Showing papers on "Elementary reaction published in 1992"

Chemical Kinetic Data Base for Propellant Combustion. II. Reactions Involving CN, NCO, and HNCO

Abstract: This paper contains evaluated chemical kinetic data on single step elementary reactions involving small polyatomic molecules which are of importance in propellant combustion. The work consists of 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 cm−3. The results of the second year’s effort add to the existing data base reactions involving CN, NCO, and HNCO with each other and the following species: H, H2, H2O, O, OH, HCHO, CHO, CO, NO, NO2, HNO, HNO2, HCN, and N2O.

Nonlocal density functional theory as a practical tool in calculations on transition states and activation energies. Applications to elementary reaction steps in organic chemistry

Abstract: Nonlocal density functional theory (NL) has been evaluated as a practical tool for theoretical studies of organic reactions. Calculations on the two abstraction reactions A ( . CH 3 +CH 4 →CH 4 + . CH 3 ) and B ( . CH 3 +CH 3 Cl→CH 4 + . CH 2 Cl) afforded the activation energies A=11.7 kcal/mol and B=8.5 kcal/mol, which compare favorably with the experimental activation energies of 14.1 kcal/mol for A and 9.4 kcal/mol for B

Hydrocarbon ignition: Automatic generation of reaction mechanisms and applications to modeling of engine knock

Abstract: A computational technique is described which automatically develops detailed chemical kinetic reaction mechanisms for large aliphatic hydrocarbon fuel molecules. This formulation uses the LISP language to apply general rules which identify the chemical species produced, the reactions between these species, and the elementary reaction rates for each reaction step. Reaction mechanisms for cetane (n-hexadecane) and most alkane fuels C{sub 7} and smaller are developed using this automatic technique, and detailed sensitivity analyses for n-heptane and cetane are described. These reaction mechanisms are then applied to calculation of knock tendencies in internal combustion engines. The model is used to study the influence of fuel molecule size and structure on knock tendency, to examine knocking properties of fuel mixtures, and to determine the mechanisms by which pro-knock and anti-knock additives change knock properties.

A study of homogeneous methanol oxidation kinetics using CSP

Abstract: The homogeneous oxidation of methanol in air at constant pressure is examined using data generated by the method of computational singular perturbation (CSP). At any moment in time, the number of exhausted fast modes and the radicals (sometimes called the intermediaries) are computationally identified. The participation index, which quantifies the participation of any elementary reaction to an equations of state of the radicals, along with the importance index, which quantifies the importance of any elementary reaction to a particular species of interest, are computed and used to assess the sensitivities of the solution to the reaction rate constants. Every elementary reaction is classified so that it either belongs to the equilibrated set which contains fast reactions already equilibrated among themselves, and/ or the rate-controlling set which contains reactions controlling the current rate of activities, or neither of the above sets—in which case it is superfluous. A number of numerical experiments were performed to verify the assessments: (a) the relative effectiveness of the reaction rate constants of two reactions (#16, #160) in breaking up the fuel indicated by the importance index is verified, (b) that fuel breakup in an early time period can actually be slowed down by increasing the reaction rate constants of certain fuel breakup reactions (#156, #159) is verified. Numerical experiments also showed that species identified as radicals responded instantly to sudden changes in reaction rates, while the non-radicals responded more smoothly. The overall response of the unknowns to perturbations is always consistent with the CSP-derived effective stoichiometric coefficients. In addition, a minimum set of species is constructed with the help of the CSP data. This minimum set, which trims the original full set of 30 species to 15 species, generates numerical solutions in excellent agreement with solutions obtained with the full set.

Transition State Spectroscopy of Bimolecular Chemical Reactions

Abstract: One of the fundamental goals of chemical physics has been to understand the nature of the potential energy surfaces on which chemical reactions occur. Much of this interest focuses on the transition state region: the region of the surface where chemical bonds are broken and reformed. The microscopic forces at play in the transition state region often control the observabk; properties of a reaction, including the reaction cross-section and the product angular and energy distributions. Indeed, the key issue in chemical reaction dynamics is to deduce the relationship between these asymptotic properties of a reaction and the detailed features of the transition state region, such as (in the case of a direct reaction) the saddle point location, barrier height, and bend potential near the saddle point. To resolve this issue successfully, one would like to measure the asymptotic properties and experimentally characterize the transition state region for a given reaction. During the last 20 years, most of the experimental emphasis has been on the former aspect; increasingly refined state-tostate scattering experiments have been developed in which final product distributions are measured as a function of wcll-defined reactant initial conditions (1). These experiments can provide a sensitive, although

