Scott A. Epstein, Department of Chemical Engineering, Carnegie Mellon University, 1207 Doherty Hall, 5000 Forbes Ave., Pittsburgh, PA 15213 and Neil M. Donahue, Department of Chemistry / Department of Chemical Engineering, Carnegie Mellon University, 1106 Doherty Hall, 5000 Forbes Ave., Pittsburgh, PA 15213.
A set of reactions with a particular importance to the chemistry of the troposphere is the reaction of alkenes with ozone. These ozonolysis reactions are a significant source and transformer of atmospheric particulate matter which has major implications on heath, visibility, and climate. Ozonolysis occurs both in the gaseous and condensed phase ultimately leading to different product distributions; however, the preliminary steps in the multi-step mechanism are common to both phases. The first step in any ozonolysis reaction involves the formation of an unstable short-lived intermediate deemed the primary ozonide (POZ). Based on the initial alkene geometry, one or more POZs can form. In addition, decomposition of each of the POZs can proceed in several different ways ultimately leading to a range of possible products. Further complicating matters, the POZ decomposes too rapidly to be observed under normal atmospheric conditions.
We have designed an apparatus to perform Temperature Programmed Reaction Spectroscopy (TPRS) of the stabilized primary ozonide on a cold surface. In this method, a selected alkene and ozone react in moderate vacuum on two liquid nitrogen cooled zinc selenide infrared-transparent windows. Surface effects are minimized with the use of macroscopic reactant volumes. A Fourier transform infrared spectrometer is used to monitor the relative concentrations of each species as the windows are warmed at a rate of 0.01 K/s to 0.3 K/s. Desorbed species are continuously pumped out of the chamber. Experiments are conducted at temperatures ranging from 90 K to 300 K.
This method allows us to track intermediate concentrations as a function of temperature on the window surface. We observe primary ozonide formation, decomposition, and the subsequent formation of decomposition products. It is possible to calculate barrier heights for the primary ozonide decomposition. This method will also be used to determine the barrier heights of competing decomposition pathways as to better predict the nature of the product distribution for these ozonolysis reactions.