Geometric parameters of the transition state in radical reactions of antioxidants

An algorithm for calculating interatomic distances in a transition state of radical abstraction reactions through the enthalpy of a reaction is developed. The algorithm is based on a combination of quantum-chemical calculations with the calculation by the intersecting-parabolas method. Using this method and experimental data (enthalpies and activation energies of reactions), interatomic distances in a reaction center are calculated for the reactions of antioxidants with a symmetrical transition state of the N...H...N and O...H...O types and a nonsymmetrical transition state of the N...H...C, O... H...C, and O...H...N types. These distances are compared with other characteristics of the reactions and reactants. The influence of such factors as the enthalpy of the reaction, triplet repulsion in the transition state, electronegativity of atoms in the reaction center, and steric repulsion of reactants on the geometric parameters of the transition state is examined. The results obtained are used to calculate the increments characterizing the influence of various factors on interatomic distances of radical reactions considered.