Meisenheimer's missing! Anionic σ‐complexes, best known as Meisenheimer complexes, have long been assumed to be intermediates in S_NAr reactions. New evidence now suggests that these intermediates may only be formed in select cases and that a concerted pathway is more common. These claims are supported by ¹²C/¹³C kinetic isotopes effects, determined using a new method based on ¹⁹F NMR, and DFT calculations.

中文翻译：

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SNAr反应中的Meisenheimer配合物：是中间态还是过渡态？

迈森海默失踪了！阴离子σ-络合物（最著名的是迈森海默络合物）长期以来一直被认为是S _N Ar反应的中间体。现在有新证据表明这些中间体可能仅在特定情况下形成，并且一致的途径更为普遍。这些主张得到了¹² C / ¹³ C动力学同位素效应的支持，该效应是使用基于¹⁹ F NMR和DFT计算的新方法确定的。