Generalized composite degradation kinetics for polymeric systems under isothermal and nonisothermal conditions

Abstract: A composite degradation methodology is extended to the conversion-dependence function in order to explain the importance of multiple reaction mechanisms which might be considered to be involved in degradation processes. Based on two elementary reaction mechanisms, a specific form of the model equation is derived, which is capable of describing various types of degradation behavior showing sigmoidal rate as well as deceleratory rate. The conversion-dependence function is derived to be independent of the Arrhenius-type reaction constant or temperature, and thus the kinetic parameters are determined by analytic methods that have been developed for isothermal and dynamic-heating experiments without any modification or additional assumptions. The developed model equation is tested by predicting the isothermal master curve of polyether-ether-ketone (PEEK), which is used as a model system in this study. The activation energies of the model system are analyzed using comparable methods for isothermal and dynamic experiments, which compare favorably in terms of the activation energy as a function of conversion. The resulting model equation, based on the kinetic parameters determined by isothermal experiments, can accurately predict both isothermal and dynamic-heating thermogravimetry utilizing the same constants and identical reaction mechanisms without additional assumption.

Shock tube ignition of ethanol, isobutene and MTBE: Experiments and modeling

Abstract: The ignition of ethanol, isobutene and methyl tert-butyl ether (MTBE) has been studied experimentally in a shock tube and computationally with a detailed chemical kinetic model. Experimental results, consisting of ignition delay measurements, were obtained for a range of fuel/oxygen mixtures diluted in Argon, with temperatures varying over a range of 1100–1900 K. Mixtures ranged from very lean to very rich, including equivalence ratios of 0.1–4.0 for isobutene, 0.25–1.5 for ethanol, and 0.15–2.4 for MTBE. The numerical model consisted of a detailed kinetic reaction mechanism with more than 400 elementary reactions, chosen to describe reactions of each fuel and the smaller hydrocarbon and other species produced during their oxidation. The overall agreement between experimental and computed results was excellent, particularly for mixtures with greater than 0.3% fuel. The greatest sensitivity in the computed results was found to falloff parameters in the dissociation reactions of isobutene, ethane, methane, and ethyl and vinyl radicals, to the C 3 H 4 and C 3 H 5 reaction submechanisms in the model, and to the reactions in the H 2 −O 2 −CO submechanism.

Proton transfer and energy coupling in the bacteriorhodopsin photocycle.

Abstract: A description of the rate constants and the energetics of the elementary reaction steps of the photocycle of bacteriorhodopsin has been helpful in understanding the mechanism of proton transport in this light-driven pump. The evidence suggests a single unbranched reaction sequence, BR-hv----K in equilibrium with L in equilibrium with M1----M2 in equilibrium with N in equilibrium with O----BR, where coupling to the proton-motive force is at the energetically and mechanistically important M1----M2 step. The consequences of site-specific mutations expressed homologously in Halobacterium halobium have revealed characteristics of the Schiff base deprotonation in the L----M1 reaction, the reorientation of the Schiff base from the extracellular to the cytoplasmic side in the M1----M2 reaction, and the reprotonation of the Schiff base in the M2----N reaction.

Skiing the reaction rate slopes.

Abstract: On the way from reactants to products, a chemical reaction passes through what chemists term the transition state-for a brief moment, the participants in the reaction may look like one large molecule ready to fall apart. The nature of this elusive state is the subject of a number of exciting recent developments that involve theory and experiment in the study of chemical reaction rates. They include time-resolved studies of molecules in the transition state region, spectrcopic observations of that region, reactive scattering state-to-state cross sections for reactions such as H + H_2 → H_2 + H, and experimental/theoretical studies of electron transfer processes in the condensed phase. The experiments of Lovejoy et al. described in this issue of Science involve the fundamental question of the quantization of vibrational energy levels of the transition state and its implications for unimolecular reaction rate theory.

Advances in chemical kinetics and dynamics

Abstract: State-to-state dynamics - do we really need all of these data?, J.J. Valentini the measurement of thermal bimolecular rate constants by the flash photolysis-shock tube (FP-ST) technique - comparison of experiment to theory, J.V. Michael diffusion controlled elementary reaction in low dimensions, R. Kopelman and Y.-E. Koo state-resolved simple bond-fission reactions - experiment and theory, H. Reisler and C. Wittig applications of chemical kinetics, D.M. Golden and J.A. Manion.

