Conservation of Bond Order during Radical Substitution Reactions: Implications for the BEBO Model

Abstract

The bond energy−bond order model has been used extensively to predict behaviors and energetics of species where ab initio calculations are still too expensive. However, the accuracy of bond order conservation, even for small polyatomic systems, is still unknown. In this paper, we use ab initio calculations at the PMP2 = (full)/6-31 g* and G-2 level to examine bond order conservation for the following gas-phase radical substitution reactions: H* + CH3OH → CH3H* + OH, H* + CH3OH → HOH* + CH3, H* + CH3OH → HH* + CH2OH, H* + CH3OH → HH* + CH3O, H* + CH3OH → H + CH2H*OH, H* + CH3OH → H + CH3OH*. We find that total bond order is approximately conserved during atom transfer reactions, but is not conserved during the more complicated hydrogenolysis reactions or during hydrogen exchange on oxygen. An early transition state is predicted for hydrogen exchange on oxygen, and late ones for the hydrogenolysis reactions. Even though the transition state structures may differ greatly from the ab initio predictions, the ...