REACTIVITY IN RADICAL ABSTRACTION REACTIONS - APPLICATION OF THE CURVE CROSSING MODEL

The curve crossing model was applied to a series of hydrogen abstraction reactions from a family of alkanes, RH (R = methyl, ethyl, isopropyl, tert-butyl) by alkyl, hydrogen and chlorine radicals. The analysis was based on quantitative data obtained from an ab initio MO study. Schematic reaction profiles for the reaction of RH with alkyl and hydrogen radicals are built up from just two configurations: reactant, DA, and product D3*A. For the Cl atom reaction, however, a significant contribution of D+A-, a charge-transfer configuration, is also shown to be present. A simple explanation for differences in the intrinsic barrier for the identity radical abstraction reaction based on the initial gap size between DA and D3*A configurations is provided. The influence of the D+A- configuration on the nature of the transition state of the Cl atom reaction and its intrinsic barrier is described. It is the D+A- configuration that is responsible for the polar character often observed in radical abstraction and addition reactions.