Kinetic Isotope Effect of the Oxidative Addition Reaction of Hydrogen to Vaska's Compound
Blumer, David James
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Permalink
https://hdl.handle.net/2142/67286
Description
Title
Kinetic Isotope Effect of the Oxidative Addition Reaction of Hydrogen to Vaska's Compound
Author(s)
Blumer, David James
Issue Date
1981
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Inorganic
Language
eng
Abstract
The rates of reaction for the oxidative addition of hydrogen and deuterium gas to Ir(CO)Cl{P(C(,6)H(,5))(,3)}(,2) (Vaska's compound) have been accurately measured over the temperature range 0-50(DEGREES)C in toluene. The kinetic isotope effect, i.e., the ratio of the rate constants k(,H(,2))/k(,D(,2)), was determined over this temperature range. The kinetic isotope effect varies substantially with temperature, being inverse (k(,H(,2))/k(,D(,2)) = 0.848 at 0(DEGREES)C, exhibits a crossover point (k(,H(,2))/k(,D(,2)) = 1) at 19.94(DEGREES)C, and becomes relatively large (k(,H(,2))/k(,D(,2)) = 1.27) at 50(DEGREES)C; a plot of ln (k(,H(,2))/k(,D(,2))) vs. l/T has a slope of -661 deg('-1) and an intercept of 2.255 and is extremely linear with a correlation coefficient r = 0.999999.
To interpret the experimental values of the kinetic isotope effect, "exact" computer calculations were carried out with a variety of transition state models. While the number of unknowns prevents a mathematically unique solution giving the transition state properties, many important properties of the transition state for the activation of hydrogen on a metal center were identified. The transition state appears early in the reaction with the loss of 33-50% of the bond order in the H-H bond and concomitant 33-50% formation of the two iridium-hydrogen bonds. The reaction coordinate involves nearly equal contributions from the H-H stretch and the two Ir-H stretch vibrations. Three motions, two bends and hindered rotation involving the M-H(,2) group were found to be a necessary part of the transition state model. The hindered rotation appears to have a rather large barrier, between 3-10 kcal/mole, when the bending frequencies of the metal-hydrogen triangle were varied between reasonable frequencies (700-300 cm('-1)). Tunnelling is not an important mechanism in the activation of hydrogen since the KIE vs. T was linear while calculations indicated curved temperature dependence with barrier curvatures of (GREATERTHEQ) 500i cm('-1). Thus, the activation barrier must be rather flat and thick.
An apparatus was designed which can automatically measure the rate of gas consumption or release at constant temperature with accuracies up to about 1 part in 10('4). A sensitive electronic differential pressure sensor monitors the gas pressure and a circuit digitally transmits the pressure data along with the reaction time to a cassette recorder. The data is later input to a PDP-11 minicomputer through a special interface and analyzed.
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