Solvent dependent structural perturbations of chemical reaction intermediates visualized by time-resolved x-ray diffraction
Solvent dependent structural perturbations of chemical reaction intermediates visualized by time-resolved x-ray diffraction
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DOI：
10.1063/1.3111401
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发表时间：
2009-04-21
影响因子：
4.4
通讯作者：
Davidsson, Jan
中科院分区：
文献类型：
--
作者：
Vincent, Jonathan;Andersson, Magnus;Davidsson, Jan
溶液中化学反应的超快时间分辨广角x射线散射是一种测定瞬态光化学物质结构动力学的强有力技术。在这里，我们研究的结构演变的光激发CH 2 I2在非极性溶剂环己烷和绘制与极性溶剂甲醇中的类似研究的比较。与早期的光谱研究一样，我们的数据证实了两种溶剂中共同的初始反应途径。光激发后，CH_2 I_2解离形成CH_2 I中心点+I中心点。残留在溶剂笼内的碘自由基与新生的CH 2 I中心点自由基再联合收割机结合形成瞬态异构体CH 2 I-I，而逃离溶剂笼的那些最终在环己烷中联合收割机结合形成I-2。此外，该瞬态异构体在非极性溶剂中的寿命长约30倍。更大的化学意义是时间分辨广角X射线衍射的性质，以准确地确定CH 2 I-I反应中间体的结构。因此，我们观察到瞬时碘-碘键在环己烷中比在甲醇中短0.07 A +/- 0.04 A。由于与极性溶剂的有利氢键相互作用，中间体的碘-碘键长在甲醇中出现。这些研究结果表明，时间分辨X射线衍射具有足够的灵敏度，使溶剂依赖的瞬态化学物质的结构扰动被准确地解决。
Ultrafast time-resolved wide angle x-ray scattering from chemical reactions in solution has recently emerged as a powerful technique for determining the structural dynamics of transient photochemical species. Here we examine the structural evolution of photoexcited CH2I2 in the nonpolar solvent cyclohexane and draw comparisons with a similar study in the polar solvent methanol. As with earlier spectroscopic studies, our data confirm a common initial reaction pathway in both solvents. After photoexcitation, CH2I2 dissociates to form CH2I center dot+I center dot. Iodine radicals remaining within the solvent cage recombine with a nascent CH2I center dot radical to form the transient isomer CH2I-I, whereas those which escape the solvent cage ultimately combine to form I-2 in cyclohexane. Moreover, the transient isomer has a lifetime approximately 30 times longer in the nonpolar solvent. Of greater chemical significance is the property of time-resolved wide angle x-ray diffraction to accurately determine the structure of the of CH2I-I reaction intermediate. Thus we observe that the transient iodine-iodine bond is 0.07 A +/- 0.04 A shorter in cyclohexane than in methanol. A longer iodine-iodine bond length for the intermediate arises in methanol due to favorable H-bond interaction with the polar solvent. These findings establish that time-resolved x-ray diffraction has sufficient sensitivity to enable solvent dependent structural perturbations of transient chemical species to be accurately resolved.