Details of potential energy surfaces involving C-C bond activation: Reactions of Fe+, Co+, and Ni+ with acetone

Article abstract of DOI:10.1021/ja00149a021

Product kinetic energy release distributions (KERDs) for reactions of Fe⁺, Co⁺, and Ni⁺ with acetone to eliminate C₂H₆ and CO have been measured. These distributions are statistical and are very sensitive to the energy of the rate-limiting transition state. We argue this transition state is most likely due to initial C-C bond insertion. The rate-limiting transition state acts to restrict high angular momentum reactant collision complexes from going on to products, thereby reducing the average kinetic energy released. By modeling the experimental KERDs, the rate-limiting transition state was determined to lie in the range of 9 ± 3 kcal/mol below the energy of the M⁺ + acetone reactants for all three metal ions. Bond energies for M⁺-CO and M⁺-C₂H₆ have also been determined: D^o₀(Co⁺-CO) = 39.1 ± 3 kcal/mol, D^o₀(Fe⁺-C₂H₆) = 17.9 ± 3 kcal/mol, and D^o₀(Ni⁺-C₂H₆) = 28.7 ± 3 kcal/mol. In addition, modeling the experimental KERDs indicates that the MC₂H₆⁺ product formed in the reaction of M⁺ with acetone is nearly exclusively an ethane adduct, with a maximum 10-15% of the dimethyl complex being formed. Finally, arguments relating the initial rate-limiting transition state to the C-H bond activation transition state in propane are made and suggest that the C-C bond activation transition state in small alkanes is 6 ± 5 kcal/mol higher in energy than C-H bond activation.