The mechanism of the reaction of ketene with methyl radical has been studied by ab initio CCSD(T)‐F12/cc‐pVQZ‐f12//B2PLYPD3/6‐311G** calculations of the potential energy surface. Temperature‐ and pressure‐dependent reaction rate constants have been computed using the Rice–Ramsperger–Kassel–Marcus (RRKM)–Master Equation and transition state theory methods. Three main channels have been shown to dominate the reaction; the formation of the collisionally stabilized CH3COCH2 radical and the production of the C2H5 + CO and HCCO + CH4 bimolecular products. Relative contributions of the CH3COCH2, C2H5 + CO, and HCCO + CH4 channels strongly depend on the reaction conditions; the formation of thermalized CH3COCH2 is favored at low temperatures and high pressures, HCCO + CH4 is dominant at high temperatures, whereas the yield of C2H5 + CO peaks at intermediate temperatures around 1000 K. The C2H5 + CO channel is favored by a decrease in pressure but remains the second most important reaction pathway after HCCO + CH4 under typical flame conditions. The calculated rate constants at different pressures are proposed for kinetic modeling of ketene reactions in combustion in the form of modified Arrhenius expressions. Only rate constant to form CH3COCH2 depends on pressure, whereas those to produce C2H5 + CO and HCCO + CH4 appeared to be pressure independent. 相似文献
For the experimental determination of the equilibrium constant of the reaction CH3 + O2 ? CH3O2 (1), the process of methane oxidation has been studied over the temperature range of 706–786 K. The concentration of CH3O2 has been measured by the radical freezing method, and that of CH3 from the rate of accumulation of ethane, assuming that C2H6 is produced by the reaction CH3 + CH3 → C2H6 (2). The equilibrium constant of reaction (1) has been obtained at four temperatures. For the heat of the reaction the value Δ?H298 = -32.2 ± 1.5 kcal/mol is recommended. 相似文献
The pyrolysis of 2% CH4 and 5% CH4 diluted with Ar was studied using both a single–pulse and time–resolved spectroscopic methods over the temperature range 1400–2200 K and pressure range 2.3–3.7 atm. The rate constant expressions for dissociative recombination reactions of methyl radicals, CH3 + CH3 → C2H5 + H and CH3 + CH3 → C2H4 + H2, and for C3H4 formation reaction were investigated. The simulation results required considerably lower value than that reported for CH3 + CH3 → C2H4 + H2. Propyne formation was interpreted well by reaction C2H2 + CH3 → P-C3H4 + H with ?? = 6.2 × 1012 exp(?17 kcal/RT) cm3 mol?1 s?1. 相似文献
The reduction of R*–SiBr2–SiBr2–R* ( 2 ) with NaR* (R* = supersilyl = SitBu3) in presence of C2H4 provides a white crystalline solid (η2‐C2H4)R*Si–SiR*(Br)(CH2–CH2–R*) ( 3 ) characterized by X‐ray diffraction analysis. Compound 3 is accompanied with an impurity of R*(Br)2Si–Si(Br)(R*)(CH2–CH2–R*) ( 4 ). The formation of 3 and 4 runs complicated because of several reactive partners. However, reduction of 2 with sodium naphthalenide in presence of ethene runs straightforward with formation of a mixture of tetrahedrane R*4Si4 ( 1 ) and bis(silirane) R*(η2‐C2H4)Si–Si(η2‐C2H4)R* ( 5 ). The latter is formed by [1+2]‐cycloaddition reaction of intermediate disilyne R*Si≡SiR* with ethene. Compound 5 has been characterized by X‐ray structure determination. The 1H NMR spectrum of the silacyclopropane ring protons shows AA′BB′ complex spectrum comprising of 2 sets each of 12 transitions. 相似文献
The ESR method is used to study the oxidation kinetics of the CH3, C2H5, n-C4H9, i-C4H9, s-C4H9, t-C4H9, n-C6H13, C6H11, C6H5CH2, CH3C6H4CH2, and C6H5CH2CH2 radicals in methanol matrix at 87 K. The reaction kinetics are shown to be describable in terms of a time-dependent rate constant k(t). The contribution from the matrix relaxation to k(t) has been determined. The oxidation rate and the shape of the kinetic curve are independent of the type of the radical. Models interpreting the experimental data are discussed. 相似文献
