Showing papers on "Reaction rate constant published in 1985"

Kinetics of Heterogeneous Photocatalytic Decomposition of Phenol over Anatase TiO2 Powder

Abstract: The photocatalytic decomposition of phenol over anatase TiO2 powder followed the first-order kinetics, upto high conversions, of which the apparent rate constant kap depended on initial concentration of phenol [phenol]0, [TiO2], O2 pressure pO2, and incident light intensity I. The dependence of the initial reaction rate Ω0 on [phenol]0 showed characteristic curvature convex to the concentration axis. The pO2 dependence of both Ω0 and kap was very similar each other and showed characteristic curvature to the pO2 axis. The dependence of both Ω0 and kap on [phenol]0 was affected by the [TiO2] and I. Both Ω0 and kap were proportional to I below ca. 1×10−5 mol m−2 s−1 and to I1⁄2 above 2×10−5 mol m−2 s−1. The activation energy was 10 kJ/mol. The results were satisfactorily explained by the equation;Ω=φ·OHIan\fracKO2pO21+KO2pO2·\frac[phenol][phenol]0+β,where Ω is the reaction rate; φ·OHIan, a parameter related to the formation rate of ·OH radicals or real reactive species; KO2, the equilibrium constant of Langm...

Dependence of rate constants for photoinduced charge separation and dark charge recombination on the free energy of reaction in restricted-distance porphyrin-quinone molecules

Abstract: Rate constants for both radical ion pair formation and recombination in porphyrin - quinone donor-acceptor molecules are reported. The ion pair formation and recombination depend on the exothermicity of the respective electron-transfer reaction in a manner previously reported. The question as to what fraction of energy change is due to relaxation processes involving the solvent vs. the porphyrin - quinone molecules has not been answered by these studies. However, the measured reorganization energy was found to be approximately solvent independent. It was observed that in these molecules the donor-acceptor distance is restricted, and the highly exothermic charge recombination reactions do not produce electronically excited states to the donor or acceptor. 11 references, 2 figures.

Free-radical copolymerization. 3. Determination of rate constants of propagation and termination for styrene/methyl methacrylate system. A critical test of terminal-model kinetics

Abstract: Reaction en masse a 40°C. Vitesse d'amorcage, de propagation et de terminaison en fonction de la composition des monomeres

Gas phase reactions of iron clusters with hydrogen. I. Kinetics

Abstract: The kinetics of the gas phase reactions of hydrogen and deuterium with iron clusters in the range Fe6 to Fe68 have been investigated. It is found that reaction rate constants are a strong function of cluster size, varying by more than five orders of magnitude in this size range. The largest rate constants correspond to approximately 3% of a hard sphere cross section. Abrupt changes in the rate constant from one cluster to the next are seen. Qualitative temperature dependencies of cluster reactivity have been determined. The more reactive clusters show decreased reactivity with increased tempeature, while the least reactive clusters become more reactive. Strong isotope effects are seen only in the Fe10 to Fe14 size range. Mechanisms for the reactions of H2 and D2 with iron clusters are discussed in light of these observations.

Unimolecular reaction rate theory for transition states of partial looseness. II. Implementation and analysis with applications to NO2 and C2H6 dissociations

Abstract: Implementation of RRKM theory for unimolecular dissociations having transition states of any degree of looseness is described for reactions involving dissociation into two fragments. The fragments may be atomic, diatomic, or polyatomic species. Action-angle and internal coordinates for the transitional modes of the reaction, transformations to Cartesian coordinates, and other calculational aspects are described. Results for the NO2-->NO+O reaction are presented, including the dependence of the microcanonical rate constant on the bond fission and bending potentials for model potential energy surfaces. Illustrative calculations for the C2H6-->2CH3 reaction are also given.

