Thermal stability of carbonyl radicals Part I. Straight-chain and branched C4 and C5 acyl radicals

Abstract

The competition between thermal decomposition (k_dis) and O₂ addition (k_O2) of linear and branched C₄ and C₅ alkanoyl (R-C(•)O, R=alkyl) radicals has been studied in a photochemical reaction chamber made from stainless steel (v=12 L). RCO radicals were prepared by continuous photolysis of Br₂–RC(O)H–O₂–NO₂–N₂ mixtures at wavelengths 420 nm. The products CO and RC(O)O₂NO₂ were analyzed by long-path IR absorption using an FT-IR spectrometer. Rate constant ratios k_dis/k_O2 were determined at 317 K for n-butyryl, n-pentanoyl, 3-methylbutyryl, 2-methylpropionyl and 2-methylbutyryl and at 6 temperatures between 293 and 317 K for 2,2-dimethylpropionyl (=pivaloyl, t-butyl-CO) radicals. Total pressures were 1 bar (M=N₂+O₂). Adopting the literature value of k_O2 for acetyl, unimolecular decomposition rate constants k_dis were derived from the measured ratios k_dis/k_O2. k_dis at 298 K, 1 bar, M=O₂+N₂ increases by factors of 35, 54 and 24 for each H atom in CH₃CO which is consecutively replaced by a methyl group (corresponding to increasing branching of R). For the unimolecular decomposition of 2,2-dimethylpropionyl radicals, the Arrhenius expression k_dis(t-butyl-CO)=6.0×10¹² exp(−41.6 kJ mol⁻¹/RT) s⁻¹ (2σ) was derived for the temperature range 293–317 K and a total pressure of 1 bar (M=N₂+O₂). The results on k_dis/k_O2 show that even for the thermally most unstable of the carbonyl radicals studied in this work, i.e. 2,2-dimethylpropionyl, only 1.8% decompose rather than add O₂ at 298 K and 1 bar in dry air.