Showing papers on "Reaction rate constant published in 1989"

Hydrogen tunneling in enzyme reactions

Abstract: Primary and secondary protium-to-tritium (H/T) and deuterium-to-tritium (D/T) kinetic isotope effects for the catalytic oxidation of benzyl alcohol to benzaldehyde by yeast alcohol dehydrogenase (YADH) at 25 degrees Celsius have been determined. Previous studies showed that this reaction is nearly or fully rate limited by the hydrogen-transfer step. Semiclassical mass considerations that do not include tunneling effects would predict that kH/kT = (kD/kT)3.26, where kH, kD, and kT are the rate constants for the reaction of protium, deuterium, and tritium derivatives, respectively. Significant deviations from this relation have now been observed for both primary and especially secondary effects, such that experimental H/T ratios are much greater than those calculated from the above expression. These deviations also hold in the temperature range from 0 to 40 degrees Celsius. Such deviations were previously predicted to result from a reaction coordinate containing a significant contribution from hydrogen tunneling.

Ab initio reaction paths and direct dynamics calculations

Abstract: A detailed study of methods for generating the minimum energy path of a chemical reaction using ab initio electronic structure calculations is presented; the convergence with respect to step size of the geometry and energy along this path is studied with several algorithms. The investigations are extended to the calculation of chemical reaction rate coefficients by interfacing the polyrate code for variational transition-state theory and semiclassical tunneling calculations with a locally modified Gaussian 82 electronic structure package that now contains reaction path following capabilities at both the Hartree-Fock and perturbation theory levels. This combined package is used to study the kinetics of the abstraction reaction CH{sub 3} + H{sub 2} {yields} CH{sub 4} + H, which is considered as a prototype organic reaction. They report calculations of reaction rates based on electronic structure theory and generalized transition-state theory, including a multidimensional tunneling correction, without performing an analytic fit to the potential surface. The calculation of dynamical processes directly from ab initio electronic structure input without the intermediary of a potential surface fit is called direct dynamics, and this paper demonstrates the feasibility of this approach for bimolecular reactions.

Kinetics and mechanism of the reaction between carbon dioxide and amines in aqueous solution

Abstract: Rate constants, ΔH‡ and ΔS‡ have been measured by the conductimetric stopped-flow technique for the reaction of carbon dioxide in aqueous solution with the primary amines 2-methoxyethylamine, 2-aminoethanol, 3-aminopropan-1-ol, 2-aminopropan-2-ol, DL-aminopropan-2-ol and the secondary amines 1,1′-iminodipropan-2-ol, 2-amino-2-(hydroxymethyl) propane-1,3-diol, 2,2′-iminodiethanol, 2,2,6,6-tetramethylpiperidin-4-ol, and morpholine. The observed first-order rate constants fit the equation kobs=kAM[R2NH]2+kW[R2NH][H2O]. The much-quoted Danckwerts mechanism is shown to be unlikely.

Measurement of protein-DNA interaction parameters by electrophoresis mobility shift assay.

Abstract: Native gel electrophoresis (mobility shift) assays may be used to obtain quantitative information about the site distribution, equilibria and kinetics of protein-DNA interactions. These applications depend on the ability of the electrophoretic system to resolve the reaction components, and on their stabilities during the separation process Factors which affect the lifetimes and mobilities of protein-DNA complexes during electrophoresis include reaction and electrophoresis buffer composition, pH, and ionic strength; the presence of low molecular weight effectors and enzymatic substrates; the nature and concentration of the gel matrix; the temperature; the molecular weights of protein and DNA; the stoichiometric ratios of their complexes; and the possibility of conformational and configurational isomerization of reaction components. We discuss how these factors influence the acquisition of quantitative data from electrophoretic patterns and band intensities, and present formulas for the estimation of equilibrium constants and rate constants for prototypical DNA-protein interactions.

Rate constants for hydrogen abstraction reactions of the sulfate radical, SO4−. Alcohols

Abstract: Rate constants have been determined for the reactions of SO4− with a series of alcohols, including hydrated formaldehyde. The SO4− radical was produced by the laser-flash photolysis of persulfate, S2O82−. Rate constants for the reactions of SO4− with alcohols range from 1.0 × 107 for methanol to 3.4 × 108 M−1 s−1 for 1-octanol. Rate constants for the reactions of SO4− with deuterated methanol and ethanol are lower by about a factor of 2.5. For methanol, ethanol, and 2-propanol, the temperature dependence of the rate constant was determined over the range 10–45°C.

