Sharma Nishant, Biswas Rupayan, Lourderaj Upakarasamy
School of Chemical Sciences, National Institute of Science Education and Research (NISER) Bhubaneswar, HBNI, P. O. Jatni, Khurda, Odisha, India.
Phys Chem Chem Phys. 2020 Nov 25;22(45):26562-26567. doi: 10.1039/d0cp05567k.
The commonly accepted mechanism of the nucleophilic aromatic substitution (SNAr) reaction has been found to be governed by the nature of the Meisenheimer structure on the potential energy surface. A stable Meisenheimer intermediate favors a stepwise mechanism, while a Meisenheimer transition state favors a concerted mechanism. Here, we show by using a detailed potential energy map (using the DFT and DLPNO-CCSD(T)/CBS methods) and ab initio classical trajectory simulations that the F- + C6H5NO2 SNAr reaction involves a Meisenheimer transition state and follows a stepwise mechanism in contrast to the expected concerted pathway. The stepwise mechanism observed in the trajectory simulations takes place by the formation of various ion-dipole and σ-complexes. While the majority of the trajectories follow the multi-step mechanism and avoid the minimum energy path, a considerable fraction exhibit a roaming atom mechanism where the F atom hovers around the phenyl ring before the formation of the products.
亲核芳香取代（SNAr）反应普遍接受的机理已被发现受势能面上迈森海默结构性质的支配。稳定的迈森海默中间体有利于分步机理，而迈森海默过渡态有利于协同机理。在此，我们通过使用详细的势能图（采用密度泛函理论（DFT）和密度拟合修正耦合簇单双激发（DLPNO-CCSD(T)）/完全基组外推（CBS）方法）以及从头算经典轨迹模拟表明，F⁻ + C₆H₅NO₂的SNAr反应涉及一个迈森海默过渡态，并且与预期的协同途径相反，遵循分步机理。轨迹模拟中观察到的分步机理是通过形成各种离子 - 偶极和σ - 配合物发生的。虽然大多数轨迹遵循多步机理并避开最低能量路径，但相当一部分表现出漫游原子机理，即F原子在产物形成之前在苯环周围徘徊。
