Showing papers on "Reaction rate constant published in 2005"

Photosensitizer Method to Determine Rate Constants for the Reaction of Carbonate Radical with Organic Compounds

Abstract: Carbonate radical (CO3•-) is a powerful oxidant that is present in sunlit surface waters and in waters treated by advanced oxidation processes. The production of CO3•- in aqueous solution through oxidation of carbonate anion by excited triplet states of aromatic ketones was investigated in this study to provide new methods for the determination of rate constants and to explore a possible photoinduced pathway of CO3•- formation in the aquatic environment. Rate constants for triplet quenching by carbonate anion of up to 3.0 × 107 M-1 s-1 and CO3•- yields approaching unity, determined using laser flash photolysis, allowed us to conclude that such a formation mechanism might be significant in sulit natural waters. Kinetic methods based on either flash photolysis or steady-state irradiation and on the use of aromatic ketones as photosensitizers gave bimolecular rate constants in the range of 4 × 106 to 1 × 108 M-1 s-1 for the reaction of CO3•- with several s-triazine and phenylurea herbicides. For various anil...

Proton electroreduction catalyzed by cobaloximes: functional models for hydrogenases.

Abstract: Cobaloximes have been examined as electrocatalysts for proton reduction in nonaqueous solvent in the presence of triethylammonium chloride. [CoIII(dmgH)2pyCl], working at moderate potentials (−0.90 V/(Ag/AgCl/3 mol·L-1 NaCl) and in neutral conditions, is a promising catalyst as compared to other first-row transition metal complexes which generally function at more negative potentials and/or at lower pH. More than 100 turnovers can be achieved during controlled-potential electrolysis without detectable degradation of the catalyst. Cyclic voltammograms simulation is consistent with a heterolytic catalytic mechanism and allowed us to extract related kinetic parameters. Introduction of an electron-donating (electron-withdrawing) substituent in the axial pyridine ligand significantly increases (decreases) the rate constant of the catalytic cycle determining step. This effect linearly correlates with the Hammet coefficients of the introduced substituents. The influence of the equatorial glyoxime ligand was also...

Methanation of CO over Nickel: Mechanism and Kinetics at High H2/CO Ratios†

Abstract: The CO methanation reaction over nickel was studied at low CO concentrations and at hydrogen pressures slightly above ambient pressure. The kinetics of this reaction is well described by a first-order expression with CO dissociation at the nickel surface as the rate-determining step. At very low CO concentrations, adsorption of CO molecules and H atoms compete for the sites at the surface, whereas the coverage of CO is close to unity at higher CO pressures. The ratio of the equilibrium constants for CO and H atom adsorption, K(CO)/K(H), was obtained from the rate of CO methanation at various CO concentrations. K(H) was determined independently from temperature programmed adsorption/desorption of hydrogen to be K(H) = 7.7 x 10(-4) (bar(-0.5)) exp[43 (kJ/mol)/RT] and hence the equilibrium constants for adsorption of CO molecules may be calculated to be K(CO) = 3 x 10(-7) (bar(-1)) exp[122 (kJ/mol)/RT]. Furthermore, the rate of dissociation of CO at the catalyst surface was determined to be 5 x 10(9) (s(-1)) exp[-96.7 (kJ/mol)/RT] assuming that 5% of the surface nickel atoms are active for CO dissociation. The results are compared to equilibrium and rate constants reported in the literature.

Switching the redox mechanism : Models for proton-coupled electron transfer from tyrosine and tryptophan

Abstract: The coupling of electron and proton transfer is an important controlling factor in radical proteins, such as photosystem II, ribinucleotide reductase, cytochrome oxidases, and DNA photolyase. This was investigated in model complexes in which a tyrosine or tryptophan residue was oxidized by a laser-flash generated trisbipyridine−RuIII moiety in an intramolecular, proton-coupled electron transfer (PCET) reaction. The PCET was found to proceed in a competition between a stepwise reaction, in which electron transfer is followed by deprotonation of the amino acid radical (ETPT), and a concerted reaction, in which both the electron and proton are transferred in a single reaction step (CEP). Moreover, we found that we could analyze the kinetic data for PCET by Marcus' theory for electron transfer. By altering the solution pH, the strength of the RuIII oxidant, or the identity of the amino acid, we could induce a switch between the two mechanisms and obtain quantitative data for the parameters that control which ...

