Showing papers on "Reaction rate published in 1985"

Effect of friction on electron transfer in biomolecules

Abstract: In biological and chemical electron transfer, a nuclear reaction coordinate is coupled to other nuclear and/or ‘‘solvent’’ coordinates. This coupling, or friction, if strong enough, may substantially slow down motion along the reaction coordinate, and thus vitiate the assumption of electron transfer being nonadiabatic with respect to the nuclei. Here, a simple, fully quantum mechanical model for electron transfer using a one mode treatment which incorporates this coupling is studied. Path integral methods are used to study the dependence of the reaction rate on friction, and the limits of the moderate and the high friction are analyzed in detail. The first limit will prevail if the reaction coordinate is, e.g., an underdamped nuclear vibration, whereas the second limit will prevail if it corresponds to a slow or diffusive degree of freedom. In the high‐friction limit, the reaction rate is explicitly shown to vary between the nonadiabatic and adiabatic expressions as the tunneling matrix element and/or the friction are varied. Starting from a path integral expression for the time evolution of the reduced density matrix for the electron and reaction coordinate, a Fokker–Planck equation is obtained which reduces in the high‐friction limit to a Smoluchowski equation similar to one solved by Zusman.

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...

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.

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.

The oxidation of graphite powder in flame reaction zones

Abstract: The rate of burning of small concentrations of fine graphite powder has been measured in a flat, laminar weak methane-air flame at sub-atmospheric pressure. Measurements were obtained with a laser doppler system, that not only measured particle velocities, but also particle concentrations and size distributions through the flame. Particle temperatures were measured by the two colour method and gas temperatures with thermocouples. An appreciable increase in the oxidation rate was observed in the flame reaction zone, attributable to the reaction zone transient species, O, H, and OH. A computational study of the flame, with comprehensive chemical kinetics and detailed representation of the transport fluxes, yielded the concentrations of all species through the flame. The observed rates of graphite oxidation are kinetically explained in terms of rates of reaction of all species with graphite. The observed elevations of particle above gas temperature in the reaction zone are higher than would be expected if the heating were due only to reactions in which the carbon surface was attacked. Amongst other possible explanations there is that of heating by exothermic radical recombination on the carbon surface.

High temperature reaction rate for H+O2=OH+O and OH+H2=H2O+H

Abstract: Shock heating together with atomic resonance absorption spectrometry (ARAS) was used to record simultaneously H- and O-atom concentration profiles in the post-shock region behind reflected shocks. The dissociation of N2O together with the reaction O + H2 = OH + H was used as a source of H and OH for the reactions H + O2 = OH + O and OH + H2 = H2O + H. The test gas mixtures consisted of argon with relative concentrations of a few ppm N2O and 100 to 500 ppm H2 and O2. The experiments were conducted in the temperature range of 1700 to 2500 K at total densities of 6 · 10−6 to 1.3 · 10−5 mol cm−3. The following rate coefficients were deduced: For temperatures below 2500 K nearly complete agreement with the value for the rate coefficient of reaction R1 as recommended by Baulch [1] was obtained. For reaction R2 a rate coefficient was deduced which is close to the value as given by Gardiner et al. [23].

An Update of and Suggested Increase in Calculated Radiative Association Rate Coefficients.

Abstract: A sizable number of radiative association reaction rate coefficients involving polyatomic molecular ions have now been calculated and utilized in ion-molecule models of the chemistry of dense interstellar clouds. Recent evidence suggests that these calculated rate coefficients may be an order of magnitude too low in many cases. The evidence is reviewed and developed and calculated rate coefficients are updated. In addition, the temperature dependence of each reaction rate coefficient in the 10--50 K range is estimated if this information has not previously appeared in the literature.

Variational transition state theory calculations of the reaction rates of F with H2, D2, and HD and the intermolecular and intramolecular kinetic isotope effects

Abstract: We use variational transition state theory to calculate rate constants and kinetic isotope effects for the reactions F+H2→HF+H (with rate constant k1), F+D2→DF+D(k2), and two other isotopic analogs as functions of temperature. The calculations are performed using a recently proposed partly empirical, partly ab initio potential energy surface, called surface No. 5, and also using a new surface, called surface No. 5A, introduced here to test the effect of a higher classical saddle point on the reaction rates, kinetic isotope effects, and reaction thresholds. The various theoretical results are compared to the available experiments to test the validity of these potential energy surfaces. For those rate constants and kinetic isotope effects for which there is more than one experimental value at a given temperature, the theoretical results for reactions on surface No. 5 agree with experiment about as well as the individual experiments agree with each other. At T>373 K where there is only one experimental measu...