Homogeneous hydrolysis rate constants for selected chlorinated methanes, ethanes, ethenes, and propanes

Abstract: Hydrolysis rate constants of 18 chlorinated methanes, ethanes, ethenes, and propanes were measured in dilute aqueous solutions within the temperature range of 0 to 180 C and at pH values of 3 to 14. Arrhenius parameters were determined for both neutral and alkaline hydrolysis reactions. Reactivity of these compounds in basic solutions increases in accord with the expected acidity of the most reactive hydrogen atom in the molecule. Neutral hydrolysis appears to depend on both the C-Cl bond strength and the degree of steric hindrance at the reaction site. Only a neutral hydrolysis process occurs for CCl{sub 4}, 1,1,1-trichloroethane, and 2,2-dichloro-propane. The chlorinated ethenes and hexachloroethane react only with hydroxide under severe conditions and exhibit no neutral hydrolysis. Some of these compounds eliminate HCl, whereas others substitute OH for Cl to form alcohols that may react further to give aldehydes or carboxylic acids as products. Environmental hydrolysis half-lives (25 C, pH 7) range from 36 h for 2,2-dichloropropane to 1850 years for CHCl{sub 3} and to more than 1,000,000 years for C{sub 2}Cl{sub 6} and the ethenes.

Dispersed polaron simulations of electron transfer in photosynthetic reaction centers

Abstract: A microscopic method for simulating quantum mechanical, nuclear tunneling effects in biological electron transfer reactions is presented and applied to several electron transfer steps in photosynthetic bacterial reaction centers. In this "dispersed polaron" method the fluctuations of the protein and the electron carriers are projected as effective normal modes onto an appropriate reaction coordinate and used to evaluate the quantum mechanical rate constant. The simulations, based on the crystallographic structure of the reaction center from Rhodopseudomonas viridis, focus on electron transfer from a bacteriopheophytin to a quinone and the subsequent back-reaction. The rates of both of these reactions are almost independent of temperature or even increase with decreasing temperature. The simulations reproduce this unusual temperature dependence in a qualitative way, without the use of adjustable parameters for the protein's Franck-Condon factors. The observed dependence of the back-reaction on the free energy of the reaction also is reproduced, including the special behavior in the "inverted region."

Reorganization free energy for electron transfers at liquid-liquid and dielectric semiconductor-liquid interfaces

Abstract: The reorganization free energy is calculated for a reaction (i) between two reactants, each in its own dielectric medium, separated by an interface, and (ii) between a reactant and some semiconductors. An expression is also given for the rate constant of an electron-transfer reaction at an interface between reactants in two immiscible phases. Under certain conditions it is shown that the reorganization energy for the two immiscible-liquid system is the sum of the electrochemical reorganization energies of the two reactants, each in its own respective solvent. The reorganization energy for a semiconductor-liquid system can differ considerably from the corresponding metal-liquid value, even a factor of 2.

Monomer-addition growth with a slow initiation step: A growth model for silica particles from alkoxides

Abstract: A simplified monomer-addition model with a first-order activation step is developed to describe the dynamics of growth of silica particles from alkoxides. In the fimit of slow hydrolysis, we obtain expressions for the evolution of the particle mass and particle polydispersity, as well as an expression for the particle size as a function of the hydrolysis rate constant, the polymerization rate constant, and the initial concentration of the orthosilicate. We find that the formation of the particles is adequately modeled by a reaction limited growth. © 1989 Academic Press, Inc.

Oxidation Studies of Crystalline CVD Silicon Nitride

Abstract: Oxidation studies of CVD were performed in dry oxygen, oxygen‐argon, and oxygen‐nitrogen‐argon gas mixtures of various oxygen and nitrogen partial pressures at a total pressure of 1 atm at 1100°–1400°C. Parallel oxidation studies of single‐crystal silicon were also conducted for direct comparison. It was observed that the oxidation of both and Si followed parabolic growth kinetics with activation energies of about 115 kcal/mol and about 30 kcal/mol, respectively. The formation of a single layer of and evolution of could not account for the much lower parabolic rate constants and much higher activation energy for than for Si during the oxidation. Detailed characterization of the oxidation scales using ellipsometry, step‐by‐step etching, SIMS, and XPS techniques indicated that a duplex oxidation scale consisting of and was formed when was oxidized. The intermediate scale was identified as a single‐phase material, not a physical mixture of and . The low oxidation rate and high activation energy for during the oxidation were attributed to the formation of and low oxygen diffusivity in this structurally dense phase.

