Additive effect of hydrocarbons on OH radical production in H2 oxidation

Mixtures of hydrocarbons (methane, allene, propyne, propene, and propane)–H₂–O₂ highly diluted with argon were heated to a temperature ranging from 1200 to 1900 K behind reflected shock waves, and the additive effects of methane, allene, propyne, propene, and propane on OH radical production in H₂ oxidation were studied by observing time‐resolved UV‐absorption (306.7 nm). It was found that, in H₂ oxidation below 1500 K, the addition of these hydrocarbons prolonged the delay time of the onset of the rapid OH radical production. An analysis using reported kinetic modeling of C₁–C₄ oxidation gave valuable information for reactions between hydrocarbons and H, O atoms and OH radicals. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 37: 50–55, 2005     

Reaction Rates of t-Butyl and Pivaloyl Radicals in Solution

The rate constants of the unimolecular decomposition of the pivaloyl radical (k_D) and of the bimolecular self terminations of pivaloyl (k₁) and t-butyl radicals (k₂) in liquid methylcyclopentane are determined by ESR.-spectroscopy: The viscosity dependence of (k₂) is analysed with respect to diffusion control of the reaction. Comparison of (k_D) values of different acyl radicals reveals a strong dependence of the activation energies on radical structure.     

Unimolekulare CH3-, CH4-, C2H4- und C3H4-Eliminierungen aus alkin- und arylsubstituierten Isopropylkationen in der Gasphase

Elimination of methyl, methane, ethylene, allene (or propyne) from two [C₃H₆Y]+ isomers (Y = p-(HC?C)? C₆H₄- and C₆H₅? C?C?) has been investigated using 13C and deuterium labelled compounds. It can be shown that the fragmentations are accompanied by extensive but not complete rearrangement of the carbon skeleton as well as by migration of the hydrogen atoms, the extent of which is determined by the reaction channel.     

Isotope Effects and Hydrogen Exchange Mechanism in Compounds Having Labile Aliphatic CÄH Bonds in Basic Liquid Ammonia Solutions

Specific features of the stepwise hydrogen exchange mechanism and transition state structure in the systems acetophenone-liquid ammonia in the absence of foreign compounds and in the presence of bases and toluene-liquid ammonia in the presence of potassium amide were studied in terms of an approach based primary and secondary kinetic isotope effects of the substrate and the solvent. The mechanisms of reactions involving acetophenone and toluene were compared. In the first case, an elementary act of CH-acid ionization is contributed to a small extent by diffusion-controlled separation of the carbanion and ammonia molecule. hydrogen exchange in toluene is characterized by complete absence of the internal ion pair return effect. The ratio k _D NH ₃/k _T NH ₃ for hydrogen exchange in acetophenone tends to decrease on addition of bases (with simultaneous increase in its rate), which may be explained by formation of an adduct via interaction between the unshared electron pair on the heteroatom in the base molecule and the carbonyl carbon atom. The anomalous temperature dependence of k _D NH ₃/k _T NH ₃ for hydrogen exchange in toluene is interpreted as a result of contribution of side metalation of the CÄH bond by potassium amide. The change in the solvent protophilicity due to replacement of the "light" solvent by deuterated one differently affects the kinetics and mechanism of hydrogen exchange in acetophenone and toluene. Measurements of the -deuterium effect gave information on the mode of angular deformation of CÄD bonds in the methyl group of toluene in the hydrogen exchange transition state.     

Intramolecular isotopic effect in the pyrolysis of ethyl-2d1 chloride

The thermal decomposition of deuterated ethyl chloride CH₂DCH₂Cl was studied in a static system in the pressure range of 0.1–26 torr, and the Arrhenius expression for the overall decomposition at the high-pressure limit and in the temperature range of 670–1100 K was found to be The intramolecular isotopic effects were first examined in the pressure range of 0.1–26 torr at 837 K, and the branching ratio k_H/k_D was found to decrease with increasing pressure. The RRKM-theory calculations describe the experimental data well. The intramolecular isotopic effect was also examined in the temperature range of 728–926 K, and the branching ratio at the high pressure limit was given by the expression when k_H and k_D are the rate constants for the HCl and DCl channels of elimination. The Arrhenius A factors obtained at the high-pressure limit together with the temperature-dependent expression of the branching ratio provided additional experimental data for an assignment (fine-tuned) of the vibrational frequencies of both activated complexes involved in the thermal decomposition of CH₂DCH₂Cl. The evaluated vibrational frequencies were then used in the RRKM calculations describing the pressure dependence of the intramolecular isotopic effect. The RRKM calculations and the experimental data were in good agreement, supporting the choice of vibrational frequencies for both the activated complexes as well as the transition-state model.     

