John M. Roscoe

The mechanism of ethylene pyrolysis at small conversions

Kinetic modelling is used in conjunction with measurements of product yields to develop a mechanism for the pyrolysis of ethylene at 896K and ethylene pressures ranging from approximately 3 to 78 kPa. An induction period was observed for all products except H2, and was followed by a steady rate, which was of second-order for all products except 1,3-C4H6, the most abundant product. The mechanism quantitatively accounts for the yields of H2, CH4, C2H6, C3H6, 1-C4H8 and 1,3-C4H6. The reaction is initiated by disproportionation of C2H4 and the product 1,3-C4H6 results from decomposition of the C4H7 radical, formed by addition of C2H3 to C2H4. The other organic products that were measured are formed as a result of reactions involving the C2H5 radical. The hydrogen is produced by abstraction from C2H4 by atomic hydrogen and its rate is controlled by the reaction C2H5 C2H4 + H which is nearly equilibrated. The main termination reaction is recombination of C2H5. The auto-acceleration which is evident particularly in the yields of H2, CH4, C2 H6, and C3H6 is accounted for by the decomposition of 1-C4H8.

Temperature Dependence of the Rate Coefficients for the Reactions of Br Atoms with Dimethyl Ether and Diethyl Ether

The rate constants for the reactions of atomic bromine with dimethyl ether and diethyl ether were measured from approximately 300 to 350 K using the relative rate method. Both isooctane and isobutane were used as the reference reactants, and the rate constants for the reactions of these hydrocarbons were measured relative to each other over the same temperature range. The kinetic measurements were made by photolysis of dilute mixtures of bromine, the reference reactant, and the test reactant in mixtures of argon and oxygen at a total pressure of 1 atm. The resulting ratios of rate constants were combined with the absolute rate constant as a function of temperature for the reference reaction of Br with isobutane to calculate absolute rate constants for the reactions of Br with isooctane, dimethyl ether, and diethyl ether. The absolute rate constant, in the units cm3 molecule(-1) s(-1), for the reaction of Br with dimethyl ether was given by k = (3.8 +/- 2.4) x 10(-10) exp(-(3.54 +/- 0.21) x 10(3)/T) while for the reaction of Br with diethyl ether the rate constant is given by k = (2.8 +/- 2.7) x 10(-10) exp(-(2.44 +/- 0.32) x 10(3)/T). On the same basis, the rate constant for the reaction of Br with isooctane is given by k = (3.34 +/- 0.59) x 10(-12) exp(-(1.80 +/- 0.11) x 10(3)/T). In each case, the activation energy of the reaction is significantly smaller than the endothermicity of the reaction. This is discussed in terms of a complex mechanism for these reactions.

Temperature Dependence of the Rate Coefficients for the Reactions of Atomic Bromine with Toluene, Tetrahydrofuran, and Tetrahydropyran

The rate coefficients for the reactions of atomic bromine with toluene, tetrahydrofuran, and tetrahydropyran were measured from approximately 295 to 362 K using the relative rate method. Iso-octane was used as the reference compound for the reaction with toluene, and iso-octane and toluene were used as the reference compounds for the reaction with tetrahydrofuran; tetrahydrofuran was used as the reference compound for the reaction with tetrahydropyran. The rate coefficients were found to be unaffected by changes in pressure and oxygen concentration. The rate coefficient ratios were converted to absolute values using the absolute rate coefficient for the reaction of Br with the reference compound. The absolute rate coefficients, in the units cm(3) molecule(-1) s(-1), for the reaction of Br with toluene are given by k(T) = (3.7 +/- 1.7) x 10(-12) exp(-(1.63 +/- 0.15) x 10(3)/T), for the reaction of Br with tetrahydrofuran by k(T) = (3.7 +/- 2.7) x 10(-10) exp(-(2.20 +/- 0.22) x 10(3)/T), and for the reaction of Br with tetrahydropyran by k(T) = (3.6 +/- 1.8) x 10(-10) exp(-(2.35 +/- 0.16) x 10(3)/T). The uncertainties represent one standard deviation. The Arrhenius parameters for these reactions are compared with results in the literature for dimethyl ether, diethyl ether, and a series of saturated hydrocarbons, and the effects of structural variation on these parameters are identified.

Experimental and theoretical investigation of the kinetics of the reaction of atomic chlorine with 1,4-dioxane.

