Showing papers on "Elementary reaction published in 1986"

Prediction of rate constants for combustion and pyrolysis reactions by bimolecular QRRK

Abstract: Bimolecular QRRK (Quantum Rice-Ramsperger-Kassel) analysis is a simple method for calculating rate constants of addition and recombination reactions, based on unimolecular quantum-RRK theory. Input parameters are readily derived, and rate constants and reaction branching can be predicted with remarkable accuracy. Such predictive power makes the method especially useful in developing mechanisms of elementary reactions. Furthermore, from the bimolecular QRRK equations, limiting forms of the rate constants in the limits of low and high pressure are developed. Addition/stabilization is pressure-dependent at low pressure but pressure-independent at high pressure, as is conventionally understood for simple decomposition, its reverse. In distinct contrast, addition with chemically activated decomposition has the opposite behavior: pressure independence at low pressure and pressure dependence [as (pressure)−1] at high pressure. The method is tested against data and illustrated by calculations for O + CO → CO2; for H + O2 → HO2 or O + OH; for H + C2H4 → C2H5 or C2H3 + H2; and for H + C2H3 → C2H4 or H2 + C2H2.

Microwave kinetic spectroscopy of reaction intermediates: O+ethylene reaction at low pressure

Abstract: A microwave spectroscopic method has been developed to study elementary reactions in real time through in situ observation of rotational spectra of reaction intermediates such as free radicals with lifetime as short as 1 ms. This method was applied to the O(3P)+ethylene reaction in order to assess the roles of (a) vinoxy+H and (b) CH3+CHO channels in the initial process. The reaction was initiated by irradiating an N2O/C2H4 mixture containing a trace amount of mercury with the 253.7 nm mercury resonance line, and the time evolution of vinoxy, HCO, and H2CO was followed by measuring their microwave absorption intensities as functions of time. The branching ratio was thus determined to be 0.4±0.1 and 0.5±0.1 for (a) and (b), respectively, at the sample pressure of 30 mTorr. The present result agrees with those obtained by Hunziker et al. [J. Photochem. 17, 377 (1981)] using much higher pressures of samples, but is not compatible with the observation of Buss et al. [J. Photochem. 17, 389 (1981)] that (a) is ...

A Detailed Mechanism for the High Temperature Oxidation of C2HCI3

Abstract: A detailed chemical kinetic mechanism describing the high temperature oxidation of C2HCI3 is presented. The mechanism involves the participation of 34 species in 73 reversible elementary reactions. The mechanism accounts for the experimental ignition delay times in shock tubes as well as species profiles for reactants, stable intermediates, and products in premixed flames.

Ketenyl radical yield of the elementary reaction of ethyne with atomic oxygen at 290-540 K

Abstract: The ketenyl radical yield of the elementary reaction between ethyne and atomic oxygen was determined at T = 287 and 535 K, as a pressure of 2 Torr (He). Use was made of the flow reactor technique, in combination with molecular beam mass spectrometry. First, a detailed investigation was made of the kinetics of formation and destruction of HCCO in C/sub 3/O/sub 2//H systems; by use of the absolute HCCO concentrations thus obtained, the sensitivity of the MBMS apparatus to HCCO could be determined. The HCCO yield of the elementary C/sub 2/H/sub 2/ + O reaction was then derived in C/sub 2/H/sub 2//O systems from the observed stationary HCCO concentration and from the known HCCO destruction rate, at a given total C/sub 2/H/sub 2/ + O reaction rate. In this way, the HCCO yield of the elementary C/sub 2/H/sub 2/ + O reaction with 2sigma error was found to be 59% +/- 20% at 298 K and 64% +/- 19% at 535 K.

Microscopic description of voltage effects on ion-driven cotransport systems.

Abstract: A microscopic model for the analysis of voltage effects on ion-driven cotransport systems is described. The model is based on the notion that the voltage dependence of a given rate constant is directly related to the amount of charge which is translocated in the corresponding reaction step. Charge translocation may result from the movement of an ion along the transport pathway, from the displacement of charged ligand groups of the ion-binding site, or from reorientation of polar residues of the protein in the course of a conformational transition. The voltage dependence of overall transport rate is described by a set of dimensionless coefficients reflecting the dielectric distances over which charge is displaced in the elementary reaction steps. The dielectric coefficients may be evaluated from the shape of the experimental flux-voltage curve if sufficient information on the rate constants of the reaction cycle is available. Examples of flux-voltage curves which are obtained by numerical simulation of the transport model are given for a number of limiting cases.