The exponential relaxation of CH3, produced by the reaction O + C2H4 → CH3 + HCO, to its steady-state concentration was quantitatively monitored after the reactants were mixed. The relaxation profiles yield the rate constant of the reaction O + CH3 → H2CO + H equal to (1.85 ± 0.28) × 10-10 cm3/molecule-sec at 300°K. Ancillary experiments yielded values for the rate constant for the reaction of O atoms with C2H4 at 300°K, the average of which is 7.7 × 10-12 cm3/molecule-sec. The experimental technique, which employs a fast-flow reactor coupled to a photoionization mass spectrometer, is described in detail and its potential discussed. 相似文献
The mechanism for the C2H3 + CH3OH reaction has been investigated by the Gaussian‐4 (G4) method based on the geometric parameters of the stationary points optimized at the B3LYP/6–31G(2df, p) level of theory. Four transition states have been identified for the production of C2H4 + CH3O (TSR/P1), C2H4 + CH2OH (TSR/P2), C2H3OH + CH3 (TSR/P3), and C2H3OCH3 + H (TSR/P4) with the corresponding barriers 8.48, 9.25, 37.62, and 34.95 kcal/mol at the G4 level of theory, respectively. The rate constants and branching ratios for the two lower energy H‐abstraction reactions were calculated using canonical variational transition state theory with the Eckart tunneling correction at the temperature range 300–2500 K. The predicted rate constants have been compared with existing literature data, and the uncertainty has been discussed. The branching ratio calculation suggests that the channel producing CH3O is dominant up to about 1070 K, above which the channel producing CH2OH becomes very competitive. 相似文献
The reaction pathways of n-butoxy and s-butoxy radicals have been investigated by TLC and HPLC analysis of end products, particularly peroxides and carbonyl compounds. The butoxy radicals were produced by the pyrolysis of very low concentrations of the corresponding dibutylperoxide in an atmosphere of oxygen and nitrogen, at atmospheric pressure. The decomposition reaction (3) s-BuO → C2H5 + CH3CHO and the reaction (2) s-BuO + O2 → HO2 + CH3COC2H5 have been studied, and the ratio k3/k2 has been determined in the temperature range 363–503 K by kinetic modeling of the formation of the observed acetaldehyde and methylethylketone. The rate constant k3 obtained was: A good agreement was observed between experimental data and RRKM theory. The implications of the results for atmospheric chemistry and combustion are discussed. At room temperature, the reaction with O2, yielding HO2 radicals and methylethylketone is, by far, the main channel for s-BuO radicals. In the field of low temperature combustion, the decomposition of s-BuO radicals producing C2H5 and CH3CHO is the main pathway; the route s-BuO + O2 decreases tremendously in importance as the temperature is raised above 393 K. 相似文献
Conventional transition-state theory is used for extrapolating rate coefficients for reactions of O atoms with alkanes to temperatures above the range of experimental data. Expressions are developed for estimating structural properties of the activated complex necessary for calculating enthalpies and entropies of activation. Particular attention is given to the problem of the effect of the O atom adduct on the internal rotations in the activated complex. Differences between primary, secondary, and tertiary attack are discussed, and the validity of representing the activated complexes of all O + alkane reactions by a fixed set of vibrational frequencies and other internal modes is evaluated. Experimental data for reactions of O atoms with 15 different alkanes (CH4, C2H6, C3H8, C4H10, C5H12, C6H14, C7H16, C8H18, i–C4H10, (CH3)4C, (CH3)2CHCH(CH3)2, (CH3)3CC(CH3)3, c–C5H10, c–C6H12, c–C7H14) are reviewed. The following approximate expressions for ΔS?(298) and E(298), the entropy and energy of activation, respectively, are consistent with the experimental data and with the calculations: where nC = number of carbon atoms in the alkane and nH = the number of “equivalent” H atoms. Using the conventional transition state theory expression, k(298) = 1015.06 exp(ΔS?/R) exp(–E(298)/298R) L mol?1s?1, one then obtains: These expressions agree with experimental values within a factor approximately 2 for alkanes larger than C3H8. 相似文献
