Showing papers on "Elementary reaction published in 2003"

Identification of general linear relationships between activation energies and enthalpy changes for dissociation reactions at surfaces.

Abstract: The activation energy to reaction is a key quantity that controls catalytic activity. Having used ab inito calculations to determine an extensive and broad ranging set of activation energies and enthalpy changes for surface-catalyzed reactions, we show that linear relationships exist between dissociation activation energies and enthalpy changes. Known in the literature as empirical Bronsted−Evans−Polanyi (BEP) relationships, we identify and discuss the physical origin of their presence in heterogeneous catalysis. The key implication is that merely from knowledge of adsorption energies the barriers to catalytic elementary reaction steps can be estimated.

Progress Highlight Catalytic CO oxidation over ruthenium--bridging the pressure gap

Abstract: The CO oxidation over Ru under UHVconditions reveals by far the lowest catalytic activity among the late transition metals, while under strongly oxidizing reaction conditions the catalytic activity of Ru turns out to be superior to Pt, Rh and Pd. This observation was taken as manifestation of the so-called pressure gap. Recent experiments have shown that this transformation from an inactive catalyst towards an active catalyst is attributed to a structural transformation of Ru to RuO2. The autocatalytic oxidation of the Ru(0 0 0 1) surface leads to the formation of an epitaxially grown RuO2(1 1 0) film whose catalytically active sites are identified with the onefold under coordinated Ru atoms (1f-cus-Ru). On the 1f-cus-Ru atoms the CO molecules adsorb strongly (120 kJ/mol), ensuring a high CO concentration on the oxide surface under reaction conditions. Experiments together with density functional theory (DFT) calculations indicate that the most important elementary reaction step during the CO oxidation is the recombination of adsorbed CO molecules with bridging O atoms from the oxide surface. Equally important for being a good oxidation catalyst is the facile replenishment of bridging O atoms on RuO2 by oxygen exposure. This process is mediated by on-top O atoms above the 1f-cus-Ru atoms. The on-top O atoms are by 1.4 eVless strongly bound than bridging O atoms. However, the activation barriers for CO recombination with on-top O and bridging O are nearly degenerated as determined by DFT calculations. Entropy effects make the oxidation von on-top CO less efficient with on-top O than with bridging O. Recent experiments provide evidence that the pressure gap for the CO oxidation reaction on RuO2 is successfully closed.

Insights into dynamics of the F+CD4 reaction via product pair correlation

Abstract: To unravel the “extra-atom” complexity of the title reaction, we exploit an experimental approach which, by taking advantage of the correlated information of coincident product pairs, allows us to peel off judiciously the intrinsic complications of a six-atom reaction, extracting the underlying backbone of three-atom dynamics. Examining the collisional energy dependencies of the pair-correlated attributes for a given state(s) of CD3 products from the title reaction, several of major observations can qualitatively be understood, whereas others await further theoretical investigations. An intriguing possibility for the existence of reactive resonances in this six-atom reaction is surmised.

Mechanism of the Diels-Alder reaction studied with the united reaction valley approach: Mechanistic differences between symmetry-allowed and symmetry-forbidden reactions

Abstract: The unified reaction valley approach (URVA) was used to investigate the mechanism of the reaction between ethene and 1,3-butadiene. The reaction valley was explored using different methods (Hartree...

Theoretical Study of Mechanisms, Thermodynamics, and Kinetics of the Decomposition of Gas-Phase α-HMX (Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine)

Abstract: We present a theoretical study of the decomposition mechanism of gas-phase α-HMX. Four distinct channels were studied using the B3LYP/cc-pVDZ level of theory. These are as follows: (i) HMX first loses an NO2 to form HMR which further breaks a C−N bond to form a chain structure and then later loses three methylenenitramines (MN, H2CNNO2) successively; (ii) the chain structure forms a 10-member ring via a rung closure step before undergoing further decomposition; (iii) HMX first eliminates an HONO, then loses two MN, and eliminates an HONO successively; (iv) HMX eliminates two HONO successively, then loses an MN, and finally eliminates an HONO. The rate constants of each elementary reaction have been calculated using the transition-state theory. The thermodynamics properties were also calculated for the stable species by employing a standard statistical thermodynamics method. Channel i was found to be the preferred decomposition pathway on the basis of the analysis of rate constants of the elementary react...

