Reaction Kinetics and Influence of Film Morphology on Propene Thin Film Oxidation

We present results detailing the oxidative reactivity of condensed propene films with particular interest towards epoxidation product formation due to their trace presence in the interstellar medium. These studies were conducted in a state-of-the-art ultra-high vacuum gas-surface chamber equipped for operation involving cryogenic substrate temperatures. We exposed condensed propene films to a supersonic beam of O(³P) generated from a radio frequency plasma source, and monitored surface reactivity with RAIRS. Interestingly, we identified significant differences in propene film crystallinity as a result of substrate deposition temperature; lower deposition temperatures (< 44 K) yield a more amorphous film whereas higher temperatures (> 59 K) yield a more ordered, crystalline film. Very little oxidative reactivity was observed in the amorphous propene suggesting that the film structure has a substantial impact on observed reactivity by either impeding or allowing efficient O(³P) diffusion. Additionally, we note significant propene reactivity towards a variety of products including propylene oxide, as well as propanal and acetone and experimentally determined the activation energy barriers. We found that not only is oxygen addition to the terminal carbon the dominating pathway, but that the addition step itself is rate limiting. Overall, this work provides fundamental mechanistic insight into the diffusion and reactivity of ground state atomic oxygen in condensed films of small, unsaturated hydrocarbons. The results also emphasize limitations of condensed-phase astrophysical reactions that rely on reactant diffusion; film composition, morphology, and thickness can significantly limit reactivity despite low reaction barriers. Acknowledgments: This work was supported by the National Science Foundation Division of Chemistry, and the NSF-Materials Research Science and Engineering Center at The University of Chicago.