Showing papers on "Reaction rate constant published in 1997"
TL;DR: In this article, the P2W15Nb3O629 polyoxoanion and Bu4N+- stabilized Ir∼190-450 (hereafter, Ir(0)∼300) nanoclusters are presented.
Abstract: Following an overview of the primitive state of mechanistic studies of the formation of nanoclusters, with a focus on LaMer's classic work on the formation of sulfur sols, kinetic and mechanistic studies of the formation of our recently reported novel P2W15Nb3O629- polyoxoanion- and Bu4N+- stabilized Ir∼190-450 (hereafter, Ir(0)∼300) nanoclusters are presented. The work reported consists of the full experimental and other details of the following eight major components: (i) development of an indirectbut easy, continuous, highly quantitative and thus powerfulmethod to monitor the formation of the Ir(0) nanoclusters via their catalytic hydrogenation activity and through the concept of pseudoelementary reaction steps; (ii) application of the appropriate kinetic equations for nucleation and autocatalysis, and then demonstration that these equations fit the observed, sigmoidal-shaped kinetic curves quantitatively with resultant rate constants k1 and k2; (iii) confirmation by a more direct, GLC method that the...
790 citations
TL;DR: The catalytic oxidation of carbon monoxide on a platinum surface was studied by scanning tunneling microscopy and the kinetic parameters were obtained, and their values agree well with data from macroscopic measurements.
Abstract: The catalytic oxidation of carbon monoxide (CO) on a platinum (111) surface was studied by scanning tunneling microscopy The adsorbed oxygen atoms and CO molecules were imaged with atomic resolution, and their reactions to carbon dioxide (CO 2 ) were monitored as functions of time The results allowed the formulation of a rate law that takes the distribution of the reactants in separate domains into account From temperature-dependent measurements, the kinetic parameters were obtained Their values agree well with data from macroscopic measurements In this way, a kinetic description of a chemical reaction was achieved that is based solely on the statistics of the underlying atomic processes
358 citations
TL;DR: Sorptive uptake of lignin and tannin from an aqueous phase by activated charcoal was investigated in the laboratory and the sorption reaction was found to be of a first order.
327 citations
TL;DR: It is demonstrated that peroxynitrite decomposes with significant rates to NO2− plus O2 at physiological pH, which may contribute to the physiology and/or cytotoxicity of NO and superoxide.
322 citations
TL;DR: The formation of the cyclic product accounts for the indefinite lag when N,N-dimethyltyramine is used as the substrate for tyrosinase in the absence of a dihydric phenol cofactor, and it is shown that the indoliumolate product is formed by cyclization of theortho-quinone, presented by pulse radiolysis studies.
317 citations
TL;DR: In this article, the authors evaluated the literature data concerning the rates of these reactions and proposed predictive schemes allowing the calculation of rate constants for these alkoxy radical reactions for atmospheric purposes.
Abstract: Alkoxy and β-hydroxyalkoxy radicals are key intermediates formed in the atmospheric degradations of alkanes and alkenes, respectively. In the troposphere, these alkoxy radicals can decompose, isomerize, and react with O2. The literature data concerning the rates of these reactions are evaluated, and predictive schemes allowing the calculation of rate constants for these alkoxy radical reactions for atmospheric purposes are proposed. Good agreement between calculated reaction rates and experimental data concerning the absolute and relative importance of these reaction pathways is obtained, and alkoxy and β-hydroxyalkoxy radical reaction rates for radicals for which experimental data are not presently available can now be calculated for use in atmospheric modeling. © 1997 John Wiley & Sons, Inc.
312 citations
TL;DR: In this paper, the reduction of nitrate to ammonia occurs with nearly complete conversion at room temperature and pressure under aerobic conditions in the presence of iron and either HCl or a pH buffer.
309 citations
TL;DR: In this paper, the chemistry of N2O5 on liquid NaCl aerosols or bulk NaCl solutions was studied at 291 K by aerosol smog chamber and wetted-wall flow tube experiments.
