection of environmentally hazardous materials
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Electrochemical deposition and chemisorption studies of iodide film have been done on the glassy carbon electrode (GCE) and polycrystalline gold electrode (poly GE). The iodide film preparation under different conditions and its stability were examined. Further, the redox activity of the iodide modified film on GCE and poly GE was examined in different pH solutions. Here, the electrochemically deposited iodide film on GCE showed good stability and was electrochemically active at pH 1.5 and 4 solutions, while the chemisorbed iodide film on poly GE was found active and stable at pH 9 and 13 solutions. The iodide modified GCEs exhibits a good electrocatalytic activity for the oxidation of As (III) and the reduction of Cr (VI), respectively. A detailed study for As (III) oxidation has been done by using the rotating ring-disk electrode technique. Meanwhile, chemisorbed iodide film on poly GE shows electrocatalytic reduction reactions of IO 3- by using different techniques like cyclic voltammetry and differential pulse voltammetry. To examine the practical analytical utility of the proposed electrode, the real sample analyses for arsenic and chromate detection have been examined and the results were found satisfactory.
- Su, Buo-Wei,Thiagarajan, Soundappan,Chen, Shen-Ming
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- Electron Transfer Reactions Between V(IV) and I(V) Catalyzed by Os(VIII) or Ru(III). A kinetic Study
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Oxidation of V(IV) by iodate, catalyzed by Os(VIII) or Ru(III) in aq perchloric acid medium, was carried out.The order with respect to oxidant is zero in both the Os(VIII)- and Ru(III)-catalyzed reactions.A unit-order dependence on V(IV) is observed in the case of Os(VIII)-catalyzed reaction and a fractional dependence on V(IV) is noticed in the case of Ru(III)-catalyzed reaction.Both Os(VIII)- and Ru(III)-catalyzed reactions exhibit an inverse unit dependence on acidity.Insensitivity to change in the dielectric constant of medium is observed in both the systems.Effects of salt and ionic strength were studied.A plausible mechanism consistent with the experimental results is postulated, rate laws being derived from the proposed mechanism.The stoichiometry of the reaction has proved to be the same for both the systems.
- Radhakrishnamurti, Patapati Subbu,Janardhana, Chelli,Mohanty, Gopal Krishna
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- Platinum-free binary Co-Ni alloy counter electrodes for efficient dye-sensitized solar cells
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Dye-sensitized solar cells (DSSCs) have attracted growing interest because of their application in renewable energy technologies in developing modern low-carbon economies. However, the commercial application of DSSCs has been hindered by the high expenses of platinum (Pt) counter electrodes (CEs). Here we use Pt-free binary Co-Ni alloys synthesized by a mild hydrothermal strategy as CE materials in efficient DSSCs. As a result of the rapid charge transfer, good electrical conduction, and reasonable electrocatalysis, the power conversion efficiencies of Co-Ni-based DSSCs are higher than those of Pt-only CEs, and the fabrication expense is markedly reduced. The DSSCs based on a CoNi0.25 alloy CE displays an impressive power conversion efficiency of 8.39 %, fast start-up, multiple start/stop cycling, and good stability under extended irradiation. A mild hydrothermal reduction strategy has been used to generate Pt-free binary Co-Ni alloys for use as counter electrode (CE) materials in dye-sensitized solar cells (DSSCs). These materials offer high electrocatalytic activity toward I3-, good electron conduction, as well as cost-effectiveness. A conversion efficiency of 8.39 % was measured under one sun irradiation, which is much higher than the 6.96 % from platinum-based DSSCs.
- Chen, Xiaoxu,Tang, Qunwei,He, Benlin,Lin, Lin,Yu, Liangmin
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- Multinuclear Transition Metal Sandwich-Type Polytungstate Derivatives for Enhanced Electrochemical Energy Storage and Bifunctional Electrocatalysis Performances
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Different transition metal (TM) units are introduced into a trivacant Keggin cluster to form three sandwich polytungstate derivatives, (H2en)[{K(H2O)0.5}2{K2(H2O)3}{Ni(H2O)(en)2}2{Ni4(H2O)2(PW9O34)2}] (1), [Cu6(Himi)6{AsIIIW9O33}2]·5H2O (2), and (H2btp)4[FeIII2FeII2(H2O)2(AsW9 O34)2]·4H2O (3) (en = ethanediamine; imi = imidazole; btp = 1,3-bis(1, 2, 4-triazol-1-yl) propane). Compound 1 is a 2,3,8-connected 3D network with {43}2{46·66·83·612·8}{6}2 topology based on bisupported tetra-Ni sandwich phosphotungstate and two kinds of potassium connection units. Compound 2 is a dense 12-connected 3D supramolecular network with {324·436·56} topology based on hexa-Cu(imi) sandwiched arsenotungstate. Compound 3 represents the first mixed valence tetra-Fe substituted sandwich arsenotungstate assembly. Compounds 1-3 show enhanced supercapacitor performance (618.2, 603.4, and 504.6 F·g-1 at a current density of 2.4 A·g-1 with 91.5%, 89.3%, and 87.8% of cycle efficiency after 5000 cycles, respectively) compared to their maternal polyoxometalates (POMs) and most reported POM-based electrode materials, which suggests that the introduction of multinuclear TM into vacant POMs is an effective method to improve the energy storage performance of POMs. In addition, compounds 1 and 3 exhibit dual-functional electrocatalytic behaviors in the reduction of iodate and the oxidation of dopamine for introduction of {Ni4} and {Fe4} units.
