Meteoric material originating primarily from cometary dust trails is injected into Titan’s atmosphere at a rate estimated at approximately 6 mt per Titanic day. Based on known meteoric compositions, the major metallic constituents of this cosmic material include Mg, Fe, Al, Ni, and Ca. Models predict that metal ion densities peak in Titan’s ionosphere at altitudes between 550 to 750 km – well within the range of altitudes where a rich environment of hydrocarbons and nitrogenic compounds has been detected; therefore, chemical interactions between meteoric ions and these species certainly occur. Here, we present results from gas-phase reactions of Fe+, FeO+, Ni+, and NiO+ with two simple nitriles, CH3CN and CH3CH2CN, known to be present in Titan’s atmosphere. All reactions were carried out in a selected-ion drift cell apparatus at room temperature in 3.5 torr of helium. The bare metal reactions were also carried out at lower temperatures (-60 to -100oC) in 2.4 torr of helium. Rate coefficients and product data were obtained for all reactions in order to evaluate reaction efficiencies and higher-order reaction sequencing. Reactions of both of the bare metals with CH3CN and CH3CH2CN resulted in association exclusively to yield M+(CH3CN)x and M+(CH3CH2CN)x adducts, where, dependent on the metal, x is as high as four. Production of higher-order adducts was enhanced at reduced temperatures. All bare metal reactions had a significant increase in overall reaction rate efficiencies when carried out at the lower temperatures. The FeO+ reactions with both neutrals resulted in association exclusively to yield FeO+(CH3CN)1-4 and FeO+(CH3CH2CN)1-3 adducts. By comparison, the NiO+ reactions exhibited greater variety and included processes such as the elimination of formaldehyde and HCN. All metal oxide reactions proceed efficiently. Product structures were modeled using ab initio calculations.

Reactions of meteoric metal ions with organic nitriles found in Titan's atmosphere

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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