Abstract: The chemistry of N2O5 on liquid NaCl aerosols or bulk NaCl solutions was studied at 291 K by aerosol smog chamber and wetted-wall flow tube experiments. The uptake of N2O5 on deliquescent aerosol was obtained to be (3.2±0.2)×10−2 (1σ error) from the aerosol experiments. In the wetted-wall flow tube we observed that nitryl chloride (ClNO2) is the main product of the reaction at NaCl concentrations larger than approximately 0.5 M and almost the only product at concentrations larger than 1 M. The ClNO2 yield does not depend linearly on the NaCl concentration, especially at small sodium chloride concentrations (i.e., smaller than 1 M). It appeared that a simple mechanism where N2O5 undergoes two reaction channels (hydrolysis and reaction with Cl−) is unable to explain the observed concentration dependence of the product yield. We propose that N2O5 dissociates to NO2+ and NO3− (rate constant kl>104 s−1) mainly. The directly hydrolysis of N2O5 (k3[H2O]) is less than 20% of the total reaction. NO2+ reacts with water to form 2H+ and NO3− (k5) or with Cl− to form ClNO2 (k4). Neglecting the influence of ionic strength we evaluate k4/k5 to be 836±32 (1σ error). Using the wetted-wall flow tube technique, we studied the uptake of nitryl chloride by aqueous solutions containing NaCl. We observed that the uptake coefficient γ decreased from (4.84±0.13)×10−6 on pure water to (0.27±0.02)×10−6 on a 4.6 M NaCl solution. The sharp decrease of γ with increasing salt concentrations is evidence of reversible hydrolysis. ClNO2 dissociates to Cl− + NO2+(k6). In the absence of Cl− we evaluate H ⋅ k61/2 to be 0.44±0.01 mol L−1 atm−1 s−1/2. Finally, we discuss that atomic Cl from photolysis of ClNO2 may play a role in the marine boundary layer at high northern latitudes.
298 citations
TL;DR: Kinetic analysis revealed that biological *NO concentrations (up to 2 microM) will significantly influence peroxidation reactions in vivo, and steady state concentrations of 30 nM *NO would effectively compete with endogenous alpha-tocopherol concentrations as a scavenger of LOO* in the lipid phase.
Abstract: The reaction between nitric oxide (*NO) and lipid peroxyl radicals (LOO*) has been proposed to account for the potent inhibitory properties of *NO toward lipid peroxidation processes; however, the mechanisms of this reaction, including kinetic parameters and nature of termination products, have not been defined. Here, the reaction between linoleate peroxyl radicals and *NO was examined using 2, 2'-azobis(2-amidinopropane) hydrochloride-dependent oxidation of linoleate. Addition of *NO (0.5-20 microM) to peroxidizing lipid led to cessation of oxygen uptake, which resumed at original rates when all *NO had been consumed. At high *NO concentrations (>3 microM), the time of inhibition (Tinh) of chain propagation became increasingly dependent on oxygen concentration, due to the competing reaction of oxygen with *NO. Kinetic analysis revealed that a simple radical-radical termination reaction (*NO:ROO* = 1:1) does not account for the inhibition of lipid oxidation by *NO, and at least two molecules of *NO are consumed per termination reaction. A mechanism is proposed whereby *NO first reacts with LOO* (k = 2 x 10(9) M-1 s-1) to form LOONO. Following decomposition of LOONO to LO* and *NO2, a second *NO is consumed via reaction with LO*, with the composite rate constant for this reaction being k = 7 x 10(4) M-1 s-1. At equal concentrations, greater inhibition of oxidation was observed with *NO than with alpha-tocopherol. Since *NO reacts with LOO* at an almost diffusion-limited rate, steady state concentrations of 30 nM *NO would effectively compete with endogenous alpha-tocopherol concentrations (about 20 microM) as a scavenger of LOO* in the lipid phase. This indicates that biological *NO concentrations (up to 2 microM) will significantly influence peroxidation reactions in vivo.
270 citations
TL;DR: This work has investigated the potential of successfully refolding reduced and denatured hen egg white lysozyme at high concentrations (1 and 5 mg/mL) and found optimum conditions which kinetically favor proper folding over inactivation were found.
