Molecular weight growth in Titan's atmosphere: branching pathways for the reaction of 1-propynyl radical (H3CCC˙) with small alkenes and alkynes

Abstract

The reaction of small hydrocarbon radicals (i.e. ˙CN, ˙C₂H) with trace alkenes and alkynes is believed to play an important role in molecular weight growth and ultimately the formation of Titan's characteristic haze. Current photochemical models of Titan's atmosphere largely assume hydrogen atom abstraction or unimolecular hydrogen elimination reactions dominate the mechanism, in contrast to recent experiments that reveal significant alkyl radical loss pathways during reaction of ethynyl radical (˙C₂H) with alkenes and alkynes. In this study, the trend is explored for the case of a larger ethynyl radical analogue, the 1-propynyl radical (H₃CCC˙), a likely product from the high-energy photolysis of propyne in Titan's atmosphere. Using synchrotron vacuum ultraviolet photoionization mass spectrometry, product branching ratios are measured for the reactions of 1-propynyl radical with a suite of small alkenes (ethylene and propene) and alkynes (acetylene and d₄-propyne) at 4 Torr and 300 K. Reactions of 1-propynyl radical with acetylene and ethylene form single products, identified as penta-1,3-diyne and pent-1-en-3-yne, respectively. These products form by hydrogen atom loss from the radical-adduct intermediates. The reactions of 1-propynyl radical with d₄-propyne and propene form products from both hydrogen atom and methyl loss, (–H = 27%, –CH₃ = 73%) and (–H = 14%, –CH₃ = 86%), respectively. Together, these results indicate that reactions of ethynyl radical analogues with alkenes and alkynes form significant quantities of products by alkyl loss channels, suggesting that current photochemical models of Titan over predict both hydrogen atom production as well as the efficiency of molecular weight growth in these reactions.