Transition metal ions (and metal oxide ions) have the ability to break C-C bonds in simple hydrocarbons and to react with C-H bonds, even in methane. We are interested in looking at the detailed mechanisms for these reactions. We do this by using electronic and vibrational spectroscopy to characterize the bonding, energetics and geometries of the reaction intermediates and the charged reactants and products. The system we have studied most extensively is the direct conversion of methane to methanol by iron oxide: FeO⁺+ CH₄ --> Fe⁺ + CH₃OH. A schematic potential energy surface for this reaction is shown below.

Calculated (CBS-QB3) FeO+ + CH4 Potential Energy Surface

Vibrational Spectra of Bare and Argon-tagged Insertion Intermediate

In order to form methanol, the reactants first form the entrance channel complex. Hydrogen abstraction leads to the critical intermediate [HO-Fe-CH₃]⁺ . This intermediate can dissociate to form the undesirable FeOH⁺ + CH₃ products or it can rearrange to form the exit channel complex, which falls apart to Fe⁺ + CH₃OH. We have studied the spectroscopy of the FeO⁺ reactant, as well as the critical [HO-Fe-CH₃]⁺ intermediate and the H₂O...FeCH₂⁺ intermediate, which leads to the minor FeCH₂⁺ + H₂O products. Vibrational spectra of the [HO-Fe-CH₃]⁺ insertion intermediate in the C-H and O-H stretching regions are shown above.

Ions such as MCH₂⁺ are important intermediates in reactions that couple small hydrocarbons to make larger ones. For example, tungsten (and several other third-row transition metals) react sequentially with methane:

W⁺ + CH₄ --> WCH₂⁺ + H₂

WCH₂⁺ + CH₄ --> WC₂H₄⁺ + H₂, etc.

producing ions as large as WC₈H₁₆⁺. We have studied MCH₂⁺ (M=Fe, Co, Ni, Au, Ta) and plan to extend these studies to tungsten and platinum.

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Calculated (CBS-QB3) FeO⁺ + CH₄ Potential Energy Surface

Time of Flight Spectra of H⁺S from photodissociation of Co²⁺(S)₄ (S=H₂O and CH₃OH)