Does Core Size Matter in the Kinetics of Ligand Exchanges of Monolayer-Protected Au Clusters?

Abstract: This paper compares the kinetics of exchanges of phenylethanethiolate ligands (PhC2S−) of the monolayer-protected clusters (MPCs) Au38(SC2Ph)24 and Au140(SC2Ph)53 with p-substituted arylthiols (p-X−PhSH), where X = NO2, Br, CH3, OCH3, and OH. First-order rate constants at 293 K for exchange of the first ca. 25% of the ligands on the molecule-like Au38(SC2Ph)24 MPC, measured using 1H NMR, vary linearly with the in-coming arythiol concentration; ligand exchange is an overall second-order reaction. Remarkably, the second-order rate constants for ligand exchange on Au38(SC2Ph)24 are very close to those of corresponding exchange reactions on the larger nanoparticle Au140(SC2Ph)53 MPCs. These are the first results that quantitatively show that the chemical reactivity of different sized nanocrystals is almost independent of size; presumably, this is because the locus of the initial ligand exchanges is a common kind of site, thought to be the nanocrystal vertexes. The rates of later stages of exchange (beyond ca....

Perchlorate Reduction by Nanoscale Iron Particles

Abstract: We report herein the near complete reduction of perchlorate (ClO $$_{4}^{-}$$ ) to chloride by nanoscale iron particles. The nanoparticles also reduce chlorate (ClO $$_{3}^-$$ ), chlorite (ClO $$_{2}^-$$ ) and hypochlorite (ClO $$^{-}$$ ) to chloride. No reaction was observed with microscale iron powder under identical conditions. The temperature sensitivity of the perchlorate-nanoparticle reaction is evidenced by progressively increasing rate constant values of 0.013, 0.10, 0.64 and 1.52 mg perchlorate per g nanoparticles per hour (mg-g-1-hr-1), respectively, at temperatures of 25, 40, 60 and 75°C. The activation energy of perchlorate-iron reaction was calculated to be 79.02 ± 7.75 kJ/mole. Despite favorable thermodynamics, the relatively large activation energy for this reaction suggests that perchlorate reduction is limited by the slow kinetics. The nanoscale iron particles may represent a potential treatment method for perchlorate-contaminated water.

One-dimensional reaction coordinates for diffusive activated rate processes in many dimensions

Abstract: For multidimensional activated rate processes controlled by diffusive crossing of a saddle point region, we show that a one-dimensional reaction coordinate can be constructed even when the diffusion anisotropy is arbitrary. The rate constant, found using the potential of mean force along this coordinate, is identical to that predicted by the multidimensional Kramers–Langer theory. This reaction coordinate minimizes the one-dimensional rate constant obtained using a trial reaction coordinate and is orthogonal to the stochastic separatrix, the transition state that separates reactants from products.

Decomposition Kinetics of the AlH3 Polymorphs

Abstract: Aluminum hydride polymorphs (α-AlH3, β-AlH3, and γ-AlH3) were prepared by organometallic synthesis. Hydrogen capacities approaching 10 wt % at desorption temperatures less than 100 °C have been demonstrated with freshly prepared AlH3. The temperature-dependent rate constants were determined by measuring the isothermal hydrogen evolution between 60 °C and 140 °C. Fractional decomposition curves showed good fits using both the second and third-order Avrami−Erofeyev equations, indicating that the decomposition kinetics are controlled by nucleation and growth of the aluminum phase in two and three dimensions. The large activation energies measured for the AlH3 polymorphs suggest that the decomposition occurs via an activated complex mechanism with complexes consisting of approximately nine AlH3 molecules (1−2 unit cells for α-AlH3).

Abnormal solvent effects on hydrogen atom abstraction. 3. Novel kinetics in sequential proton loss electron transfer chemistry.