The rate coefficients for the reaction of 1,4-dioxane with atomic chlorine were measured from T = 292-360 K using the relative rate method. The reference reactant was isobutane and the experiments were made in argon with atomic chlorine produced by photolysis of small concentrations of Cl2. The rate coefficients were put on an absolute basis by using the published temperature dependence of the absolute rate coefficients for the reference reaction. The rate coefficients for the reaction of Cl with 1,4-dioxane were found to be independent of total pressure from p = 290 to 782 Torr. The experimentally measured rate coefficients showed a weak temperature dependence, given by k(exp)(T) = (8.4(-2.3)(+3.1)) × 10(-10) exp(-(470 ± 110)/(T/K)) cm3 molecule (-1) s(-1). The experimental results are rationalized in terms of statistical rate theory on the basis of molecular data obtained from quantum-chemical calculations. Molecular geometries and frequencies were obtained from MP2/aug-cc-pVDZ calculations, while single-point energies of the stationary points were computed at CCSD(T) level of theory. The calculations indicate that the reaction proceeds by an overall exothermic addition-elimination mechanism via two intermediates, where the rate-determining step is the initial barrier-less association reaction between the chlorine atom and the chair conformer of 1,4-dioxane. This is in contrast to the Br plus 1,4-dioxane reaction studied earlier, where the rate-determining step is a chair-to-boat conformational change of the bromine-dioxane adduct, which is necessary for this reaction to proceed. The remarkable difference in the kinetic behavior of the reactions of 1,4-dioxane with these two halogen atoms can be consistently explained by this change in the reaction mechanism.

A theoretical analysis of the kinetics of the reaction of atomic bromine with tetrahydrofuran

The kinetic behaviour for the reaction of atomic bromine with tetrahydrofuran has been analysed using the information from quantum chemical calculations. Structures and energy profiles were first o...

The photolysis of acetone at 308 nm

Abstract The photolysis of acetone using an excimer laser (XeCl transition at 308 nm) was studied to determine its potential as a source of CH3 radicals for kinetic measurements. The data were analysed by kinetic modelling and the potential of the system for kinetic measurements was examined by using it to study the reaction of CH3 with propylene at temperatures in the range 298–431 K. The model can accurately describe the yields of CH4, C2H6 and CO both in the presence and absence of propylene. The results obtained when mixtures of propylene and acetone were photolysed are consistent with current data on the reactions of propylene with CH3 and suggest that the 308 nm photolysis of acetone is a viable source of CH3 for kinetic measurements using end product analysis combined with kinetic modelling.

Theoretical study of the reaction kinetics of atomic bromine with tetrahydropyran.

A detailed theoretical analysis of the reaction of atomic bromine with tetrahydropyran (THP, C5H10O) was performed using several ab initio methods and statistical rate theory calculations. Initial geometries of all species involved in the potential energy surface of the title reaction were obtained at the B3LYP/cc-pVTZ level of theory. These molecular geometries were reoptimized using three different meta-generalized gradient approximation (meta-GGA) functionals. Single-point energies of the stationary points were obtained by employing the coupled-cluster with single and double excitations (CCSD) and fourth-order Møller-Plesset (MP4 SDQ) levels of theory. The computed CCSD and MP4(SDQ) energies for optimized structures at various DFT functionals were found to be consistent within 2 kJ mol(-1). For a more accurate energetic description, single-point calculations at the CCSD(T)/CBS level of theory were performed for the minimum structures and transition states optimized at the B3LYP/cc-pVTZ level of theory. Similar to other ether + Br reactions, it was found that the tetrahydropyran + Br reaction proceeds in an overall endothermic addition-elimination mechanism via a number of intermediates. However, the reactivity of various ethers with atomic bromine was found to vary substantially. In contrast with the 1,4-dioxane + Br reaction, the chair form of the addition complex (c-C5H10O-Br) for THP + Br does not need to undergo ring inversion to form a boat conformer (b-C4H8O2-Br) before the intramolecular H-shift can occur to eventually release HBr. Instead, a direct, yet more favorable route was mapped out on the potential energy surface of the THP + Br reaction. The rate coefficients for all relevant steps involved in the reaction mechanism were computed using the energetics of coupled cluster calculations. On the basis of the results of the CCSD(T)/CBS//B3LYP/cc-pVTZ level of theory, the calculated overall rate coefficients can be expressed as kov.,calc.(T) = 4.60 × 10(-10) exp[-20.4 kJ mol(-1)/(RT)] cm(3) molecule(-1) s(-1) for the temperature range of 273-393 K. The calculated values are found to be in excellent agreement with the experimental data published previously.