Showing papers on "Elementary reaction published in 1991"

THERM: Thermodynamic property estimation for gas phase radicals and molecules

Abstract: We have developed a computer code for an IBM PC/XT/AT or compatible which can be used to estimate, edit, or enter thermodynamic property data for gas phase radicals and molecules using Benson's group additivity method. The computer code is called THERM (THermo Estimation for Radicals and Molecules). All group contributions considered for a species are recorded and thermodynamic properties are generated in old NASA polynomial format for compatibility with the CHEMKIN reaction modeling code. In addition, listings are created in a format more convenient for thermodynamic, kinetic, and equilibrium calculations. Polynomial coefficients are valid from 300–5000 K using extrapolation methods based upon the harmonic oscillator model, an exponential function, or the Wilhoit polynomials. Properties for radical and biradical species are calculated by applying bond dissociation increments to a stable parent molecule to reflect loss of H atom. THERM contains a chemical reaction interpreter to calculate thermodynamic property changes for chemical reactions as functions of temperature. These include equilibrium constant, heat release (required heat, ΔHr), entropy change (ΔSr), Gibbs free energy change (ΔGr), and the ratio of forward to reverse Arrhenius A-factors (for elementary reactions). This interpreter can also process CHEMKIN input files. A recalculation procedure is incorporated for rapid updating of a database of chemical species to reflect changes in estimated bond dissociation energies, heats of formation, or other group values. All input and output files are in ASCII so that they can be easily edited, expanded, or updated.

Chemical Kinetic Data Base for Propellant Combustion I. Reactions Involving NO, NO2, HNO, HNO2, HCN and N2O

Abstract: This publication contains evaluated chemical kinetic data on a number of single step elementary reactions involving small polyatomic molecules which are of importance in propellant combustion. The work involves the collection and evaluation of mechanistic and rate information and the use of various methods for the extrapolation and estimation of rate data where information does not exist. The conditions covered range from 500–2500 K and 1017–1022 particles/cm3. The results of the first years effort lead to coverage of all pertinent reactions of the following species; H, H2, H2O, O, OH, OCHO, CHO, CO, NO, NO2, HNO, HNO2, HCN, and N2O.

Comparison between Experimental Measurements and Numerical Calculations of the Structure of Heptane-Air Diffusion Flames

Abstract: Detailed numerical calculations are performed to determine the structure of heptane-air diffusion flames, and the results are compared with experimental measurements. The configuration used is the diffusion flame stabilized in the vicinity of a stagnation plane, which is formed by directing an oxidi2ing gas flow onto the vaporizing surface of a pool of heptane. Profiles of the concentration of various stable species and of the temperature have been measured by gas chromatography and by thermocouples. respectively. To evaluate the influence of strain on the structure or the flame, the measurements taken at a fixed composition of the oxidizer stream and at two values of the strain rate were chosen for comparison with the calculated results. The computations were performed using a chemical kinetic mechanism consisting of forty-two elementary reactions involving eighteen species. To simplify the chemical kinetic mechanism, it was assumed that heptane is attacked by radicals to form the heptyl radical...

A numerical study of silane combustion

Abstract: A kinetic model of the mechanism of silane combustion has been developed, using a system of 70 elementary reaction steps and 25 chemical species. The model was used to examine silane ignition at ordinary pressures under both shock tube conditions and low-temperature constant-volume conditions. The agreement between model predictions and experimental data is very good, both with respect to shock tube ignition delay times and to the pronounced, nonlinear variation of autoignition time with initial pressure at near-ambient temperatures. The model also reproduces observed trends in H 2 /H 2 O product yield as a function of the initial SiH 4 /O 2 ratio. One key to this mechanism is competition between thermal stabilization and chain-branching decomposition reactions of an excited-state silylperoxy radical, and a second key is the reaction of water vapor with the intermediate species SiH 2 =O.