- Cui, Liping,Dai, Wenting,Lv, Jinghua,Ma, Xinyue,Yu, Kai,Yuan, Jie,Zhou, Baibin
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- A solvent-free strategy to realize the substitution of I- for IO3- in a BiOIO3 photocatalyst with an opposite charge transfer path
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Substitutional doping is an effective way to improve photocatalytic activity, but substituting an individual ion for an atomic group has always been difficult to achieve. Herein, a simple in situ solvent-free strategy is reported for the synthesis of a photocatalyst by replacing IO3- in BiOIO3 with I- (BiOIO3-P42) via regulating the oxidation-reduction environment, which has been realized by changing the viscosity of the reaction system. Exhilaratingly, the change in the viscosity of the reaction system facilitated the phase composition change from pure BiOIO3 to I--doped BiOIO3 and then to pure BiOIO3, which could be attributed to the synergistic effect of acidity and reducibility for various viscosities. This interesting phenomenon has been identified by XRD, XPS, SEM and HRTEM. Benefiting from the different valence states of iodine in I- and IO3-, the photogenerated electrons and holes could move in opposite directions due to the driving force provided by electrostatic attraction. I--doped BiOIO3 therefore demonstrates an optimal photocatalytic removal rate in the degradation of methyl orange and bisphenol A, which is about two times higher than that of pure BiOIO3. We anticipate that such an in situ substitutional doping technique can be further investigated as a convenient method to design nanomaterials with boosted photocatalytic performance.
- Xie, Jing,Cao, Yali,Hu, Jindou,Tang, Yakun,Jia, Dianzeng
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- Colloidal synthesis of wurtz-stannite Cu2CdGeS4 nanocrystals with high catalytic activity toward iodine redox couples in dye-sensitized solar cells
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Wurtz-stannite Cu2CdGeS4 nanocrystals were synthesized via a facile hot-injection method at a low temperature. They exhibited low charge transfer resistance at the electrolyte-electrode interface and high electrocatalytic activity for the reduction of I3- in dye-sensitized solar cells (DSSCs). Moreover, this DSSC showed a power conversion efficiency of 7.67%, comparable to the Pt-based device (7.54%).
- Huang, Shoushuang,Zai, Jiantao,Ma, Dui,He, Qingquan,Liu, Yuanyuan,Qiao, Qiquan,Qian, Xuefeng
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- Effect of dissolved oxygen on the oxidation of dithionate ion. Extremely unusual kinetic traces
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The effect of dissolved oxygen on the acid-dependent disproportionation of dithionate ion (S2O62-) and its oxidation with a number of reagents (I2, Br2, I3-, Br3-, IO3-, BrO3 -, IO4-, MnO4-, Cr(VI), Ce(VI), and H2O2) were studied. Dithionate ion is remarkably inert at room temperature, and heating to elevated temperatures (50-90 °C) was necessary to observe detectable reaction rates in all of these processes. It was confirmed that dithionate ion is never oxidized directly; its redox reactions are zeroth-order with respect to the oxidizing agent and proceed through disproportionation and subsequent fast oxidation of the sulfur(IV) formed. The effect of dissolved oxygen is attributable to its reaction with sulfur(IV) produced in the disproportionation. This autoxidation occurs only with a catalyst. Cerium(III) and iodide ions were shown to catalyze the autoxidation, and their effect on the kinetic traces was studied. In a few cases, highly unusual kinetic traces, such as straight lines with sharp break points, inverted parabolas, and a combination of these, were detected and successfully interpreted in terms of simple kinetic and stoichiometric considerations.
- Lente, Gabor,Fabian, Istvan
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- High-Amplitude Hydrogen Ion Concentration Oscillation in the Iodate-Thiosulfate-Sulfite System under Closed Conditions
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In the closed system of iodate, hydrogen sulfite, and thiosulfate, high-amplitude, greatly damped pH oscillation occurs in unusually narrow concentration ranges of reactants.A semiquantitative description of the kinetic curves has been made in terms of the empirical rate laws of the subsystems by making allowance for cross effects.The sceleton model is closely related to the Lotka model, the archetype of chemical oscillators.A possible mechanism is suggested to give an account of experimental findings.
- Rabai, Gy.,Beck, M. T.
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- EXOTIC KINETIC PHENOMENA AND THEIR CHEMICAL EXPLANATION IN THE IODATE-SULFITE-THIOSULFATE SYSTEM
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An extremely sharp minimum has been found on the pH versus time curves in the iodate-sulfite-thiosulfate system in a limited range of initial concentrations of the reactants in a static system, while sustained oscillation in pH occurs in a CSTR.This system seems to be the simplest chemical oscillator that can be described by two empirical rate laws, one acid-base equilibrium, and the flow terms.For the extremely sharp change of pH the catalytic effect of thiosulfate on the iodate oxidation of sulfite and the inhibiting effect of sulfite on the oxidation of thiosulfate by iodate are responsible.No explanation could be offered for the extreme temperature sensitivity of the oscillatory behavior in the flow system.
- Rabai, Gy.,Beck, M. T.
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- ELECTROCHEMICAL STUDY OF 2 AND Fe(NO)2IPPh3 IN DMF AND EVIDENCE FOR THEIR COMPLETE IONIZATION
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2 and Fe(NO)2IPPh3 are completely ionized in DMF; the resulting Fe(NO)+2 and Fe(NO)2PPH+2 can be reduced at a mercury electrode giving rise to the neutral Fe(NO)2 which is known to be a powerful catalyst.
- Piazza, G.,Innorta, G.
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- Growth of Silver Halides from the Molecule to the Crystal. A Pulse Radiolysis Study
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Halide ions were produced in situ by pulse radiolysis, via electron transfer to dihalomethanes from solvated electrons or hydrogen atoms, and were then used to generate halide molecules and larger aggregates.This evolution of silver halide aggregates was studied for the case of silver iodide on the time scale of 1E-6-1E2 s.Conductivity detection allows determination of the rate of formation of the first AgI molecule.Spectrophotometric detection of the growth of the particles from the stage of the molecular species to colloidal particles of bulk electronic properties is then possible.When the aggregates thus formed approach sizes of ca. 50 Angstroem, their detection by light scattering provides an independent method of size determination.In the region where such measurements are possible, the sizes determined by light scattering agree with sizes calculated assuming confinement of excitons in small particles and their electron-hole coulomb screening.A similar approach is suggested for growth studies of other insoluble materials.
- Schmidt, K. H.,Patel, R.,Meisel, D.
