No abstract

The competition between thermal decomposition (k dis ) and reaction with O 2 (k O2 ) has been studied for the 2butoxyl 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 O 2 and NO, using N 2 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 ¼ O 2 + N 2 ), k dis /k O2 was determined at O 2 partial pressures between 100 and 1000 mbar. At all temperatures, there was a systematic increase of () with the partial pressure of O 2 which possibly is the result of an additional O 2 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 O 2 independent yield of acetaldehyde results in k dis /k O2 ¼ (6.8 AE 1.4) Â 10 17 , (2.3 AE 0.5) Â 10 18 , and (5.5 AE 1.1) Â 10 18 molecule cm À3 at 279.8, 298.2, and 313.5 K, respectively, leading to the Arrhenius expression k dis /k O2 ¼ (2.0 AE 0.5) Â 10 26 exp(À45.4 kJ mol À1 /RT) molecule cm À3 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 O 2 is the major loss process of 2-butoxyl radicals under the conditions of the lower troposphere. Using literature values for k O2 , k dis ¼ 3.9 Â 10 12 exp(À47.1 kJ mol À1 /RT) s À1 is derived for a total pressure of 1 bar (M ¼ N 2 + O 2 ), which compares very favourably with a recent theoretical estimate (ab initio + RRKM) by R. Zellner, Phys. Chem. Chem. Phys., 2000, 2, 1907).y Electronic supplementary information (ESI) available: See

No abstract

No abstract

No abstract