Mechanism of the silane decomposition. I. Silane loss kinetics and rate inhibition by hydrogen. II. Modeling of the silane decomposition (all stages of reaction)

Abstract: Part I: Kinetic data for the static system silane pyrolysis (from 640–703 K, 60–400 torr) are presented. For conversion from 3–30%, first-order kinetics are obtained, with silane loss rates equal to half the hydrogen formation rates. At conversions greater than 40%, rate inhibition attributable to the back reaction of hydrogen with silylene occurs. Overall reaction rates are not surface sensitive, but disilane and trisilane yield maxima under some conditions are. A nonchain mechanism capable of describing quantitatively all stages of the silane pyrolysis is proposed. Post 1.0% initiation is both homogeneous (gas phase) and heterogeneous (on the walls), and reaction intermediates are silylenes and disilenes. Free radicals are not involved at any stage of the reaction. Rate data at high conversions and with added hydrogen provide kinetics for the addition of silylene to hydrogen [reaction (−1)1] relative to its addition to silane [reaction (2)]: k−1,/k2 = 10−0.65 × e−3200 cal/RT. With E2 = 1300 cal, this gives a high pressure activation energy for silylene insertion into hydrogen of E−1 = 8200 cal. Part II: An analysis is made of each rate constant of the silane mechanism and the modeling results are compared with experimental results. Agreement is excellent. It is concluded that the dominant sink reaction for silylene intermediates is 1,2—H2 elimination from disilane (followed by Si2H4 polymerization and wall deposition). The model is in accord with slow isomerization between disilene and silylsilylene and near exclusive 1,2—H2 elimination from Si2H6. It is also concluded that disilene is about 10 kcal/mol more stable than silylsilylene and that the activation energy for isomerization of silylsilylene to disilene is greater than 26 kcal/mol.

Termolecular reaction of nitrogen monoxide and oxygen: A still unsolved problem

Abstract: The oxidation of nitrogen monoxide has been studied extensively between 226 and 758 K at pressures of NO and O2 ranging from about 0.2 to 30 torr. It has been shown that (i) the reaction is properly first order against oxygen and second order against nitrogen monoxide, as well under initial conditions as during the course of the reaction; (ii) the termolecular rate constant, k, first decreases with increasing temperature and reaches a minimum value at 600 K; (iii) the transition state theory is unable to describe this behavior correctly, (iv) under the present experimental conditions k can be represented either by (Formula Presented.) or by (Formula Presented.) The latter equation is compatabile with a multiple‐step mechanism. Copyright © 1985 John Wiley & Sons, Inc.

A continuous stirred tank reactor investigation of the gas‐phase reaction of hydroxyl radicals and toluene

Abstract: A continuous stirred tank reactor (CSTR) was used to study the gas-phase reaction between HO⋅ and toluene. HO⋅ was generated by the in situ photolysis of nitrous acid. Flow reactor operation at steady-state conditions with a residence time of 20 min allowed investigation of primary and very rapid secondary reactions. CSTR and batch reactor experiments were also performed with selected products. Both gas-phase and aerosol products were identified by chromatography and mass spectroscopy, with total product yields between 55 and 75% of reacted carbon. Toluene reaction products included cresols, nitrocresols, nitrotoluenes, 3,5-dinitrotouluene, benzaldehyde, benzyl nitrate, nitrophenols, methyl-p-benzoquinone, glyoxal, methylglyoxal, formaldehyde, methyl nitrate, PAN, and CO. The fraction of HO⋅ methyl hydrogen abstraction was calculated to be 0.13 ± 0.04. The ratio of reaction rate constants for nitrotoluene versus cresol formation from the HO⋅-adduct was calculated to be about 3.3 × 104. Also, the ratio of cresol formation versus O2 addition to the HO⋅-adduct was estimated to be ≥0.5 for atmospheric conditions. Comparisons of these measurements with previous values and the implications with respect to photochemical kinetics modeling of the atmosphere are discussed.

