C26. Computational Study of the Mechanism and Product Yields in the Reaction System C₂H₃ + CH₃ ó C₃H₆ ó  H + C₃H₅

a) The Catholic University of America, Department of Chemistry, Washington, DC 20064, USA
^b) National Institute of Standards and Technology, Physical and Chemical Properties Division,Gaithersburg, MD 20899, USA.

Radical-radical cross-combination reactions constitute an integral part of the overall mechanisms of oxidation and pyrolysis of hydrocarbons.^1,2 Reactions of this type are difficult to study experimentally due to the high reactivity of the chemical species involved and, consequently, reliable experimental data are sparse. Unlike simple reactions of recombination of alkyl radicals that proceed via formation of stable molecules, cross-radical reactions that involve unsaturated species can possess complex mechanisms including chemically activated rearrangements and decomposition. Such reactions are characterized by complex distributions of products displaying both temperature and pressure dependencies.

The reaction between vinyl and methyl radicals

is the simplest of the class of reactions between alkenyl and alkyl radicals. This reaction plays an important role in mechanisms of evolution of planetary atmospheres.^3,4 Both the methyl and the vinyl radicals are also critical intermediates in the oxidation and pyrolysis of hydrocarbons.

The kinetics of reaction 1 has been studied experimentally by several groups.^5-9 In our recent experimental work,⁷ overall rate constants and product yields of reaction 1 were determined in the temperature region 300-900 K and bath gas (He) density (3-12)´10¹⁶ molecule cm^-3. Kinetics of the C₂H₃ and CH₃ decay and that of product formation were monitored in real-time direct experiments using Laser Flash Photolysis / Time Resolved Photoionization Mass Spectrometry.

The experimental data on reaction 1 are consistent with a mechanism which consists of two processes: (1) addition with the formation of chemically activated propene that can stabilize in collisions with the bath gas or decompose to H atom and allyl radical (reaction channels 1a and 1b) and (2) disproportionation where the CH₃ radical abstracts a hydrogen atom from vinyl resulting in the product channel 1c:

In the current computational work, the mechanism of reaction 1 is studied by ab initio quantum chemical methods. In particular, the pathways of reaction channels 1a, 1b, and 1c, as well as other potential channels, are investigated. The results of the ab initio study and of the earlier experimental work are used to create a model of the chemically activated route in reaction 1 (channels 1a and 1b). In this model, energy- and angular momentum-dependent rate constants k(E,J) are calculated using the RRKM method in combination with the microcanonical variational selection of the transition state. Pressure effects are described by the solution of the master equation using the approach described by Bedanov et al.¹⁰ and the virtual components algorithm of Knyazev and Tsang.¹¹ Qualitative behavior of the calculated pressure dependencies and the sensitivity of the modeling results to the critical parameters of the model are investigated. The model is used to predict the rate constants and branching fractions of reaction 1 at temperatures and pressures outside the experimental ranges.

Finally, the same model is used to analyze the data on two other important reactions which occur on the same potential energy surface: the thermal decomposition of propene

and the reaction of H atom with allyl radical

The results demonstrate the increasing importance of the CH₃ + C₂H₃ channels in both reactions 2 and 3 at high temperatures (above ~1500 K).

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