High Temperature Reactions of CH3 1. The Reaction CH3 + H2 → CH4 + H

Abstract: The rate coefficient of the reaction CH3 + H2 → CH4 + H was determined at high temperatures behind incident shock waves by time-resolved measurements of the absorption of the methyl radical at 216.5 nm. The resulting Arrhenius expression k3 = 1013.3±0.1 exp(−(7200 ± 400) K/T) cm3 mol−1 s−1 is in good agreement with earlier indirect measurements. Several methyl radical sources are compared.

Mass spectrometric investigation and computer modeling of the methane-oxygen-ozone reaction from 480 to 830 K

Abstract: The reaction of methane with ozonized oxygen was investigated in a molecular beam source reactor, consisting of a heated 1-mm-diameter alumina flow tube equipped with a 0.2-mm nozzle. Typical values for pressure and residence time were 600 mbar and 16 ms, respectively, and the temperature range covered was from 480 to 830 K. The gas mixture expanding from the reactor was transformed into a molecular beam and analyzed by a mass spectrometer. H/sub 2/O, CO, CH/sub 2/O, CH/sub 3/OH, H/sub 2/O/sub 2/, CO/sub 2/, and CH/sub 3/OOH were found as reaction products. The experimental results could be fairly well modeled by a reaction mechanisms consisting of 47 elementary reactions with 21 species. A kinetic sensitivity analysis and investigation of the reaction pathways yields the main mechanistic features of the reaction. The reaction is initiated by the thermal decomposition of ozone. Very important in the reaction are secondary reactions of ozone with methyl radicals and hydrogen atoms. Besides that, radical-radical reactions of methyl and methylperoxy radicals play a dominant role in the course of the reaction. 37 references, 5 figures, 2 tables.

High Temperature Photochemistry (HTP): Kinetics and mechanism studies of elementary combustion reactions over 300-1700 K

Abstract: The HTP technique makes it possible to study the kinetics and mechanisms of elementary reactions of important combustion-related free radicals over wide temperature ranges. It utilizes standard flash photolysis methods to generate the radicals and resonance fluorescence diagnostics to follow their time-dependent concentrations, from which kinetic and mechanistic information is derived. The use of a single technique to explore the kinetics of reactions over wide temperature ranges gives high confidence in the experimentally determined temperature dependences and provides a precise data base for both theoretical and empirical extrapolation of kinetic parameters to other temperatures of interest. Results obtained in HTP studies of the reactions O+CH3, H+H2O, OH+CH4, and OH+C6H6 are discussed.

A Direct Study of the Reaction CH2(X3B1) ± C2H4 in the Temperature Range 296 K ≤ T ≤ 728 K

Abstract: The reaction between CH2-radicals in the triplet electronic ground state X˜3B1 (3CH2) and C2H4 was studied in the gas phase at temperatures between 296 K ≤ T ≤ 728 K in an isothermal discharge flow system. CH2-radicals were generated either in the reaction O + CH2CO or by exciplex laser photolysis of CH2CO. Their concentration was monitored with a far infrared Laser Magnetic Resonance spectrometer. The experimental rate constant obtained from the pseudo first order decay of 3CH2 in the presence of a large excess of C2H4 was found to be Correcting for the small contribution to the depletion of 3CH2 due to collisional excitation to the singlet first excited state a1A1(1CH2) followed by consecutive reactions of 1CH2, the rate constant for the direct reaction of 3CH2 with C2H4 was obtained to be A reaction mechanism describing both the present data as well as the results of earlier related-studies in the CH2 + C2H4 system is proposed. The mechanism and product yields are discussed in terms of unimolecular rate theory.

Kinetic analysis of complex reactions

Abstract: TG and DSC techniques have been extensively used to study complex solid state reactions. For complex reactions (constituting of all exothermic or all endothermic one step first order individual reactions), it has been shown that the results of TG and DSC instruments may not be identical. This is because the TG instrument is incapable of correctly recording the true effective reaction rate of complex reactions (if the reaction rate is not proportional to the total amount of reactants). This may happen when the reaction rates of the individual reactions in the composite reaction mixture are significantly different. In this communication it has also been suggested that the Friedman analysis of obtaining activation energy (E) may be inapplicable for complex or composite reaction due to the fact that, there may be no unique effective constant conversion at various heating rates.

