Relative-rate study of thermal decomposition of the 2-butoxyl radical in the temperature range 280–313 KElectronic supplementary information (ESI) available: See http://www.rsc.org/suppdata/cp/b1/b111474c/
作者:H. G. Libuda、O. Shestakov、J. Theloke、F. Zabel
DOI:10.1039/b111474c
日期:2002.5.29
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 (Mâ=âO2â+âN2), kdis/kO2 was determined at O2 partial pressures between 100 and 1000 mbar. At all temperatures,
there was a systematic increase of (kdis/kO2)effââ¡â(Î[CH3CHO]corrâÃâ[O2])
/
(2âÃâÎ[CH3C(O)CH2CH3]) with the partial pressure of O2 which possibly is the result of an additional O2 independent source of acetaldehyde (â8% 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 6â10% chemically activated 2-butoxyl radicals in the temperature range 280â313 K. Correction of (kdis/kO2)eff for the O2 independent yield of
acetaldehyde results in kdis/kO2â=â(6.8â±â1.4)âÃâ1017, (2.3â±â0.5)âÃâ1018, and (5.5â±â1.1)âÃâ1018 molecule cmâ3 at 279.8, 298.2, and 313.5 K, respectively, leading to the Arrhenius expression kdis/kO2â=â(2.0â±â0.5)âÃâ1026exp(â45.4 kJ molâ1/RT) molecule cmâ3 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, kdisâ=â3.9âÃâ1012âexp(â47.1 kJ molâ1/RT) sâ1 is derived for a total pressure of 1 bar (Mâ=âN2â+âO2), which compares very favourably with a recent theoretical estimate (ab initioâ+âRRKM) by Somnitz and Zellner (H. Somnitz and R. Zellner, Phys. Chem. Chem. Phys., 2000, 2, 1907).
在由石英制成的新建210升光反应器中,对2-丁氧基自由基的分解(kdis)与O2反应(kO2)的竞争进行了研究。2-丁氧基自由基是在存在O2和NO的情况下,通过持续的254纳米光解2-丁氧基碘化物产生的,使用N2作为缓冲气体。反应物和产物通过使用傅里叶变换红外光谱仪的长路径(29米)红外吸收进行分析。根据乙醛和丁酮的产品比例,修正了少量副产物的影响,得到了kdis/kO2的比例。在280、298和313 K以及总压力为1巴(M=O2+N2)的条件下,kdis/kO2在O2的分压介于100到1000毫巴之间被确定。在所有温度下,(kdis/kO2)eff≡(Δ[CH3CHO]corr×[O2])/(2×Δ[CH3C(O)CH2CH3])随O2分压的增加有系统性的增加,这可能是由于来自O2独立来源的乙醛的额外贡献(在298 K,1巴的条件下,大约8%的2-丁氧基自由基通过这两种竞争路径中的任何一种反应)。在100到1000毫巴的压力依赖性研究支持了这样一个假设,即额外的乙醛来源于在280到313 K的温度范围内形成的化学激活的2-丁氧基自由基,其比例为6到10%。修正(kdis/kO2)eff对于O2独立产生的乙醛的产率,得到了kdis/kO2=(6.8±1.4)×1017,(2.3±0.5)×1018,和(5.5±1.1)×1018分子cm-3在279.8,298.2,和313.5 K,分别,导致Arrhenius表达式kdis/kO2=(2.0±0.5)×1026exp(-45.4 kJ mol-1/RT)分子cm-3在总压力为1巴。这个kdis/kO2的温度依赖性意味着,取决于温度,在较低对流层的条件下,要么是热分解,要么是与O2反应是2-丁氧基自由基的主要损失过程。使用文献中的kO2值,对于总压力为1巴(M=N2+O2),导出kdis=3.9×1012exp(-47.1 kJ mol-1/RT) s-1,这与Somnitz和Zellner的最近理论估计(从头计算+RRKM)非常吻合。