Analysis of the Surface-Enhanced Homogeneous Reaction during Oxidative Dehydrogenation of Propane over a V-Mg-O Catalyst

Abstract: The nature of the desorbed species that caused surface-initiated gas-phase reactions on a V-Mg-O catalyst during the oxidation of propane was investigated by modeling the reactions in the post-catalytic reactor (reactor volume downstream of the catalyst) with elementary reactions of the oxidative pyrolysis of propane. The elementary reactions reported in the literature that were used consisted of 37 elementary reactions and 20 chemical species. The rate constants were chosen so that the computed and experimental propane conversions and product distributions in the oxidative pyrolysis reaction were in good agreement at conversion up to 35% at 574 C and 25% at 558 C. With the use of these elementary reactions and rate constants, the conversions and product distributions in the postcatalytic reactor were calculated assuming different desorbed reactive intermediates which included propyl, ethyl, methyl, and OH radicals. After comparison of the results of the calculation with those from experiments, it was concluded that propyl radicals were the most likely species that desorbed from a V-Mg-O catalyst surface, although OH radicals were also possible candidates. The rate of propyl radical desorption was estimated to be 23% of the rate of reaction of propane on the catalyst surface at 570 C and 5%more » at 556 C.« less

Conformational fluctuations and protein reactivity. Determination of the rate-constant spectrum and consequences in elementary biochemical processes.

Abstract: When a protein's active site happens to be strongly coupled with the protein structure, the rate constant of the reaction may eventually be modulated by the conformational fluctuations Evidence for this effect has long been provided by extensive flash photolysis investigations of liganded hemoproteins and more recently of the nonheme respiratory protein hemerythrin in hydro-organic solvents Within a given protein conformational substate, an elementary reaction step is characterized by one single free energy barrier and by a first-order rate constant, k, which changes with temperature according to an Arrhenius law At physiological temperature and low viscosity, ultrafast conformational relaxation causes efficient averaging of the reaction rates and the protein displays exponential kinetics with and average rate constant 〈k〉 Under sufficiently general conditions, it can be shown that 〈k〉 also follows a simple Arrhenius law with ‘effective’ values of the pre-exponential factor Aeff and activation enthalpy Heff It is found that Aeff strongly depends on the overall shape of the rate constant distribution and that Heff actually corresponds to the lower limit of the enthalpy of activation, ie the value associated with the highest possible reaction rate The underlying distribution of rate constants can be reconstructed from a set of experiments in which the kinetics depart from an exponential, ie at low temperature and high viscosity The most probable distribution of exponentials consistent with the observed kinetics of the geminate recombinations of oxygen with photodissociated hemerythrin has been determined by using a new approach, known as the maximum entropy method The results are consistent with a single pre-exponential value and a distributed enthalpy spectrum As expected, Heff does not coincide either with the most probable nor with the average value of the enthalpy The most salient findings are that the probability for any protein molecule to have an enthalpy of activation equal to the effective value Heff vanishes and that Aeff differs by nearly three orders of magnitude from the true value Ao Biochemical reaction rates are actually average values, since protein reactions are measured under physiological conditions, where conformational relaxation is always fast Our understanding of the significance of Aeff and Heff is therefore entirely dependent on the knowledge of the distribution function of the rate constants In particular, enthalpy and entropy terms of similar reactions performed by different proteins cannot be compared as long as the distribution of the rate constants remains unknown

The chemical reaction molecular dynamics method and the dynamic transition state: Proton transfer reaction in the formamidine and water solvent system

Abstract: The chemical reaction mechanism in solution is analyzed by using the chemical reaction molecular dynamics (CRMD) method where a solute molecule and a few solvent molecules are regarded as a supermolecule and the chemical reaction dynamics can be analyzed in a time-dependent way on the intrasupermolecular potential surface. We have examined a proton transfer reaction, the formamidine-water system, and focused on the dynamic effect in the chemical reaction after considering the static «electronic» solvent effect. Two schemes, the constant-temperature scheme (CTS) and the constant-energy scheme (CES), have been employed, and a new type of critical state, named the dynamic transition state (DTS), was found by the appearance of a cusp in the hydrogen-bonding correlation function (HBCF). The cusp is due to the stopping of change in the O-H bond length, which produces a water molecule in the product region. In the CES, alternately modulated oscillation appeared, which is a characteristic in triatomic systems and should play an important role in energy flow in solution

