Conservation of bond order during radical substitution reactions: Implications for the BEBO model

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 + CH₃-OH*. 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 barrier heights predicted with bond order conservation are only incorrect by 1-2 kcal/mol. This behavior arises because the potential energy surfaces are relatively flat in the region where the transition states are found. Consequently, the energies of the transition state predicted with either method are in close agreement, even though the structures are poorly represented by bond order conservation methods.