THEORETICAL-STUDY OF MECHANISMS OF AROMATIC NUCLEOPHILIC-SUBSTITUTIONIN THE GAS-PHASE

Mechanisms of aromatic nucleophilic substitution of halogenobenzenes ( Hal = F-I) by hard nucleophiles (OCH3-, F-) in the gas phase have been studied by the semiempirical quantum chemical MNDO and AM1 methods. T he roles and structural stabilities of the anion sigma complexes and d ipole-charge complexes and positional selectivity and proton transfer problems are discussed. The conclusions from the MNDO and AM1 calculat ions are in some cases inconsistent. Proton transfer may play a crucia l role in the course of the reaction under study. The anion sigma comp lex formed in the reaction of fluorobenzene with the methoxide anion p ossesses a coplanar bent conformation. The concept of the negative hyp erconjugation effect was found to be useful for the interpretation of the structural stability of Meisenheimer-type anion a complexes.