Silicon nitride formation by low energy N + and N + 2 ion beams

Abstract: Reactions of N+ and N+2 ions with Si(100) surface are examined as a function of both ion kinetic energy and dose using a low energy ion beam instrument. The Si surface is exposed to low energy (1–300 eV) ion beams in an ultrahigh vacuum environment and the resulting surface species are characterized by Auger electron spectroscopy and ultraviolet photoelectron spectroscopy. The absolute reaction probability Pr is measured for nitridation processes. Pr(N+) has a value of ∼0.25 and stays constant in the energy range of 1–25 eV. Pr(N+2) increases from zero to ∼0.25 in the same range. Continued exposure of the ion beams to a dose ≳5×1015 ions/cm2 leads to a saturation and formation of a dense and stable silicon nitride layer. Variation of Pr with energy and dose is explained in terms of elementary reaction steps such as charge neutralization of the projectile ion, collisional dissociation of N+2, nitridation reaction, and chemically induced desorption of surface nitrogen species. A mechanism is proposed to exp...

Detailed Chemical Kinetic Modeling: Chemical Reaction Engineering of the Future

Abstract: Publisher Summary A chemical reaction can be viewed as occurring via the formation of an excited state that can be any one of the degrees of freedom of the collection of N atoms. That is, translational, rotational, vibrational, and electronic excitation can lead to a chemical reaction. Most elementary chemical reactions can be categorized as unimolecular or bimolecular events. However, further phenomenological classification is useful for the development of detailed chemical kinetic models to estimate rate parameters for new reactions by analogy to similar reactions in the same phenomenological class. The transition state would be stable to all geometric deformations except the reaction coordinate, which may be a bond distance, bond angle, or dihedral angle, depending on the nature of the reaction for simple molecules. However, when large molecules are involved, such as those observed in biochemical reactions, complex reaction coordinates are possible. That is, the minimum energy path between the reactants and the products may not be represented by a simple reaction coordinate, but may involve a concerted deformation of bonds, bond angles, and dihedral angles.

High Temperature Reactions of Phenylacetylene

Abstract: Hydrogen atom abstraction from phenylacetylene (C6H5-C2H) like its reaction with hydrogen atoms have been studied at elevated temperatures behind reflected shocks. - The unimolecular decomposition of very low initial concentrations (3 - 22 ppm) of phenylacetylene was investigated in the temperature range 1600 to 1900 K by monitoring the temporal H-atom production. - For C6H5-C2H + H, the thermal decomposition of very low concentrations (1 - 3 ppm) of C2H5I served as H-atom source. Atomic resonance absorption spectrometry (ARAS) was used to record simultaneously H-atom and I-atom profiles. The experiments covered the temperature range 1190 to 1530 K. - For both series of experiments, the total pressure was about 2.3 (+/-0.3) bar. For the unimolecular reaction R1: C6H5-C2H C6H4-C2H + H a rate constant expression of: was found. For the bimolecular reaction R2: C6H5-C2H + H products a rate constant of: was deduced. From the available thermochemical data two product channels have to be discussed: Detailed evaluation of existing thermodynamic data enables the calculation of an equilibrium constant for reaction R 2a. From this and under the assumption that reaction R 2a is the dominant product pathway at elevated temperatures, a rate constant expression of is deduced for the reaction of phenyl readicals with acetylene, which is an important process in sooting flames: This rate constant agrees within a factor of 2 with the data from recent studies executed by Fahr et al. [1a, 1b].

Surface catalysis studied by in situ positron emission

Abstract: A COMPLETE understanding of heterogeneous catalytic processes requires quantitative in situ information on the concentrations of reactants present on the catalyst surface, but few techniques are available to supply this information. Here we show that positron-emitter labelling and scanning, using the isotopes11C, 13N and15O, can be used to study the reactions taking place during the catalytic conversion of automotive exhaust. By introducing small pulses of labelled molecules into a reactant stream passing through the catalyst, in situ quantitative information on the concentrations and residence times of reactants in the reactor is obtained. The labels are detected using a positron camera, an imaging device adapted from nuclear medicine, and the recorded data are presented as 'reaction images', showing quantitatively the distribution of the label in the catalyst bed as a function of position and time. These data can then be used to quantify reaction kinetics by serving as the input to mathematical simulations based on elementary reaction steps.

