Abstract
Mechanisms of aromatic nucleophilic substitution of halogenobenzenes (Hal = F-I) by hard nucleophiles (OCH 3 −, F −) in the gas phase have been studied by the semiempirical quantum chemical MNDO and AMI methods. The roles and structural stabilities of the anion σ complexes and dipole-charge complexes and positional selectivity and proton transfer problems are discussed. The conclusions from the MNDO and AM1 calculations are in some cases inconsistent. Proton transfer may play a crucial role in the course of the reaction under study. The anion σ complex formed in the reaction of fluorobenzene with the methoxide anion possesses a coplanar bent conformation. The concept of the negative hyperconjugation effect was found to be useful for the interpretation of the structural stability of Meisenheimer-type anion σ complexes.
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