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The commonly accepted mechanism of the nucleophilic aromatic substitution reaction (S N Ar) 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, a concerted mechanism. Here, we show by detailed potential energy map (using DFT and DLPNO-CCSD(T)/CBS methods) and ab initio classical trajectory simulations that the F - C 6 H 5 NO 2 S N Ar 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 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.
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