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- Kinetics and mechanism of the oxidation of tetrathionate by iodine in a slightly acidic medium
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The iodine-tetrathionate reaction has been reinvestigated spectrophotometrically at T = 25.0 ± 0.1°C and at an ionic strength of 0.5 M adjusted by sodium acetate as a buffer component in both the absence and presence of the iodide ion in the pH range of 4.25-5.55. The reaction was found to be independent of pH within the range studied, and it was clearly demonstrated that the reaction proceeds via an intermediate S4O 6I- formed in a pre-equilibrium. Iodide dependence of the kinetic curves strongly suggests that the iodide ion has to be involved in this equilibrium. Further reactions of the intermediate, including its hydrolysis and reaction with iodide, leads to the strict stoichiometry characterized by S 4O62- + 7I2 + 10H2O → 4SO42- + 14I- + 20H+. A seven-step kinetic model with three fitted kinetic parameters is suggested and discussed. A rate equation is also derived from which a sound explanation of the iodide dependence of the apparent rate coefficient is presented. Furthermore, it has also been pointed out that formation of the triiodide ion alone is not sufficient to take the retardation effect of the iodide ion into account quantitatively.
- Kerek, Attila,Horvath, Attila K.
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- Temperature Dependence of the Equilibrium Constant for Iodine Hydrolysis at Temperatures between 25 and 120 deg C
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The equilibrium constant for iodine hydrolysis at elevated temperatures has been investigated using absorption spectroscopy and a simulation computer program.The temperature dependence of the aqueous I2 maximum extinction coefficient for visible light (at 460 nm) was first evaluated in a preliminary experiment using saturated I2 solution.The time dependence of I2 concentrations in dilute solution has subsequently been measured using the temperature-dependent extinction coefficient.The results were analysed by the simulation program, in which 11 normally accepted react ions were considered, and the equilibrium constant, K, was estimated.It was concluded that the K value at temperatures ranging between 25 and 120 deg C may be expressed by logK(T) = 13880/T - 0.2445T + 308.4 log T - 749.1 where the units of K and T are mol2 dm-6 and K, respectively.
- Burns, William G.,Matsuda, Masami,Sims, Howard E.
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- A Novel Single-Atom Electrocatalyst Ti1/rGO for Efficient Cathodic Reduction in Hybrid Photovoltaics
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Single-atom catalysts (SACs) are a frontier research topic in the catalysis community. Carbon materials decorated with atomically dispersed Ti are theoretically predicted with many attractive applications. However, such material has not been achieved so far. Herein, a Ti-based SAC, consisting of isolated Ti anchored by oxygen atoms on reduced graphene oxide (rGO) (termed as Ti1/rGO), is successfully synthesized. The structure of Ti1/rGO is characterized by high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure spectroscopy, being determined to have a five coordinated local structure TiO5. When serving as non-Pt cathode material in dye-sensitized solar cells (DSCs), Ti1/rGO exhibits high electrocatalytic activity toward the tri-iodide reduction reaction. The power conversion efficiency of DSCs based on Ti1/rGO is comparable to that using conventional Pt cathode. The unique structure of TiO5 moieties and the crucial role of atomically dispersed Ti in Ti1/rGO are well understood by experiments and density functional theory calculations. This emerging material shows potential applications in energy conversion and storage devices.
- Liang, Suxia,Zhu, Chao,Zhang, Naitian,Zhang, Shuo,Qiao, Botao,Liu, Hua,Liu, Xiaoyan,Liu, Zheng,Song, Xuedan,Zhang, Heming,Hao, Ce,Shi, Yantao
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- Mechanistic Studies on the Palladium-Catalyzed Direct C-5 Arylation of Imidazoles: The Fundamental Role of the Azole as a Ligand for Palladium
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An in-depth mechanistic study on the palladium-catalyzed direct arylation of imidazoles at the C-5 position is presented. The interactions of triphenylphosphine (PPh3)-ligated aryl-Pd species with 1,2-dimethyl-1H-imidazole (dmim) have been studied in detail. In contrast with previous suggestions, phosphine-ligated organo-Pd species are not active and the reaction proceeds through imidazole-ligated organo-Pd intermediates. The kinetics of the oxidative addition of aryl halides with dmim-ligated Pd(0) species have been characterized in a Pd(dba)2/dmim model system. A thorough study of the equilibria involving novel [ArPd(dmim)2X] complexes (X=I, OAc) and the unexpected cationic [ArPd(dmim)3]+ is also reported. The ability of these species to effect the C-H arylation of dmim at room temperature in the presence of acetate is also demonstrated.
- Perego, Luca Alessandro,Grimaud, Laurence,Bellina, Fabio
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p. 597 - 609
(2016/02/27)
-
- Enhancing the anion affinity of urea-based receptors with a Ru(terpy) 22+ chromophore
-
Covalent linking of a Ru(terpy)22+ substituent improves recognition and sensing properties of the urea subunit toward anions. Urea's anion affinity is enhanced by the electrostatic attraction exerted by the RuII cation and by the electron-withdrawing effect exerted by the entire polypyridine-metal complex. Such an enhancement of the anion affinity, which results from the combination of a through-space and a through-bond effect, is greater than that exerted by the classical neutral electron-withdrawing substituent nitrophenyl. Small yet significant modifications of π-π* and MLCT bands of the Ru(terpy)22+ chromophore, detected through UV-vis spectrophotometric titrations, allowed the determination of the constants for the formation of receptor-anion H-bond complexes in diluted MeCN solution. On 1H NMR titration experiments, carried out under more concentrated conditions, the interaction of a second Cl- ion was observed, taking place through an outer-sphere mechanism. The Ru(terpy) 22+ substituent favors the deprotonation of a urea N-H fragment on addition of a second equivalent of fluoride, with formation of HF2-.
