Showing papers on "Elementary reaction published in 1974"

Gas‐phase reaction of Ba with N2O. II. Mechanism of reaction

Abstract: A reaction mechanism and a correlation diagram argument are presented to account for the high photon yield of the chemiluminescent reaction of Ba atoms with N2O. Most BaO molecules appear to be formed initially in the long‐lived a3π state and to be transferred subsequently by collisions into the shorter‐lived A1Σ+ state. This mechanism accounts for numerous kinetic and spectroscopic observations of BaO. The arguments suggest that reactions of alkaline earth atoms with N2O and with O3 may be suitable for electronic‐transition chemical lasers.

Rate constant of OH + OH = H2O + O from 1500 to 2000 K

Abstract: It was shown several years ago that concentration profiles of the OH radical in the shock‐initiated combustion of lean ([H2]/[O2] = 0.1) hydrogen‐oxygen‐argon mixtures at low pressures (≃ 30 kPa) and high temperatures (1200–2000 K) exhibit transient maxima prior to attainment of partial equilibrium. At that time, the maxima could not be accounted for quantitatively in terms of the accepted mechanism of the H2–O2 reaction. The profiles have been reanalyzed utilizing more sophisticated computational techniques and increased knowledge of the reaction mechanism. The occurrence of maxima at temperatures above 1500 K was found to depend upon the ratio of the rate constants of the elementary reactions O+H2→OH+H and OH+OH→H2O+O. Using the rate constant expression 1.6×1014 exp(−56.6 kJ/R T) cm3 mol−1 s−1 for the former reaction, the rate constant expression for the latter was found to be 5.5×1013 exp(−29 kJ/R T) cm3 mol−1 s−1. This latter expression does not extrapolate linearly on an Arrhenius plot to the availab...

Detailed balance in chemical kinetics as a consequence of microscopic reversibility

Abstract: The idea that the chemical principle of detailed balance is a consequence of microscopic reversibility is examined in terms of the stochastic theory of chemical reaction rates, particularly as developed by Snider. It is emphasized that detailed balance is a purely macroscopic requirement for all cases except those in which the chemical reaction mechanism contains a closed loop, or where forward and reverse rate constants for an elementary reaction step are measured in different experimental situations, each designed to isolate one unidirectional elementary reaction from its counterpart. Snider's theory is extended to a simpe triangular (closed‐loop) isomerization mechanism, and it is shown that detailed balance is not obtained as a result; this is in contrast to local‐equilibrium theories, for which the relationship between microscopic reversibility and detailed balance is well established. An attempt to overcome this difficulty by reformulating Snider's theory, in terms of the theory of lumping error, was only partly successful. The limitations of the present approach are discussed, and consequences for universally valid interpretations of phenomenological chemical kinetics are suggested.

The kinetics and mechanism of the slow oxidation of hydrogen

Abstract: A low-temperature matrix isolation method has been developed which makes possible the isolation and detection of HO2 radicalsfrom a gas-phase radical chain-reaction system containing atomic H, O and OH radicals as well. The formation of HO2 radicals is demonstrated in the rarified hydrogen flame. The same radicals are observed to exist during the slow oxidation reaction of hydrogen between the second and third inflammation limits The kinetics of accumulation of HO2, H2O2, and H2O has been studied as a function of total pressure, composition of reacting mixtures, temperature, and diameter and the treatmentof the surface of the reaction vessel. It has been established that in a reactor treated with boric acid, the concentration of HO2 radicalsis in the order of 101 2–101 4 particles/cm3. The kinetic relationship obtained confirms the necessity of including the following step in the mechanism of slow oxidation of hydrogen: For a quantitative description of the experimental results, calculationsof the complete mechanism of the slow reaction are performed by computer methods. The values of the rate constants of the elementary reactions that enter into the mechanism proposed were taken from published data. Some of them were varied by small intervals to give a better agreement with experimental values. The agreement justifies the mechanism proposed for the experimental conditions employed.

Frequency of Reaction Events in a System with Unimolecular Chemical Reaction

Abstract: Eine reversible monomolekulare Reaktion A ^ wird als ein Abzählprozeß betrachtet, der durch die Gesamtzahl von Reaktionsereignissen (Umwandlungen von Teilchen A in und in A) charakterisiert wird. Aus den Grundpostulaten des stochastischen Modells der Reaktion ist es möglich, die Wahrscheinlichkeitsverteilung, den Mittelwert und die Varianz der Zufallsgröße M(t) abzuleiten. Für eine große Teilchenzahl nähert sich die wahrscheinliche Verteilung der Reaktionsereignisse der normalen. Die Häufigkeit der Reaktionsereignisse im System (definiert als zeitliche Ableitung der mittleren Zahl von Ereignissen) ist gleich der Summe zweier Terme, deren Differenz die Änderung der mittleren Teilchenzahl von A mit der Zeit ergibt (Reaktionsgeschwindigkeit). Der Koeffizient der Variation der Zahl der Ereignisse an einem genügend weit vom Reaktionsbeginn entfernten Zeitpunkt ist umgekehrt proportional der Wurzel aus t und der Gesamtzahl von Teilchen im System.