Abstract: Newly synthesized cloned gene proteins expressed in bacteria frequently accumulate in insoluble aggregates or inclusion bodies. Active protein can be recovered by solubilization of inclusion bodies followed by renaturation of the solubilized (unfolded) protein. The recovery of active protein is highly dependent on the renaturation conditions chosen. The renaturation process is generally conducted at low protein concentrations (0.01-0.2 mg/mL) to avoid aggregation. We have investigated the potential of successfully refolding reduced and denatured hen egg white lysozyme at high concentrations (1 and 5 mg/mL). By varying the composition of the renaturation media, optimum conditions which kinetically favor proper folding over inactivation were found. Solubilizing agents such as guanidinium chloride (GdmCl) and folding aids such as L-arginine present in low concentrations during refolding effectively enhanced renaturation yields by suppressing aggregation resulting in reactivation yields as high as 95%. Quantitatively the kinetic competition between lysozyme folding and aggregation can be described using first-order kinetics for the renaturation reaction and third-order kinetics for the overall aggregation pathway. The rate constants for both reactions have been found to be strongly dependent on denaturant and thiol concentration. This strategy supercedes the necessity to reactivate proteins at low concentrations using large renaturation volumes. The marked increase in volumetric productivity makes this a viable option for recovering biologically active protein efficiently and in high yield in vitro from proteins produced as inclusion bodies within microbial cells.
236 citations
TL;DR: In this article, the ability of aqueous micelles and similar association colloids to control reaction rates is treated in terms of pseudophase models in which micells and water are regarded as distinct reaction media, with consideration of reactant concentrations and rate constants in the two regions.
TL;DR: In this paper, a new equation has been deduced from which a direct determination of the standard rate constant from linear sweep voltammograms can be made, and some questions related with the detection limit of irreversibility by means of the linear voltammetric method are also discussed.
Abstract: A new equation has been deduced from which a direct determination of the standard rate constant from linear sweep voltammograms can be made. Some questions related with the detection limit of irreversibility by means of the linear voltammetric method are also discussed.
TL;DR: In this paper, the authors determined the rate constant of hydrogen peroxide decomposition in supercritical water at pressures ranging from 5.0 to 34.0 MPa and for temperatures up to 450°C.
Abstract: This work determines the rate constant of hydrogen peroxide decomposition in supercritical water. Experiments were conducted at pressures ranging from 5.0 to 34.0 MPa and for temperatures up to 450°C. The rate of the homogeneous decomposition reaction is distinguished from the rate of the catalytic decomposition on the reactor surface by conducting experiments at different surface-to-volume ratios. The rate constant of H2O2 decomposition follows the expression k(s−1) = 1013.4 ± 1.2 exp[(− 180 ± 16 kJ/mol)/RT], providing a higher value than that derived from RRKM calculations. Comparison between experiments at supercritical conditions (P = 24.5 and 34.0 MPa) and experiments in high-pressure steam (P = 5.0 and 10.0 MPa) show that the efficiency of the catalytic surface reactions decreases significantly under supercritical conditions. The rate constant just mentioned was incorporated into a methanol oxidation model and the new prediction is compared with the experimental data of Rice et al. (1996), showing a distinct improvement compared to the original prediction.
TL;DR: In this article, the effect of reactant composition on the reaction rate was specifically studied by changing the PH20/PCO20 ratio between 9.0 and 0.3, and it was found that different reagents become rate limiting depending upon pressure.
Abstract: The kinetics of the reverse water-gas shift (RWGS) reaction over CuO/ZnO/Al2O3 catalysts was studied by use of CO2H2 cycles, hydrogen chemisorption and catalytic tests performed in both differential and integral plug flow reactors. The effect of the reactant composition on the reaction rate was specifically studied by changing the PH20/PCO20 ratio between 9.0 and 0.3. It was found that different reagents become rate limiting depending upon pressure. While in a H2-rich region the rate increases strongly with CO2 partial pressure and is zero order in hydrogen, under low PH20/PCO20 ratios the reaction is less active and is strongly positive order in hydrogen and low order in carbon dioxide. The experimental data were modeled by considering that the reaction proceeds through a surface redox mechanism, copper being the active metal. A good agreement between experimental and calculated data was obtained by assuming that in the redox mechanism either the dissociative CO2 adsorption (H2-rich region) or both the CO2 dissociation and the water formation (H2-lean region) determine the rate of the overall reaction. Based on previous studies performed on copper crystal surfaces, such a change in kinetics may be explained by assuming that under H2-rich atmosphere a surface structural or phase transition occurs involving a change in reactivity with respect to CO2 dissociation.
