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

The photoionization of N2plays a key role ininitiating the formation of complex organic molecules in thenitrogen-rich atmosphere of Saturn's largest moon, Titan. To date,only a handful of laboratory studies have explored the reactivity ofN2+ions with hydrocarbons???limited to methane, acetylene, andethylene???at the low temperatures relevant to Titan. Here, the ratecoefficients, product identity, and branching ratios of the ion-molecule reactions of N2+with C3H4isomers, namely, propyneCH3CCH and allene CH2CCH2, were measured between 24 and72 K in uniform supersonicflows. The rate coefficients arecollisional and their temperature dependence is in remarkableagreement with capture models. The outcomes of both reactionsare 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, nondissociativecharge-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, cyclopropenylc-C3H3+, by following the lowest-energy pathwayfor the decomposition of allene and propyne cations. The measured rate coefficients and branching ratios were included in aphotochemical model of Titan's atmosphere. The results point toward a secondary role of N2++C3H4reactive pathways in theproduction ofc-C3H3+.