Reaction of azide radicals with aromatic compounds. Azide as a selective oxidant

Abstract: In basic aqueous solution the N/sub 3/.radical is found to oxidize aromatic systems such as aniline and phenoxide ions and their derivatives at rate constants of (3-5) x 10/sup 9/ M/sup -1/s/sup -1/. In contrast to the reactions of OH radicals, where addition to the ring dominates, oxidation appears to be directly by electron transfer. Compounds such as benzene, toluene, and anisole are not observably oxidized by N/sub 3/. Phenol, in its neutral form in acidic solution, is oxidized several orders of magnitude more slowly than is the phenoxide anion. The rate of oxidation of phenols is strongly dependent on substitution, with activating groups increasing the rate in the order para > ortho > meta. N/sub 3/. can be readily prepared radiolytically by OH oxidation of azide. Being a neutral radical which does not absorb significantly above 300 nm it is very promising as a selective oxidant for pulse radiolysis studies. 27 references, 6 figures, 3 tables.

Rotational‐level‐dependent quenching of A 2Σ+ OH and OD

Abstract: The role of rotation in the collisional quenching of the A2Sigma(+) v' = 0 states of OH and OD is investigated experimentally by laser-induced-fluorescence measurements in a two-chamber differentially pumped flow chamber similar to that of McDermid and Laudenslager (1982) The experimental apparatus is described in detail, and the results are presented in extensive tables and graphs Collisional-quenching rate constants are determined for 21 different quenchers at various specific N' rotational levels and shown to decrease as the rotational quantum number increases, with no significant role for the internal levels of the colliding pairs and reasonable agreement with theoretical calculations incorporating attractive forces

Kinetics of the gas phase reaction of Cl atoms with a series of organics at 296±2 K and atmospheric pressure

Abstract: Using a relative rate technique, rate constants have been determined for the gas phase reactions of Cl atoms with a series of organics at 296 ± 2 K and atmospheric pressure of air. Using a rate constant of 1.97 × 10−10 cm3 molecule−1 s−1 for the reaction of Cl atoms with n-butane, the following rate constants (in units of 10−11 cm3 molecule−1 s−1) were obtained: ethane, 6.38 ± 0.18; propane, 13.4 ± 0.5; isobutane, 13.7 ± 0.2; n-pentane, 25.2 ± 1.2; isopentane, 20.3 ± 0.8; neopentane, 11.0 ± 0.3; n-hexane, 30.3 ± 0.6; cyclohexane, 31.1 ± 1.4; 2,3-dimethylbutane, 20.7 ± 0.6; n-heptane, 34.1 ± 1.2; acetylene, 6.28 ± 0.18; ethene, 10.6 ± 0.3; propene, 24.4 ± 0.8; benzene, 1.5 ± 0.9; and toluene, 5.89 ± 0.36. These data are compared and discussed with the available literature values.

Standard Gibbs energy of formation of the hydroxyl radical in aqueous solution. Rate constants for the reaction chlorite (ClO2-) + ozone .dblarw. ozone(1-) + chlorine dioxide

Abstract: Constantes de vitesse determinees par radiolyse pulsee. Energie libre standard de formation de OH en solution aqueuse

Mathematical models for potassium release kinetics in calcareous soils

Abstract: Potassium release from the coarse medium and fine silt and the coarse and medium-fine clay fractions of six Great Plain soils was determined by successive extraction with Ca-saturated cation exchange resins. All soils contained primarily montmorillonite-mica minerals. Results indicated that 65 to 80% of the total K released in 7000 h of extraction time occurred in the clay fraction. Four mathematical models (first-order rate, parabolic diffusion, power function, and Elovich) were used to describe cumulative K release. Comparisons of coefficients of determination (r) and standard errors of the estimate (SE) indicated that the Elovich, power function, and parabolic diffusion equations adequately described cumulative K release, whereas the first-order rate equation did not. Rate constants for the three equations were highly correlated with mica content and relative alfalfa yield and K uptake. In the past, others have used complex equations containing three simultaneous first-order rate terms to describe K release; however, results reported herein show that simple one-term equations can be used. Additional Index Words: nonexchangeable K release, resin extractable K, diffusion, kinetics, Elovich equation View complete article To view this complete article, insert Disc 4 then click button8

Electron affinity of SF6 and perfluoromethylcyclohexane. The unusual kinetics of electron transfer reactions A−+B=A+B−, where A=SF6 or perfluorinated cyclo‐alkanes