TL;DR: In this paper, the data were treated as a pseudo first-order reaction for the pyrolysis of polypropylene, and several previously published interpretation methods were applied to the data, but the wide temperature range used in this work (45-580 °C) encompassed a change in the decomposition mechanism and this greatly limited the utility of the methods.
TL;DR: This work exploited the distinct spectroscopic properties of myeloperoxidase and its oxidized intermediates, compounds I and II, to establish two successive one-electron oxidations of TMB, and applies the results to a new, more accurate assay based upon the formation of the charge-transfer complex between TMB and its diimine final product.
Abstract: Earlier investigations of the oxidation of 3,5,3',5'-tetramethylbenzidine (TMB) using horseradish peroxidase and prostaglandin H-synthase have shown the formation of a cation free radical of TMB in equilibrium with a charge-transfer complex, consistent with either a two- or a one-electron initial oxidation. In this work, we exploited the distinct spectroscopic properties of myeloperoxidase and its oxidized intermediates, compounds I and II, to establish two successive one-electron oxidations of TMB. By employing stopped-flow techniques under transient-state and steady-state conditions, we also determined the rate constants for the elementary steps of the myeloperoxidase-catalyzed oxidation of TMB at pH 5.4 and 20 degrees C. The second-order rate constant for compound I formation from the reaction of native enzyme with H2O2 is 2.6 x 10(7) M-1 s-1. Compound I undergoes a one-electron reduction to compound II in the presence of TMB, and the rate constant for this reaction was determined to be (3.6 +/- 0.1) x 10(6) M-1 s-1. The spectral scans show that compound II accumulates in the steady state. The rate constant for compound II reduction to native enzyme by TMB obtained under steady-state conditions is (9.4 +/- 0.6) x 10(5) M-1 s-1. The results are applied to a new, more accurate assay for myeloperoxidase based upon the formation of the charge-transfer complex between TMB and its diimine final product.
TL;DR: In this paper, the authors investigated the reaction of precipitated silica with this organosilane and the influence of various parameters on the reaction type and the reaction kinetics was considered, and the results of the investigation obtained using 29Si-CP/MAS solid state NMR spectroscopy agree well with a horizontal reaction model.
Abstract: Silica in combination with organosilanes (e.g. [bis(3-triethoxysilylpropyl)tetrasulfane] = TESPT) has recently become more important in tire applications. Their use in tire treads leads to an improvement in rolling resistance and wet traction. The requirements for the attainment of these properties are, that the triethoxysilyl groups of TESPT react with the silanol groups on the silica surface during compounding, and the polymer active groups react with the polymer during cure. The reaction of precipitated silica with this silane was investigated. The influence of various parameters on the reaction type and the reaction kinetics was considered. The results of the investigation obtained using 29Si-CP/MAS solid state NMR spectroscopy agree well with a horizontal reaction model in which a single siloxane bond is first formed with the silica surface (primary reaction). It is followed by condensation reactions between silanol groups of silane molecules which are already bound to the silica surface (se...
TL;DR: In this article, an investigation of the treatability of a commonly used azo dye in the textile dyeing and finishing operations, Reactive Black 5, by an advanced oxidation process, to achieve an acceptable degree of destruction of the target pollutant was performed.
Abstract: Treatment of textile dyeing wastewaters by conventional methods have recently proved to be inadequate. Such methods consist of various combinations of biological, physical and chemical methods including coagulation/flocculation and carbon adsorption. The study described herein is an investigation of the treatability of a commonly used azo dye in the textile dyeing and finishing operations, Reactive Black 5, by an advanced oxidation process, to achieve an acceptable degree of destruction of the target pollutant. The research was focused on a UV/H2O2 oxidation process, where the individual effects of H2O2 concentration, initial dye concentration, and alkalinity under neutral pH on the pseudo-first order reaction rate constant were investigated. Relative removal rates of color with respect to chemical oxygen demand and total organic carbon in dye-distilled water mixture were compared; effect of bicarbonate alkalinity at neutral pH was tested. Pseudo-first order reaction rate constants and electrical energy r...