Thermal Decomposition of Cyanogen Measured in C2N2/O2 and C2N2/H2 Reaction Systems by Atomic Resonance Absorption

Abstract: The thermal decomposition of cyanogen has been measured behind reflected shock waves in the temperature range 1900–2650 K at pressures of about 18–20 bar using Atomic Resonance Absorption Spectroscopy (ARAS) C2N2/O2 and C2N2/H2 mixtures highly diluted in argon were shock heated and O and H atom concentrations were monitored in the post shock reaction zone Because of the fast secondary reactions of the dissociation product CN, the measured concentrations of O or H were strongly dependent on the rate of dissociation of cyanogen The experimentally determined rate coefficient data for the dissociation of cyanogen are closely fit by the Arrhenius expression: k = · 297 10−7 exp(−53665 K/T)cm3/s The present rate coefficient data is compared with the results of previous investigators and also with the estimates obtained from the weak collision unimolecular reaction rate theory

A kinetic study of bulk thermal polymerization of styrene

Abstract: A kinetic study was performed on the bulk thermal polymerization of styrene. The conversion and average molecular weights were measured. Rate equations were derived on the basis of a model proposed in a previous paper which introduced the gel effect into each elementary reaction by considering the decrease of segmental jump frequency during polymerization. The model could successfully simulate the conversion and the average molecular weight.

Investigation of radical reactions in C2Cl4 and C2Cl4/O2 discharges by EPR, mass, and emission spectroscopy

Abstract: The reaction of tetrachloroethylene, C2Cl4, with O(3P) atoms as well as the plasma decomposition of C2Cl4 and C2Cl4/O2 mixtures have been investigated by combined application of electron paramagnetic resonance (EPR) and emission and mass spectroscopies. C2Cl4 plasma decomposition is shown to proceed primarily to the formation of CCl3 radicals and chlorine-deficient products, which are ultimately involved in the formation of carbonaceous layers. A simple reaction model accounts for all the detected stable and radical species, encountered during the plasma decomposition. The model also enables order-of-magnitude estimates of decomposition rate constants to be made. The suppression of the formation of both carbonaceous layers and products CmCln (m⩾3) in C2Cl4/O2 discharges is explained using results of an investigation of elementary reactions in the system C2Cl4/O(3P)/O2. The stable products of C2Cl4/O2 discharges, i.e., COCl2, CCl4, and C2Cl6, respectively, are shown to originate from recombination of the peroxy radicals CCl3OO and C2Cl5OO.

Addition of 2,3-dimethylbutane to slowly reacting mixtures of hydrogen and oxygen at 480 °C. Rate constants of the elementary reactions involved

Abstract: The oxidation of 2,3-dimethylbutane (DMB) has been studied by adding traces of the alkane to slowly reacting H2–O2 mixtures in an aged boric acid coated vessel at 480 °C. Rate constants have been obtained for H and HO2 attack on DMB, and Arrhenius parameters of log10(A/dm3 mol–1 s–1)= 9.45 (per C—H bond) and E= 67 kJ mol–1 are suggested for HO2 attack at the tertiary C—H bond in DMB. Combination with data at room temperature gives log10(A22/dm3 mol–1 s–1)= 11.45 and E22= 30 kJ mol–1 for the overall reaction H + DMB → H2+ C6H13. (22)Methane, propene, 2,3-dimethylbut-1-ene, 2,3-dimethylbut-2-ene, 2-methylbut-2-ene, and the two O-heterocyclic compounds, 2,2,3-trimethyl-oxetane and tetramethyloxirane, are the major initial products. The proportions of the two species of dimethylbutyl radicals formed in each mixture used has been estimated by computer treatment in order to facilitate interpretation of the product distribution. Detailed mechanisms for the two radicals are discussed quantitatively. Rate constants are determined for the 1,4t H atom transfer involved in reaction (5A) and for the 1,5t transfer involved in reaction (5B). No other experimentally derived value is available for a 1,5t H atom transfer in alkylperoxy radicals (CH3)2C(O2)CH(CH3)2→(CH3)2C(O2H)C(CH3)2(5A), (CH3)2CHCH(CH3)CH2O2→(CH3)2CCH(CH3)CH2O2H. (5B)The role of reaction (X) in the mechanism of autoignition of alkanes is discussed R + O2→ conjugate alkene + HO2. (X)

