Showing papers on "Elementary reaction published in 1996"

Bond-Selected Chemistry: Vibrational State Control of Photodissociation and Bimolecular Reaction

Abstract: Controlling chemical reactions with light rests on the idea of exciting a vibration that becomes the reaction coordinate in subsequent chemistry. Vibrational excitation techniques such as infrared or stimulated Raman excitation of fundamental vibrations or vibrational overtone excitation of higher levels permit the preferential cleavage of a bond in a photodissociation or bimolecular reaction. The key to bond-selected chemistry is the initial preparation of a suitable vibrational state followed, in the case of bond-selected photodissociation, by electronic excitation or, in the case of bond-selected bimolecular reaction, by collision with a reactive atom. Such experiments demonstrate bond-selected chemistry, permit detailed comparison to theory, and reveal general principles of vibrational state control of chemical reactions.

Reaction mechanisms in aromatic hydrocarbon formation involving the C5H5 cyclopentadienyl moiety

Abstract: The quantum chemical BAC-MP4 and BAC-MP2 methods have been used to investigate the reaction mechanisms leading to polycyclic aromatic hydrocarbon (PAH) ring formation. In particular we have determined the elementary reaction steps in the conversion of two cyclopentadienyl radicals to naphthalene. This reaction mechanism is shown to be an extension of the mechanism occurring in the H atomassisted conversion of fulvene to benzene. The net reaction involves the formation of dihydrofulvalene, which eliminates a hydrogen atom and then rearranges to form naphthalene through a series of ring closures and openings. The importance of forming the -CR(·)-CHR-CR′=CR″- moiety, which can undergo rearrangement to form three-carbon atom ring structures, is illustrated with the C4H7 system. The ability of hydrogen atoms to migrate around the cyclopentadienyl moiety is illustrated both for methyl-cyclopentadiene, C5H5CH3, and dihydrofulvalene, C5H5C5H5, as well as for their radical species, C6H7 and C5H5C5H4. The mobility of hydrogen in the cyclopentadienyl moiety plays an important role both in providing resonance-stabilized radical products and in creating the -CR(·) CHR-CR′=CR″- moiety for ring formation. The results illustrate the radical pathway for converting five-membered rings to aromatic six-membered rings. Furthermore, the results indicate the important catalytic role of H atoms in the aromatic ring formation process.

Analysis of the Thermochemistry of NOx Decomposition over CuZSM-5 Based on Quantum Chemical and Statistical Mechanical Calculations

Abstract: CuZSM-5 is the most active catalyst known for the direct decomposition of NOx We have performed first-principles quantum mechanical calculations to evaluate the electronic and structural properties of species adsorbed to Cu sites that might be involved in NOx decomposition Using statistical mechanics, we have calculated ΔU°, ΔH°, and ΔG° of possible elementary reactions in order to evaluate the stability of adsorbates on Cu sites and the ease of their interconversion On the basis of these calculations, we propose a reaction pathway for NOx decomposition This scheme involves only single, isolated copper sites, is internally consistent, and is consistent with experimental observations

Quantumchemical study of the isobutane cracking on zeolites

Abstract: The ab initio quantumchemical investigation of the elementary steps of the catalytic isobutane cracking is presented. A reasonable agreement between experimental and theoretical activation energies is found. The obtained results demonstrate that the adsorbed carbenium and carbonium ions represent not the really existing reaction intermediates, but the high-energy transition states of the corresponding elementary reactions. This results in much higher activation energies than for the similar reactions in homogeneous super-acid solutions.

Kinetics and Mechanism of Methanol Oxidation in Supercritical Water

Abstract: We oxidized methanol in supercritical water at 246 atm and temperatures between 500 and 589 °C. Pseudo-first-order rate constants calculated from the data led to Arrhenius parameters of A = 1021.3±5.3 s-1 and Ea = 78 ± 20 kcal/mol. The induction time for methanol oxidation decreased from 0.54 s at 525 °C to 0.093 s at 585 °C and the reaction products were formaldehyde, CO, and CO2. Formaldehyde was a primary product, while CO and CO2 were secondary products. Formaldehyde was more reactive than methanol and its yield was always less than 24%. The temporal variation of the CO yield exhibited a maximum, whereas the CO2 yield increased monotonically. The experimental data were consistent with a set of consecutive reactions (CH3OH → CH2O → CO → CO2) with pseudo-first-order global kinetics. The experimental data were also used to validate a detailed chemical kinetics model for methanol oxidation in supercritical water. With no adjustments, this elementary reaction model quantitatively predicts the product distr...

