INTRINSIC ACTIVATION BARRIERS FOR A PROTOTYPE HYDROGENOLYSIS REACTIOND-]DCH3+CH3 IN C-3-NU SYMMETRY(C2H6)

The presence of intrinsic barriers to reaction are very important to t he theory of reactions, but at present accurate values of the intrinsi c barriers to reaction for hydrogenolysis reactions do not appear in t he open literature. In this paper we use ab initio methods to estimate the intrinsic barrier for a prototype hydrogenolysis reaction for the abstraction of a methyl group from ethane: D + CH3CH3 --> DCH3 + CH3. A potential energy surface for this reaction was calculated at the UM P2/6-31G level of theory. Then a variety of methods up to QCISD(T)/6- 311G(d,p) were used to estimate the activation barrier and intrinsic a ctivation energy for this reaction. It was found that this reaction ha s an intrinsic barrier of 48 +/- 2 kcal/mol. By comparison experimenta l results suggest that the intrinsic barrier to hydrogen abstraction f rom ethane, D + CH3CH3 --> DH + CH2CH3, is only 13 kcal/mol. These res ults show that the intrinsic barriers to hydrogen abstraction are much lower than the intrinsic barriers to methyl abstraction, in agreement with previous experiments.