High-Temperature Reaction of NH3–N2O System in Shock Waves

Abstract: The high-temperature reaction of NH3 with N2O in shock waves was investigated by measuring ultaviolet absoption of NH3 and OH, and infrared emission of N2O, in the temperature range of 1700–2300 K on the mixture of NH3–N2O diluted with large amount of argon. A comparison of the observed decays of NH3 and N2O and the formation of OH with those obtained by computer simulation behind the incident shock wave was carried out. From the comparison, the rate constants of the following elementary reactions were determined: NH3+O→NH2+OH[k4=1012.5exp(−25.5 kJ⁄RT) cm3 mol−1 s−1]; NH3+OH→NH2+H2O[k5=1012.5exp(−8.4 kJ⁄RT) cm3 mol−1 s−1].

Experimental Investigation and Computer Simulation of Products During the Induction Phase of Methane Oxidation from 1170 to 1460 K

Abstract: Abstract– A molecular beam source reactor is used to study the oxidation of methane at temperatures from 1170 to 1460 K, pressures above 1000 mbar and residence times of about 3 msec. The mixtures investigated range from lean to rich conditions. The products detected in the induction phase or the reaction up to the ignition point arc H2O, H2, CH2O, C2H6, CO, C2H4. CO2, C2H2 and C2H2O. A recent flame chemistry reaction model is extended by elementary reactions important for the induction phase and used to simulate the experimental data. The general agreement is good, although quantitative discrepancics remain. Analysis of the major reaction pathways shows that the reactions of the methyl radical in the induction phase up to high conversions are distinctly different from the ones in flames: CH3 is mostly oxidized by the HO2 radical, with minor contributions from the reactions with O2 and O.

Kinetics and Energetics of the Reaction ClO ± O2 (1Δg) → ClO3

Abstract: The rate coefficient for reaction between ClO (2II) and O2 (1Δg) has been measured using a flow/flash photolysis technique. An upper limit of k1 ≤ 3·10−15 cm3/s at 298 K was found. The formation of symmetrical ClO3 (2A1) as a product is spin and orbital symmetry allowed but is probably prevented by an activation barrier and by equilibrium constraints. The rate coefficient for quenching of O2(1Δ) by Cl2 was found to be 2·10−17 cm3/s at 298 K.

Lithium-induced reaction.

Abstract: References five years previously. She also suffered from mild acne in her teenage years. She had never taken lithium prior to this course. In early November 1985she was started on lithium car bonate (250mg t.d.s.). Physical examination, including thyroid and renal function tests, were normal. Within two weeks lithium was increased to 250mg q.i.d., and kept at this dose as her serum lithium levels were within accept able limits (0.7—0.9 mEq/L). Her mental state improved considerably over the next month—so much so that she was discharged from hospital following a nine-month admission. On the sixth week after commencement,she complained of sudden generalised hair loss. This was quite severe, but eased by the ninth week. Serum lithium levels remained within normal limits during this time. As the hair loss subsided, the patient noticed small acneiform eruptions over her neck and face. These became painful, enlarged and pusy, and this continued in varying degrees of severity until the thirteenth week—at which time the drug was discontinued. Within two weeks her skin con dition had almost completely recovered, but unfortunately herdepression had returned and readmission was necessary.

Quenching processes in electronically excited sulfur dioxide generated by chemical reaction

Abstract: Luminescence from SO/sub 2/ was studied as the molecule was formed in the steady-state reactions of ozone with methyl mercaptan and with dimethyl disulfide. The bulk of the electronically excited SO/sub 2/ was generated in the elementary reaction SO + O/sub 3/. A chemical timing method was used to analyze the quenching of SO/sub 2/*. Effects of increasing noble gas pressure on the fluorescence and phosphorescence were observed. By use of a presently accepted photophysical model for SO/sub 2/, the quenching of the fluorescence allowed the spectroscopically unresolvable long-lived and short-lived fluorescent singlet states to be distinguished. The /sup 3/B/sub 1/ state that yields the phosphorescence was not observed to be created in the elementary reaction of SO with O/sub 3/. It was also difficult to account for the formation of the /sup 3/B/sub 1/ state by a collisionally induced intersystem crossing from one of the singlet states. Instead, the participation of an unspecified intermediate state producing the /sup 3/B/sub 1/ state on collision is proposed. The photophysical effect and the participation of the unspecified state are suggested to be similar to those discussed by other workers.

