UnimolecularRate Expression for Cyclohexene Decompositionand Its Use in Chemical Thermometry under Shock Tube Conditions.

Themethods used in deriving the rate expressions from comparativerate single-pulse shock tube studies, recent direct shock tube studies,and high-pressure flow experiments bearing on the data for the reverseDiels–Alder decomposition of cyclohexene to form ethylene and1,3-butadiene are reviewed. This current interest is due to the increasingneed for accurate kinetics and physical data (particularly the temperature)for realistic simulations in practical areas such as combustion. Therate constants derived from the direct shock tube studies and high-pressureflow experiments are somewhat larger than those used in comparativerate single-pulse shock tube experiments. For the latter, it is shownthat they have been derived from a variety of independent experimentsthat include rate constants for unimolecular decomposition and isomerizationprocesses that are considered to be well understood. The possibilityof non-Arrhenius behavior in the unimolecular rate constants as aconsequence of the large range covered in rate constants (as muchas 12 orders of magnitude) for the comparative rate experiments hasbeen examined and ruled out as a source of the discrepancy. Our analysisshows that there is the need to consider the possibility of radical-induceddecompositions for verifying the correctness of the reaction mechanismsin studying unimolecular reactions. In the case of cyclohexene decomposition,recent experiments demonstrating the presence of residual amountsof H atoms in shock tube experiments suggest that addition to thedouble bond can also lead to the formation of ethylene and 1,3-butadieneand hence to rate constants larger than the true values. This possibilityis even more likely to occur in high-pressure flow experiments. Asa result, the internal standard method must be used with care anda radical inhibitor should always be present in sufficiently largequantities to suppress possible chain reactions. The present analysisresults have important implications for the determination of temperaturesin shock tubes. [ABSTRACT FROM AUTHOR]