Mechanistic details of the heterogeneous decomposition of ammonia on platinum

Abstract: Absolute reaction rates have been measured for the catalytic decomposition of NH/sub 3/ and ND/sub 3/ and for the NH/sub 3/ + D/sub 2/ exchange reaction over a polycrystalline platinum wire at pressures between 5 x 10/sup -7/ and 0.5 torr and temperatures between 400 and 1200 K in a continuous flow microreactor. At relatively low pressures and/or high temperatures, a primary isotope effect was observed for the decomposition of ND/sub 3/. Under these conditions, the order of the decomposition reaction is unity with respect to ammonia pressure with an apparent activation energy of 4.2 kcal/mol. As coverages increase, corresponding to relatively high pressures and/or low temperatures, the order of the decomposition reaction is zero with respect to ammonia, and the reaction rate becomes controlled by nitrogen desorption, with an activation energy of the decomposition reaction of 22 kcal/mol. The kinetics of the NH/sub 3/ + D/sub 2/ exchange reaction have been used, together with data concerning the adsorption-desorption parameters of NH/sub 3/, H/sub 2/, and N/sub 2/ as well as the reaction intermediates NH and NH/sub 2/, to develop a mechanistic model which describes the reaction rate over a wide range of experimental conditions. 35 references, 6 figures.

Reaction of ethane with deuterium over platinum(111) single-crystal surfaces

Abstract: Deuterium exchange and hydrogenolysis of ethane were studied over (111) platinum surfaces under atmospheric pressures and a temperature range of 475-625 K. Activation energies of 19 kcal/mol for exchange and 34 kcal/mol for hydrogenolysis were obtained. The exchange reaction rates displayed kinetic orders with respect to deuterium and ethane partial pressures of -0.55 and 1.2, respectively. The exchange production distribution was U-shaped, peaking at one and six deuterium atoms per ethane molecule, similar to results reported for other forms of platinum, e.g., supported, films, and foils. The pressure of ethylidyne moieties on the surface was inferred from low-energy electron diffraction and thermal desorption spectroscopy. A mechanism is proposed to explain the experimental results, in which ethylidyne constitutes an intermediate in one of two competitive pathways. 31 references, 9 figures, 3 tables.

STS-8 atomic oxygen effects experiment

Abstract: A flight experiment was performed on the eighth Space Shuttle mission to measure reaction of surfaces with atomic oxygen in the low earth orbital environment. More than 300 individual samples were exposed to ram (normal to surface) conditions for 41.75 hr leading to a total atomic oxygen fluence of 3.5 x 10 to the 20th atoms/sq cm. Reaction rates for surface recession measured primarily by mass change of several organic films were in the range of 3.0 x 10 to the -24th cu cm/atom, and less than 5 x 10 to the -26th cu cm/atom for Teflon. Effects of parameters such as temperature and solar radiation were assessed, as was the importance of atmospheric ionic species on surface recession. In an experiment performed on the fifth Space Shuttle flight, no temperature dependence of reaction rate for the organic films studied was found in the temperature range of 25 to 125 C. Preliminary findings indicate that the reactivity of organic films is not affected by temperature (in the range of 65 to 125 C), solar radiation, or ionic species. Significant surface morphology changes led to a carpet-like appearance also consistent with previous findings.

Effect of intraparticle mass transfer resistance on reactivity of immobilized yeast cells

Abstract: Effect of diffusion resistance on reaction by using an immobilized yeast entrapped by Ca-alginate gel was studied. Intraparticle effective diffusivity of substrate, De, depends upon cell density, cc, i.e. De/Do = k2(1 - k1cc)2. Here, D0 is a reference diffusivity, e.g. diffusivity in water; k1 and k2 are constants. Overall reaction rates of ethanol production by immobilized resting-yeast were measured. Experimental rates coincide well with calculated results using the effective diffusivity and reaction rate of free cells. Diffusion seemed to be restricted by cells. Thus large cell density does hot necessarily mean high reactivity. Finally, the curvature of the Lineweaver-Burk plot is pointed out.