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

Physical Chemistry Chemical Physics Pub Date : 2002/5/7 0:00:00 , DOI:10.1039/B111474C
Abstract

The competition between thermal decomposition (kdis) and reaction with O2 (kO2) 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 kdis/kO2 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), kdis/kO2 was determined at O2 partial pressures between 100 and 1000 mbar. At all temperatures, there was a systematic increase of (kdis/kO2)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 (kdis/kO2)eff for the O2 independent yield of acetaldehyde results in kdis/kO2 = (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 kdis/kO2 = (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 kdis/kO2 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 kO2, kdis = 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).

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