Reaction Kinetics and Influence of Film Morphology on the Oxidation of Propene Thin Films by O(3P) Atomic Oxygen

We present results detailing the oxidative reactivity of condensed propene thin films, with particular attention to epoxide product formation because of its importance in the industrial production of polyurethane plastics and the trace presence of these species in the interstellar medium. These studies were conducted in a state-of-the-art ultrahigh vacuum scattering instrument equipped for operation with cryogenic substrate temperatures. After exposing films to a supersonic beam of ground-state atomic oxygen, O­(3P), generated from a radio frequency plasma source, reflection absorption infrared (RAIR) spectra confirm significant propene reactivity, yielding products including propylene oxide, propanal, and a small amount of acetone. In addition to identifying these primary products, we discuss experimentally determined activation energy barriers for reaction in the condensed propene system. Interestingly, we identify significant differences in propene film crystallinity as a result of substrate deposition temperature; lower deposition temperatures (59 K) yield a more ordered, crystalline film. Very little oxidative reactivity is observed in the amorphous propene film, suggesting that the film structure has a substantial impact on observed reactivity by impeding or allowing efficient O­(3P) diffusion. Overall, this work provides fundamental mechanistic insights into the diffusion and reactivity of atomic oxygen in condensed films of small, unsaturated hydrocarbons. The results also emphasize limitations of condensed-phase reactions that rely on reactant diffusion; film composition, morphology, and thickness can significantly limit reactivity despite low reaction barriers.