Metal ion assisted unimolecular decomposition of gaseous organometallic complexes : acquisition of reaction rate constants and dynamics of the dissociative mechanism.




Villarroel, Otsmar J.

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Reaction rate constants have been acquired for the transition metal ion assisted decomposition of various organic molecules, and their deuterium labeled analogs in the gas phase. The metal ion activates organic bonds and mediates the formation of products. Thus, the transition metal cation lowers the bond activation energy requirements in these decomposition reactions making these systems model for catalysis. Catalytic reaction kinetics are not well understood and it is hoped that the resolved study of simpler catalytic models will further the development of the theoretical tools necessary to describe such mechanistic behavior at the molecular level. Reaction rate constants for these model systems are measured using a custom-built molecular beam apparatus. The clusters are formed under supersonic expansion conditions and are bound by the charge-dipole electrostatic interaction between a transition metal cation and a polar organic molecule. The unimolecular decomposition occurs upon laser photon absorption by the jet-cooled cluster yielding a stable neutral molecule and corresponding ion. This dissertation will focus on the unimolecular decomposition kinetics of the Co⁺-Acetone cluster and its deuterium labeled analog. Rate constants are measured at well resolved cluster internal energies. The kinetic isotope effect (KIE) for each measurement was determined. Results are compared to the similar Ni⁺-Acetone decomposition reactions, where the KIE was also measured. These two similar systems present rather different dissociation dynamics. Arguments based on the electronic structure of each ion explain this unique behavior between these similar systems. DFT calculations are made on most systems presented in this dissertation. The most likely geometries and relative energies of the reactants, intermediates and products are determined. Such information specifies aspects of the reaction coordinate and leads to suggestions of mechanisms. This was primarily applied in the final chapter of this dissertation where preliminary results of Ni⁺-assisted decomposition of cyclopentanone are presented. This system represents the group’s first study of a ring-opening reaction.



Castleberry, V.A.; Dee, S. J.; Villarroel, O. J.; Laboren, I. E.; Frey, S. E.; Bellert, D. J. "The Low Energy Unimolecular reaction Rate Constants for the Gas Phase, Ni+-mediated Dissociation of the C-C sigma bond in Acetone." J. Phys. Chem. A. 2009, 113: 10417-10424.
Dee, S. J.; Castleberry, V. A.; Villarroel, O. J.; Laboren, I. E.; Frey, S. E.; Ashley, D.; Bellert, D. J. "Rate limiting step in the low-energy unimolecular decomposition Reaction of Ni+-acetone into Ni+CO + ethane." J. Phys. Chem. A. 2009, 113: 14074-14080.