The reaction C2H5 + O2 → C2H5O2 in glassy methanol-d4 and the H-atom abstraction by CH3, C2H5, and n-C4H9 radicals in C2H5OH + C2D5OH and CD3CH2OH + C2D5OH glassy mixtures have been studied by electron spin resonance. The analysis of the dependence of the reaction rates on the concentration of O2 (oxidation) and C2H5OH, CD3CH2OH (H-atom abstraction) has shown that the √t law is not conditioned by the existence of regions characterized by different rate constants. 相似文献
Summary Temperature-programmed desorption (TPD) of CH4, C2H6, C2H4, and CO and temperature-programmed pulse surface reactions (TPSR) of CH4, C2H6, C2H4, CO, and CO/H2 over a Co/MWNTs catalyst have been investigated. The TPD results indicated that CH4 and C2H6 mainly exist as physisorbed species on the Co/MWNTs catalyst surface, whilst C2H4 and CO exist as both physisorbed and chemisorbed species. The TPSR results indicated that CH4 and C2H6 do not undergo reaction between room temperature and 450oC. Pulsed C2H4 can be transformed into CH4 at 400 oC whilst pulsed CO can be transformed into CO2 at 100 or 150oC. In gaseous mixtures of CO and H2 containing excess CO, the products of pulsed reaction were CH3CHO and CH3OH. When the ratio of CO and H2 was 1:2, pulsed CO and H2 were transformed into CH3CHO, CH3OH and CH4. In H2 gas flow, pulsed CO was transformed into a mixture of CH3CHO and CH4 between 200 and 250oC and was transformed into CH4 only above 250oC. 相似文献
A polydimethylsiloxane (PDMS) membrane was synthesized and permeation behavior of ternary gas mixtures including C3H8, CH4 and H2 through it was studied as a function of operating parameters. Mixed gas permeability values were also compared with pure gas data as well as literature to validate experimental results. The aim was to predict separation factor (SF) of C3H8 as a function of feed temperature, pressure, flow rate and C3H8 concentration with the aid of artificial neural network (ANN) technique. Multilayer perceptron (MLP), which is the most common type of feedforward neural network (FFNN), was used for prediction. The Levenberg–Marquardt training method was initially employed to train the net. Then, optimum numbers of hidden layers and nodes in each layer were determined. The selected structure (4:4:5:1) was finally used to predict SF of C3H8 for different inputs in the domain of training data. The modeling results showed that there is an excellent agreement between the experimental data and the predicted values, with mean absolute errors of less than 1%. Both modeling and experimental results confirmed that increasing feed temperature, feed pressure and C3H8 concentration in feed debilitates separation performance; however, SF increases with increasing feed flow rate. As a result, ANN can be recommended for the modeling of mixed gas transport through dense membranes such as PDMS. 相似文献
High-temperature (>1000°K) pyrolysis of acetaldehyde (~1% in an atmosphere of pure nitrogen) was examined in a turbulent flow reactor which permits accurate determination of the spatial distribution of the stable species. Results show that the products in order of decreasing importance are CO, CH4, H2, C2H6, and C2H4. Rates of formation were consistent with the Rice–Herzfeld mechanism by including reactions to explain C2H4 formation and the possible presence of ketene. A steady-state treatment of the complete mechanism indicates that the overall reaction order decreases from \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{3}{2} $\end{document} to 1, which is supported by the new experimental data. Using earlier low-temperature results, the rate constant for the reaction CH3CHO → CH3 + CHO (1) was found as k1=1015.85±0.21 exp (?81,775±1000/RT) sec?1. Also, data for the ratio of rate constants for reactions CH3CHO + CH3 → CH4 + CH3CO (4) and 2CH3 → C2H6(6) were fitted to the empirical expression k4/k61/2=10?13.89±0.03T6.1 exp(?1720±70/RT) (cm3/mole·sec)1/2 and causes for the curvature are discussed. The noncatalytic effect of oxygen on acetaldehyde pyrolysis at high temperature is explained. 相似文献