Relative-rate study of thermal decomposition of the 2-butoxyl radical in the temperature range 280–313 K

The competition between thermal decomposition (k_dis) and reaction with O₂ (k_O2) has been studied for the 2-butoxyl radical in a newly built 210 L photoreactor constructed of quartz. 2-Butoxyl radicals were generated by continuous 254 nm photolysis of 2-butoxyl iodide in the presence of O₂ and NO, using N₂ as a buffer gas. Reaction educts and products were analysed by long-path (29 m) IR absorption using an FTIR spectrometer. The ratio k_dis/k_O2 was derived from the product ratios of acetaldehyde and butanone, corrected for small amounts of side products. At 280, 298, and 313 K and a total pressure of 1 bar (M = O₂ + N₂), k_dis/k_O2 was determined at O₂ partial pressures between 100 and 1000 mbar. At all temperatures, there was a systematic increase of (k_dis/k_O2)_eff ≡ (Δ[CH₃CHO]_corr × [O₂]) / (2 × Δ[CH₃C(O)CH₂CH₃]) with the partial pressure of O₂ which possibly is the result of an additional O₂ independent source of acetaldehyde (≈8% of the 2-butoxyl radicals reacting by either of the two competing pathways at 298 K, 1 bar). Pressure-dependence studies between 100 and 1000 mbar support the hypothesis that the additional acetaldehyde originates from the formation of 6–10% chemically activated 2-butoxyl radicals in the temperature range 280–313 K. Correction of (k_dis/k_O2)_eff for the O₂ independent yield of acetaldehyde results in k_dis/k_O2 = (6.8 ± 1.4) × 10¹⁷, (2.3 ± 0.5) × 10¹⁸, and (5.5 ± 1.1) × 10¹⁸ molecule cm⁻³ at 279.8, 298.2, and 313.5 K, respectively, leading to the Arrhenius expression k_dis/k_O2 = (2.0 ± 0.5) × 10²⁶exp(−45.4 kJ mol⁻¹/RT) molecule cm⁻³ at a total pressure of 1 bar. This temperature dependence of k_dis/k_O2 implies that, depending on temperature, either thermal decomposition or reaction with O₂ is the major loss process of 2-butoxyl radicals under the conditions of the lower troposphere. Using literature values for k_O2, k_dis = 3.9 × 10¹² exp(−47.1 kJ mol⁻¹/RT) s⁻¹ is derived for a total pressure of 1 bar (M = N₂ + O₂), which compares very favourably with a recent theoretical estimate (ab initio + RRKM) by Somnitz and Zellner (H. Somnitz and R. Zellner, Phys. Chem. Chem. Phys., 2000, 2, 1907).