Abstract: A prolonged search involving several dozen phenols, each in numerous solvents, for an ArOH/2,2-diphenyl-1-picrylhydrazyl (dpph•) reaction that is first-order in ArOH but zero-order in dpph• has reached a successful conclusion. These unusual kinetics are followed by 2,2‘-methylene-bis(4-methyl-6-tert-butylphenol), BIS, in five solvents (acetonitrile, benzonitrile, acetone, cyclohexanone, and DMSO). In 15 other solvents the reactions were first-order in both BIS and dpph• (i.e., the reactions followed “normal” kinetics). The zero-order kinetics indicate that in the five named solvents the BIS/dpph• reaction occurs by sequential proton loss electron transfer (SPLET). This mechanism is not uncommon for ArOH/dpph• reactions in solvents that support ionization, and normal kinetics have always been observed previously (see Litwinienko, G.; Ingold, K. U. J. Org. Chem. 2003, 68, 3433 and Litwinienko, G.; Ingold, K. U. J. Org. Chem. 2004, 69, 5888). The zero-order kinetics found for the BIS/dpph• reaction in five s...

Photocuring kinetics of UV‐initiated free‐radical photopolymerizations with and without silica nanoparticles

Abstract: We used photodifferential scanning calorimetry to investigate the photocuring kinetics of UV-initiated free-radical photopolymerizations of acrylate systems with and without silica nanoparticles. Two kinetics parameters-the rate constant (k) and the order of the initiation reaction (m)-were determined for hybrid organic-inorganic nanocomposite systems containing different amounts of added silica nanoparticles (0-20 wt %) and at different isothermal temperatures (30-100 °C) using an autocatalytic kinetics model. The kinetic analysis revealed that the silica nanoparticles apparently accelerate the cure reaction and cure rate of the UV-curable acrylate system, most probably due to the synergistic effect of silica nanoparticles during the photopolymerization process. However, a slight decrease in polymerization reactivity that occurred when the silica content increased beyond 15 wt % was attributed to aggregation between silica nanoparticles. We also observed that the addition of silica nanoparticles lowered the activation energy for the UV-curable acrylate system, and that the collision factor for the system with silica nanoparticles was higher than that obtained for the system without silica nanoparticles, indicating that the reactivity of the former was greater than that of the latter.

Role of iron surface oxidation layers in decomposition of azo-dye water pollutants in weak acidic solutions

Abstract: While decomposition of water pollutants in the presence of metallic iron can be strongly influenced by the nature and structure of the iron surface layer, the composition and structure of the layer produced and transformed in the decomposition process, have been meagerly investigated. The studies presented here establish strong relationships between the composition and structure of the iron oxidized surface layer and the kinetics and reaction pathways of orange II decomposition. The most striking observation is a dramatic difference between dye decomposition at pH 3 and 4. Orange decomposition at pH 2 and 3 is a very fast process with pseudo-first-order kinetics, with a surface normalized rate constant kSA = 0.18 L/m2 min at pH 3 and 30 °C. Whereas at pH 4 and 5 the rate is lower with pseudo-zero-order kinetics, with normalized rate constant kSA = 1.4 × 10−5 mol/m2 min at pH 5 and 30 °C. At pH 3 the iron surface is covered by a polymeric Fe(OH)2 mixed with FeO very thin layer whose thickness remains almost constant with reaction time. There is a slow formation of an additional surface product with akaganeite-like structure. At pH 3 almost all oxidized iron is detected in solution, whereas at pH 5 almost total oxidized iron is cumulated on iron surface in the form of a lepidocrocite, γ-FeOOH, layer. The thickness of the layer increases continuously with time. The quantitative evaluation of the produced surface lepidocrocite and its surface distribution were performed by means of infrared reflection spectroscopy and spectral simulation methods. At higher temperature 40–50 °C, other surface products such as goethite, α-FeOOH, and feroxyhite, β-FeOOH, are also observed. Decomposition of orange is a multi-step process, at pH 3 the orange molecule is at first adsorbed on the very thin iron oxidized layers through SO3 group and then undergoes reduction. Discoloration of orange II in aerobic solution takes place by reduction of the single bondNdouble bond; length as m-dashNsingle bond bond at the iron surface. The major intermediate is 1-amino-2-naphtol, which undergoes further decomposition without forming any aromatic species. The previously suggested sulfanilic acid as intermediate was not detected in solution. At pH 3 orange reduction and reduction of intermediates are governed by the combination of an electron transfer reaction, with the thin oxide surface layer as a mediator, and the catalytic hydrogenation reaction. At pH 4 and 5 continuous growing of lepidocrocite surface layer demonstrates the importance of the layer as a mediator in the electron transfer reaction. The layer shows a good conductivity, which results from adsorption and absorption of iron ions in the surface structure. It is observed that the decomposition reaction becomes significant at open circuit potential (OCP) below −120 mV (SHE). At pH 3 this condition is fulfilled almost immediately after introduction of iron to aqueous solution, whereas at pH 4 and 5 the OCP of iron decreases very slowly. Iron surface layer composition and structure can be modified by an addition of Fe2+ to solution, which increases the dye decomposition rate. The performed observations make the treatment of waste water in the presence of metallic iron a promising environmental solution.