Mechanism and kinetics of the reaction 1 dolomite + 2 quartz = 1 diopside + 2 CO 2 investigated by powder experiments

Abstract: Experimental investigation of the forward reaction I dolomite + 2 quartz = I diopside + 2 C02was carried out in a conventional hydrothermal apparatus, in order to determine the mechanism and the kinetics at a total pressure of 500 MPa and 680'C. The CO2 contents of the fluid phase, consisting of carbon dioxide and water, was found to vary from 80 to nearly 100 mol9o. The experiments lasted up to 365 days (8,760 hours). SEM examination of the reaction mixtures clearly shows a dissolution - crystallization mechanism operating within the whole range of X(CO), even at almost "dry" conditions. The data on degree of conversion ta,'sas time, results of SEM studies of experiments, and a variation of surface area of reactants and products, have shown that rateJimiting processes are always interface-controlled as long as the diopside crystals do not cover the dolomite surfaces completely. During the first 6@-800 hours, the rate of the heterogeneous reaction is controlled by the interplay of dolomite dissolution and diopside crysta! lization (including nucleation and growth). Compared with these processes, quanz dissolution appears to be rapid in any case under the experimental conditions applied. All of these dissolution and crystallization "steps" are themselves complex processes that consist of several elementary reactions. The experimental results show that the rate-limiting process changes during the course of reaction, even if P-T-X(CO) conditions are constant, or nearly constant. At the beginning of the reaction, diopside crystallization is rate-limiting, but later, dolomite dissolution becomes rate-determining; finally, when the dolomite-armoring diopside rim is complete (-800 hours, -3590 conversion), the transport of

Directions of theoretical and experimental investigations into the mechanisms of heterogeneous catalysis

Abstract: The roles of the atomic structure and the electronic structure of the active surface sites in bonding of reactants and causing bond breaking or bond formation have been the focus of theoretical studies. In addition to calculations on static systems, usually clusters, modelling of the transition states and the dynamics of elementary reaction steps (adsorption, dissociation, surface diffusion, desorption) have been performed. Variations of electronic structure of elements across the periodic table have been shown to be responsible for the unique importance of transition metals in catalysis. Experimental studies utilize catalysts with well-characterized structure (zeolites, crystal surfaces) and information about surface structure, composition and chemical bonding of adsorbates becomes available on the molecular level. Deliberate alteration of catalyst structure, surface composition by alloying and electronic structure by addition of electron donor and electron acceptor promoters have been utilized to modify reaction rates and selectivity. This way many of the molecular ingredients of heterogeneous catalytic reactions have been identified. In recent years evidence has been accumulating that indicates periodic and long term restructuring of the catalyst surface as necessary for chemical change and reaction turnover. These findings point to the need of time resolved studies and in-situ investigations of both the substrate and the adsorbate sides of the surface chemical bonds simultaneously on a time scale shorter than the reaction turnover frequency. Close collaboration between theorists and experimentalists is essential if we are to succeed in designing heterogeneous catalysts.

An Investigation of the Reaction of CH3 Radicals with O2 at High Temperatures

Abstract: The reaction of CH3 radicals with O2 was investigated behind incident shock waves at temperatures between 1400 and 2300 K and total densities between 2 · 10−6 and 8 · 10−6 mol/cm3. The formation of O and H atoms was followed by atomic resonance absorption spectroscopy (ARAS) at 130.5 nm and 121.6 nm. A rate constant of 2.3 · 1013 exp(–15400 K/T) cm3/mol s for the reaction CH3 O2 + CH3O + O was determined.

