Single- and two-state reactivity in the gas-phase C-H band activation of norbarnane 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 stru ctures 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 ga s-phase ion-cyclotron resonance experiments (Helv. Chim. Acta 1995, 78, 101 3). Interesting features of the reaction profiles are crossovers of the hig h-spin sextet (S=5/2) and low-spin quartet (S=3/2) states en route to the t ransition structures (TS); this type of behavior has been termed two-state reactivity (Helv. Chim. Acta 1995, 78, 1393). The branchings between the en do and exo pathways are simulated by Rice-Ramsperger-Kassel-Marcus (RRKM) t heory with the calculated harmonic frequencies. Additionally, hydrogen/deut erium kinetic isotope effects are computed using RRKM theory and compared w ith the experimental data. The simulated KIEs differ for high-spin and low- spin TSs, suggesting that isotope effects can be used as sensitive probes f or diagnosing spin-crossover mechanisms.