Activation of methane by FeO⁺: Determining reaction pathways through temperature-dependent kinetics and statistical modeling

The temperature dependences of the rate constants and product branching ratios for the reactions of FeO⁺ with CH₄ and CD ₄ have been measured from 123 to 700 K. The 300 K rate constants are 9.5 × 10^-11 and 5.1 × 10^-11 cm³ s^-1 for the CH₄ and CD₄ reactions, respectively. At low temperatures, the Fe⁺ + CH₃OH/CD ₃OD product channel dominates, while at higher temperatures, FeOH⁺/FeOD⁺ + CH₃/CD₃ becomes the majority channel. The data were found to connect well with previous experiments at higher translational energies. The kinetics were simulated using a statistical adiabatic channel model (vibrations are adiabatic during approach of the reactants), which reproduced the experimental data of both reactions well over the extended temperature and energy ranges. Stationary point energies along the reaction pathway determined by ab initio calculations seemed to be only approximate and were allowed to vary in the statistical model. The model shows a crossing from the ground-state sextet surface to the excited quartet surface with large efficiency, indicating that both states are involved. The reaction bottleneck for the reaction is found to be the quartet barrier, for CH ₄ modeled as -22 kJ mol^-1 relative to the sextet reactants. Contrary to previous rationalizations, neither less favorable spin-crossing at increased energies nor the opening of additional reaction channels is needed to explain the temperature dependence of the product branching fractions. It is found that a proper treatment of state-specific rotations is crucial. The modeled energy for the FeOH⁺ + CH ₃ channel (-1 kJ mol^-1) agrees with the experimental thermochemical value, while the modeled energy of the Fe⁺ + CH ₃OH channel (-10 kJ mol^-1) corresponds to the quartet iron product, provided that spin-switching near the products is inefficient. Alternative possibilities for spin switching during the reaction are considered. The modeling provides unique insight into the reaction mechanisms as well as energetic benchmarks for the reaction surface.