Detailed Kinetic Modelling of Toluene Combustion

Abstract: A detailed chemical kinetic mechanism for the combustion of toluene has been assembled and evaluated for a wide range of combustion regimes. The latter include counterflow diffusion flames, plug flow reactors, shock tubes and premixed flames. The reaction mechanism features 743 elementary reactions and 141 species and represents an attempt to develop a chemical kinetic mechanism applicable to intermediate and high temperature oxidation. Toluene thermal decomposition and radical attack reactions leading to oxygenated species are given particular attention. The benzyl radical sub-mechanism is expanded to include izomerization and thermal decomposition reactions, which are important at flame temperatures, and a molecular oxygen attack path to form the benzylperoxy radical, which is found to be relevant at lower temperatures, The final toluene kinetic model results in excellent fuel consumption profiles in both flames and plug flow reactors and sensible predictions of the temporal evolution of the hydrogen ra...

Fundamentals of Chemical Kinetics

Abstract: 1. The empirical framework of chemical kinetics. 2. The experimental study of reaction kinetics. 3. Reaction mechanism and reaction order. 4. Theories of bimolecular reaction. 5. The interpretation of bimolecular reactions in solution. 6. Unimolecular gas phase reactions. 7. Chain reactions. 8. Heterogeneous catalysis. 9. Homogeneous catalysis. 10. Relaxation and other advanced techniques. 11. Photochemistry and radiation chemistry. 12. Reaction dynamics. Appendices. Answers to problems. Index.

Kinetic modeling of the oxidation of large aliphatic hydrocarbons

Abstract: Because of the complexity of low-temperature oxidation, a detailed reaction scheme of higher hydrocarbons (which are components of practical fuels) typically involves several hundred chemical species taking part in thousands of elementary reactions. Nevertheless, only a very limited number of different reaction types is appearing, for example, alkane thermal decomposition, H-atom abstraction to form an alkyl radical, alkyl radical isomerization, and β decomposition of the alkyl radical for the high-temperature range and a few additional reaction types at low temperature. A LISP program developed for the automatic generation of reaction mechanisms is able to produce mechanisms for the oxidation of aliphatic hydrocarbons. In contrast to earlier attempts described in the literature, a rather complete description of the aldehyde oxidation is included. The transition between low-and high-temperature range with a negative temperature dependence is well reproduced. With the help of newly available experiments for n-decane, the reaction mechanisms for n-heptane, and n-decane are validated for a wide range of pressures, temperatures, and equivalence ratios, covering conditions dictated by potential applications. This is a severe test case, because calculated ignition-delay times are very sensitive with respect to the quality of the reaction mechanism used. Additional sensitivity analysis based on the OH concentration, shows the principal rate-determining reactions. However, more kinetic data for high hydrocarbons and oxygenated species are necessary to validate the reaction mechanism, especially with respect to chain-length dependencies of rate coefficients and the behavior of fuels with multiple C-C bonds. Furthermore, some results on flame velocity are given for n-heptane.