TL;DR: In this paper, the authors used a Langmuir competitive adsorption model to fit the data and showed that the effect of inhibition by adsorbed cations should be greater at higher temperature (> 50°C, due to the positive value of the adsorbing enthalpy of cation adsorptions on oxide surfaces.
TL;DR: In this article, the half-order kinetics in the partial pressure of O 2 (P O 2 ) were obtained for the soot-O 2 reaction, which was complicated in the temperature dependence of CO 2 and N 2 formation as well as in the reaction order with respect to P NO.
Abstract: Kinetics of soot-O 2 , soot-NO and soot-O 2 -NO reactions over CuFe 2 O 4 were investigated by using temperature programmed reaction (TPR) technique in which a soot-catalyst mixture was loaded in a reactor and exposed to a fluent stream containing gaseous reactant(s) under a constant heating rate. Within the temperature range where a substantial amount of the charged soot remained and the reaction can be regarded to be the zero order in the amount of soot, the kinetic analysis of non-steady TPR results was possible. The half-order kinetics in the partial pressure of O 2 ( P O 2 ) was obtained for the soot-O 2 reaction. The soot-NO reaction was complicated in the temperature dependence of CO 2 and N 2 formation as well as in the reaction order with respect to P NO ; the reaction order was first at lower temperatures and increased with increasing temperature. In the soot-O 2 -NO reaction, i.e. the so-called simultaneous soot-NO x removal, the rate of CO 2 formation and those of both N 2 and N 2 O formation depended on partial pressures as P 0.6 NO P 0.6 O 2 and P 1.0 NO P 0.4 O 2 , respectively. Based on the kinetic results, possible reaction mechanisms were discussed.
TL;DR: In this paper, a model based on penetration theory has been developed to calculate the dynamic absorption rate of sulfur dioxide into a droplet of limestone slurry, which includes both instantaneous equilibrium reactions and reactions with finite rates; limestone dissolution, sulfite oxidation, gypsum crystallization and the hydrolysis reaction of CO2.
TL;DR: In this paper, the first-order rate coefficient for the SO3 loss by gas-phase reaction shows no significant pressure dependence and can be expressed as kI(s-1) = 3.90 × 10-41 exp(6830.6/T)[H2O]2 where H2O is in units of molecule cm-3 and T is in Kelvin.
Abstract: The gas-phase reaction of SO3 with H2O and the heterogeneous reaction of SO3 with H2O−H2SO4 surfaces have been studied in a fast flow reactor coupled to a chemical ionization mass spectrometer (CIMS) for species detection. The gas-phase reaction was studied under turbulent flow conditions over the pressure range from 100 to 760 Torr N2 and the temperature range from 283 to 370 K. The loss rate of SO3 was measured under pseudo-first-order conditions; it exhibits a second-order dependence on water vapor concentration and has a strong negative temperature dependence. The first-order rate coefficient for the SO3 loss by gas-phase reaction shows no significant pressure dependence and can be expressed as kI(s-1) = 3.90 × 10-41 exp(6830.6/T)[H2O]2 where [H2O] is in units of molecule cm-3 and T is in Kelvin. The overall uncertainty of our experimentally determined rate coefficients is estimated to be ±20%. At sufficiently low SO3 concentrations (<1012 molecule cm-3) the rate coefficient is independent of the init...
TL;DR: These data, together with analysis of end products by high-performance liquid chromatography, show that the major fluorescent product formed by radiation-induced or chemical hydroxylation of 3-CCA is 7-OHCCA.