The kinetics of the gas-phase reaction between ozone and alkenes

The rate law ? d[O₃]/ dt = k₁[A][O₃] + k₃[A][O₃]²/ (k₄ + k₅[O₂]) has been found to obtain for the reaction of ozone with allene and with 1,2-butadiene. We now find that this rate law also applies to the reaction of ozone with ethylene and presumably with all lower alkenes. This generalizes the inhibiting effect of oxygen and accounts for the simplifed rate law found in the presence of excess oxygen. Oxygen itself is a product of the ozone–ethylene reaction, and we find that as [O₃]₀ increases, the (O₂ formed)/(O₃ used) ratio approaches 1.5. Values of k₁, k₃/k₅ for ethylene are compared with those for allene, 1,3-butadiene, and propene. A generalized mechanism is postulated for the reaction of ozone with alkenes involving a chain sequence that produces oxygen and which accounts for the observed rate law. A specific mechanism is postulated for the reaction of O₃ with ethylene, and the thermochemistry of the chain sequence is examined in detail.     

Reactions of methyl radicals with acetaldehyde and acetaldehyde-d1. I. relative rates of atom abstraction reactions at 785°K

The role of ethenoxy radicals in the pyrolysis of CH₃CDO was studied by mass spectrometric analysis of the isotopic composition of the methane, ethane, and recovered aldehyde. Experimental evidence was obtained for the formation of ethenoxy radicals and for their reaction with acetaldehyde. Mixtures of CH₃CHO and CH₃CDO were pyrolyzed in order to minimize H-D scrambling in the methyl group of the aldehyde. A kinetic treatment of the methyl radical reactions and furnished the rate constant ratios (k_2a + k_2b)/k_1a = 2.7 and k_1b/k_1a = 0.62 at 785°K. It is concluded that at the usual temperatures of CH₃CHO pyrolysis the rate of alkyl hydrogen capture is comparable to that of formyl hydrogen abstraction. The results and conclusions are discussed and compared with previous work.     

Kinetics and mechanism of the reaction H + CH3ONO

The reaction of hydrogen atoms with methyl nitrite was studied in a fast-flow system using photoionization mass spectrometry and excess atomic hydrogen. The associated bimolecular rate coefficient can be expressed by in the temperature range of 223-398°K. NO, CH₃OH, CH₄, C₂H₆, CH₂O, and H₂O are the main products; OH and CH₃ radicals were detectable intermediates. The mechanism was deduced from the observed product yields using normal and deuterated reactants. The primary reaction steps were identified as followed by a rapid unimolecular decomposition of CH₂ONO into CH₂O and NO. Since the extent of reaction channel (1b) could not be determined independently, only extreme limits could be obtained for the individual contributions of the two channels of reaction (3) which follows the generation of CH₃O radicals: The most probable values, k_3a/k₃ = 0.31 ± 0.30 and k_3b/k₃ = 0.69 ± 0.30, support the previous results on this reaction, although the range of uncertainties is much greater here.     

Reaction of H and D atoms with deuterated propylenes

Effects of deuterium substitution in propylene on the relative rates of H(D) atom abstraction from and addition to the olefin, and on the orientation of H(D) atom addition, have been studied in the gas phase at room temperature. Effects of isotopic substitution of the olefinic hydrogen atoms on abstraction could not be observed, but abstraction is reduced five- to tenfold by deuteration of the methyl group. Deuteration of either olefinic position enhances the rate of addition to the substituted carbon atom. Disproportionation-combination ratios for deuterium-substituted propyl radicals are not greatly different from those for unsubstituted radicals, the largest effect being for C₃D₇ radicals, for which the overall k_d/k_c is reduced 10–15%.     

Dynamics of the enzymatic oxidation of methane

Kinetic isotope effect data for the oxidation of deuterium-substituted methane molecules with methane monooxygenase (MMO) are analyzed in the framework of a multistep nonradical mechanism. New evidence is obtained in favor of the hypothesis of the intermediate formation of a complex containing pentacoordinated carbon. A kinetic scheme whose first step involves two hydrogen molecules of the substrate being oxidized is considered. For coincidence between the calculated and experimental distributions of the oxidation products of partially deuterated methane, the formation of the intermediate complex containing pentacoordinated carbon must be reversible and the rate of the back decomposition of this complex must be substantially higher than the rate of its formation (w _?1 ? w ₁). The experimental distribution of the products of deuterated methane (CH₃D, CH₂D₂, and CHD₃) hydroxylation with MMO, which could not earlier be explained within the widely accepted oxygen rebound mechanism, is quantitatively explained for the first time in terms of the dynamics of a nonradical mechanism using parameters having a simple physical meaning and plausible values.     