- Baggi, Giorgio,Boiocchi, Massimo,Ciarrocchi, Carlo,Fabbrizzi, Luigi
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p. 5273 - 5283
(2013/06/26)
-
- On the complexity of kinetics and the mechanism of the thiosulfate- periodate reaction
-
The thiosulfate-periodate reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 ± 0.1 °C in an acetate/acetic acid buffer by monitoring the absorbance in the 250-600 nm wavelength range at a constant ionic strength adjusted by the buffer component sodium acetate. In agreement with a previous study, we found that the reaction cannot be described by a single stoichiometric equation, tetrathionate and sulfate are simultaneously formed, and its ratio strongly depends on the pH. As expected at certain initial concentration ratios of the reactants, the reaction behaves as a clock reaction, but after its appearance, iodine is slowly consumed mainly because of the moderate tetrathionate-iodine reaction. It is also enlightened that the initial rate of the reaction is completely independent of the pH, which apparently contradicts a previous study, which postulates a "supercatalytic" behavior of the hydrogen ion on the title reaction. Significant buffer assistance that may change the absorbance-time profiles was also observed. On the basis of the kinetic data, a robust 28-step kinetic model with 22 fitted parameters is proposed and discussed to explain adequately all of the important characteristics of the kinetic curves.
- Rauscher, Evelin,Cseko, Gyoergy,Horvath, Attila K.
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p. 5793 - 5802
(2011/07/29)
-
- Organosulfur oxoacids. Part 2. A novel dimethylthiourea metabolite - Synthesis and characterization of the surprisingly stable and inert dimethylaminoiminomethane sulfonic acid
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A new metabolite of the biologically active thiocarbamide dimethylthiourea (DMTU) has been synthesized and characterized. DMTU's metabolic activation in the physiological environment is expected to be dominated by S-oxygenation, which produces, successively, the sulfenic, sulfinic, and sulfonic acids before forming sulfate and dimethylurea. Only the sulfinic and sulfonic acids are stable enough to be isolated. This manuscript reports on the first synthesis, isolation, and characterization of the sulfonic acid: dimethylaminoiminomethanesulfonic acid (DMAIMSOA). It crystallizes in the orthorhombic Pbca space group and exists as a zwitterion in its solid crystal form. The negative charge is delocalized over the sulfonic acid oxygens and the positive charge is concentrated over the planar N-C-N framework rather than strictly on the sp2-hybridized cationic carbon center. As opposed to its sulfinic acid analogue, DMAIMSOA is extremely inert in acidic environments and can maintain its titer for weeks at pH 6 and below. It is, however, reasonably reactive at physiological pH conditions and can be oxidized to dimethylurea and sulfate by mild oxidants such as aqueous iodine.
- Petersen, Jeffrey L.,Otoikhian, Adenike A.,Morakinyo, Moshood K.,Simoyi, Reuben H.
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p. 1247 - 1255
(2011/02/24)
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- Oxyhalogen-sulfur chemistry - Kinetics and mechanism of oxidation of methionine by aqueous iodine and acidified iodate
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The oxidation of methionine (Met) by acidic iodate and aqueous iodine was studied. Though the reaction is a simple two-electron oxidation to give methionine sulfoxide (Met-S=O), the dynamics of the reaction are, however, very complex, characterized by clock reaction characteristics and transient formation of iodine. In excess methionine conditions, the stoichiometry of the reaction was deduced to be IO3- + 3Met → I- + 3Met-S=O. In excess iodate, the iodide product reacts with iodate to give a final product of molecular iodine and a 2:5 stoichiometry: 2IO3 - + 5Met + 2H+ → I2 + 5Met-S=O + H 2O. The direct reaction of iodine and methionine is slow and mildly autoinhibitory, which explains the transient formation of iodine, even in conditions of excess methionine in which iodine is not a final product. The whole reaction scheme could be simulated by a simple network of 11 reactions.
- Chikwana, Edward,Davis, Bradley,Morakinyo, Moshood K.,Simoyi, Reuben H.
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p. 689 - 697
(2009/12/04)
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- Formation of organozincate anions in LiCl-mediated zinc insertion reactions
-
Tetrahydrofuran solutions of the products formed in LiCl-mediated zinc insertion reactions into various organic halides RHal were analyzed by anion-mode electrospray ionization (ESI) mass spectrometry. In all cases, organozincate anions were observed. The
- Koszinowski, Konrad,Boehrer, Petra
-
p. 771 - 779
(2009/06/19)
-
- Pulse radiolysis study of the oxidation of the I- ions with the radical anions Br2 >=- In an aqueous solution: Formation and properties of the radical anion BrI>=-
-
The radiation chemical redox transformations in solutions of bromides in the presence of minor additives of iodides were studied by pulse radiolysis. The change in the concentrations of the Br- and I- ions changes the ratio of the formed short-lived radical anions Br2 ?-, BrI?-, and I2 ?-. The spectrum of the mixed radical anion BrI?- contains a broad optical band at 370 nm with 370 = 9650 L mol-1 cm -1. The reduction potential of the BrI?-/Br -, I- pair is 1.25 V. The rate constants for the forward and backward reactions Br2 ?- + I- BrI?- + Br- are k f = 4.3?10 9 and k r = 1.0?105 L mol-1 s-1, respectively; for the reactions BrI?- Br - + I?, k f = 5.7?108 s-1 and k r = 1.0?1010 L mol-1 s-1.
- Ershov,Janata,Gordeeva
-
p. 1821 - 1826
(2009/12/02)
-
- Molybdenum and copper catalysis of reductions by titanium(II) and titanium(III)
-
Reductions of vanadium(iv), benzoquinone, and tri-iodide, both by titanium(iii) and by titanium(ii), are catalyzed by molybdenum(vi). The VO 2+-Ti(ii) reaction is catalyzed by copper(ii) as well. Reactions of Ti(ii) with the oxidant in excess y
- Yang, Zhiyong,Gould, Edwin S.
-
p. 396 - 398
(2007/10/03)
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- Two- and three-body photodissociation of gas phase I3 -
-
The photodissociation dynamics of gas phase I-3 by using a fast beam photofragment translational spectrometer was examined. It was stated that the photofragment translational spectrometer was coupled to a coincidence imaging detector that enabled the direct detection and analysis of two and three neutron or anion fragments from single dissociation events. The three-body dissociation yielding I-+2I(2P3/2) photofragments was also seen throughout the energy range probed. Analysis shows that the three-body decay dynamics was dominated by synchronous concerted dissociation.
- Hoops, Alexandra A.,Gascooke, Jason R.,Faulhaber, Ann Elise,Kautzman, Kathryn E.,Neumark, Daniel M.