Inhibition of moist carbon monoxide oxidation by trace amounts of hydrocarbons

Abstract: The moist carbon monoxide oxidation reaction perturbed by small quantities of hydrocarbons is studied to yield information on the reactions of H, O, OH, and HO 2 radicals with hydrocarbons (RH) and on general mechanistic inhibition behavior at temperatures near 1000 K. In particular, the inhibiting action of CH 4 , C 2 H 6 , C 3 H 8 , C 2 H 4 , C 3 H 6 , and C 2 H 2 at pressures below the second explosion limit of CO/H 2 O/O 2 mixtures are reported over the temperature range 1026–1140 K at 1 atm . The kinetics of these mixtures are shown to complement mechanistic studies on RH/O 2 mixtures for the development and validation of hierarchical hydrocarbon oxidation reaction mechanisms. Considering all the hydrocarbons studied, the general ranking of effectiveness as an inhibitor was found to follow the order: propene>propane >methane>ethane>ethene>acetylene. In fact, acetylene was observed to always promote the oxidation of moist CO, thus emphasizing the importance of O-atom radical attack rather than OH attack on acetylene. Methodologies are also described which permit assessment of elementary reaction rates from inhibition effects. In order to evaluate the potential of these techniques to yield information on rates of H and HO 2 attack on hydrocarbons, these methods were applied to constrained moist CO oxidation inhibitions where RH+OH is most important. In the case of the well studied methane reaction and the lesser studied propene reaction, inhibition studies lead to rate constants of 1.6×10 12 cm 3 /mol-s at 1026 K for CH 4 and 8×10 12 cm 3 /mol-s at 1020 K for C 3 H 6 , respectively. These values are in good agreement with literature data from other more direct techniques. These results show promise of the approach for determining lesser studied elementary reactions at these temperatures, particularly under reaction conditions where HO 2 and H reactions dominate the reaction inhibition characteristics.

Molecular dynamics studies of elementary surface reactions of acetylene and ethynyl radical in low-pressure diamond-film formation

Abstract: Molecular dynamics studies of some of the important elementary reactions involved in the low-pressure synthesis of diamond films are reported. The C(111) surface is modeled with an ensemble of 127 atoms and a velocity reset procedure to incorporate the thermal effects of the bulk. The hydrocarbon potential developed by Brenner is employed in all calculations for both the surface and the incident gas-phase molecules. 25 refs., 10 figs., 1 tab.

A model of diamond growth in low pressure premixed flames

Abstract: Recognizing the potential importance of diamond thin film growth from combustion environments, a computational investigation of diamond synthesis in low pressure premixed flames has been conducted. The model employed solves the two‐dimensional continuity, momentum, global energy, and species conservation equations in stagnation point flow geometry, and accounts for gas phase and surface reaction kinetics. The heterogeneous mechanism employed to describe diamond growth assumes that the methyl radical is the primary growth precursor. The gas phase mechanism includes elementary reaction pathways which generate methyl radicals from acetylene and in addition, includes a mechanism for cyclization (the formation of benzene) via acetylene and ethylene precursors. In this way, the pathway towards soot formation, which is believed to be a consequence of the formation of fused polycyclic aromatics, is shown to be a possible explanation for an eventual decrease in diamond growth rates at increasing fuel to oxygen flo...

The Formation of O and H Atoms in the Reaction of CH2 with O2 at High Temperatures

Abstract: The formation of O and H atoms in the reaction of CH2 with O2 was investigated behind incident shock waves at temperatures between 1000 K and 1700 K and at total densities between 5 · 10−6 and 1 · 10−5 mol/cm3 by means of Atomic Resonance Absorption Spectroscopy (ARAS) at 130.5 nm and 121.6 nm, respectively. The contribution of O and H forming channels was found to be about 20%. The rates of formation of O and H atoms in this reaction were determined to be k1a = (4 ± 2) · 1010 cm3/mol · s k1e = (5 ± 2) · 1010 cm3/mol · s In the temperature range investigated an over all rate constant can be represented by k1 = (5 ± 3) · 1011 cm3/mol · s. A comparison with low temperature data leads to a change of the apparent energy of activation of the overall reaction in the temperature range between 300 and 1000 K.

Direct investigations of the kinetics of the reactions of CN radicals with N atoms and 3CH2 radicals with NO

Abstract: The kinetics of two elementary reactions of radicals important in fuel nitrogen chemistry were investigated by means of the laser photolysis -laser induced fluorescence and molecular beam mass spectrometry methods. Reaction (−1) CN+N=C+N2 was investigated at 900 Pa total pressure. The rate constants decreased by one order of magnitude between room temperature and 534 K and could be fitted with the following Arrhenius expression: k−1(294 K≤T≤534 K)=10(11.29±0.06)exp((1770±170)/T) cm3mol−1 At 750 K an upper limit of 5·1011 cm3mol−1 was determined for reaction (−1). In the reaction of 3CH2 radicals with NO the addition complex CH2NO as well as the final products OH and HCN were detected and the branching ratios (α) for the following channels have been determined: CH2+NO→(CH2NO)* (CH2NO)*(+M)→NCO+H2 α300–1025 K

