A temperature and pressure dependent kinetics study of the gas-phase reactions of bromine (2P3/2) and chlorine (2PJ) atoms with methylvinyl ketone
Huskey, Dow T.
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A laser flash photolysis resonance fluorescence (LFP-RF) technique has been employed to study the kinetics of the reactions of methylvinyl ketone (MVK) with atomic bromine (Br) and atomic chlorine (Cl) as a function of temperature (203 755 K) and pressure (12 600 Torr) in nitrogen bath gas. The results of this study are also compared to published kinetics studies for similar reactions. Over the temperature range 200 K < T < 250 K for the reaction of Br with MVK, measured rate coefficients were pressure dependent suggesting the formation of an adduct. The adduct undergoes dissociation on the time scale of the experiments (< 0.1 s) and establishes an equilibrium between Br, MVK, and MVK Br. At temperatures above 298 K no reaction of Br with MVK was observed. Similarly, over the temperature range 405 K < T < 510 K, the reaction of Cl with MVK shows similar kinetics to that of Br and MVK suggesting an equilibrium is established. Equilibrium constants for adduct dissociation and formation are determined for the forward and reverse rate coefficients in both reactions. Second and third-law analyses are carried out to obtain information about the thermochemistry of the equilibrium reactions for Br with MVK and Cl with MVK. Adduct bond strengths of Br and Cl reactions with MVK are reported and compared to reactions with other unsaturated species. Ab initio calculations for these reactions are also presented in this study. Excellent agreement is observed between theory and experiment. Additionally, a reaction of Cl with MVK was observed over the temperature ranges 600 K < T < 760 K and 210 K < T < 365 K. At the lower temperatures, measured rate coefficients are also pressure dependent, however, the adduct remained stable. At the highest temperatures, the measured rate coefficients were pressure independent, suggesting hydrogen abstraction as the dominant reaction pathway. Energetics obtained from ab initio calculations suggest that only abstraction of the methyl hydrogen is likely to occur at a measurable rate in the temperature range investigated.