J. Phys. Chem. 1995, 99, 9801-9810
9801
Ab Initio Computation of Combustion Kinetics. 1. Vinyl Radical
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Barry K. Carpenter Deparment of Chemistry, Baker Laboratory, Come11 University, Ithaca, New York 14853-1301 Received: November 2, 1994; In Final Form: February 13, 1995@
The purpose of the work described in this paper is to investigate the utility of ab initio molecular orbital calculations for the prediction of rate constants and activation parameters of reactions occumng in hydrocarbon combustion. The reaction of vinyl radical with oxygen has been chosen because there exist reliable experimental data against which the calculations can be calibrated. The results suggest that good agreement (within a factor of 2) between observed and calculated rate constants can be achieved but only i f a mechanism different from the one previously assumed is employed. The new mechanism involves cyclization of the first-formed vinylperoxy radical to a three-membered-ring dioxiranylmethyl radical rather than the four-membered-ring dioxetanyl radical that was assumed in earlier mechanisms. The agreement of the computed rate constants with existing experimental data, as well as the identification of the new mechanism, would appear to imply that ab initio calculations of the type described can have a useful role in the analysis of combustion processes. Predictions of results expected in shock-tube studies of the reaction are presented. It is shown that shocktube experiments should provide definitive distinction between the old and new mechanisms for the reaction.
Introduction Numerical simulation of the kinetic steps involved in combustion is of considerable importance in the design of optimal systems for the utilization of hydrocarbon fuels.' However, the mechanisms of oxidative degradation of hydrocarbons under combustion conditions are generally extremely complex-commonly involving hundreds or even thousands of elementary steps.* Considerable effort has been expended to obtain reliable kinetic data for these elementary reactions, but the task is challenging because many of the proposed reactive intermediates are difficult to generate cleanly-a condition of paramount importance for kinetic studies. In this paper, the performance of ab initio quantum theory, in combination with RRKM kinetic modeling, is evaluated as a potential supplement to the experimental determination of kinetic parameters. The premise, explored also by other^,^ is that while it will probably never be preferable to use computed rate constants and activation parameters in place of experimental ones, it might be preferable to use computed values in place of empirically estimated ones for those reactions where experimental determination has proven impossible. In order to evaluate such a procedure, it is, of course, necessary to explore the performance of the computational model for a system where the facts are known by experimental measurement. The system chosen for this purpose is the low-temperature (