Weak Collision Effects in the Reaction CH3CO → CH3 + CO

Abstract: Rate constants for the unimolecular decomposition of CH3CO have been obtained as a function of temperature (420 - 500 K) and helium density (3 - 18 · 1016 atom cm−3), conditions which are in the second order region of the fall-off curve. An Arrhenius expression for the low-pressure limit unimolecular rate constant was obtained from the results, k1±(He) = (6.7 ± 1.8) · 10−9 exp[(-6921 ± 126 K)/T] cm3 molecule−1 s−1. Using a Master Equation formalism to calculate values of k1, a set of the two energy parameters needed in the calculations, E± and ΔE)down (including its temperature dependence), was found that is within the range of expected values (including a temperature dependence in the case of ΔEdown), which, when incorporated into the Master Equation, provides calculated rate constants which agree well with the measured ones. They are E± = 65.3 ± 4.0 kJ mol−1 and ΔEdown = 65.6 + 0.271 T cm−1 (the latter, a parameterized expression, is valid only in the temperature range of this study). A transition state model for the unimolecular decomposition of CH3CO was produced which provides high-pressure limit rate constants for this reaction (k1±(CH3CO ± CH3 + CO) = 2.50 · 1013 exp(-8244 K/T) s−1 and k−1±(CH3 + CO ± CH3CO) = 7.64 · 10−13 exp(-3073 K/T) cm3 molecule−1 s−1) and k(E) values for solving the Master Equation for reaction conditions that are in the fall-off region. Fall-off behavior of k1 and k−1 reported by others for several different bath gases was reproduced within the uncertainty limits of the experimental results using the Master Equation formalism incorporating the transition state model, the energy parameter E± given above, and reasonable values for ΔEdown for the different bath gases used. This Master Equation formalism and transition state model should provide unimolecular rate constants for reaction (1, - 1) in the fall-off region for additional bath gases using reasonable estimates of ΔEdown (e. g., values obtained for this energy-transfer parameter for collisions between other polyatomic radicals and the bath gases of interest).

Model studies on the kinetics and mechanism of polyarylate synthesis by acidolysis

Abstract: Model reactions were carried out to simulate the acidolysis process for polyarylate synthesis by using p-tert-butylphenyl acetate (ptBuPhOAc) and benzoic acid in diphenyl ether. p-tert-Butylphenol was formed in the reaction mixture and its concentration stayed constant throughout the reaction. Acetic benzoic anhydride and benzoic anhydride were detected by NMR. Based on this experimental evidence, a mechanism for the acidolysis was proposed involving the mixed anhydride. The kinetics of the acidolysis reaction was studied for this model reaction. The overall reaction order is two and the reaction order with respect to each reactant is one. Second-order reaction rate constants were measured at different reaction conditions (200–250°C). The activation energy (Ea), activation enthalpy (ΔH≠), and activation entropy (ΔS≠) were calculated from these data. The thermodynamic parameters of the acidolysis reaction were also measured for the analogous reaction of p-tert-butylphenyl pivalate (ptBuPhOPiv) and benzoic acid. The kinetics of two other elementary reactions involved in the acidolysis reaction were also studied: p-tert-butylphenol with acetic anhydride or benzoic anhydride, and p-tert-butylphenyl pivalate with benzoic acid.

Kinetic and product distribution analysis of the reaction of atomic hydrogen with vinyl chloride

Abstract: An elementary reaction mechanism has been developed to model the experimentally observed loss of vinyl chloride by reaction with atomic hydrogen, as well as the observed products. At the low-pressure, room temperature experimental conditions the consumption of C{sub 2}H{sub 3}Cl by reaction with H occurs primarily by nonipso attack by H on the =CH{sub 2} group to form (CH{sub 3}C{center_dot}HCl){sup {double_dagger}}. This energized complex then undergoes an H shift to form (C{center_dot}H{sub 2}CH{sub 2}Cl){sup {double_dagger}}, which decomposes to form Cl + CH{sub 2}=CH{sub 2}. Collisional stabilization of the original adduct is also important. Abstraction of Cl by H is negligible in these conditions. The authors` mechanism is based on quantum Rice-Ramsperger-Kassel (QRRK) analysis of the reactions of the energized adducts from the separately considered ipso and nonipso additions. The authors also utilized transition-state theory for the isomerization reaction, evaluated with literature rate constants and barriers. The authors extend the QRRK calculations to higher pressures and temperatures for use by the modeling community. A mechanistic pathway is presented to explain the formation of the various reaction products observed. 26 refs., 13 figs., 7 tabs.