Model of the radical addition reaction as the superposition of three potential curves

Abstract

A new semiempirical model of the reaction of radical addition to molecules with multiple bonds has been developed. In the framework of this model, the transition state (TS) of the reaction X^. + Y=Z → XYZ^. is considered as the result of the intersection of the potential curve of the stretching vibration of the forming bond X-Y with the curve that is the difference between the amplitudes of stretching vibrations of the Y-Z and Y=Z bonds and the stretching vibrations are considered harmonic. The kinetic parameters describing the activation energy as a function of the enthalpy of the reaction were calculated for 34 classes of addition reactions using the new model. The factors determining the activation energy of the addition reactions are analyzed: triplet repulsion in the TS, the π electrons in the α position to the reaction center, the electronegativity of atoms of the reaction center of the TS, the steric factor, the interaction of polar groups in the TS, and the force constants of the reacting bonds. The increments characterizing the contribution of these factors to the activation energy are calculated. The model is also used to describe the energy of 12 classes of cyclization reactions and 16 classes of radical decomposition reactions. The parameters that make it possible to estimate the activation energy of the reaction from its enthalpy are calculated for these classes of reactions.