Elusive anion growth in Titan’s atmosphere: low temperature kinetics of the C3N−− + HC3N reaction

International audienceIon chemistry appears to be deeply involved in the formation of heavy molecules in the upper atmosphere of Titan. These large species form the seeds of the organic aerosols responsible for the opaque haze surrounding the biggest satellite of Saturn. The chemical pathways involving individual anions remain however mostly unknown. The determination of the rates of the elementary reactions with ions and the identification of the products are essential to the progress in our understanding of Titan’s upper atmosphere. We have taken steps in that direction through the investigation of the low temperature reactivity of C3N−,−, which was tentatively identified in the spectra measured by the CAPS-ELS instrument of the Cassini spacecraft during its high altitude flybys. The reaction of this anion with HC3N, one of the most abundant trace organics in the atmosphere, has been studied over the 49–294 K temperature range in uniform supersonic flows using the CRESU technique. The proton transfer is found to be the main exit channel ( > 91%) of the CView the MathML source315N−− + HC3N reaction. It remains however indistinguishable with the non-isotopically labeled CView the MathML source314N−− reactant. The T−1/2T−1/2 temperature dependence of this proton transfer reaction and its global rate are reasonably well reproduced theoretically using an average dipole orientation model. A minor exit channel, reactive detachment ( < 9%), has also been uncovered, although the nature of the neutral products has not been determined. It is concluded that the CView the MathML source314N−− + HC3N reaction cannot contribute to the growth of molecular anions in the upper atmosphere of Titan. Due to the low branching into the neutral exit channel, it cannot contribute either to the growth of neutrals even assuming a complete mass transfer