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p. 7901 - 7909
(2007/10/03)
-
- Formation of palladium(0) complexes from Pd(OAc)2 and a bidentate phosphine ligand (dppp) and their reactivity in oxidative addition
-
A Pd0 complex is spontaneously generated from Pd(OAc)2 and a bidentate phosphine such as dppp (1,3-bis(diphenylphosphino)propane). dppp is the reducing agent and is oxidized to the hemioxide dppp(O). The intramolecular reduction step is reversible. A stable Pd0 complex is quantitatively formed in the presence of 2 equiv of dppp, water, and a base (NEt3). The oxidative addition of PhI gives a cationic complex, PhPd(dppp)(dppp(O))+, in which dppp(O) behaves as a monodentate ligand. PhPd(OAc)(dppp) is formed in the presence of added AcO-. The oxidative addition of PhI is an intricate reaction whose kinetics has been investigated only in the presence of added AcO-. It involves reactive dimeric or/and monomeric Pd0 complexes ligated by AcO- whose relative reactivity is a function of the PhI concentration.
- Amatore, Christian,Jutand, Anny,Thuilliez, Audrey
-
p. 3241 - 3249
(2008/10/08)
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- Aqueous solutions of unipositive cadmium; reactions of (CdI)22+(aq)
-
Aqueous solutions 10-3 mol dm-3 in (Cd1)2, prepared by treating O2-free solutions of Cd(ClO4)2 or Cd(O3SCF3)2 Cd powder at 65 °C, can be handled
- Babich,Gould
-
p. 998 - 999
(2007/10/03)
-
- Influence of external electric fields on reaction fronts in the iodate-arsenous acid system
-
The propagation of arsenous acid-iodate reaction fronts of different net stoichiometries in externally applied dc electric fields is studied for a range of both electric field intensities and initial compositions of the reacting mixture (represented by the stoichiometric factor S0). Regions of three different types of net stoichiometry in the parametric space E/V vs S0, where E is the intensity of the applied electric field and V the reaction front propagation velocity, are determined both experimentally and by analyzing a reaction-diffusion-migration model that includes a realistic kinetic scheme of the reaction studied. Both agreement with and discrepancies between the theoretical predictions and experimental findings are discussed.
- Forstova, Lenka,Sevcikova, Hana,Marek, Milos,Merkin, John H.
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p. 9136 - 9143
(2007/10/03)
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- Aqueous solutions of hypovalent gallium; reductions using gallium(I)
-
Solutions 0.2 mol dm-3 in Ga1, prepared by dissolving Ga2Cl4 in dry acetonitrile, are stable for more than seven days and may be diluted 300- to 1000-fold with O2-free water to give Ga(I) preparations
- Swavey,Gould
-
p. 2159 - 2160
(2007/10/03)
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- Kinetics and mechanisms of the oxidation of hydrazinium ion (N2H5+) by ICI and IBr; determination of the protonation constant of N2H5+
-
The kinetics of N2H5+ oxidation by ICl and IBr in 0.1-1.0 M [H+] is investigated by following the formation of I2Cl- and I2Br- with excess ICl2- and IBr2-, respectively, when total hydrazine is the limiting reactant. Highly acidic solutions are needed to suppress the rates of N2H4 reactions and to avoid hydrolysis of the interhalogens. Protonation constants of N2H5+, KP2 = [N2H62+]/[N2H5 +][H+], measured spectrophotometrically at 25.0 °C with picric acid as an indicator, are 1.61 M-1 (μ = 0.50 M) and 1.72 M-1 (μ = 1.00 M). In the proposed mechanisms, ICl and IBr react by an I+ transfer process to N2H5+ with loss of H+ (k1) to form a steady-state species, IN2H4+, that eliminates I- and H+ to give N2H3+ (k2). Subsequent reactions consume a second interhalogen, as diazine (N2H2) is rapidly oxidized to N2. Rate constants (25.0 °C, μ = 1.00 M) for the ICl/N2H5+ reaction are k1 = 4.12 × 106 M-1 s-1, k-1/k2 = 1.9 M-2 and for the IBr/N2H5+ reaction are k1 = 7.6 × 104 M-1 s-1, k-1/k2 = 28 M-2. The relative rates of oxidation of N2H5+ are ICl ? IBr ? I2.
- Jia, Zhongjiang,Margerum, Dale W.
-
p. 5374 - 5378
(2008/10/08)
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- Spin-orbit relaxation and recombination dynamics in I-2(CO2)n and I-2(OCS)n cluster ions: A new type of photofragment caging reaction
-
We report a new type of photofragment caging reaction that is only possible because of the strong solvent-induced perturbation of the inherent electronic structure of the chromophore. The photoexcitation of I-2 at 395 nm promotes it to a dissociative state correlating with I-+I*(2P1/2), the only near-ultraviolet dissociation channel for unsolvated I-2. In I-2 (CO2)n and I-2(OCS)n clusters, interaction with the solvent is observed to result in extremely fast spin-orbit relaxation. In general, we detect three reaction pathways: (1) direct dissociation of the chromophore to I-+I*(2P1/2); (2) the I-2→I-+I* dissociation, followed by spin-orbit quenching leading to I-+I(2P3/2) products; and (3) the I-2→I-+I* dissociation, followed by spin-orbit quenching and I-+I(2P3/2)→I-2 recombination and vibrational relaxation. We present experimental evidence of the spin-orbit relaxation and caging and discuss possible mechanisms. The results include: the measured translational energy release in 395 nm photodissociation of unsolvated I-2, indicating that solvation-free dissociation proceeds exclusively via the I-I* channel; ionic product distributions in the photodissociation of size-selected I-2(CO2)n and I-2(OCS)n clusters at the same wavelength, indicating the above three reaction channels; and ultrafast pump-probe measurements of absorption recovery, indicating picosecond time scales of the caging reaction. We rule out the mechanisms of spin-orbit quenching relying on I*-solvent interactions without explicitly considering the perturbed electronic structure of I-2. Instead, as described by Delaney et al. (companion paper), the spin-orbit relaxation occurs by electron transfer from I- to I*(2P1/2), giving I(2P3/2)+I-. The 0.93 eV gap between the initial and final states in this transition is bridged by differential solvation due to solvent asymmetry. Favorable comparison of our experimental results and the theoretical simulations of Delaney et al. yield confidence in the mechanism and provide understanding of the role of cluster structure in spin-orbit relaxation and recombination dynamics.