Etching of GaAs (100) with gaseous H/CH3 mixtures

GaAs (100) wafers were etched in mixtures of hydrogen atoms and methyl radicals. The atoms were formed in a remote hydrogen plasma, and a fraction of these were converted into methyl radicals by introducing methane into the flow system upstream from the semiconductor surface. The flux of hydrogen atoms into the reaction chamber was determined by isothermal calorimetry. The methyl radical flux passing over the substrate was then calculated using previously determined rate parameters for the reaction between atomic hydrogen and methane, and a simple modeling program. The GaAs etch rates were about an order of magnitude faster when methyl radicals were present in the hydrogen atom stream, and were found to follow a first-order dependence on the partial pressure of methyl radicals. Absolute rate constants were determined and an Arrhenius activation energy of 1.2 kcal mol^?1 was calculated. The values of k and E_a are consistent with a diffusion-controlled process. SEM photographs of the surface revealed small crystallographic features that made the surface appear very rough. XPS analysis indicated that these surfaces were arsenic deficient. A mechanism is proposed for the etching of GaAs by a combination of methyl radicals and hydrogen atoms. © 1994 John Wiley & Sons, Inc.     

Mechanism of electrophilic fluorination of aromatic compounds with NF-reagents

Kinetic isotope effects H/D in electrophilic fluorination of aromatic compounds with NF-reagents were investigated. The small values of k _H/k _D (0.86–1.00) are in agreement with the polar reaction mechanism where the Wheland complex decomposition is not the limiting stage. The fluorination of 1,3,5-trideuterobenzene was established by ¹H and ¹⁹F NMR spectroscopy to occur with a 1,2-migration of a hydrogen (deuterium) atom. The analysis of Brown-Stock relationship demonstrated that the activity of NF-reagents exceeded that of many known electrophilic systems including halogenation, but it was essentially less than the activity of elemental fluorine.     

Kinetic simulation of the enzymatic oxidation of methane

The kinetics of methane oxidation by methane monooxygenase is simulated numerically. Literature data on the distribution of products of the oxidation of deuterated methane CH_{4 ? n} D _n (CH₃D, CH₂D₂, and CHD₃), as well as on the kinetic isotope effect in the competitive oxidation of CH₄ and CD₄ by methane monooxygenase, are analyzed in the framework of a nonradical multistep mechanism. Kinetic schemes whose first step involves two hydrogen atoms of the oxidation substrate are considered. The kinetic models suggested for methane oxidation are in good agreement with experimental data.     

Acetylene-allene rearrangement of propargyl systems X—CH2—C≡CH (X = H,Me, NMe2, OMe,F, SMe): an ab initio study

Acetylene—allene rearrangement in propargyl systems XCH₂CHCH (X = H, Me, NMe₂, OMe, F, and SMe) was studied using the ab initio approach. The relative stabilities of the starting and final propyne structures and the corresponding allenes as well as the structure of intermediate carbanions were considered. n--Conjugation was shown to dominate in allene stabilization while the inductive effect of heteroatomic substituents makes at least comparable contribution to stabilization (or destabilization) of the propynyl structure. In particular, relative instability of 1-methoxypropyne can be rationalized by high electronegativity of O atom, which leads to dramatic decrease in the total electron density in the region of the neighboring CC triple bond. The influence of substituents on the mobility of the migrating proton was considered for the gas phase and with solvation effects included. Calculations involving electron correlation at the MP2 level of theory were shown to be insufficient for correct reproduction of the energy differences between the corresponding propynes and allene structures. The results of MP4 calculations with inclusion of ZPE correction are in good agreement with the available experimental data.     

Mechanism of olefin polymerization on supported Ziegler-Natta catalysts based on data on the number of active centers and propagation rate constants

