Single- and two-state reactivity in the gas-phase C-H bond activation of norbornane by 'bare' FeO+

The potential-energy surface for C-H bond activation of norbornane by 'bare' FeO⁺ is examined at the B3LYP/6-31G** level of theory. The free reactants combine to form norbornane/FeO⁺ ion-dipole clusters in which the FeO⁺ unit can bind at either the exo or endo face of norbornane. The transition structures for insertion of FeO⁺ into the exo and endo C-H bonds are located at least 9 kcal · mol^-1 below the entrance channel, thus accounting for the observed unit efficiency of the C-H bond activation reported in previous gas-phase ion-cyclotron resonance experiments (Helv. Chim. Acta 1995, 78, 1013). Interesting features of the reaction profiles are crossovers of the high-spin sextet (S = 5/2) and low-spin quartet (S = 3/2) states en route to the transition structures (TS); this type of behavior has been termed two-state reactivity (Helv. Chim. Acta 1995, 78, 1393). The branchings between the endo and exo pathways are simulated by Rice- Ramsperger-Kassel-Marcus (RRKM) theory with the calculated harmonic frequencies. Additionally, hydrogen/deuterium kinetic isotope effects are computed using RRKM theory and compared with the experimental data. The simulated KIEs differ for high-spin and low-spin TSs, suggesting that isotope effects can be used as sensitive probes for diagnosing spin-crossover mechanisms.