Multistep asymptotic analyses of flame structures

Abstract: Aspects pertaining to rate-ratio asymptotic analyses of the structure of laminar, unstretched premixed flames and nonpremixed flames are described. The rate-ratio asymptotic analyses employ reduced chemical-kinetic mechanisms derived from detailed mechanisms. The rates of the global steps of the reduced chemical-kinetic mechanisms are related to the rates of elementary reactions. The outer structure of premixed flames is presumed to comprise an intert preheat zone of thickness of the order of unity, a thin reaction zone where all chemical reactions take place, and a postflame zone where the products are in equilibrium. For methane, n-heptane, and methanol flames the reaction between the radicals and the fuel is chain breaking. For iso-octane flames, the intermediate species iso-butene rather than the fuel deplets the radicals. This property is used to match the structure of the reaction zone with the structure of the chemically inert preheat zone. To obtain the burning velocity, it is necessary to analyze the structure of at least two reactive layers within the reaction zone: the inner layer and the oxidation layer. Qualitative features of the analyses of these layers in premixed methane flames are discussed. The characteristic temperature at the inner layer, To, is found to play a central role in asymptotic analyses. It is calculated by comparing the rates of chain-breaking and chain-branching reactions and found to depend mainly on pressure. The burning velocity is found to be proportional to the difference between the adiabatic flame temperature and To. Therefore, flame propagation cannot take place if the adiabatic flame temperature is lower than To. This shows that rate-ratio asymptotic analyses can provide a fundamental explanation for the existence of flammability limits. Characteristics of the analyses of the structure of hydrogen and wet carbon monoxide flames are also addressed. Rate-ratio asymptotic analyses of the structure of these flames are fundamentally different from those of hydrocarbon flames, because the reactions between these fuels and radicals are not chain breaking. Rate-ratio asymptotic analyses of nonpremixed flames are also described. The discussion is focused mainly on methane flames. The parameters that appear in the expression for the scalar dissipation rate at extinction are found to be similar to those for the burning velocity of premixed flames.

State-resolved studies of reactions in the gas phase

Abstract: During the first years of The Journal of Physical Chemistry, chemists were just beginning to understand chemical reactions in gases as sequences of elementary reaction steps. Basic models of reaction dynamics were developed in succeeding decades which provided powerful qualitative insight into reaction mechanisms and rate constants. In recent decades, the development of molecular beam and laser technologies has allowed us to look into the transition state region at the dynamics of chemical bond formation and breaking. State-selective preparation of reactants and state-resolved detection of products with velocity and angle resolution permit exacting quantitative test of dynamical theories. Thanks to the ability to calculate accurate ab initio potential energy surfaces and solve Schroedinger's equation for reactive scattering dynamics, the simplest reactions are now understood quantitatively at the most fundamental level possible. The concepts and models thus developed, along with ever more powerful experim...

Environmental Chemistry (Gas and Gas−Solid Interactions): The Role of Physical Chemistry

Abstract: This paper reviews two physical chemistry topics related to atmospheric chemistry. First, we describe semiempirical models, based on thermochemistry and transition state theory, which are employed to estimate gas phase reaction rates over a wide range of temperatures and pressures. We also review briefly the experimental techniques utilized for measurements of elementary reaction rate constants, which provide the primary input to these models. We then address chemical reactions which take place in the atmosphere on the surface of solid, ice-like aerosol particles, discussing some current views on the mechanisms of these reactions and describing some laboratory techniques for the study of these heterogeneous processes.

Kinetic evidence for the dependence of surface reaction rates on the distribution of reactants on the surface

Abstract: Two kinetically distinct types of oxygen atoms can be identified during the isothermal oxidation of CO on Pt(111) even though they all sit in identical sites at the start of the reaction. This is explained by a lowering in the reaction activation barrier with oxygen islanding.

Thermal decomposition of benene. Single-pulse shock-tube investigation

Abstract: The thermal decomposition of benzene was studied behind reflected shocks in a pressurized driver single-pulse shock tube. The temperature range covered was 1400–2000 K at overall densities of ∼3.5×10 −5 mol/cm 3 . Gas chromatography (GC) analyses of post-shock samples revealed the presence of the following decomposition products: C 2 H 2 , C 4 H 2 , C 6 H 5 -C≡CH, C 6 H 5 -C 6 H 5 , and small quantities of C 6 H 2 and C 6 H 4 . The main decomposition product at low temperatures is biphenyl, which is formed in the reaction C 6 H 5 +C 6 H 6 →C 6 H 5 -C 6 H 5 +H. At higher temperatures, the opening of the phenyl radical becomes a major reaction that competes with its attack on benzene. At these temperatures, the main products are C 2 H 2 and C 4 H 2 . A reaction scheme containing 15 species and 22 elementary reactions reproduces very well the experimental product distribution. Kinetic modeling indicates that at low temperatures the overall decomposition of benzene is second order with respect to benzene. At high temperatures, the decomposition is close to a first-order reaction with a rate constant given by k total =2.14×10 10 exp(−63.0×10 3 / RT ) s −1 , where R is expressed in units of cal/(K mol). Arrhenius rate parameters for the production rates of the decomposition products are given, and a discussion of the mechanism is presented.

