Simple quantum-mechanical model of the elementary event of organic nucleophilic substitution reactions in the condensed phase

A semiempirical model is proposed for the elementary event of nucleophilic S_N2 substitution in alkyl halides. An analysis was made of the factors which determine the activation energies of the processes, i.e., the energy of approach of the reagents, the energy of extension of the chemical bonds, the energy of reorganization of the medium, and the equilibrium energy of the elementary event. The contributions from the first three energy parameters are commensurable, and the energy of approach has a predominant role. The role of the equilibrium energy, determined by the solvation energy of the reacting ions and the energy of the formed and broken chemical bonds, involves the creation of thermally neutral conditions for the reactions. The effective potential surfaces of the repulsive interaction between the halide ions and the methyl halides were constructed in terms of the model. The tunneling factor and the kinetic isotope effect for the lightest reacting particle in the reaction series (the fluoride ion) were also determined.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 4, pp. 417–424, July–August, 1985.