Mechanism for the Coupled Photochemistry of Ammonia and Acetylene: Implications for Giant Planets, Comets and Interstellar Organic Synthesis.

Laboratory studies provide a fundamental understanding of photochemical processes in planetary atmospheres. Photochemical reactions taking place on giant planets like Jupiter and possibly comets and the interstellar medium are the subject of this research. Reaction pathways are proposed for the coupled photochemistry of NH3 (ammonia) and C2 H2 (acetylene) within the context Jupiter's atmosphere. We then extend the discussion to the Great Red Spot, Extra-Solar Giant Planets, Comets and Interstellar Organic Synthesis. Reaction rates in the form of quantum yields were measured for the decomposition of reactants and the formation of products and stable intermediates: HCN (hydrogen cyanide), CH3 CN (acetonitrile), CH3 CH = N-N = CHCH3 (acetaldazine), CH3 CH = N-NH2 (acetaldehyde hydrazone), C2 H5 NH2 (ethylamine), CH3 NH2 (methylamine) and C2 H4 (ethene) in the photolysis of NH3 /C2 H2 mixtures. Some of these compounds, formed in our investigation of pathways for HCN synthesis, were not encountered previously in observational, theoretical or laboratory photochemical studies. The quantum yields obtained allowed for the formulation of a reaction mechanism that attempts to explain the observed results under varying experimental conditions. In general, the results of this work are consistent with the initial observations of Ferris and Ishikawa (1988). However, their proposed reaction pathway which centers on the photolysis of CH3 CH = N-N = CHCH3 does not explain all of the results obtained in this study. The formation of CH3 CH = N-N = CHCH3 by a radical combination reaction of CH3 CH = N• was shown in this work to be inconsistent with other experiments where the CH3 CH = N• radical is thought to form but where no CH3 CH = N-N = CHCH3 was detected. The importance of the role of H atom abstraction reactions was demonstrated and an alternative pathway for CH3 CH = N-N = CHCH3 formation involving nucleophilic reaction between N2 H4 and CH3 CH = NH is advanced.