Analysis of external activation systems with multiple isomerizations and decompositions

Carter, W. P. L.; Tardy, D. C.

doi:10.1021/j100609a002

Cited by 11 publications

(8 citation statements)

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“…Various solutions for the steady-state equations have been reported [47,48,50,51]; in this work an iterative procedure [50] was used in that each iteration corresponds to the time between collisions: The population is distributed according to the probabilities for collisions and reaction. The stabilization and decomposition products are accumulated until S 1 ϩ S 2 ϩ D 1 ϩ DЈ has converged to the initial population.…”

Section: F(e) ϭ Kј(e)b(e)/⌺kј(e)b(e)mentioning

confidence: 99%

Rate constant and RRKM product study for the reaction between CH3 and C2H3 at T = 298 K

Thorn

Payne

Chillier

et al. 2000

Int. J. Chem. Kinet.

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The total rate constant k 1 has been determined at P ϭ 1 Torr nominal pressure (He) and at T ϭ 298 K for the vinyl-methyl cross-radical reaction: (1) CH 3 ϩ C 2 H 3 : Products. The measurements were performed in a discharge flow system coupled with collision-free sampling to a mass spectrometer operated at low electron energies. Vinyl and methyl radicals were generated by the reactions of F with C 2 H 4 and CH 4 , respectively. The kinetic studies were performed by monitoring the decay of C 2 H 3 with methyl in excess, 6 Ͻ [CH 3 ] 0 / [C 2 H 3 ] 0 Ͻ 21. The overall rate coefficient was determined to be k 1 (298 K) ϭ (1.02 Ϯ 0.53) ϫ 10 Ϫ10 cm 3 molecule Ϫ1 s Ϫ1 with the quoted uncertainty representing total errors. Numerical modeling was required to correct for secondary vinyl consumption by reactions such as C 2 H 3 ϩ H and C 2 H 3 ϩ C 2 H 3 . The present result for k 1 at T ϭ 298 K is compared to two previous studies at high pressure (100-300 Torr He) and to a very recent study at low pressure (0.9-3.7 Torr He).Comparison is also made with the rate constant for the similar reaction CH 3 ϩ C 2 H 5 and with a value for k 1 estimated by the geometric mean rule employing values for k(CH 3 ϩ CH 3 ) and k(C 2 H 3 ϩ C 2 H 3 ). Qualitative product studies at T ϭ 298 K and 200 K indicated formation of C 3 H 6 , C 2 H 2 , and C 3 H 5 as products of the combination-stabilization, disproportionation, and combination-decomposition channels, respectively, of the CH 3 ϩ C 2 H 3 reaction. We also observed the secondary C 4 H 8 product of the subsequent reaction of C 3 H 5 with excess CH 3 ; this observation provides convincing evidence for the combination-decomposition channel yielding C 3 H 5 ϩ H. RRKM calculations with helium as the deactivator support the present and very recent experimental observations that allylic C-H bond rupture is an important path in the combination reaction. The pressure and temperature dependencies of the branching fractions are also predicted. . Reaction rate constants and product channel information are required for all such models that attempt to account quantitatively for the observed atmospheric composition and structure. Hydrocarbon photochemistry is initiated by the photodissociation of methane in the upper

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Section: F(e) ϭ Kј(e)b(e)/⌺kј(e)b(e)mentioning

confidence: 99%

Rate constant and RRKM product study for the reaction between CH3 and C2H3 at T = 298 K

Thorn

Payne

Chillier

et al. 2000

Int. J. Chem. Kinet.

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“…The yields of S and D are computed by solving the master equation with the appropriate model for P ( E ‘, E ) and RRKM theory for the k ( E )'s . If the experiments are in the high-pressure region, then a simple correction to the strong collision model may be used …”

Section: Approachesmentioning

confidence: 99%

Pressure Effect on CH₃ and C₂H₃ Cross-Radical Reactions

1999

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The effect of pressure on the cross-radical reactions of vinyl and methyl radicals has been investigated. These radicals were produced by excimer laser photolysis of methyl vinyl ketone (CH 3 COC 2 H 3 ) at 193 nm. The reaction products were detected and analyzed using a sensitive gas chromatograph and mass spectrometer. The study covered a pressure range from about 0.28 kPa (2.1 Torr) to 27 kPa (200 Torr) at 298 K. The yield of propylene (C 3 H 6 ), the cross-combination product of methyl and vinyl radicals, was compared to the yield of ethane (C 2 H 6 ), the methyl radical combination product. At 27 kPa [C 3 H 6 ]/[C 2 H 6 ] ) 1.28 was derived. This ratio was reduced to about 0.75 when the pressure was reduced to about 0.28 kPa. Kinetic modeling results indicated that the contribution of the combination reaction C 2 H 3 + CH 3 + M f C 3 H 6 + M to the total cross-radical reactions is reduced from 78% at high pressures (27 kPa) to about 39% at low pressures (0.28 kPa). At low pressures an additional reaction channel, C 2 H 3 + CH 3 f C 3 H 5 + H, becomes available, producing a host of allyl radical reaction products including 1,5-hexadiene, the allyl radical combination product. The observed 1,5-hexadiene is strong evidence for allyl radical formation at low pressures, presumably from the decomposition of the chemically activated C 3 H 6 . Macroscopic and microscopic modeling of product yields and their pressure dependencies were used to interpret the experimental observations. Results of master equation calculations using weak colliders and RRKM theory are in agreement with the observed pressure dependence of the combination reactions. It has been shown that the chemically activated species can undergo unimolecular processes that are competitive with collisional stabilization. The pressure dependence for the unimolecular steps appears as a pressure dependence of the combination/disproportionation ratio. The apparent pathological behavior in this unsaturated system is attributed to the formation of a stronger C-C bond as contrasted to the weaker C-C bond formed from combination of saturated hydrocarbon radicals. This C-C bond strength is sufficiently high for the chemically activated propylene, produced from the methyl and vinyl cross-combination reaction to cleave the allyl C-H bond or isomerize to cyclopropane.