Kinetics and mechanism of hydroxyl radical reaction with methyl hydroperoxide

Abstract: The reaction of hydroxyl radical with methyl hydroperoxide, CH{sub 3}OOH, was investigated in the temperature range 203-423 K by pulsed photolytic generation of OH and detection by laser-induced fluorescence. The rate coefficient for the overall reaction, OH + CH{sub 3}OOH {yields} products (k{sub 1}) was measured by using {sup 18}OH and OD in place of OH. The rate coefficient for the CH{sub 3}O{sub 2} production channel OH + CH{sub 3}OOH {yields} CH{sub 3}O{sub 2} + H{sub 2}O (k{sub 1a}) was obtained by using OH. The channel that yields CH{sub 2}OOH, OH + CH{sub 3}OOH {yields} CH{sub 2}OOH + H{sub 2}O (k{sub 1b}), is not observed when monitoring OH since CH{sub 2}OOH rapidly falls apart to give back OH (and CH{sub 2}O) but is observed when studying the {sup 18}OH or OD reaction with CH{sub 3}OOH. By monitoring OH production in OD + CH{sub 3}OOH reaction at 249 K, the two-channel mechanism was confirmed, and the values for k{sub 1} and k{sub 1a} were also determined. Both reaction 1 and channel 1a show negative activation energies, with k{sub 1} = (2.93 {plus minus} 0.30) {times} 10{sup {minus}12} exp((190 {plus minus} 14)/T) cm{sup 3} molecule{sup {minus}1} s{sup {minus}1} (average of {sup 18}OHmore » and OD studies) and k{sub 1a} = (1.78 {plus minus} 0.25) {times} 10{sup {minus}12} exp((220 {plus minus} 21)/T) cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}, where the indicated error is 1 {sigma}, including estimated systematic errors and {sigma}{sub A} = A{sigma}{sub lnA}. The rate coefficient for the reaction of OD with CH{sub 3}OOD is at least a factor of 2 smaller than that for reaction 1a. The thermal decomposition lifetime for CH{sub 2}OOH to give OH + CH{sub 2}O is deduced to be shorter than 20 {mu}s at 205 K. The mechanism of reaction 1 and the implications of our kinetic and mechanistic results to Earth's atmospheric chemistry are discussed.« less

Fourier transform infrared kinetic studies of the reaction of HONO with HNO3, NO3 and N2O5 at 295 K

Abstract: The kinetics of the reaction of nitrous acid (HONO) with nitric acid (HNO3), nitrate radicals (NO3) and dinitrogen pentoxide (N2O5) have been studied using Fourier transform infrared spectroscopy. Experiments were performed at 700 torr total pressure using synthetic air or argon as diluents. From the observed decay of HONO in the presence of HNO3 a rate constant of k<7×10-19 cm3 molecule-1 s-1 was derived for the reaction of HONO with HNO3. From the observed decay of HONO in the presence of mixtures of N2O5 and NO2 we have also derived upper limits for the rate constants of the reactions of HONO with NO3 and N2O5 of 2×10-15 and 7×10-19 cm3 molecule-1 s-1, respectively. These results are discussed with respect to previous studies and to the atmospheric chemistry of HONO.

Advanced oxidation processes. Test of a kinetic model for the oxidation of organic compounds with ozone and hydrogen peroxide in a semibatch reactor

Abstract: Experimental data are presented to test a kinetic model of the OE/H{sub 2}O{sub 2} process in a semibatch reactor. The effect of bicarbonate and carbonate ions is measured and found to be in concurrence with model predictions. The effect of pH in the ozone mass-transfer-limited region was examined in bicarbonate-spiked distilled water. Since the reaction is mass transfer limited, the primary effect above pH 7 is the result of changes in the distribution of inorganic carbon species which are OH-radical scavengers. Below pH 7, there is a lag period during which ozone and peroxide increase until the chain reaction begins. The effects of chloride ion and the concentration of radical scavengers other than carbonate species in ground waters are also measured. The mass-transfer/reaction rate model has been used to estimate rate constants for the reaction of hydroxyl radicals with trichloroethylene, 1,2-dibromoethane, 1,2-dibromo-3-chloropropane, carbon tetrachloride, and two bicyclic alcohols, 2-methylisoborneol and geosmin. While the model developed for the distilled water system was successful in predicting the rate of tetrachloroethylene (PCE) oxidation and the concentration of residual ozone and peroxide in regions I and III, respectively, there are several features of the model that remain unresolved when the matrix is changed tomore » a real surface or ground water. This and subsequent papers will investigate these effects.« less

