Methane-to-methanol conversion by first-row transition-metal oxide ions: ScO⁺, TiO⁺, VO⁺, CrO⁺, MnO⁺, FeO⁺, CoO⁺, NiO⁺, and CuO⁺

The reaction pathway and energetics for methane-to-methanol conversion by first-row transition-metal oxide ions (MO⁺s) are discussed from density functional theory (DFT) B3LYP calculations, where M is Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. The methane-to-methanol conversion by these MO⁺ complexes is proposed to proceed in a two-step manner via two transition states: MO⁺ + CH₄ → OM⁺(CH₄) → [TS] → OH-M⁺-CH₃ → [TS] → M⁺(CH₃OH) → M⁺ + CH₃OH. Both high-spin and low-spin potential energy surfaces are characterized in detail. A crossing between the high-spin and the low-spin potential energy surfaces occurs once near the exit channel for ScO⁺, TiO⁺, VO⁺, CrO⁺, and MnO⁺, but it occurs twice in the entrance and exit channels for FeO⁺, CoO⁺, and NiO⁺. Our calculations strongly suggest that spin inversion can occur near a crossing region of potential energy surfaces and that it can play a significant role in decreasing the barrier heights of these transition states. The reaction pathway from methane to methanol is uphill in energy on the early MO⁺ complexes (ScO⁺, TiO⁺, and VO⁺); thus, these complexes are not good mediators for the formation of methanol. On the other hand, the late MO⁺ complexes (FeO⁺, NiO⁺, and CuO⁺) are expected from the general energy profiles of the reaction pathways to efficiently convert methane to methanol. Measured reaction efficiencies and methanol branching ratios for MnO⁺, FeO⁺, CoO⁺, and NiO⁺ are rationalized from the energetics of the high-spin and the low-spin potential energy surfaces. The energy diagram for the methane-to-methanol conversion by CuO⁺ is downhill toward the product direction, and thus CuO⁺ is likely to be an excellent mediator for methane hydroxylation.