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

Abstract

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( 3 P), 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 (<44 K) yield a more amorphous film, whereas higher 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( 3 P) 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.

A New Pathway for Intersystem Crossing: Unexpected Products in the O(3P) + Cyclopentene Reaction

“…Studies of O­( 3 P) reactions with acyclic unsaturated hydrocarbons − show bimolecular pathways where products are formed on both the singlet PES and the triplet PES. Whereas the reaction of O­( 3 P) with acetylene (C 2 H 2 ) was found to occur exclusively on the triplet surface, − in the reaction of O­( 3 P) with ethene (C 2 H 4 ), almost half of the products formed on the singlet PES after ISC, suggesting that in this simplest case, the rate of ISC competes with the rate of the reaction on the triplet surface.…”

Acetone–Water Interactions in Crystalline and Amorphous Ice Environments

“…Since the 1711 cm –1 peak is attributed to multilayer acetone or acetone–acetone interactions, this suggests that when the ASW film is thinner, fewer acetone molecules interact with water in the acetone–water interfacial region and that the water film is not yet self-similar; by self-similarity we mean that the structure of the interface is no longer varying with changes in thickness. Films less than 100 layers contain small islands that with increasing thickness converge to form a uniform film structure. ,,− Thus, in contrast to the ultrathin films grown in previous studies, − we chose to grow ice films of at least 150 layers to ensure that any differences observed with how acetone interacts with the CI, np-ASW, and p-ASW D 2 O ices are due to the underlying water coordination and not simply a film thickness effect.…”

Exploiting Photoionization Reflectron Time-of-Flight Mass Spectrometry to Explore Molecular Mass Growth Processes to Complex Organic Molecules in Interstellar and Solar System Ice Analogs

“…The identification of glycerol and ethylene glycol in irradiated methanol ices exploiting PI-ReTOF-MS represents a particular success since both molecules have essentially indistinguishable infrared spectra, and classical FTIR cannot be utilized in their identification in complex mixtures . Fourth, our studies also revealed the mechanisms leading to the formation of the epoxides (cyclic ethers) ethylene oxide (c-C 2 H 4 O) and propylene oxide (c-C 3 H 6 O) via addition of suprathermal ground state oxygen (O­( 3 P)) or electronically excited oxygen (O­( 1 D)) to carbon–carbon double bonds of ethylene and propylene, respectively. Racemic propylene oxide represents the first chiral molecule detected in the interstellar medium .…”