Reaction paths for the conversion of methane to methanol catalyzed by FeO⁺

We propose possible theoretical reaction paths for the conversion of methane to methanol catalyzed by FeO⁺. The geometric and electronic structures for the reactant, product, intermediates, and transition states were calculated and analyzed in detail by means of a hybrid Hartree Fock/density functional method. Sextet and quartet spin states were taken into consideration in the analysis of the reaction paths. The conversion of methane to methanol was shown to proceed through basic concerted hydrogen- and methyl-shift reactions. A fragment molecular orbital analysis for the formation of the reactant complex, OFe⁺ CH₄, which plays an important role in the initial stage of methane activation, was carried out in order to understand the nature of the interesting Fe⁺C bond. The five-coordinate methane in the reactant complex was calculated to have a C(3r)-type geometry. Each reaction path presented in this paper includes an important insertion species. HO-Fe⁺ CH₃ or H-Fe⁺ -OCH₃, and two transition states. Thus, there are several kinds of reaction paths, if the high-spin sextet and low-spin quartet states are taken into consideration. A reaction towards the hydroxy intermediate, HO-Fe⁺ -CH³, was found to be more favorable in both the sextet and quartet spin states from the viewpoint of activation energy, and this intermediate is extremely stable. It was found from intrinsic reaction coordinate (IRC) analyses that two basic reactions coexist, namely, hydrogen or methyl migration between the reactant and the methoxy intermediate, H-Fe⁺ -CH₃. This transition state is interesting, because the two transition states resulting from C-H bond cleavage and methyl migration are located in the same region of space on the potential energy surfaces IRCs are partially shown for the complicated first halves of the total reaction paths.