Abstract: Coumarin-3-carboxylic acid (3-CCA) was used as a detector for hydroxyl radicals (.OH) in aqueous solution. The .OH was generated by gamma irradiation or chemically by the Cu2+-mediated oxidation of ascorbic acid (ASC). The excitation and emission spectra of 3-CCA, hydroxylated either chemically or by gamma irradiation, were nearly identical to those of an authentic 7-hydroxycoumarin-3-carboxylic acid (7-OHCCA). The pH-titration curves for the fluorescence at 450 nm (excitation at 395 nm) of 3-CCA, hydroxylated either chemically or by gamma radiation, were also identical to those of authentic 7-OHCCA (pK = 7.4). Time-resolved measurements of the fluorescence decays of radiation- or chemically hydroxylated 3-CCA, as well as those of 7-OHCCA, indicate a monoexponential fit. The fluorescence lifetime for the product of 3-CCA hydroxylation was identical to that of 7-OHCCA (approximately 4 ns). These data, together with analysis of end products by high-performance liquid chromatography, show that the major fluorescent product formed by radiation-induced or chemical hydroxylation of 3-CCA is 7-OHCCA. Fluorescence detection of 3-CCA hydroxylation allows real-time measurement of the kinetics of .OH generation. The kinetics of 3-CCA hydroxylation by gamma radiation is linear, although the kinetics of 3-CCA hydroxylation by the Cu2+-ASC reaction shows a sigmoid shape. The initial (slow) step of 3-CCA hydroxylation is sensitive to Cu2+, but the steeper (fast) step is sensitive to ASC. Analysis of the kinetics of 3-CCA hydroxylation shows a diffusion-controlled reaction with a rate constant 5.0 +/- 1.0 x 10(9) M(-1) s(-1). The scavenging of .OH by 3-CCA was approximately 14% for chemical generation with Cu2+-ASC and approximately 50% for gamma-radiation-produced .OH. The yield of 7-OHCCA under the same radiation conditions was approximately 4.4% and increased linearly with radiation dose. The 3-CCA method of detection of .OH is quantitative, sensitive, specific and therefore accurate. It has an excellent potential for use in biological systems.
TL;DR: In this paper, a detailed quasiclassical trajectory study of the O2+N→NO+O reaction is performed based on ab initio potential-energy surfaces of the 2A′ and 4A′ states.
Abstract: A detailed quasiclassical trajectory study of the O2+N→NO+O reaction is performed based on ab initio potential-energy surfaces of the 2A′ and 4A′ states. The study is aimed at generating a database of thermally averaged and O2 state-specific rate constants needed for accurate simulations of NO kinetics in high-temperature flow processes. The rate constants obtained show good agreement with the available experimental data and with other quasiclassical trajectory calculations. It is found that the reactant internal energy of the O2+N→NO+O reaction is less effective in enhancing the rate than in the N2+O→NO+N reaction. An analysis of the product vibrational energy shows that NO formed by the O2+N→NO+O reaction has a non-Boltzmann distribution. It is also found that the most populated NO vibrational level is determined by the reactant vibrational energy, while the terminal slope of the NO vibrational distribution is a strong function of the reactant translational temperature.
TL;DR: In this article, a poly(aniline) and poly(vinylsulfonate) composite-coated glassy carbon electrodes were found to give stable and reproducible electrocatlytic responses to NAD(P)H in citrate-phosphate buffer at pH 7.
Abstract: Poly(aniline)–poly(vinylsulfonate) composite-coated glassy carbon electrodes are found to give stable and reproducible electrocatlytic responses to NAD(P)H in citrate–phosphate buffer at pH 7. Analysis of these amperometric responses as a function of NAD(P)H concentration, film thickness, electrode potential and rotation speed show that the reaction occurs within the film, rather than just at the outside, and that the reaction is reversibly inhibited by the oxidation product [NA(P)D
+
]. Based on a full kinetic analysis, rate constants for the various processes are determined.
TL;DR: In this article, a chemiluminescent probe, luminol, was successfully applied to monitoring superoxide ions (O2•-) and hydrogen peroxide (H2O2) produced on photocatalytic reaction in aqueous TiO2 suspension.
Abstract: A chemiluminescent probe, luminol, was successfully applied to monitoring superoxide ions (O2•-) and hydrogen peroxide (H2O2) produced on photocatalytic reaction in aqueous TiO2 suspension. Two chemiluminescent reactions were distinguished from the decay profile after the end of the irradiation, and the reaction mechanism was analyzed. The fast decay component gives information about O2•- and the slow one provides the amount of H2O2. The rate constant for the reaction of O2•- with luminol was found to be 1 × 104 M-1 s-1. The amount of O2•- during the irradiation on TiO2 in alkaline solution could be estimated to be on the order of 10-13 M. Detection of H2O2 in concentrations as small as 10-9 M was demonstrated in the photocatalytic water oxidation.
