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Abstract

The gas-phase reaction of NiAl₂O₄⁺ with CH₄ is studied by mass spectrometry in combination with vibrational action spectroscopy and density functional theory (DFT). Two product ions, NiAl₂O₄H⁺ and NiAl₂O₃H₂⁺, are identified in the mass spectra. The DFT calculations predict that the global minimum-energy isomer of NiAl₂O₄⁺ contains Ni in the +II oxidation state and features a terminal Al−O^.- oxygen radical site. They show that methane can react along two competing pathways leading to formation of either a methyl radical (CH₃⋅) or formaldehyde (CH₂O). Both reactions are initiated by hydrogen atom transfer from methane to the terminal O^.- site, followed by either CH₃⋅ loss or CH₃⋅ migration to an O^2- site next to the Ni²⁺ center. The CH₃⋅ attaches as CH₃⁺ to O^2- and its unpaired electron is transferred to the Ni-center reducing it to Ni⁺. The proposed mechanism is experimentally confirmed by vibrational spectroscopy of the reactant and two different product ions.