Activation of Methane by FeO⁺: Determining Reaction Pathways through Temperature-Dependent Kinetics and Statistical Modeling

Abstract

The temperature dependences of the rate constants and product branching ratios for the reactions of FeO + with CH 4 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 cm 3 s −1 for the CH 4 and CD 4 reactions, respectively. At low temperatures, the Fe + + CH 3 OH/CD 3 OD product channel dominates, while at higher temperatures, FeOH + /FeOD + + CH 3 /CD 3 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 3 OH 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.

Further Insight into the Reaction FeO+ + H2 → Fe+ + H2O: Temperature Dependent Kinetics, Isotope Effects, and Statistical Modeling

“…Furthermore, we note that the TS energy is primarily a function of the current thermal data, whereas the J max parameter is primarily determined by the higher energy range data (GIB), highlighting the complementary nature of the two experiments; the fitted curves of Figure 2 depend weakly on J max up to about 700 K. Figure 4 illustrates the sensitivity of the fit on the two parameters. Like for the FeO + + CH 4 reaction in ref 22, the restriction of the range of contributing J increasingly matters only with increasing T trans such that the GIB data could not be reproduced with J max = ∞ at all. Increasing J max for D 2 beyond the fitted value of 40, analogous to the GIB data for FeO + + CH 4 from ref 22, would result in a more pronounced upturn of the modeled k tot (T trans ) with increasing temperature.…”

“…The aim of the present work is to expand upon this, employing a combination of advanced statistical modeling, electronic structure calculations, as well as a significantly broader set of experimental data. We are motivated to analyze reaction 1.1 after our successful modeling of the bond activation of methane 22 (1. 2) in which we applied the statistical adiabatic channel model (SACM) 23 to the primary complex formation 24 and general statistical rate theory 25 to the subsequent intrinsic dynamics.…”

The Kinetic Isotope Effect as a Probe of Spin Crossover in the CH Activation of Methane by the FeO+ Cation

“…This result means that RC and TS1 must have different spin states, the sextet and quartet, respectively, which requires the spin crossover that reduces the barrier height and catalyzes the CH activation 9c. Recently, Ard et al 20. reproduced experimental rate constants and product branching ratios using a statistical model with TS energies as adjustable parameters under the assumption of the spin crossover.…”

Statistical modeling of the reactions Fe+ + N2O → FeO+ + N2 and FeO+ + CO → Fe+ + CO2

“…After completion of our work on reactions (1) and (2), an alternative analysis of rate constants for similar ion-molecule reactions was developed and applied to the reactions 10 (3) FeO + + CH 4 It was confirmed that "rotational channel switching" was important, i.e. that different Jdependences of the energies of the loose entrance (E 0a (J)) and rigid exit (E 0b (J)) transition states into and out of the primary adducts (with E 0a (J) > E 0b (J) at small J and E 0a (J)<E 0b (J) at large J) had to be accounted for.…”

“…2should be made. It was shown in ref 10. for the reaction FeO + + CH 4 (3) that the rate constant at large translational temperatures stops decreasing and finally increases with temperature.…”