Solvent Effects on the Stability and Delocalization of Aryl Dicyanomethyl Radicals: The Captodative Effect Revisited
Solvent Effects on the Stability and Delocalization of Aryl Dicyanomethyl Radicals: The Captodative Effect Revisited
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溶剂对芳基二氰甲基自由基稳定性和离域的影响：重新审视俘获效应
DOI：
10.1021/jacs.9b06576
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发表时间：
2019
影响因子：
15
通讯作者：
Winter, Arthur H.
中科院分区：
文献类型：
--
作者：
Peterson, Joshua P.;Winter, Arthur H.
捕获效应假设，自由基取代的电子给予和接受基团享有一个特殊的增强的稳定性，一个模型的理论支持，由简单的MO和共振参数。从理论上的一个关键预测是，在极性溶剂中比在非极性溶剂或气相中更大的自由基的捕获稳定化，这可以在共振模型中被视为电荷分离共振形式的溶剂稳定化。然而，一些实验研究未能观察到溶剂对自由基稳定性的影响，对捕获效应的关键方面产生了怀疑。在这里，我们详细研究了溶剂对结构相关的captodative芳基二氰基甲基自由基的稳定性的影响。这些自由基的一个有吸引力的特点是，它们存在的自由基的平衡与他们的二聚体的稳定的稳态人口，使我们能够直接从实验中表征其热力学稳定性和自旋离域在不同极性的溶剂。与之前的研究相反，我们发现捕获自由基确实被极性溶剂稳定，这是通过从非极性甲苯到更极性的溶剂时自由基-二聚体缔合常数向自由基移动高达100倍来测量的。此外，在极性溶剂中，自旋转移到供体取代基上，远离苄基碳。在共振模型中，这些结果可以通过两性离子共振结构对整个混合物的贡献增加来解释。这些结果为先前被驳回的理论关键预测提供了实验支持。
The captodative effect postulates that radicals substituted with both electron donating and accepting groups enjoy a special enhanced stabilization, a model given theoretical support by simple MO and resonance arguments. A key prediction from theory is that captodative stabilization of radicals is larger in polar solvents than in nonpolar solvents or the gas phase, which can be viewed in the resonance model as solvent stabilization of charge-separated resonance forms. Yet, several experimental studies have failed to observe a solvent effect on radical stability, casting doubt on key aspects of the captodative effect. Here, we examine in detail the effect of solvent on the stability of structurally related captodative aryl dicyanomethyl radicals. An attractive feature of these radicals is that they exist as stable steady state populations of radicals in equilibrium with their dimers, allowing us to directly characterize from experiment their thermodynamic stabilities and spin delocalization in solvents of varying polarity. In contrast to the prior studies, we find that captodative radicals are indeed stabilized by polar solvents, as measured by a shift in the radical–dimer association constants by up to 100-fold toward the radical upon going from nonpolar toluene to more polar DMF. Moreover, in polar solvents, the spin is shifted onto the donor substituent and away from the benzylic carbon. Within the resonance model, these results can be explained by the increased contributions of the zwitterionic resonance structures to the overall hybrid. These results provide experimental support to a key prediction from theory that had previously been dismissed.