Chemical Kinetic Modeling of Dimethyl Carbonate in an Opposed-Flow Diffusion Flame

Abstract: Dimethyl carbonate (DMC) has been of interest as an oxygenate additive to diesel fuel because of its high oxygen content. In this study, a chemical kinetic mechanism for DMC was developed for the first time and used to understand its combustion under conditions in an opposed flow diffusion flame. Computed results were compared to previously published experimental results from an opposed flow diffusion flame. It was found that the decomposition rate DMC ⇒ H3COC( O)O + CH3 in the flame was much slower than originally thought because resonance stabilization in the H3COC( O)O radical was less than expected. Also, a new molecular elimination path for DMC is proposed, and its rate is calculated by quantum chemical methods. In the simulations of DMC in the flame, it was determined that much of the oxygen in dimethyl carbonate goes directly to CO2. This characteristic reduces the effectiveness of DMC for soot reduction in diesel engines. In an ideal oxygenate additive for diesel fuel, each oxygen atom stays bonded to one carbon atom in the products thereby preventing the formation of carbon–carbon bonds that can lead to soot. When CO2 is formed directly, two oxygen atoms are bonded to one carbon atom thereby wasting one oxygen atom in the oxygenate additive. To determine how much CO2 is formed directly, the branching ratio of the key reaction, CH3OC O going to the products CH3 + CO2 or CH3O + CO, was determined by ab initio methods. The A-factors of the rate constant of this reaction were found to be about 10–20 times higher than previous estimates. The new reaction rate constants obtained can be used as reaction rate rules for all oxygenates that contain the ester moiety including biodiesel.

Ruthenium-catalyzed propargylic substitution reactions of propargylic alcohols with oxygen-, nitrogen-, and phosphorus-centered nucleophiles.

Abstract: The scope and limitations of the ruthenium-catalyzed propargylic substitution reaction of propargylic alcohols with heteroatom-centered nucleophiles are presented. Oxygen-, nitrogen-, and phosphorus-centered nucleophiles such as alcohols, amines, amides, and phosphine oxide are available for this catalytic reaction. Only the thiolate-bridged diruthenium complexes can work as catalysts for this reaction. Results of some stoichiometric and catalytic reactions indicate that the catalytic propargylic substitution reaction proceeds via an allenylidene complex formed in situ, whereby the attack of nucleophiles to the allenylidene C(gamma) atom is a key step. Investigation of the relative rate constants for the reaction of propargylic alcohols with several para-substituted anilines reveals that the attack of anilines on the allenylidene C(gamma) atom is not involved in the rate-determining step and rather the acidity of conjugated anilines of an alkynyl complex, which is formed after the attack of aniline on the C(gamma) atom, is considered to be the most important factor to determine the rate of this catalytic reaction. The key point to promote this catalytic reaction by using the thiolate-bridged diruthenium complexes is considered to be the ease of the ligand exchange step between a vinylidene ligand on the diruthenium complexes and another propargylic alcohol in the catalytic cycle. The reason why only the thiolate-bridged diruthenium complexes promote the ligand exchange step more easily with respect to other monoruthenium complexes in this catalytic reaction should be that one Ru moiety, which is not involved in the allenylidene formation, works as an electron pool or a mobile ligand to another Ru site. The catalytic procedure presented here provides a versatile, direct, and one-step method for propargylic substitution of propargylic alcohols in contrast to the so far well-known stoichiometric and stepwise Nicholas reaction.