Numerical and asymptotic studies of the structure of premixed iso-octane flames

Abstract: Numerical calculations and rate-ratio asymptotic analysis are performed to obtain the structure and burning velocities of premixed iso-octane flames. The numerical calculations employ a detailed chemicalkinetic mechanism comprising 967 elementary reactions, a skeletal chemical-kinetic mechanism comprising 47 elementary reactions, and a reduced chemical-kinetic mechanism for lean to stoichiometric conditions made up of six overall reactions among nine species including the hydrogen radical. The values of burning velocities calculated numerically using the detailed, skeletal, and reduced chemical-kinetic mechanisms are found to agree well with each other as well as with previous measurements. The asymptotic structure of stoichiometric and lean flames is analyzed using a reduced chemical-kinetic mechanism comprising five overall reactions. This mechanism is deduced from the reduced chemical kinetic mechanism employed in the numerical calculations after introducing steady-state approximation for the hydrogen radical. In the analysis, the flame structure is presumed to consist of three zones—a preheat zone of thickness of order unity, a thin reaction zone, and a postflame zone. In the reaction zone, the chemical reactions are presumed to take place in three layers—an inner layer, a C3H4-consumption layer, and a H2-CO oxidation layer. Within the inner layer, the fuel iso-octane is consumed in a thin sublayer and i-C4H8 is formed, which subsequently reacts with radicals to form the intermediate hydrocarbon compound C3H4. This intermediate hydrocarbon is consumed in the C3H4-consumption layer. In the inner layer and the C3H4-consumption layer, H2 and CO are formed. Most of the final products CO2 and H2O are formed in the H2-CO oxidation layer. In this layer, H2 is presumed to be in steady state everywhere except in a thin sublayer called the H2-consumption layer. The burning velocities calculated using the results of the asymptotic analysis are found to agree reasonably well with those calculated numerically using the detailed, skeletal, and reduced chemical-kinetic mechanisms and with previous measurements.

The Effect of Gas‐Phase Additives C 2 H 4 , C 2 H 6, and C 2 H 2 on SiH4 / O 2 Chemical Vapor Deposition

Abstract: The effects of adding and on the chemical vapor deposition of from were studied using a hot‐wall tubular reactor operated at a temperature of 873 K. Without additives, rough films with poor step coverage were obtained. Adding resulted in clear films with good step coverage. However, adding did not improve the quality of the film or the step coverage. Numerical simulations of the gas‐phase elementary reaction kinetics of the reaction system were made for the same conditions as the experiments. The simulations show rapid conversion (i.e., within 0.006s) of into , resulting in high concentrations of , which might form clusters that deposit to form rough films. Numerical simulations, including , and , showed that and reduced the gas‐phase reaction rate and that did not affect it. These numerical results agree with our experimental results and show that simulations of gas‐phase, elementary chemical reactions are sufficiently accurate to reproduce the behavior of some aspects of CVD processes.

Mechanism for the electrocatalyzed oxidation of glycerol deduced from an analysis of chemical instabilities

Abstract: The electrocatalyzed oxidation of glycerol in alkaline solution is compared with the oxidations of ethylene glycol and methanol. An analysis of behaviors caused by instabilities provides strong evidence that the elementary reactions that dominate the oxidation of glycerol are the same as those that dominate the oxidation of methanol. These reactions include the formation of surface bonded CO and its reaction with surface bonded hydroxyl radicals. The reactions that precede the dominant reactions in the oxidation of glycerol are relatively fast and must involve cleavage of C−C bonds. Evidence from phase diagrams indicates that the most probable sequence for the fast reactions requires a sufficient number of neighboring vacant sites to produce three surface-bonded CO complexes for each glycerol molecule. Reaction sequences that lead to zero, one, or two CO complexes occur with small probability.