- Sanov, Andrei,Sanford, Todd,Nandi, Sreela,Lineberger, W. Carl
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p. 664 - 675
(2007/10/03)
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- The iodine-thiosulfate reaction: A pulse-radiolysis study and spectrophotometric evidence for the intermediates I2S2O32- and IS2O3-
-
Spectroscopic evidence for I2S2O32- and IS2O3-, postulated as intermediates in the iodine-thiosulfate reaction, has been obtained by using the technique of pulse radiolysis to oxidize iodide to I3- in the presence of thiosulfate. The spectra show blue shifts with respect to I3- and I2 respectively. The kinetics of the reaction over a range of concentrations different from those reported by Scheper and Margerum1 have also been studied and further evidence for their quantitative scheme has been obtained; a change predicted by their model, but not detected by them, in kinetic order in S2O32- at low thiosulfate and high iodide concentrations has been found.
- Packer, John E.,Anderson, Robert F.
-
p. 435 - 438
(2007/10/03)
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- Kinetics and Mechanisms of the Oxidation of Hydroxylamine by Aqueous Iodine
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First-order rate constants (kf, 25.0°C, μ = 0.50 M) for the loss of I3(1-)/I2 in the presence of excess NH3OH(1+)/NH2OH are measured from pH 2.0 to 6.8 with variation of the reactant concentrations as wellas the concentrations of I(1-) and various buffers. The kf values rangefrom 5 x 10**-4 to 2 x 10**4 s**-1 and depend on the NH2OH concentration. A multistep mechanism is proposed where I2 and NH2OH react rapidly toform an I2NH2OH adduct (KA = 480 M**-1) that undergoes general-base (B)assisted deprotonation to give INHOH + I(1-) + BH(1+). At higher pH, hydroxylamine acts as a general base as well as a reductant. Rate constants for various bases (H2O, CH3COO(1-), NH2OH, HPO4(2-), and OH(1-)) fit aBroensted β value of 0.58. The rates decrease greatly with increases of H(1+) and I(1-) concentrations due to NH3OH(1+) and I3(1-) formation, loss of general-base assistance, and the reverse reaction of BH(1+) + I(1-) + INHOH to re-form I2NH2OH. The INHOH species is a steady-state intermediate that decays to form I(1-) + HNO + H(1+). Subsequent rapid dehydrative dimerization of HNO gives N2O as the final product. The hydroxylamine oxidation process proceeds entirely by I(1+) transfer to nitrogen followed by I(1-) loss, as opposed to electron-transfer pathways. Kinetic evidence is given for I2NH2OH as an intermediate and for INHOH as asteady-state species.
- Liu, Rong Ming,McDonald, Michael R.,Margerum, Dale W.
-
p. 6093 - 6099
(2008/10/08)
-
- Kinetics of the Reactions of F- with CF3Br and CF3I as a Function of Temperature, Kinetic Energy, Internal Temperature, and Pressure
-
The reactions of F- with CF3Br and CF3I have been studied at several temperatures as functions of pressure and ion-neutral average center-of-mass kinetic energy (cm>).Rate constants and product branching fractions were measured by using a variable temperature-selected ion flow drift tube apparatus.Both title reactions involve two product pathway: displacement and association, the latter being the major channel under most experimental conditions.The rate constants for displacement are governed by total energy, regardless of the relative contributions by translational and internal energy, indicating that the displacement reactions behave statistically.In contrast, the rate constants for association depend strongly on the internal energy of CF3X (rotations and vibrations) compared with the relatively weak effect of translational collision energy.For CF3Br the rate constants for the association channel increase with increasing pressure while those for displacement are independent of pressure, indicating that the additional reactivity leading to more association at higher pressure appears not to compete with the displacement pathway.This lack of competition may be the result of two different reaction intermediates.The more efficient CF3I reaction is at or near the high-pressure limit for association, showing no pressure dependence in both the association and displacement channels.Therefore, there appears to be no competition between these channels for CF3I as well, but the data are less definitive in this case.
- Morris, Robert A.,Viggiano, A. A.
-
p. 3740 - 3746
(2007/10/02)
-
- Nucleophilic Displacement of Halides from Dihalogenoplatinum(II) Complexes containing a Chelating Thioether. Kinetics and Equilibria
-
Kinetic and equilibrium measurements on the displacement of halides from the substrates (X, Y = Cl, Cl; Cl, Br; Br, Br; Br, I; I, I; Cl, I) with the nucleophiles Cl(1-), Br(1-) and I(1-) have been carried out in methanol at 25 deg C, I = 0.5 mol dm3 (LiClO4).The results are discussed in terms of the relative stability of the planar ground and five-co-ordinate transition states, and the lability and kinetic cis effects of co-ordinated halides are compared.All substrates appear to be characterized by a sort of stereoelectronic hindrance leading to retardation effects, mainly in the second-order substitution pathway.
