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Kinetics and Branching for the Reactions of N2+ with C3H4 Isomers at Low Temperatures and Implications for Titan’s Atmosphere

Abstract : The photoionization of N2 plays a key role in initiating the formation of complex organic molecules in the nitrogen-rich atmosphere of Saturn’s largest moon, Titan. To date, only a handful of laboratory studies have explored the reactivity of N2+ ions with hydrocarbons─limited to methane, acetylene, and ethylene─at the low temperatures relevant to Titan. Here, the rate coefficients, product identity, and branching ratios of the ion–molecule reactions of N2+ with C3H4 isomers, namely, propyne CH3CCH and allene CH2CCH2, were measured between 24 and 72 K in uniform supersonic flows. The rate coefficients are collisional and their temperature dependence is in remarkable agreement with capture models. The outcomes of both reactions are similar: they proceed primarily via dissociative charge transfer, leading to the formation of C3H3+ (main product, >70%) and C3H2+ (between 9 and 17%), whereas a second, nondissociative charge-transfer mechanism leading to C3H4+ becomes slightly more prominent as the temperature decreases (from 3 to 12%). C3H3+ is plausibly formed predominantly as the smallest aromatic cation, cyclopropenyl c-C3H3+, by following the lowest-energy pathway for the decomposition of allene and propyne cations. The measured rate coefficients and branching ratios were included in a photochemical model of Titan’s atmosphere. The results point toward a secondary role of N2+ + C3H4 reactive pathways in the production of c-C3H3+.
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https://hal.archives-ouvertes.fr/hal-03634107
Contributor : Sándor Demes Connect in order to contact the contributor
Submitted on : Saturday, April 23, 2022 - 10:51:23 AM
Last modification on : Thursday, May 19, 2022 - 4:36:07 PM

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Ahmad Mortada, Sophie Carles, S. Demes, Baptiste Joalland, François Lique, et al.. Kinetics and Branching for the Reactions of N2+ with C3H4 Isomers at Low Temperatures and Implications for Titan’s Atmosphere. ACS Earth and Space Chemistry, ACS, 2022, 6 (5), pp.1227-1238. ⟨10.1021/acsearthspacechem.1c00347⟩. ⟨hal-03634107⟩

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