Abstract: Measurements of the equilibria (1): A−+B=A+B− with a pulsed electron high pressure mass spectrometer lead to ΔG01, ΔH01, and ΔS01. Equilibria involving SF6 as A and perfluoromethylcyclohexane C7F14 provide electron affinities E.A.(SF6)=1.05±0.1 eV and E.A.(C7F14)=1.06±0.15 eV. The kinetics of reactions (1) involving the above two compounds were studied. The rate constants kf for SF−6 +B=SF6+B− were found to increase with exothermicity of the reaction. The temperature dependence for kf was determined. B compounds leading to high exothermicities were associated with kf at the collision limit and essentially no temperature dependence. B of progressively lower electron affinity led to kf below the collision limit and negative temperature dependence, while B with the lowest E.A. but still leading to exothermic reaction produced lowest kf with positive temperature dependence. A model is provided explaining the above behavior. The known large change of geometry between SF−6 and SF6 introduces an internal barrier...

Chemical kinetics of the reaction of carbon dioxide with ethanolamines in nonaqueous solvents

Abstract: The kinetics of the reaction of carbon dioxide with mono- and diethanolamine in such nonaqueous solvents as methanol, ethanol, and 2-propanol and in water were studied using a stirred tank absorber with a plane gas-liquid interface at 303 K. The reaction was found to be of first order with respect to carbon dioxide for every solvent. The order of reaction with respect to ethanolamine was found to be unity only for an aqueous solution of monoethanolamine and for the other solutions, ranged from 1.4 to 2, depending on the solvent species. The reaction order was increased in the order of water, methanol, ethanol, and 2-propanol. The variation of the reaction order with the solvent species could be explained in terms of a reaction scheme via a zwitterion. Almost linear dependence of the logarithm of the reaction rate constant on the solubility parameter of the solvent was derived.

Investigation of the effect of metal ions on the reactivity of thiol groups in human 5-aminolaevulinate dehydratase.

Abstract: The reaction of human 5-aminolaevulinate dehydratase with 5,5'-dithiobis-(2-nitrobenzoic acid) (Nbs2) results in the release of 4 molar equivalents of 5-mercapto-2-nitrobenzoic acid (Nbs) per subunit. Two of the thiol groups reacted very rapidly (groups I and II), and their rate constants were determined by stopped-flow spectrophotometry; the other two thiol groups (groups III and IV) were observed by conventional spectroscopy. Titration of the enzyme with a 1 molar equivalent concentration of Nbs2 resulted in the release of 2 molar equivalents of Nbs and the concomitant formation of an intramolecular disulphide bond between groups I and II. Removal of zinc from the holoenzyme increased the reactivity of groups I and II without significantly affecting the rate of reaction of the other groups. The reactions of the thiol groups in both the holoenzyme and apoenzyme were little affected by the presence of Pb2+ ions at concentrations that strongly inhibit the enzyme, suggesting that Zn2+ and Pb2+ ions may have independent binding sites. Protein fluorescence studies with Pb2+ and Zn2+ have shown that the binding of both metal ions results in perturbation of the protein fluorescence.

Determination of the adsorption behaviour of ‘overpotential-deposited’ hydrogen-atom species in the cathodic hydrogen-evolution reaction by analysis of potential-relaxation transients

Abstract: Despite its major importance in electrocatalysis and mechanistic aspects of electrode processes, the experimental determination of the adsorption behaviour of kinetically involved intermediates, e.g. H in H2 evolution, in electrode reactions proceeding at appreciable Faradaic currents has hitherto remained little developed. A new method of analysis of the kinetics of the decay of overpotential on open-circuit, following interruption of a polarizing current, has been applied to the study of the adsorption behaviour of the ‘overpotential-deposited’(o.p.d.) H generated as an intermediate in the cathodic evolution of H2 at appreciable currents. Results are described for Ni and electrodeposited Ni–Mo–Cd composite cathode materials.The method enables, for the first time, the H coverage to be reliably determined as a function of potential, at appreciable overpotentials, and the corresponding adsorption pseudocapacitance behaviour to be directly and accurately evaluated for the o.p.d. species in a continuous Faradaic reaction. The results show that H coverage at the surfaces of these metals attains limiting values at overpotentials of –0.15 to –0.25 V, so that the Faradaic process of H2 evolution proceeds, at these metals, on a surface incompletely but appreciably covered by H atoms. By application of a steady-state analysis to a discharge and electrochemical-desorption mechanism this behaviour is explained theoretically and the ratios of rate constants required to account for the observed H-adsorption behaviour are evaluated. The Tafel slopes for the H2-evolution reaction at the metals studied are discussed in terms of the observed H-adsorption behaviour and steps in the reaction mechanism.

