Kinetics of the (CH3)2CHCO and (CH3)3CCO radical decomposition: temperature and pressure dependences

Abstract

Flash photolysis coupled to UV absorption spectrometry was used for a relative measurement of the (CH₃)₂CHCO and (CH₃)₃CCO radical decomposition rate constants. The reference reaction was the association reaction of (CH₃)₂CHCO and (CH₃)₃CCO with O₂, for which the same rate constant as that of the CH₃CO+O₂ reaction was assumed: k₂=3.2×10⁻¹² cm³ molecule⁻¹ s⁻¹ (R. Atkinson, D. L. Baulch, R.A. Cox, R. F. Jr. Hampson, J. A. Kerr, M. J. Rossi and J. Troe, J. Phys. Chem. Ref. Data, 1999, 28, 191), independent of temperature. The rate expressions obtained at atmospheric pressure (N₂+O₂) for decomposition are k[(CH₃)₂CHCO]=3.8×10^(12±1) exp[−(5970±600) K/T] s⁻¹ (413 K<T<503 K) and k[(CH₃)₃CCO]=1.65×10^(12±1) exp[−(4870±310) K/T] s⁻¹ (323 K<T<453 K). RRKM calculations were also performed in order to assess the pressure dependence of rate constants. The reactions are in the fall-off region at atmospheric pressure and in the temperature range investigated. Calculations were fitted to experimental data and have yielded at the high pressure limit k_∞[(CH₃)₂CHCO)=3.3×10¹⁴ exp(−7145 K/T) s⁻¹ and k_∞[(CH₃)₃CCO]=1.1×10¹⁴ exp(−6090 K/T) s⁻¹, in fairly good agreement with the expressions derived from transition state theory calculations. The detailed results are given in the form of the Troe expression. The atmospheric implications of the results are discussed. In particular, the results suggest that the class of RCO radicals containing only alkyl R groups cannot decompose in the troposphere and thus, add to O₂ to form acylperoxy radicals RC(O)O₂.