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 O-2 (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 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 = 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 (k(dis)/k(O2))(eff) (Delta[CH3CHO](corr) x [O-2]) / (2 x [CH3C(O)CH2CH3]) with the partial pressure of O-2 which possibly is the result of an additional O-2 independent source of acetaldehyde (approximate to8% 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) = (6.8 +/- 1.4) 10(17), (2.3 +/- 0.5) x 10(18), and (5.5 +/- 1.1) x 10(18) 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) x 10(26) exp(-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 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) = 3.9 x 10(12) exp(-47.1 kJ mol(-1) / RT) s(-1) is derived for a total pressure of 1 bar ( M = N-2 + O-2), which compares very favourably with a recent theoretical estimate (ab initio + RRKM) by Somnitz and Zellner.