Mechanism of Two Homogeneous Reactions; CO Self Exchange and N2 Self Exchange

• Main Author: Rockwood, Alan L.
• Format: Others
• Published: DigitalCommons@USU 1981
• Subjects: Mechanism; Homogeneous Reactions; CO; N2; Chemistry
• Online Access: https://digitalcommons.usu.edu/etd/7217; https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=8298&context=etd

The two atom switching reactions referred to in the title were originally studied at temperatures greater than 2000°K in shock tubes by other investigators. For each reaction they proposed a direct four-center exchange mechanism in which one of the reactant molecules must be vibrationally excited, (the vibrational excitation mechanism or VEM). One of the predictions of the VEM is that molecules which are vibrationally hot but translationally cold should react through the four center transition state that leads to exchange. Using a mercury photosensitization technique, it is shown in the present work that excitation of CO to high vibrational levels is not sufficient to cause the CO self-exchange reaction. Similar attempts were also made to verify the VEM for the N2 reaction, but no exchange was observed. Kinetic modeling studies show that an atomic chain mechanism triggered by traces of oxygen impurity is responsible for all or much of the CO exchange observed in the shock tubes. Modeling studies show that many of the observed features of the N2 reaction are also correctly predicted by an atomic chain mechanism; however, the critical step in the mechanism, the N + N2 exchange reaction, has never been observed. Potential surface calculations show that at the restricted Hartree-Fock level of approximation the N3 potential surface has an energy barrier for exchange of over 80 kcal/mole, which is much too high if the atomic mechanism is to operate in the shock tubes. By comparison with similar calculations on N2o+, it is argued that the RHF calculations probably overestimate the true barrier height by about 80 kcal/mole, so the barrier to exchange on the N3 potential surface is probably no more than a few kcal/mole, and the N + N2 reaction is probably fast at high temperatures. Potential surface calculations on N4 show that the barrier to exchange through the four-center transition state is almost certainly much too high to account for the exchange observed in the shock tubes. Certain limitations on the rate law and energy barrier to exchange are derived for the VEM. It is concluded that both exchange reactions can be explained by atomic chain mechanisms and there is no need to invoke the VEM for either reaction.