Reaction rate analysis of complex kinetic systems

Abstract: Using the elementary sensitivity densities, a reaction rate sensitivity gradient is obtained which is the derivative of the rate of species concentration change with respect to the rate coefficient. The method is used to analyse the mechanism of high-temperature formaldehyde oxidation and high-temperature propane pyrolysis

How rapid are the internal reactions of the ubiquinol:cytochrome c2 oxidoreductase?

Abstract: The temperature dependence of the partial reactions leading to turn-over of the UQH2:cyt c2 oxidoreductase of Rhodobacter sphaeroides have been studied. The redox properties of the cytochrome components show a weak temperature dependence over the range 280–330 K, with coefficients of about 1 m V per degree; our results suggest that the other components show similar dependencies, so that no significant change in the gradient of standard free-energy between components occurs over this temperature range. The rates of the reactions of the high potential chain (the Rieske iron sulfur center, cytochromes c1 and c2, reaction center primary donor) show a weak temperature dependence, indicating an activation energy < 8 kJ per mole for electron transfer in this chain. The oxidation of ubiquinol at the Qz-site of the complex showed a strong temperature dependence, with an activation energy of about 32 kJ mole−1. The electron transfer from cytochrome b-566 to cytochrome b-561 was not rate determining at any temperature, and did not contribute to the energy barrier. The activation energy of 32 kJ mole−1 for quinol oxidation was the same for all states of the quinone pool (fully oxidized, partially reduced, or fully reduced before the flash). We suggest that the activation barrier is in the reaction by which ubiquinol at the catalytic site is oxidized to semiquinone. The most economical scheme for this reaction would have the semiquinone intermediate at the energy level indicated by the activation barrier. We discuss the plausibility of this simple model, and the values for rate constants, stability constant, the redox potentials of the intermediate couples, and the binding constant for the semiquinone, which are pertinent to the mechanism of the ubiquinol oxidizing site.

Reorganization free energy for electron transfers at liquid-liquid and dielectric semiconductor-liquid interfaces

Abstract: The reorganization free energy is calculated for a reaction (i) between two reactants, each in its own dielectric medium, separated by an interface, and (ii) between a reactant and some semiconductors. An expression is also given for the rate constant of an electron-transfer reaction at an interface between reactants in two immiscible phases. Under certain conditions it is shown that the reorganization energy for the two immiscible-liquid system is the sum of the electrochemical reorganization energies of the two reactants, each in its own respective solvent. The reorganization energy for a semiconductor-liquid system can differ considerably from the corresponding metal-liquid value, even a factor of 2.

Deprotonation of tertiary amine cation radicals. A direct experimental approach

Abstract: Le sel radical cationique, p-An 2 NCH 3 . +AsF 6 − (An=anisyl) a ete prepare par oxydation de l'amine correspondante avec l'hexafluoroarseniate de dioxygenyle. Quatre mecanismes possibles sont envisages pour la reaction de ce radical cationique avec la quinuclidine

Thermal decomposition of propyne and allene in shock waves

Abstract: Propyne (p-C3H4) or allene (a-C3H4) mixtures, highly diluted with Ar, were heated to the temperature range 1200–1570 K at pressures of 1.7–2.6 atm behind reflected shock waves. The thermal decompositions of propyne and allene were studied by both measuring the profiles of the IR emission at 3.48 μm or 5.18 μm and analyzing the concentrations of reacted gas mixtures. The mechanism and the rate constant expressions were discussed from both the profiles and the concentrations of reactant and products obtained. The rate constant expressions for reactions, (1) p-C3H4 → a-C3H4, (−1) a-C3H4 → p-C3H4, and (5) p-C3H4 + H → CH3 + C2H2 were evaluated.