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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 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.



Imaging Ion Photodissociation


In photodissociation, some of the available energy goes towards breaking the chemical bond, some is internal (rotational, vibrational and electronic) energy of the fragments and the remainder is relative translational energy of the fragments - kinetic energy release (KER). In our time-of-flight instruments, large kinetic energy release can lead to broadening in fragment time-of-flight peaks, as we observe in photodissociation of M2+(H2O)4 (M=Ni, Co) to form H3O+ and MOH+(H2O)2 (see next section). However, it is difficult to accurately reconstruct the three-dimensional product velocity distribution from a one-dimensional measurement. Imaging the products yields a two-dimensional projection, which is much more informative. Measurements of the kinetic energy release (KER) and anisotropy in a photodissociation reveal whether the excited states are repulsive or attractive, the extent of coupling between excited states, energy partitioning between translational and internal degrees of freedom, and bond dissociation energies.



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 photodissociation spectra of the ions (below, left). 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, Co2+(H2O)4 + hv --> CoOH+(H2O)2 + H3O+ We learn a great deal about the mechanism for this reaction by measuring the relative kinetic energy of the two fragments (below, right). We've also studied the spectroscopy and photodissociation dynamics of Co2+ and Ni2+ solvated by methanol, water, and acetonitrile.


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)