http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Reaction mechanisms are generally probed by looking at the energetics of observable intermediates and the transition states that have observable effects on rates or selectivity. Mechanisms are complex, however, and there are often many transition states and intermediates hidden from observations. We described here how hidden mechanistic steps can be probed by observing their effect on a later product selectivity. This is possible because molecules that pass through a high energy-barrier retain their excess energy for a short time. It only takes a few dozen femtoseconds to pass into the area of a subsequent intermediate. This is short compared to the picoseconds it takes for energy to dissipate out to the environment. In this way, the intermediates possess excess energy that is neither thermally equilibrated nor initially statistically distributed within the molecule. We have observed this effect in reactions of short-lived alkoxy radical intermediates. Depending on how a methylcyclobutoxy radical is chemically generated, the kinetic isotope effects (KIE) for next C-C bond cleavage step changes. We observed KIE in-turn allows us to read-out the amount of excess energy the alkoxy radical gained during its generation. This provides a new kind of mechanistic probe. In recent years, many new methods have been developed for oxidation of alcohols that have shown alkoxy radicals as the key intermediates. These methodologies often involve steps like electron transfer or electronic excitation that cannot be studied by ordinary mechanistic probes. Herein, the process of energy read-out can be utilized to point out crucial steps and investigate hidden transition states. The electronic version of this dissertation is accessible from https://hdl.handle.net/1969.1/198542
Doctor of Philosophy Texas A&M University 2022 https://hdl.handle.net/1969.1/198542