TL;DR: In this paper, the second-order rate constants of HOBr reduction by H2O2 were determined and it was shown that nucleophilic attack of HO2- on HOBr must be the dominant reaction in the system.
TL;DR: In this paper, the effects of positive ame stretch on the laminar burning velocities of CO/H2/air mixtures were studied both experimentally and computationally for outwardly propagating spherical ames having concentrations of hydrogen in the fuel mixture of 3 − 50% by volume, fuel-equivalence ratios of 0.6 − 5.0, and pressures at 0.5 − 4.0 atm.
Abstract: Effects of positive ame stretch on the laminar burning velocities of CO/H2/air mixtures were studied both experimentally and computationally for outwardly propagating spherical laminar premixed ames having concentrations of hydrogen in the fuel mixture of 3 – 50% by volume, fuel-equivalence ratios of 0.6 – 5.0, and pressures of 0.5 – 4.0 atm. Both measured and predicted ratios of unstretched to stretched laminar burning velocities varied linearly with Karlovitz numbers, yielding constant Markstein numbers for each reactant mixture and pressure. Effects of stretch on laminar burning velocities were modest at low hydrogen concentrations, but approached earlier results for hydrogen/air ames as hydrogen concentrations increased. Predicted and measured ame properties were in reasonably good agreement using several contemporary chemical reaction mechanisms.
TL;DR: In this paper, an isotopic scrambling reaction was studied in a high-pressure flow reactor and the measured rate constants were (2.3 ± 1.0) × 10-13 exp[−(2100 ± 250)/T] cm3 molecule-1.
Abstract: Gas-phase hydrogen (H) abstractions from molecules by free radicals have been studied extensively. They form the simplest class of elementary reactions and also play a key role in atmospheric chemistry and so are the centerpiece of models of reactivity. Despite intense scrutiny, two fundamental mechanistic issues remain unresolved: (1) Do H abstractions proceed directly or indirectly? (2) Do thermodynamic or electronic interactions determine their reaction barrier? The thermoneutral identity reaction, OH + H2O → H2O + OH, provides an excellent opportunity to answer these questions. Several theoretically predicted H2O−HO complexes raise the possibility of an indirect mechanism, while no thermodynamic forcing influences the reaction barrier. To examine the various reactivity models, the isotopic scrambling reactions 18OH + H216O → H218O + 16OH and 16OD + H216O → H16OD + 16OH are studied in a high-pressure flow reactor. The measured rate constants are (2.3 ± 1.0) × 10-13 exp[−(2100 ± 250)/T] cm3 molecule-1 ...
TL;DR: In this article, the authors investigated the kinetics of the inclusion of ionene-6,10 (3b) dibromide and of its monomeric model, 1,10-bis(trimethylammonium)decane (5) diiodide, by α-cyclodextrin (1a) in an aqueous solution.
Abstract: The kinetics of the inclusion of ionene-6,10 (3b) dibromide and of its monomeric model, 1,10-bis(trimethylammonium)decane (5) diiodide, by α-cyclodextrin (1a) were investigated by 1H NMR spectroscopy in an aqueous solution. The inclusion of the monomer 5 is unusually slow and shows a high activation energy, Ea = 63 kJ mol-1, which was attributed to a high steric hindrance for the threading caused by the terminal trimethylammonium groups. Microcalorimetric titration of 5 with 1a revealed a stability constant, KS = 1540 M-1, of the inclusion compound. Because of its high kinetic stability, 5·1a was termed a rotaxane. The inclusion of the ionene polymer by α-cyclodextrin takes days, months, or even years depending on the temperature. The kinetics could be described quantitatively by a Monte Carlo type of computer simulation of a consecutive hopping process of the rings along the polymer chain. The rate constants and activation energies for the polymer 3b were found to be similar to those for the monomer 5. A...