- Pitteri, Bruno,Marangoni, Giampaolo,Cattalini, Lucio,Canovese, Luciano
-
p. 169 - 174
(2007/10/02)
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- Non-metal redox kinetics: Hypobromite and hypoiodite reactions with cyanide and the hydrolysis of cyanogen halides
-
Pulsed-accelerated-flow spectroscopy is used to measure second-order rate constants (where the initial half-lives are 3-9 μs) for the reactions of cyanide ion with OBr- and with OI- (25.0°C, μ = 1.00 M). The proposed mechanism includes parallel paths with halogen-cation transfer to CN- by solvent-assisted reaction with OX- (X = Br, I) and by direct reaction with HOX: OX- + CN- + H2O → kox XCN + 2OH- OX- + H2O ? HOX + OH- HOX + CN- → kHOX XCN + OH- The relative reactivities of the hypohalites with CN- (kox) are as follows: OI- (6 × 107 M-1 s-1) ≈ OBr- (5.7 × 107 M-1 s-1) ? OCl- (310 M-1 s-1). The rate constants for the hypohalous acid reactions with CN- (kHOX) are as follows: HOBr (4.2 × 109 M-1 s-1) > HOCl(1.22 × 109 M-1 s-1). The base hydrolysis of ICN is studied spectrophotometrically by the appearance of I- at 225 nm (ε = 12 070 M-1 cm-1). Saturation kinetics are observed with increased OH- concentration. This is attributed to rapid equilibration to give HOICN- (KOH = 3.2 M-1), which inhibits the OH- attack at the carbon atom in ICN to form OCN- (k4 = 1.34 × 10-2M-1s-1). The base hydrolysis of BrCN is studied by following the disappearance of the 105 amu peak with membrane introduction mass spectrometry. Rate constants for the reactions of BrCN with OH- (kOH = 0.53 ± 0.01 M-1 s-1) and with CO32- are determined (kCO3 = (7.5 ± 0.3) × 10-3 M-1 s-1). The relative reactivities of cyanogen halides for the base hydrolysis are as follows: ClCN ? BrCN ? ICN.
- Gerritsen, Cynthia M.,Gazda, Michael,Margerum, Dale W.
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p. 5739 - 5748
(2008/10/08)
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- Methyl transfers. 15. CoIPc- as a nucleophile and leaving group
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Many complexes of cobalt in the +1 oxidation state are excellent nucleophiles. The complex with phthalocyanine, Pc2-, is an example; the rates of reaction of complex (CoIPc)-, 1-, with MeI and other alkylating agents in dimethylacetamide (DMA) solution are reported. In contrast to previous studies with other Co(I) complexes, the rate of reaction of the methylated product, (MeCoIIIPc, 3) with I- is also readily measured, allowing a kinetic evaluation of the equilibrium constant K = [1-][MeI]/[3][I-] = 0.004 in DMA at 25 °C. Similarly, Br- also reacts with 3 reversibly, and the rate and equilibrium constants are reported. Thus 1- is also a good leaving group. With CN, 3 gives complex MeCoIIIPcCN- reversibly and slowly gives MeCN in a reaction zero order in CN-. The fast identity rate constants for I- attack on MeI as well as that for Br- attack on MeI are given. Two different paths to the identity methyl transfer rate constant, kCoCo, for the 1- + 3 reaction with use of the Marcus equation gave kCoCo = 4.4 M-1 s-1 from the 3 + I- data but kCoCo = 0.088 M-1 s-1 from the 3 + Br- data. This discrepancy, which is outside of experimental error, constitutes a deviation from the Marcus treatment; it is discussed.
- Galezowski, Wlodzimierz,Ibrahim, Prabha N.,Lewis, Edward S.
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p. 8660 - 8668
(2007/10/02)
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- A new type of oxyhalogen oscillator: The bromite-iodide reaction in a continuous flow reactor
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The reaction between bromite and iodide ions in a flow reactor shows two steady states and sustained oscillations in pH, redox potential, and [I-]. The oscillatory behavior is extremely sensitive to the inflow ratio [BrO2-]/[I-], the flow rate, the initial pH of the input solutions, and the stirring rate. Batch experiments suggest that the system behaves in several respects like the analogous chlorite-iodide oscillator. The bromite-iodide system, however, is also a pH oscillator and is the first oxyhalogen oscillator to oscillate in the neutral and basic pH range.
- Orbán, Miklós,Epstein, Irving R.
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p. 1252 - 1256
(2007/10/02)
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- A pulse radiolysis study of the reaction of OH with I2 and the decay of I2-
-
The rate constant for the reaction of the hydroxyl radical with iodine was estimated to be (9.5 +/- 1.0) * 10-9 dm3 mol-1 s-1 at 20 deg C from fitting the time dependence of the decrease in the absorption of iodine at 460 nm.A number of rate constants associated with the decay of I2- that were required for this fitting process were also determined.They were the reaction of I2- with I2- (2k = (4.60 +/- 0.05) * 10-9 dm3 mol-1 s-1) at an ionic strength of 1.2 * 10-2 mol dm-3; I2- with I (k = 4.6 +/- 1.6) * 10-9 dm3 mol-1 s-1); and I with I (2k = (3.0 +/- 1.0) * 10-10 dm3 mol-1 s-1).
- Elliot, A. John
-
p. 1658 - 1661
(2007/10/02)
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- Kinetics and mechanism of oxidation of oxalic acid by iodate in aqueous sulphuric acid medium
-
The oxidation of oxalic acid by iodate in aqueous sulphuric acid medium proceeds with a measurable rate only at 60 deg C and above.Two paths (1) involving HIO3 and OXH2 and (2) HIO3 and OXH- have been found to be significant at +> less than 1.0 mol dm-3 and path (1) only at +> above 1.0 mol dm-3.A single step two electron transfer mechanism is suggested.
- Ramana, P V,Rao, R V Appa
-
p. 971 - 973
(2007/10/02)
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- Temperature Dependence of the Rate Constants for Reaction of Dihalide and Azide Radicals with Inorganic Reductants
-
Rate constants for several reactions of inorganic radicals with inorganic reductants in aqueous solutions have been measured by pulse radiolysis as a function of temperature, generally between 5 and 75 deg C.The reactions studied were of the dihalide radicals, Cl2.-, Br2.-, and I2.-, the (SCN).- radical, and the neutral radical N3., reacting with the substitution-inert metal complexes, Fe(CN)6(4-), Mo(CN)8(4-), and W(CN)8(4-), and with the anions SO3(2-), HSO3(1-), NO2(1-), and ClO2(1-).The rate constants measured were in the range of 1E6 to 5E9 M-1 s-1and the calculated Arrhenius activation energies ranged from 5 to 35 kJ mol-1.The preexponential factors also varied considerably, with log A ranging from 8.9 to 13.1.The temperature dependence of the reaction rate constant is correlated to the reaction exothermicity for the metal complexes, which apparently react by outer-sphere electron transfer.The simple anions, however, have lower activation energies, which do not correlate well with the exothermicities, suggesting that these anions probably react by inner-sphere mechanism.