Simulation of the diffusion-controlled reaction between superoxide and superoxide dismutase. I. Simple models.

Abstract: A two-stage Brownian dynamics simulation method is used to study the diffusion-influenced bimolecular reaction between superoxide and superoxide dismutase (SOD). The crystal structure of the dimeric enzyme is used in constructing detailed topographical and electrostatic models. Several electrostatic models are considered. In the most realistic, the excluded volume of the protein, which is impermeable to penetration by mobile ions, is assigned a dielectric constant of 2 and the surrounding “solvent” is assigned a value of 78. A finite difference method is used to solve the linearized Poisson–Boltzmann equation. For native SOD, the simulations reproduce the pronounced salt dependence of the rate constant observed experimentally. This salt dependence is attributed to electrostatic interactions between enzyme and substrate that are inherently attractive and amplified by the low dielectric constant of the protein interior. The simulation method is also applied to a modified enzyme, acylated SOD.

‘‘Direct’’ calculation of quantum mechanical rate constants via path integral methods: Application to the reaction path Hamiltonian, with numerical test for the H+H2 reaction in 3D

Abstract: The method recently proposed by Miller, Schwartz, and Tromp for determining Boltzmann rate constants ‘‘directly’’—by the path integral evaluation of a reactive flux correlation function—is developed within the framework of the reaction path Hamiltonian model for a general polyatomic reaction. The expression for the correlation function, the time integral of which is the rate constant, is reduced to a single path integral over only one degree of freedom (the reaction coordinate). Effects of tunneling, ‘‘frictional’’ effects on the reaction coordinate due to coupling to other degrees of freedom, and the effects of recrossing the transition state dividing surface are all correctly accounted for in the approach. Numerical tests of the formulas for the 3D version of the H+H2 reaction (on the Porter–Karplus potential surface) gives excellent agreement with the (known) accurate results for this system.

Kinetics of the daunomycin--DNA interaction.

Abstract: The kinetics of the interaction of daunomycin with calf thymus DNA are described. Stopped-flow and temperature-jump relaxation methods, using absorption detection, were used to study the binding reaction. Three relaxation times were observed, all of which are concentration dependent, although the two slower relaxations approach constant values at high reactant concentrations. Relaxation times over a wide range of concentrations were gathered, and the data were fit by a minimal mechanism in which a rapid bimolecular association step is followed by two sequential isomerization steps. The six rate constants for this mechanism were extracted from our data by relaxation analysis. The values determined for the six rate constants may be combined to calculate an overall equilibrium constant that is in excellent agreement with that obtained by independent equilibrium measurements. Additional stopped-flow experiments, using first sodium dodecyl sulfate to dissociate bound drug and second pseudo-first-order conditions to study the fast bimolecular step, provide independent verification of three of the six rate constants. The temperature dependence of four of the six rate constants was measured, allowing estimates of the activation energy of some of the steps to be made. We speculate that the three steps in the proposed mechanism may correspond to a rapid "outside" binding of daunomycin to DNA, followed by intercalation of the drug, followed by either conformational adjustment of the drug or DNA binding site or redistribution of bound drug to preferred sites.

Kinetics of the isotope exchange reaction of 18O with NO and O2 at 298 K

Abstract: The rates of 18O isotope exchange reactions were measured at 298 K using a discharge‐flow, modulated molecular beam mass spectrometer apparatus whose detection limit for NO and O was in the 109–1010 cm−3 range. The NO exchange is very fast, k1, x=(3.7±0.5)×10−11 cm3 s−1, and, assuming statistical breakdown of a long‐lived complex, the rate constant for the formation of the NO°2 complex, k1, c f is (7.4±1.0)×10−11 cm3 s−1. The slower O2 exchange was measured three different ways, yielding three rate constants whose average is k2, x=(2.9±0.5)×10−12 cm3 s−1. The results are compared with earlier isotope exchange experiments and discussed in the context of measured and calculated high pressure recombination and vibrational relaxation rate constants.