Metz Research Group Projects

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Studies of C-H and C-C Bond Activation by M⁺ and MO⁺

Transition metal ions (and metal oxide ions) have the ability to break C-C bonds in simple hydrocarbons (typically producing methane) 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 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.

In order to form methanol, the reactants first form the entrance channel complex; (1). Hydrogen abstraction leads to the critical intermediate [HO-Fe-CH₃]⁺ (2). This intermediate can dissociate to form the undesirable FeOH⁺ + CH₃ products or it can rearrange to form the exit channel complex (3), 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.

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.

Vibrational Raman Spectroscopy of Transient Ions

Our studies to date are based on electronic spectroscopy, and thus usually give little information on the vibrations of the electronic ground state of the molecule. By combining stimulated Raman excitation of a molecular vibration with selective photodissociation of the vibrationally excited molecules we plan to obtain the vibrational Raman spectrum of ions such as FeO⁺ and intermediates of the reactions shown above.

Solvation of Multiply Charged Transition Metal Ions

In solution, transition metal ions typically have a +2 or +3 formal charge. By using an electrospray source to produce multiply-charged ions we can study the spectroscopy of solvated, multiply-charged ions. We have confirmed that Ni²⁺(H₂O)₆ and Co²⁺(H₂O)₆ are responsible for the characteristic colors observed in aqueous nickel (II) and cobalt (II) solutions, respectively by measuring the photofragment spectra of the ions. A minimum number of solvent molecules is required to solvate the +2 charge. Ions containing few solvent molecules photodissociate to form two +1 ions. For example, Ni²⁺(H₂O)₄ + hv --> NiOH⁺(H₂O)₂ + H₃O⁺ We learn a great deal about the mechanism for this reaction by measuring the relative kinetic energy of the two fragments. We've also studied the spectroscopy of NiOH⁺ and NiOH⁺(H₂O). We're extending these studies to other ions and other solvents (CH₃OH, CH₃SH, CH₃CN).

Zeolite Thin Films

Zeolite thin films have extremely useful catalytic and transport properties. Professor Michael Tsapatsis of the UMass Chemical Engineering Deptartment has developed techniques to grow these films, controlling their thickness and morphology. We're trying to extend these techniques by using laser ablation to deposit microcrystalline zeolites onto a substrate. Subsequent layers of the zeolite are grown from solution.