Chemical kinetics : fundamentals and new developments

Abstract: General Problems of Chemical Kinetics: Basic concepts of chemical kinetics. Elementary Gas Phase Reactions: Theory of elementary reactions Methods of investigation of elementary reactions Experimental investigation of gas phase reactions. Chemical Reactions in Liquid Phase: Diffusion-controlled reactions in solution Bimolecular reactions in solutions Influence of media Free radical reactions Reactions of ions and radical ions Reactions of molecules. Chain Reactions: Chain non-branched reactions Oxidation of organic compounds by dioxygen Chain branched reactions Methods to study chain reactions. Homogeneous Catlysis: General kinetics Regularities in Homogeneous catalysis Acid-base catalysis Redox catalysis Catalysis by metal complexes Enzymes as catalysts.

Ab initio/RRKM study of the potential energy surface of triplet ethylene and product branching ratios of the C(3P) + CH4 reaction

Abstract: Calculations of the lowest triplet state potential energy surface for the C(3P) + CH4 reaction have been performed using the CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G(d,p) method, and the microcanonical RRKM approach has been used to compute rate constants for individual reaction steps and product branching ratios. The results show that the reaction can occur by abstraction and insertion mechanisms. The abstraction pathway producing CH(2Π) + CH3(2A2‘ ‘) has a barrier of 26.9 kcal/mol relative to the reactants. The insertion leading to the HC−CH3(3A‘ ‘) intermediate via a 12.2 kcal/mol barrier followed by its isomerization to H2C−CH2(3A1) (through a 1,2 H shift) and/or by dissociation with an H-atom loss is found to be a more favorable mechanism. At a low excess internal energy originating from the collision energy (12.2 kcal/mol), the sole reaction products are C2H3 + H, where 90% of them are formed through the fragmentation of HC−CH3 and the rest (10%) are produced via the H2C−CH2 intermediate. At the higher...

Application of Computational Kinetic Mechanism Generation to Model the Autocatalytic Pyrolysis of Methane

Abstract: An automated computational mechanism-generation technique is applied to construct elementary-step chemical kinetic reaction models for the pyrolysis of methane at 1038 K and 0.58 atm. Under these conditions, the pyrolysis process is extremely complex and exhibits autocatalysis. The mechanism-generation approach constructs a detailed set of elementary reactions, retrieves or estimates required reaction rates and thermochemistry, and constructs a kinetic model that gives excellent agreement with experimental data for several species. Key to the success is a newly developed capability of the algorithm to identify pressure-dependent chemically activated reactions. A rate-based species-selection methodology is used to determine kinetically significant species, and the algorithm is demonstrated to identify critical low-concentration byproducts. Multistep chemically activated reactions involving the formation of cyclopentadiene and subsequent hydrogen atom production are found to be important reactions, agreeing...

Elementary reactions of the phenyl radical, C6H5, with C3H4 isomers, and of benzene, C6H6, with atomic carbon in extraterrestrial environments

