Kinetic energy release distributions reactions

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

The purpose of this article is to review some of the current endeavors in this developing field. To maintain brevity, the focus is on recent studies carried out in our own laboratory and in conjunction with Professor M.T. Bowers at the University of California at Santa Barbara, with emphasis on the use of kinetic energy release distributions and infrared laser multiphoton excitation to probe potential energy surfaces for the reactions of atomic metal ions with alkenes and alkanes. 

Studies of kinetic energy release distributions have implications for the reverse reactions. Notice that on a Type II surface, the association reaction of ground state MB+ and C to form MA+ cannot occur. In contrast, on a Type I potential energy surface the reverse reaction can occur to give the adduct MA+. Unless another exothermic pathway is available to this species, the reaction will be nonproductive. However, it is possible in certain cases to determine that adduct formation did occur by observation of isotopic exchange processes or collisional stabilization at high pressures. 

Figure 9. Kinetic energy release distributions for several dehydrogenation reactions. Data from reference 38.

In contrast to the results obtained for dehydrogenation reactions, kinetic energy release distributions for alkane elimination processes can usually be fit with phase space theory. Results for the loss of methane from reaction 9 of Co + with isobutane are shown in Figure 10b. In fitting the... 

Figure 10. Comparison of experimetnal kinetic energy release distributions to phase-space calculations for (a) dehydrogenation of n-butane by Co+ and (b) loss of methane in reaction of Co+ with isobutane. Data from reference 38.

The success of the phase space theory in fitting kinetic energy release distributions for exothermic reactions which involve no barrier for the reverse reaction have led to the use of this analysis as a tool for deriving invaluable thermochemical data from endothermic reactions. This is an important addition to the studies of endothermic reactions described above. As an example of these studies, consider the decarbonylation reaction 11 of Co+ with acetone which leads to the formation of the... 

The application of newer methods to studies of gas phase organometallic reactions will lead to the development of routine techniques for determination of the thermochemistry of organometallic species. The examples discussed above demonstrate that an analysis of kinetic energy release distributions for exothermic reactions yields accurate metal ligand bond dissociation energies. This can be extended to include neutrals as well as ions. For example, reaction 15 has been used to determine accurate bond dissociation energies for Co-H and C0-CH3 (57). 

Organometallic Reaction Energetics from Product Kinetic Energy Release Distributions... 

Figures, t othetical potential energy surfaces for the reaction M+ + A + C and the corresponding product kinetic energy release distributions in the center-oi-mass fiame.

Practical Considerations in Calculations. In order to calculate kinetic energy release distributions, structures and vibrational frequencies for the various species are required. These are taken from the literature where possible, or estimated from literature values of similar species. The details of the kinetic energy release distributions are found to va only weakly with structure or vibrational frequencies over the entire physically reasonable range for these quantities. The distributions are strongly dependent on the total energy available to the dissociating complex, and hence in our model to the AH of reaction. Often all heats of formation of product and reactants are well known except one, the organometallic product ion. This quantity can then be used as a parameter and varied until the best fit with experiment is obtained. 

It should be apparent that phase space fitting of kinetic energy release distributions yields important thermochemical information for exothermic reactions with no reverse activation barrier. As another example, Co+ ions decarbonylate acetone (reaction 10) yielding a dimethyl cobalt ion as... 

In accordance with the considerations outlined in the previous section, a statistical kinetic energy release distribution should be observed for this system. As shown in Figure 7, the experimental distribution for this process can be fit very closely using statistical phase space theory, which yields a bond dissociation energy Do°(Fe -C5H5) = 55 + 5 kcal/mol. A reaction coordinate diagram for... 

Another example of a hydrogen elimination process which is both reversible and exhibits a statistical kinetic energy release distribution is the dissociation of the cyclopentadienyl rhodium isopropyl ion, reaction 13(12). In this case the... 

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