- Shoute, L. C. T.,Alfassi, Z. B.,Neta, P.,Huie, R. E.
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p. 3238 - 3242
(2007/10/02)
-
- Synthesis and electrosynthesis of bis-silylated cyclopentadienylniobium(V) cationic complexes. Their reactions and structural characterization
-
Chemical oxidation of complexes Nb(η5-C5H4SiMe3)2X2, X = Cl (1), Br (2), I (3), with ferrocenium hexafluorophosphate in dichloromethane gives the cationic complexes 5-C5H4SiMe3)2X2>+ PF6-, X = Cl (4), Br (5
- Antinolo, Antonio,Fajardo, Mariano,Otero, Antonio,Mugnier, Yves,Nabaoui, Hassane,Mourad, Houssef
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p. 155 - 164
(2007/10/02)
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- Structural studies of organometallic compounds in solution. II. Magnesium bromide and iodide in diethyl ether and tetrahydrofuran; an extended X-ray absorption fine structure (EXAFS) and large angle X-ray scattering(LAXS) study
-
The structures of the magnesium iodide complexes formed in concentrated diethyl ether and tetrahydrofuran solutions have been determined by the large angle X-ray scattering (LAXS) technique.Those of the more dilute solutions of magnesium bromide in diethyl ether and tetrahydrofuran solutions have been determined by the extended X-ray absorption fine structure (EXAFS) method.The investigation of the MgI2 diethyl ether solution was carried out at 44 deg C since this solution cystallizes at about 30 deg C.This solution is probably best considered a melt, the structure of which can be regarded as a close-packing of iodide ions with magnesium ions occupying some of the holes.The coordination around a specific magnesium ion depends on whether it occupies a tetrahedral or an octahedral hole.The average Mg-I bond distance is 2.75 Angstroem.Solvated MgI+ is the dominating complex in MgI2-tetrahydrofuran solution.Magnesium is probably six-coordinate in this complex, and the Mg-I bond distance is 2.52(5) Angstroem.The Mg-Br bond distance is 2.49 and 2.66 Angstroem in diethyl ether and tetrahydrofuran, respectively.
- Wellmar, Agneta,Persson, Ingmar
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p. 143 - 153
(2007/10/02)
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- Large Amplitude pH Oscillation in the Oxidation of Hydroxylamine by Iodate in a Continuous-Flow Stirred Tank Reactor
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Large amplitude pH oscillation occurs in the iodate oxidation of hydroxylamine in a continuous-flow tank reactor (CSTR) in a narrow range of flow rates and input concentrations.In addition to pH, the redox potential, the iodide ion concentration, the rate of gas (N2O) formation, and the color of the solution (I2) change periodically.The oscillatory period is extremely long, 2-6 h.The dynamical behaviour of the reaction was modeled by a scheme that takes into account the following component processes: the protonation equilibrium of NH2OH, the dissociation of water,the direct redox reaction between IO3- and NH3OH+, the Dushman reaction between IO3- and I-, the autoinhibitory reaction between I2 and NH3OH+, the fast dimerization of NOH to N2O, the reaction of NOH with NH2OH, and the volatility of iodine.Empirical rate laws of the component reactions were used successfully to calculate the kinetic behaviour in a closed system, as well as in the CSTR.
- Rabai, Gyula,Epstein, Irving R.
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p. 6361 - 6365
(2007/10/02)
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- Non-metal redox kinetics: Reactions of iodine and triiodide with sulfite and hydrogen sulfite and the hydrolysis of iodosulfate
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The reactions of I3- and I2 with SO3H- and SO32- (in the p[H+] range 4.1-6.6) are studied by the pulsed-accelerated-flow technique. Pseudo-first-order rate constants (excess [SO32-]T and [I-]) are measured in the range from 6300 to 74000 s-1 (25.0°C, μ = 0.50). The proposed mechanism is I3- + SO3H- →k1 ISO3- + 2I- + H+ I2 + SO3H- →k2 ISO3- + I- + H+ I3- + SO32- →k3 ISO3- + 2I- I2 + SO32- →k4 ISO3- + I- where the second-order rate constants (M-1 s-1) are k1 = 1.5 × 107, k2 = 1.7 × 109, k3 = 2.9 × 108, and k4 = 3.1 × 109. The iodosulfate that is formed hydrolyzes with a first-order rate constant of 298 s-1 at 25.0°C. ISO3- + H2O →kh I- + SO42- + 2H+ A temperature-dependent stopped-flow study gives ΔH≠ = 65 kJ mol-1 and ΔS≠ = 21 J mol-1 K-1 for the hydrolysis reaction. The positive ΔS≠ value indicates a dissociative mechanism without the addition of H2O in the transition state for ISO3- hydrolysis.
- Yiin, Boudin S.,Margerum, Dale W.
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p. 1559 - 1564
(2008/10/08)
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- Experimental and modeling study of oscillations in the chlorine dioxide-iodine-malonic acid reaction
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At pH 0.5-5.0, a closed system containing an aqueous mixture of chlorine dioxide, iodine, and a species such as malonic acid (MA) or ethyl acetoacetate, which reacts with iodine to produce iodide, shows periodic changes in the light absorbance of I3-. This behavior can be modeled by a simple scheme consisting of three component reactions: (1) the reaction between MA and iodine, which serves as a continuous source of I-; (2) the reaction between ClO2? and I-, which acts as a source of ClO2-; and (3) the self-inhibited reaction of chlorite and iodide that kinetically regulates the system. The fast component reaction between chlorine dioxide and iodide ion was studied by stopped-flow spectrophotometry. The rate law is -[ClO2?]/df = 6 × 103 (M-2 s-1)[ClO2?][I-]. A two-variable model obtained from the empirical rate laws of the three component reactions gives a good description of the dynamics of the system. The oscillatory behavior results not from autocatalysis but from the self-inhibitory character of the chlorite-iodide reaction.
- Lengyel, István,Rábai, Gyula,Epstein, Irving R.
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p. 9104 - 9110
(2007/10/02)
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