Abstract: Binary collisions of ground state carbon atoms, C( 3 Pj), with benzene, C6H6(X 1 A1g), and of phenyl radicals, C6H5(X 2 A1), with methylacetylene, CH3CCH(X 1 A1), were investigated in crossed beam experiments, ab initio calculations, and via RRKM theory to elucidate the underlying mechanisms of elementary reactions relevant to the formation of polycyclic aromatic hydrocarbons (PAHs) in extraterrestrial environments. The reactions of phenyl radicals with allene, H2CCCH2, and with cyclopropene, cyc-C3H4, as well as the reaction of benzyl radicals, C6H5CH2, with acetylene, HCCH, were also inves- tigated theoretically. The C( 3 Pj) atom reacts with benzene via complex formation to a cyclic, seven membered C7H5 doublet radical plus atomic hydrogen. Since this pathway has neither an entrance nor an exit barrier and is exoergic, the benzene molecule can be destroyed by carbon atoms even in the coldest molecular clouds. On the other hand, the reaction of phenyl radicals with methylacetylene has an entrance barrier; at high collision energies, the dynamics are at the boundary between an osculating complex and a direct pathway. Statistical calculations on the phenyl plus methylacetylene reaction demonstrate dramatic energy/temperature dependencies: at lower temperatures, the bicyclic indene isomer is the sole reaction product. But as the temperature increases to 2000 K, formation of indene diminishes in favor of substituted acetylenes and allenes, such as PhCCH, PhCCCH3, PhCHCCH2, and PhCH2CCH. Also, direct H-abstraction channels become accessible, forming benzene and C3H3 radicals, including propargyl. Similar conclusions were reached for the reactions of phenyl radicals with the other C3H4 isomers, as well as for the benzyl + acetylene reaction. The strong temperature dependence emphasizes that distinct product isomers must be included in reaction networks modeling PAH formation in extraterrestrial environments.

Theoretical Study of the Reaction Mechanism and Role of Water Clusters in the Gas-Phase Hydrolysis of SiCl4

Abstract: The energies and thermodynamic parameters of the elementary reactions involved in the gas-phase hydrolysis of silicon tetrachloride were studied using ab initio quantum chemical methods (up to MP4//MP2/6-311G(2d,2p)), density functional (B3LYP/6-311++G(2d,2p)), and G2(MP2) theories. The proposed mechanism of hydrolysis consists of the formation of SiCl 4-x(OH)x (x ) 1-4), disiloxanes Cl4-x(OH)x-1Si-O-SiCl4-x(OH)x-1, chainlike and cyclic siloxane polymers [-SiCl2sO-]n, dichlorosilanone Cl2SidO, and silicic acid (HO)2SidO. Thermodynamic parameters were estimated, and the transition states were located for all of the elementary reactions. It was demonstrated that the experimentally observed kinetic features for the hightemperature hydrolysis are well described by a regular bimolecular reaction occurring through a four-membered cyclic transition state. In contrast, the low-temperature hydrolysis reaction cannot be described by the traditionally accepted bimolecular pathway for SisCl bond hydrolysis because of high activation barrier (Ea ) 107.0 kJ/mol, ¢G q ) 142.5 kJ/mol) nor by reactions occurring through three- or four-molecular transition states proposed earlier for reactions occurring in aqueous solution. The transition states of SiCl 4 with oneand two-coordinated water molecules were located; these significantly decrease the free energy of activation ¢G q (to 121.3 and 111.5 kJ/mol, correspondingly). However, this decrease in ¢G q is not sufficient to account for the high value of the hydrolysis rate observed experimentally under low-temperature conditions.

In Situ Diagnostics and Modeling of Methane Catalytic Partial Oxidation on Pt in a Stagnation-Flow Reactor

Abstract: The effect of catalyst temperature on the partial oxidation reaction pathways of methane over platinum is investigated in a stagnation-flow reactor. A new experimental method is described that uses Raman spectroscopy to measure the concentration of CH 4 along the centerline of the reactor with the Pt surface at 900-1100 °C. The method permits the direct comparison of the measured reactivity with that predicted by the CHEMKIN SPIN stagnation-flow code when combined with recently developed partial oxidation elementary reaction mechanisms. A significant increase in the reactivity of CH 4 on Pt is observed between 1000 and 1100 °C, concurrent with an increase in the selectivity to H 2 and CO. No single mechanism available in the literature correctly models the increase in reactivity or the change in selectivity in this temperature range. The experimental results are interpreted by examining the competitive adsorption between CH 4 and O 2 and the two pathways by which CH 4 can undergo dissociative adsorption. The temperature dependence of the sticking coefficient for the direct dissociative adsorption of CH 4 is specifically identified as an important yet highly uncertain parameter in the reaction mechanism.

New approaches to the deduction of complex reaction mechanisms.

Abstract: The formulation of a macroscopic reaction mechanism, the sequence of elementary reaction steps by which reactants are turned into products, is difficult. We review several new methods of determining the causal connectivity of chemical species, the reaction pathway (the sequence of chemical species), and the reaction mechanisms of complex reaction systems from prescribed measurements and theories.