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“…The energy transfer model for the collisional stabilization of the excited radicals is also needed in the calculation. 25 Heptane was assumed to be a strong collider. Energy transfer involving weak colliders is specified by probabilities of the excited species going from one energy state to another upon collision.…”

Section: Reactionmentioning

confidence: 99%

“…Energy transfer involving weak colliders is specified by probabilities of the excited species going from one energy state to another upon collision. [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] In these calculations, collision with hydrogen was assumed to be described by a step ladder model with a step size of 400 cm-1 (1.144 kcal/mol), and nitrogen by a step ladder model with a step size of 800 cm-1 (2.29 kcal/mol).22 (A step ladder model assumes that upon collision the amount of energy transferred is always equal to the step size.) It was found that if energy transfer in hydrogen was described by an exponential model with average transferred energy = 525 cm"1 IIIII*V(1.50 kcal/mol) exactly the same ratios were obtained as with the step ladder model, despite the significantly different energy transfer probabilities obtained by the two models.…”

Section: Reactionmentioning

confidence: 99%

“…It was found that if energy transfer in hydrogen was described by an exponential model with average transferred energy = 525 cm"1 IIIII*V(1.50 kcal/mol) exactly the same ratios were obtained as with the step ladder model, despite the significantly different energy transfer probabilities obtained by the two models. [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] The manner the detailed energy transfer probabilities are obtained for the step ladder or exponential models is described elsewhere. 38 Watkins and O'Deen's studies8 were done with acetylene as the bath gas.…”

Section: Reactionmentioning

confidence: 99%

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Reactions of chemically activated pentenyl radicals. Kinetic parameters of 1,4 hydrogen shifts and the cis-trans isomerization of homoallylic radicals

Carter¹,

Tardy²

1974

J. Phys. Chem.

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The reactions resulting from adding H atoms to 2-pentyne in the gas phase have been studied using several deactivating gases. Measured decomposition products are propyne, 1-butyne, and 1,2-butadiene, resulting from decompositions of initially formed radicals at rates consistent with thermal results. 1,3-Butadiene, resulting from isomerization of initially formed 2-penten-2-yl first via a 1,4 H shift and then via a cis-trans isomerization of homoallylic 2-penten-5-yl, was also formed. Information about the rates of these isomerizations was obtained. 1,4 H shift activated complex models could be tested using this work and literature results, and the use of looser activated complex models is supported. Using loose 1,4 H shift activated complex frequencies and a 0°K primary vinylic C-H bond dissociation energy of 110.2 kcal/mol, a 1,4 H shift critical energy of 18.9 kcal/mol, and a cis-trans homoallylic isomerization critical energy of 23.4 kcal/mol are obtained. In addition, there is evidence of a possible 1,3 H shift of 2-penten-3-yl occurring with a critical energy of about 30 kcal/mol.

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Analysis of external activation systems with multiple isomerizations and decompositions

Abstract: A general method for the steady-state analysis of complex external activation systems with multiple isomerization and decompositions is developed. This method is used for both "strong" and "weak" collisions. The algorithm for such an analysis is discussed.

Cited by 11 publications

References 5 publications

Rate constant and RRKM product study for the reaction between CH3 and C2H3 at T = 298 K

Rate constant and RRKM product study for the reaction between CH3 and C2H3 at T = 298 K

Pressure Effect on CH₃ and C₂H₃ Cross-Radical Reactions

Reactions of chemically activated pentenyl radicals. Kinetic parameters of 1,4 hydrogen shifts and the cis-trans isomerization of homoallylic radicals

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Analysis of external activation systems with multiple isomerizations and decompositions

Abstract: A general method for the steady-state analysis of complex external activation systems with multiple isomerization and decompositions is developed. This method is used for both "strong" and "weak" collisions. The algorithm for such an analysis is discussed.

Cited by 11 publications

References 5 publications

Rate constant and RRKM product study for the reaction between CH3 and C2H3 at T = 298 K

Rate constant and RRKM product study for the reaction between CH3 and C2H3 at T = 298 K

Pressure Effect on CH3 and C2H3 Cross-Radical Reactions

Reactions of chemically activated pentenyl radicals. Kinetic parameters of 1,4 hydrogen shifts and the cis-trans isomerization of homoallylic radicals

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Product

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Pressure Effect on CH₃ and C₂H₃ Cross-Radical Reactions