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 behav iors and energetics of species where ab initio calculations are still too e xpensive. However, the accuracy of bond order conservation, even for small polyatomic systems, is still unknown. In this paper, we use ab initio calcu lations at the PMP2 = (full)/6-31 g* and G-2 level to examine bond order co nservation for the following gas-phase radical substitution reactions: H* CH3OH --> CH3H* + OH, H* + CH3OH --> HOH* + CH3, H* + CH3OH --> HH* + CH2O H H* + CH3OH - HH* + CH3O, H* + CH3OH - H + CH2H*OH, H* + CK3OH - H + CH3OH *. We find that total bond order is approximately conserved during atom tra nsfer reactions, but is not conserved during the more complicated hydrogeno lysis reactions or during hydrogen exchange on oxygen. An early transition state is predicted for hydrogen exchange on oxygen, and late ones for the h ydrogenolysis reactions. Even though the transition state structures may di ffer greatly from the ab initio predictions, the barrier heights predicted with bond order conservation are only incorrect by 1-2 kcal/mol. This behav ior arises because the potential energy surfaces are relatively flat in the region where the transition states are found. Consequently, the energies o f the transition state predicted with either method are in close agreement, even though the structures are poorly represented by bond order conservati on methods.