Conservation of bond order during hydrogenolysis and dehydrogenation reactions

Previous investigations have used bond-order conservation to make useful predictions about surface phenomena, but the accuracy of bond-order conservation for complicated processes is unknown. In this paper, we use ab-initio calculations at the MP2= (full)/6-31g* level to investigate bond-order conservation, and its implications, for the following gas-phase 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 bond order is approximately conserved during the atom transfer reactions, but is not conserved during hydrogenolysis reactions. The transition state is predicted to be too early for hydrogen exchange on oxygen, too late for the hydrogenolysis reactions, and about right for the hydrogen transfer reactions. Even though the transition-state structures are not well represented using conserved total bond order, the energies of the barriers predicted this way are off by only 1-2 kcal mol(-1). Physically, the potential energy surfaces are so hat near the transition state that errors in geometry do not produce significant errors in energy. Consequently, although bond-order conservation does not predict the correct transition states geometry the energies of the transition state predicted with bond-order conserved pathways are very close to their ab initio values. (C) 1998 Elsevier Science B.V. All rights reserved.