Ismail Huzeifa, Abel Paul R, Green William H, Fahr Askar, Jusinski Leonard E, Knepp Adam M, Zádor Judit, Meloni Giovanni, Selby Talitha M, Osborn David L, Taatjes Craig A
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Phys Chem A. 2009 Feb 19;113(7):1278-86. doi: 10.1021/jp8096132.
The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent methods. The rate constant as a function of temperature at 20 Torr has been determined by a laser-photolysis/laser absorption technique. Vinyl iodide is photolyzed at 266 nm, and both the vinyl radical and the iodine atom photolysis products are monitored by laser absorption. The vinyl radical concentration is derived from the initial iodine atom concentration, which is determined by using the known absorption cross section of the iodine atomic transition to relate the observed absorption to concentration. The measured rate constant for the self-reaction at room temperature is approximately a factor of 2 lower than literature recommendations. The reaction displays a slightly negative temperature dependence, which can be represented by a negative activation energy, (E(a)/R) = -400 K. The laser absorption results are supported by independent experiments at 298 K and 4 Torr using time-resolved synchrotron-photoionization mass-spectrometric detection of the products of divinyl ketone and methyl vinyl ketone photolysis. The photoionization mass spectrometry experiments additionally show that methyl + propargyl are formed in the vinyl radical self-reaction, with an estimated branching fraction of 0.5 at 298 K and 4 Torr.
已通过两种独立方法测量了乙烯基自由基自反应的速率系数。利用激光光解/激光吸收技术测定了20托下速率常数随温度的变化。碘化乙烯在266纳米处发生光解，乙烯基自由基和碘原子光解产物均通过激光吸收进行监测。乙烯基自由基浓度由初始碘原子浓度推导得出，初始碘原子浓度通过利用碘原子跃迁的已知吸收截面将观测到的吸收与浓度相关联来确定。在室温下测得的自反应速率常数比文献推荐值低约2倍。该反应表现出略微的负温度依赖性，可用负活化能（E(a)/R）=-400 K来表示。在298 K和4托下使用时间分辨同步辐射光电离质谱检测二乙烯基酮和甲基乙烯基酮光解产物的独立实验支持了激光吸收结果。光电离质谱实验还表明，在乙烯基自由基自反应中会形成甲基 + 炔丙基，在298 K和4托下估计的分支比为0.5。