The reaction of acetaldehyde and propionaldehyde with hydroxyl radicals: experimental determination of the primary H2O yield at room temperature

Abstract: The branching fraction of H-abstraction in the elementary reactions of acetaldehyde and propionaldehyde with OH at 290 K was determined directly using a fast-flow reactor coupled to a molecular beam sampling mass spectrometry apparatus. The primary-product H 2 O yield of the title reactions was quantified relative to that of the isobutane+OH reaction, and found to be 89±6 and 100±10% for the reactions of acetaldehyde+OH and propionaldehyde+OH, respectively. Furthermore, an upper limit of 3% could be determined for the yield of formic acid in the hypothetical addition/elimination reaction pathway. We conclude that the reaction of OH radicals with aldehydes proceeds predominantly, if not exclusively, via H-abstraction, forming H 2 O and RCO.

A many-particle derivation of the Integral Encounter Theory non-Markovian kinetic equations of the reversible reaction A+B⇄C in solutions

Abstract: The non-Markovian binary kinetic equations for a rather realistic model of association–dissociation reaction A+B⇄C in liquid solutions are derived using a newly developed method based on a many-particle consideration. The method leads to an infinite hierarchy for correlation patterns that can be truncated at the first step on a two-particle level to give the Integral Encounter Theory result. It is shown that although being restricted in time, the theory predicts true equilibrium concentrations of reactants and the equilibrium constant corresponding to the thermodynamic detailed balancing principle.

Theory of intrapolymer excimer-formation kinetics

Abstract: We generalize the Wilemski–Fixman theory for reversible polymer cyclization to treat the kinetics of intrachain excimer-formation reactions. While most previous theories for intrachain reactions dealt with the end-to-end reaction case, we consider the general situation in which the reacting groups are located at any place on the chain backbone. Various aspects of the reaction kinetics, such as the effect of hydrodynamic interaction and the dependence of reaction rate on the positions of reacting groups as well as on the chain length, are investigated.

Calculating Reversible Potentials for Elementary Reactions in Acid and Base from Model Reaction Energies

Abstract: Reversible potentials, U°, have been calculated based on reaction energies, Er, for the intermediate steps in the outer-sphere oxygen reduction reaction to water. The working formula is U° = (−Er eV-1 + c) V, where U° is the reversible potential and the reference energy of the electron is −4.6 eV on the physical (vacuum) or 0 V on the hydrogen scale. Results using a 6-31G** basis in the B3LYP hybrid density functional theory are shown to be comparable to ab initio MP2 results obtained previously for acid solution with c = 0.49 for the former and 0.50 for the latter. Both methods are shown to work for basic solution but, unlike in acid, the c values are different, 0.58 for B3LYP and 0.76 for MP2, reflecting differences in the two computational methods in treating anions.

Possible molecular hydrogen formation mediated by the radical cations of anthracene and pyrene

Abstract: Hydrogen molecules cannot be formed readily by the association of gaseous hydrogen atoms. Possible H2 formation mediated by the radical cations of typical polycyclic aromatic hydrocarbons (PAHs), anthracene and pyrene, was studied at the B3LYP/6-31G** level of theory. We presumed that H2 is formed by way of two elementary reactions: the addition of an H atom to a PAH molecular cation, and the H abstraction from the resulting monohydro-PAH cation (i.e., arenium ion) by a second H atom to yield H2. The first reaction takes place without any activation energy. The second reaction is also predicted to proceed along almost barrierless pathways, although it is far from being a typical ion–molecule reaction. There is a possibility that these reactions might constitute one of the mechanisms for H2 formation in extremely cold interstellar space. Deuterium enrichment in PAH cations is possibly accompanied by such H2 formation because deuteration lowers the energies of polyatomic PAH cations appreciably. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1378–1382, 2003

Kinetics of Elementary Reactions in the Chain Chlorination of Cyclopropane

Abstract: The kinetics of elementary reactions involved in the chain chlorination of cyclopropane are examined using a combination of absolute and relative rate constant measurements and first principles ele...