Reaction rates and energy barriers for elementary reactions of organometallic molecules in the gas phase

Dawn Marie Kollar-Becher, Purdue University

Abstract

This thesis explores the kinetic and thermochemical behavior of two separate organometallic systems, Mo(CO)$\sb6$/C$\sb2$H$\sb4$ and Fe(CO)$\sb5$/H$\sb2$. The first chapter chronicles the study and importance of organometallic complexes from a historical perspective. Chapter Two details the construction of the stainless steel, thermally controlled, cross cell used in these investigations. The third chapter is devoted to the development of a new technique of Time-Resolved Fourier Transform Infrared (TRFTIR) spectroscopy which interfaces an excimer laser with the moving interferometer mirror. In the fourth chapter gas phase samples of Mo(CO)$\sb5$(C$\sb2$H$\sb4$) are prepared in situ by laser irradiation of quantitative mixtures of Mo(CO)$\sb6$, CO and C$\sb2$H$\sb4$. In the presence of CO and C$\sb2$H$\sb4$, MO(CO)$\sb5$(C$\sb2$H$\sb4$) decays thermally to reform Mo(CO)$\sb6$ by the mechanism of dissociative substitution. Systematic study of the rate of this reaction as function of partial pressures of CO and C$\sb2$H$\sb4$ yields the elementary high-pressure limiting thermal rate constant for unimolecular dissociation of Mo(CO)$\sb5$(C$\sb2$H$\sb4$), k$\sb1$, and the relative rate constant for recombination of Mo(CO)$\sb5$ with CO and C$\sb2$H$\sb4$ (k$\sb2$/k$\sb3$). These measurements are extended to determine k$\sb1$ and k$\sb2$/k$\sb3$ over a range of precisely controlled temperatures to determine Arrhenius parameters reflecting energetic and statistical properties of these elementary rate processes. The activation energy for elementary ethylene loss corresponds reasonably with expected bond-strength trends. The corresponding isolated-molecule preexponential factor, however, is many orders of magnitude smaller than comparable quantities measured previously in solution, suggesting that detailed dynamics of decomposition in the liquid phase differ fundamentally from elementary ligand loss in the isolated molecule. The fifth and final chapter explores the dissociative substitution kinetics of Fe(CO)$\sb4$(H)$\sb2$ and a complex we initially assigned as Fe(CO)$\sb3$(H)$\sb2$H$\sb2$. Both of these compounds are created in situ by irradiating gas phase mixtures of Fe(CO)$\sb5$/H$\sb2$/CO with 308 nm laser light. Studies of the rate of reaction versus the partial pressure of reactants are reported, with the corresponding rate constants for the dissociative loss of H$\sb2$ from the unsaturated fragments Fe(CO)$\sb4$ and Fe(CO)$\sb4$(H)$\sb2$. Branching ratios (k$\sb2$/k$\sb3$) measuring the preference these fragments have for recombination with either CO or H$\sb2$ were also measured. In addition, the Arrhenius dependence of the rate of these reactions on temperature was explored, yielding the first gas phase measurements of the activation energies and preexponential factors for the dissociative loss of hydrogen. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Grant, Purdue University.

Subject Area

Chemistry

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