Nonclassical kinetics in three dimensions: Simulations of elementary A+B and A+A reactions.

Abstract: Monte Carlo simulations are employed to study the rate laws of A+A\ensuremath{\rightarrow}0 and A+B\ensuremath{\rightarrow}0 diffusion-limited elementary reactions in three dimensions (3D). Using reflective instead of cyclic boundary conditions we do observe the Zeldovich regime in 3D for the A+B reaction. The time and density for the crossover into the Zeldovich regime in 3D agree with the existing scaling laws and provide the hitherto missing scaling coefficient. We show that the behavior of the A+A reaction rates in 1D, 2D, and 3D and the early time behavior of the A+B reaction rate map the rate of distinct sites visited by a single random walker, giving nonclassical kinetics at early times in all cases. We also determine a simple scaling law for crossover to finite size effects, which depends only on the linear lattice length except when the crossover to finite size effects and the crossover to the Zeldovich regime are concomitant. \textcopyright{} 1996 The American Physical Society.

Selectivity as a measure of “livingness” of the polymerization of cyclic esters

Abstract: The selectivity parameter, defined as the ratio of the rate constant of desired elementary reaction (i.e. initiation or propagation) to the rate constant of the competing side reaction, should be used as a quantitative measure of the livingness of polymerization. The most important side reactions in the anionic and pseudoanionic polymerization of cyclic esters are presented and the importance of selectivity in initiation and propagation is discussed, taking into account the relation between selectivity and the structure of monomers, initiators, and active species. Reasons for the low selectivity of ions and the high selectivity of aluminium-based initiators, namely dialkylaluminium alkoxides (R2AlOR') and aluminium trialkoxides (Al(OR)3) are given. The relatively low reactivity and steric hindrance, created by the presence of bulky substituents at the Al atom, are responsible for the enhanced selectivity. Thus, Al(OR)3 found recently to carry three chains, provides particularly crowded and therefore selective active species. The structure of the aluminium alkoxide active species is described, as established on the basis of 1H NMR, 27Al NMR, and molecular weight measurements by Multi Angle Laser Light Scattering (MALLS) of the living polymers.

Primary charge separation. The primary processes of bacterial photosynthesis — ultrafast reactions for the optimum use of light energy

Abstract: Femtosecond spectroscopy is used to study the molecular mechanisms of the primary electron transfer. Data on native and mutated reaction centers of Rhodopseudomonas viridis show, that primary electron transfer is an ultrafast stepwise reaction. The electron is transferred via a chain of pigments: In a first reaction step the electron is transported from the special pair to the accessory bacteriochlorophyll with a time constant of ≈ 3 ps. A second, faster reaction carries the electron with 0.65 ps to a bacteriopheophytin. Experiments on mutated reaction centers with strongly modified reaction times yield additional information on energetics, reorganisation energies and electronic coupling of the reaction center. A consistent theoretical treatment of the data shows that standard non-adiabatic theory describes well the primary electron transfer process. It also reveals that the realised reaction parameters optimise the reaction centers for highest possible quantum yield.

A Gaskinetic Investigation of HOBr Reactions with CI(2P), O(3P) and OH(2II). The Reaction of BrCl with OH(2II)

Abstract: The reactions of HOBr with atomic chlorine (1), with oxygen (2) and with hydroxyl radicals (3) have been investigated at 300 K using the discharge flow technique with mass-spectrometric detection. The rate constants based on HOBr consumption have been determined to be k 1 = (8.0±0.4).10 -11 and k 2 = (3.1±0.2).10 -11 . For the reaction with OH radicals k 3 < 5.10 -13 was obtained. All rate constants are given in units cm 3 molecule -1 s -1 . The reactions of HOBr with Cl and with O are shown to proceed via bromine abstraction with formation of BrCI and BrO, respectively. The predominant channel in the reaction of OH with BrCl was identified to be step (4a) OH+BrCl → HOBr+Cl (4a) Based on the present kinetic investigation the heat of formation for HOBr at T= 300 K was evaluated to be ΔH f 0 (HOBr) = - (60.22 ± 2) kJ.mol -1 .