Spectroscopic study on the elementary reactions of oxidative polymerization

Abstract: The catalytic cycle of the Cu-pyridine complex catalyzed polymerization of 2,6-xylenol was divided into four elementary processes: the coordination of the substrate, the electron transfer from the substrate to the cupric ion, the dissociation of the activated substrate, and the reoxidation of the catalyst. Rate constants of each elementary reaction, k1, ke, kd, and ko, were determined spectroscopically by the stopped-flow method. The approximate magnitudes of each rate constant were k1: 10 (1·mole−1·minp−1), ke :10−2 (min−1), and ko, 102 (min−1), respectively. Thus, it was considered that the ke or the kd process was the rate-determining step, but the latter could not be measured in this procedure. The rate constants were affected by the species of the solvent. With a few exceptions of the solvent which had coordinating ability, k1 and ke increased and ko decreased with the polarity of the solvent. The effects of the polymer-ligand on each elementary reaction were also studied in the polymerization catalyzed by the Cu-partially-quaternized poly(vinylpyridine) complex. k1 and ko were smaller and ke was larger than those of the monomeric analog. It was interpreted that the ke step was accelerated by the electrostatic field of the polymer-ligand.

Kinetics of Formation of SrWO4 in the Solid State Reaction Between WO3 and SrCO3

Abstract: The solid state reaction WO3 + SrCO3 → SrWO4 + CO2 was studied with the reactants in form both of separate pellets and of powder mixtures. As for pellets, by an application of Wagner’s method it was possible to state that the governing process is the cation counterdiffusion involving W6+ and Sr2+. Thermogravimetric measurements in the temperature range 663° - 755 °C on equimolar powder mixtures and with SrCO3 in a specially prepared spherulitic form were interpreted using Dünwald- Wagner’s equation

Effect of 5-ethylidene-2-norbornene on elementary reaction constants of copolymerization of ethylene and propylene in the presence of a VoCl3Al(C2H5)1·5Cl1·5 catalytic system☆

Abstract: Elementary constants were derived for copolymerization of ethylene, propylene, and ethylene, propylene and 5-ethylidene-2-norbornene (ENB) dissolved in heptane with VOCl3Al(C2H5)1·5Cl1·5 catalytic system at 20°. It was found that there is a difference between these values in binary and ternary copolymerization. Results confirm the participation of ENB in the formation of active centres.

銅錯体触媒による2,6-キシレノールの酸化重合素過程の分光学的解析

Abstract: The catalytic cycle of the polymerization of 2, 6-xylenol catalyzed by the copper-pyridine complex is divided into four elementary processes, the coordination of the substrate(1), the electrontransfer from the substrate te the cupric ion (k), the dissociation of the aetivated substrate (kd) and the reoxidation of the catalyst (ke). The rate constant of each elementary reaction was determined by means of the stoppedfiow spectrophotometric study of the copper complex, and the detailed mechanism of the catalysis was discussed. From the comparison of rate constants (Table 1), the process involning, either k. or kd was considered to be the ratedetermining step but the latter could not be measured in the present procedure. It was found that the oxygen molecule was involved in the cupric substrate omplex, promoting the k. process. The rate constants were affected by the components of the solvent. Except for a few cases where solvents having coordinating ability are used, k and ke increased and ke decreased with the increasing solvent polarity.

Automated Estimation of Kinetic Rate Parameters and Mechanisms of Gas Phase Chemical Reactions.

Abstract: : Automated computational techniques were developed and used to evaluate the specific kinetic properties of molecules and to define chemically reacting systems. The Arrhenius rate parameters were calculated for over 100 simple fission, atom transfer, and hot molecule reactions. These data have been used to model the thermal decomposition complex gas phase reactions of organic molecules. As many as 30 elementary reactions were used in the reaction simulation analyses. Concentration/rate time histories were computed for each species represented in the mechanism scheme. The effective rate coefficients and kinetic order of some of the reactions were also determined from the basic data. The data are interpreted and correlated in some detail with experimental findings. (Author)

Kinetics and mechanism on the aquation and anation reactions of monochloroacetatopentaammineruthenium(iii) complex in aqueous solution

Abstract: The aquation and anation reactions of monochloroacetatopentaammineruthenium(III) complex are studied to know the elementary reaction mechanism under the same reaction conditions. The aquation reaction seems to take the SN 2 mechanism, through the process which involves the seven-coordinated intermediate while the anation reaction is well illustrated by the limiting SN 1 mechanism. The elementary rate constants for the anation reaction was determined.