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 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.

ReaxFF: A Reactive Force Field for Hydrocarbons

“…Originally used for the H + H 2 reaction surface, it is a very good approximation to more complicated empirical forms such as LEPS surface. [12][13] While it has recently been extended to more complex reactions, [14][15] it remains mainly useful for H atom transfer reactions in a linear collision geometry.…”

Extensions of the Marcus equation for the prediction of approximate transition state geometries in hydrogen transfer and methyl transfer reactions

“…Figure 1 compares the values of r t B and r t F calculated at the MP2/6-31G* level to those predicted by Eqs. (11) and (12). Notice that the values of r t B and r t F predicted from Eqs.…”

“…(13) into Eqs. (11) and (12) to calculate r t B and r t F . Table 2 shows the bond lengths we calculate in this way.…”

“…Blowers and Masel [12] found that in MP2/6-31G(d), C A is about 0.5 for hydrogen transfer reactions and 1.0 for methyl transfer reactions. For the hydrogen transfer reactions we will consider later in this paper, E 0 A is about 10 5 kcal/mol [13] while the bond energies are on the order of 100 kcal/mol.…”

“…In the next section we will use ab initio calculations at various levels to ®nd the geometry of the transition state for 26 hydrogen transfer reactions and 6 methyl transfer reactions to see how well they ®t Eqs. (5), (11), and (12).…”