The number of active centers (C _g) and propagation rate constants (k _g) for the polymerization of propylene and ethylene on highly active titanium-magnesium catalysts (TMCs) of different compositions at 70°C were determined using the method of ¹⁴CO inhibition of polymerization. In the polymerization of propylene on the TiCl₄/D₁/MgCl₂-AlEt₃/D₂ system (D₁ is dibutyl phthalate or 2,2-diisobutyl-1,3-dimethoxypropane; D₂ is a silane), the effects of D₁ and D₂ donors on the values of C _g and k _g were studied. It was found that the donors decreased the values of k _g for nonstereospecific centers, had no effect on the values of k _g for stereospecific centers, and increased the fraction of stereospecific centers, as well as the fraction of sleeping centers regardless of their stereospecificity. The rate constants of isotactic-chain transfer with C₃H₆, AlEt₃, H₂,and ZnEt₂ were determined. In the polymerization of ethylene, a number of TMCs exhibited strong diffusion limitations, which manifested themselves in a dramatic decrease in the determined values of k _g. It was demonstrated that diffusion limitations can be removed by decreasing the particle size and the concentration of active centers and by performing prepolymerization with propylene. The values of k _g in ethylene polymerization were similar for stereospecific and nonstereospecific centers.__________Translated from Kinetika i Kataliz, Vol. 46, No. 2, 2005, pp. 180–190.Original Russian Text Copyright © 2005 by Bukatov, Zakharov, Barabanov.     

Correlation of activation energies of radical–molecule metathesis reactions with enthalpic,polar, and steric parameters

It is shown that the activation energies E oF chlorine atom abstraction by cyclohexyl radicals and hydrogen atom abstraction by Cl atoms from polychloroalkanes can be correlated with the bond dissociation energies D and the Taft polar and steric substituent constants σ* and E_s by the expression: where ΔE and ΔD represent the differences between the E and D values of a given substrate and those of a reference compound (CH₃ substituted substrate) and α, ρ, and δ are the corresponding correlation coefficients. The use of this expression allows quantitative evaluation of the relative contribution of the various factors affecting the activation energies of these reactions and estimation of related thermochemical data.     

The influence of hydrogen on the kinetics of plasmapyrolytic methane conversion

The plasma pyrolysis of methane in the presence of excess hydrogen was kinetically modeled by means of a single pulse shock tube (SPST) technique. Pyrolytic decomposition of methane and the product formation including quenching was investigated as a function of temperature and dwell time in order to elucidate the kinetic role of hydrogen. Methane pyrolysis with an excess of hydrogen leads to lower conversion degrees and lower product yields as compared to pure methane. The decay curves may be described kinetically by first- and second-order ate equations, depending on reaction conditions. An excess of hydrogen causes a decrease in the overall rate constants for the methane decomposition and a straightening of the Arrhenius plots. According to the reaction mechanisms the recombination of primary radicals, especially CH₃, with H₂ molecules and H atoms is significant.     

Disproportionation-to-combination ratios for cyclohexyl-h11 and -d11 radicals in the gas phase as determined by the radiolysis of water vapor–cyclohexane mixtures

In the radiolysis of water vapor containing small concentrations of cyclohexane, the principal products which account for about 98% of all end products are found to be hydrogen, cyclohexene, and bicyclohexyl. Cyclohexene and bicyclohexyl yields were determined over a range of temperatures (70–200°C), total pressures (50–2400 torr), and total doses (0.15–2.0 Mrad). The disproportionation–combination ratio k/k for c-C₆H₁₁ radicals could be determined as 0.56 ± 0.01 from the ratio of cyclohexene to bicyclohexyl yield. By using c-C₆D₁₂, the ratio k/k for c-C₆D₁₁ radicals is found to be 0.38 ± 0.01. Comparison of the reactivity pattern of C₆H₁₁ and C₆D₁₁ radicals leads to (k)/(k)/(k/k) = 1.47 ± 0.02. The corresponding values for the reactions of c-C₆H₁₁ with c-C₆D₁₁ were also determined.     

Observation of Spontaneous C=C Bond Breaking in the Reaction between Atomic Boron and Ethylene in Solid Neon

A ground‐state boron atom inserts into the C=C bond of ethylene to spontaneously form the allene‐like compound H₂CBCH₂ on annealing in solid neon. This compound can further isomerize to the propyne‐like HCBCH₃ isomer under UV light excitation. The observation of this unique spontaneous C=C bond insertion reaction is consistent with theoretical predictions that the reaction is thermodynamically exothermic and kinetically facile. This work demonstrates that the stronger C=C bond, rather than the less inert C?H bond, can be broken to form organoboron species from the reaction of a boron atom with ethylene even at cryogenic temperatures.     

Oxidation of propene in the gas phase

A series of laboratory and modelling experiments on the oxidation of propene in the gas phase has been undertaken to determine conditions which give high yields of propene oxide. The conditions under which the experiments were conducted were 505–549 K and up to 4 bar pressure. It is proposed that propene oxide is formed from propene by reaction with several peroxy radicals including HO₂ and CH₃CO₃. However, one of the more important radicals is hydroxypropylperoxy. Its reaction with propene, under these conditions is more important than concerted decomposition to formaldehyde and acetaldehyde. © 1995 John Wiley & Sons, Inc.