Chlorine Perchlorate Formation in the Gas Phase Photolysis of Chlorine Dioxide

Abstract: OClO/O2/N2 mixtures were photolyzed in a temperature controlled 4201 reaction chamber at temperatures between 249 and 300 K and total pressures between 0.5 and 1000 mbar. Initial OClO concentrations were in the range (1.7–5.7) · 1015 molecule/cm3. Reaction mixtures were analyzed in situ via long-path IR absorption using a Fourier-transform spectrometer. In some experiments product spectra were simultaneously monitored in the IR and the UV. Depending on reaction conditions, the product IR spectra were dominated by absorption bands of Cl2O3 or Cl2O4 or a mixture of both. Evidence is presented for the crucial role of O atoms in the Cl2O4 formation, suggesting either of the two mechanisms: (I) OClO + O + M ClO3 + M ClO3 + ClO + M Cl2O4 + M, or (II) OClO + ClO + M Cl2O3 + M, Cl2O3 + O + M Cl2O4 + M. Both the weak temperature dependence and the strong pressure dependence of the Cl2O4 yield support mechanism (I). In addition, Cl2O6 was detected as a minor product of OClO photolysis under certain reaction conditions, both by its IR and UV absorption.

An Investigation of the Reactions of NO3 Radicals with Cl and ClO

Abstract: The reaction (1) Cl + NO3 ClO + NO2 was studied at 298 K by observing the kinetic behaviour of NO3, Cl and ClO using discharge-flow mass-spectrometric technique. A rate constant k(1) = (2.26 ± 0.17) · 10–11 cm3 molecule−1 s−1 was obtained. The secondary processes between NO3 and ClO were also investigated and OClO was detected as a reaction product. For the rate constant of the reaction (2a) ClO + NO3 ClOO + NO2 an upper limit of k(2a) = 1 · 10−13 cm3 molecule−1 s−1 was found. In contrast to previous findings it is shown that reaction (2b) ClO + NO3 OClO + NO2 is the predominant channel with k(2b) = (4.3 ± 1.0) · 10−13 cm3 molecule−1 s−1.

Desulfurization kinetics of molten copper by gas bubbling

Abstract: Molten copper with 0.74 wt pct sulfur content was desulfurized at 1523 K by bubbling Ar-O2 gas through a submerged nozzle. The reaction rate was significantly influenced not only by the oxygen partial pressure but also by the gas flow rate. Little evolution of SO2 gas was observed in the initial 10 seconds of the oxidation; however, this was followed by a period of high evolution rate of SO2 gas. The partial pressure of SO2 gas decreased with further progress of the desulfurization. The effect of the immersion depth of the submerged nozzle was negligible. The overall reaction is decomposed to two elementary reactions: the desulfurization and the dissolution rate of oxygen. The assumptions were made that these reactions are at equilibrium and that the reaction rates are controlled by mass transfer rates within and around the gas bubble. The time variations of sulfur and oxygen contents in the melt and the SO2 partial pressure in the off-gas under various bubbling conditions were well explained by the mathematical model combined with the reported thermodynamic data of these reactions. Based on the present model, it was anticipated that the oxidation rate around a single gas bubble was mainly determined by the rate of gas-phase mass transfer, but all oxygen gas blown into the melt was virtually consumed to the desulfurization and dissolution reactions before it escaped from the melt surface.

On the application of Kramers' theory to elementary chemical reactions.

Abstract: Kramers' diffusion model in the energy controlled low viscosity and the momentum controlled high viscosity range is confronted with reality for elementary unimolecular reactions and radical associations in dense media. Collisional energy transfer appears to be much more complicated than described by the idealized model. On the other hand, there are examples where the Kramers-Smoluchowski equation well describes the transition to high viscosity behavior. In other cases, solvent shifts of the reaction barriers are pronounced and superimposed on the transport effects described by Kramers' model.

Understanding Chemical Reactivity Through the Intersecting-State Model

Abstract: A novel theoretical model for the understanding of chemical reactivity is presented. This intersecting-state model allows the estimation of the energy barrier of elementary reactions in terms of reaction energy, force constants and lengths of the reactive bonds, bond order of the transition state, n‡ and a parameter λ associated with the energy distribution in the molecular products. For a synchronous reaction A+B-C → A-B+C where the resonance effects at the transition state can be neglected n‡=1/2; significant resonance effects are present when there are mobile electrons in the reactants and are responsible for higher n‡ values. The model has been applied to the study of hydrogen, hydride, proton and methyl transfers, free-energy relationships, sigmatropic shifts and electron transfer reactions and new perspectives for chemical reactivity have been found. It is shown that ISM encompasses several current models of chemical reactivity and constitutes a useful interpretative method for organic reactions.