Inhibitory features of the thermal oxidation of carbon monoxide. A kinetic foundation to dynamic instabilities in closed vessels

Abstract: Unusual experimental features of the oxidation of carbon monoxide in closed vessels, in which successive ‘ignitions’(non-periodic oscillations) result from a continuous increase of the vessel temperature, are revisited. An isothermal kinetic basis for the interpretation of these events, deduced by numerical methods, is presented. It provides a foundation from which (i) the incomplete spontaneous combustion of CO, discovered more than fifty years ago but not explained hitherto, and (ii) the isothermal oscillatory glows that accompany CO oxidation, may also be interpreted.The isothermal kinetic scheme, related specifically to CO + O2 to which some H2 has been added, comprises 21 elementary reactions. It contains the generally accepted minimum description of H2 oxidation in vessels in which surface termination of radicals is relatively inefficient. Three additional elementary reactions are the interactions of CO with O, OH and HO2. The strong inhibitory features of the ignition are traced to the intervention of the termination step: CO + O + M → CO2+ M coupled to a quadratic interaction that generates oxygen atoms: HO2+ H → H2O + O. The successive ignitions are explained very satisfactorily, but a quantitative discrepancy between the predicted and measured relative proportions of CO and H2 that are consumed in each of the successive ignitions cannot be rationalized solely on the basis of the homogeneous 21-step mechanism. It is suggested that additional surface interactions may play a part. The water-gas equilibrium is used to illustrate this.

Applications of Kinetic CARS-Spectroscopy to Elementary Reactions of Large Molecules in the Gas Phase: Direct k(E) Measurements of Cycloheptatriene-Isomerizations

Abstract: The unimolecular isomerization of rovibrationally highly excited cycloheptatriene (CHT) and its methyl and ethyl derivatives have been studied in the gas phase by means of coherent anti-Stokes Raman scattering (CARS) as detection method. Reactants were prepared in the electronic ground state with a well defined internal energy using single photon excitation at 266 nm, followed by a very fast internal conversion, Si -* S0. Pump and probe measurements at low pressures allowed to obtain specific rate constants k(E), among them the first directly measured value for the isomerization for CHT, fc(460 kJ mol-1) (2.7 + 0.6) 107s_1. The results agree well with the available indirectly and directly determined rate data and the calculations on the basis of unimolecular rate theories. CARS spectra at frequencies around 3000 cm-1 are reported for the CHT's, toluene, o-, m-, and p-xylene as well as the transient spectra observed during the isomerization reaction. In the kinetic work electronically resonance CARS signals were utilized of C2 radicals which were produced in the CARS laser probe pulse itself. The conditions of C2 formation are discussed also with respect to the usability of resonance CARS of fragments as a method for low pressure kinetic CARS studies of larger organic molecules.

The rate constant of a quantum-diffusion-controlled bimolecular reaction

Abstract: A quantum-mechanical equation is derived in the tight-bond approximation which describes the motion and chemical interaction of a pair of species A and B when their displacement in the matrix is caused by tunnelling. Within the framework of the discrete model of random walks, definitions are given of the probability and rate constant of a reaction A + B → P (products) proceeding in a condensed medium. A method is suggested for calculating the rate constant of a quantum-diffusion-controlled bimolecular reaction. By this method, an expression is obtained for the rate constant in the stationary spherically symmetrical case. An equation for the density matrix is also proposed which describes the motion and chemical interaction of a pair of species when the quantum and classical diffusion are competitive.

Shock‐tube Studies of N2O Decomposition and N2O ‐ H2 Reaction.

Abstract: N2O decomposition and N2O–H2 reaction were studied behind incident and reflected shock waves in the temperature range 1450–2200 K and pressure range 0.6–3.5 atm using both single-pulse and time-resolve techniques. A computer-simulation study was performed to determine the rate-constant expressions of important elementary reactions. Computer calculations showed that the values of the rate constant for N2O+O\xrightarrowk2N2+O2 and N2O+O\xrightarrowk2NO+NO (cited in current papers) are too low. The rate-constant expressions were found to be k2=7.0×1014exp(−28 kcal/RT) cm3mol−1 s−1 and k3=5.6×1014exp (−28 kcal/RT) cm3 mol−1 s−1. The rate constant expression for N2O+H\xrightarrowk2N2+OH, was found to be k4=1.5×1014exp (−15 kcal/RT) cm3 mol−1 s−1.