Photoreduction of thioxanthen-9-one and benzophenone by amines. Electrical conductivity and light emission studies

Abstract: The reactions of thioxanthen-9-one (TX) triplets with various amines (1,4-diazabicyclo[2,2,2]octane, DABCO, diphenylamine, DPA, triethylamine, TEA, triisobutylamine, TIBA and aniline, AN), in acetonitrile solution at room temperature have been investigated, and in some cases compared with the reactions of benzophenone (BP) triplets. In all cases the formation of free ions has been evidenced by photocurrent measurements. Apart from the system TX–DABCO free ions are formed in a complex process (involving more than one elementary reaction) which implies that the quantum yield Φ(ion) cannot be simply related to the oxidation potential of the amines. In the system TX–DABCO and BP–DABCO the same rate constant (1.2 × 1010 dm3 mol–1 s–1) has been found for the formation of free ions and the decay of triplets (determined by luminescence measurements).

Pressure effects on carbon monoxide rebinding to the isolated alpha and beta chains of human hemoglobin.

Abstract: The effects of pressure on the recombination kinetics of carbon monoxide binding to the isolated alpha and beta chains of human adult hemoglobin at pH 7, approximately 20 degrees C, were studied by the use of millisecond and nanosecond laser photolyses. The kinetic data were analyzed on the basis of a simple three-species model, which assumes two elementary reaction processes of bond formation and ligand migration steps. The activation volume for each elementary step was obtained from the pressure dependence of the rate constants. A pressure-dependent activation volume change from negative to positive values in the bimolecular carbon monoxide association reaction was observed for both of the isolated chains. This finding is attributed to a change of the rate-limiting step from the bond formation step to the ligand migration step. For both of the isolated chains, the activation volumes for ligand migration into and from the protein were estimated as +12-16 and +7-11 cm3 mol-1, respectively. These positive activation volumes for the ligand migration process may be caused by conformational fluctuations of proteins, that is, the conformational changes from "closed" to "open" structure. In the iron-ligand bond formation process, the activation volumes are -15 to -22 cm3 mol-1, which are almost identical to that for the model heme complexes [Taube, D. J., Projahn, H.-D., van Eldik, R., Magde, D., & Traylor, T. G. (1990) J. Am. Chem. Soc. 112, 6880-6886]. Accordingly, the surrounding protein contributions to the activation volumes for the bond formation process could be small.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitivity analysis of laminar premixed CH 4 -air flames using full and reduced kinetic mechanisms

Abstract: In this chapter we have calculated atmospheric pressure, stoichiometric premixed laminar methane-air flames with an initial temperature of 300 Kelvin using both a detailed mechanism of elementary reactions and a systematically reduced mechanism consisting of only four global reactions. We have discussed numerical difficulties associated with the functional form of the global reaction rates and have applied first-order sensitivity analysis to identify and interpret characteristics of the reduced scheme which make it difficult to obtain numerical solutions for flame structures using this particular reduced reaction scheme. In particular, we have concluded that Newton's method may not be the ideal method to solve the governing equations on the basis of a systematically reduced reaction mechanism.

Kinetic Polynomial: A New Concept of Chemical Kinetics

Abstract: A system of quasi-steady-state equations for a single pathway mechanism of a catalytic reaction can always be reduced to a polynomial in terms of the steady state reaction rate, a kinetic polynomial. The coefficients of this polynomial are polynomials in the parameters of the elementary reaction rates. The form of the lowest coefficient of the polynomial ensures the thermodynamic validity of this form of representation of quasi-steady-state equations. The properties of the kinetic polynomial are discussed in connection with such concepts of chemical kinetics as “molecularity”, “stoichiometric number”.