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 C₂H₂ (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), CH₃CN (acetonitrile), CH₃CH = N-N = CHCH₃ (acetaldazine), CH₃CH = N-NH₂ (acetaldehyde hydrazone), C₂H₅NH₂ (ethylamine), CH₃NH₂ (methylamine) and C₂H₄ (ethene) in the photolysis of NH₃/C₂H₂ 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 CH₃CH = N-N = CHCH₃ does not explain all of the results obtained in this study. The formation of CH₃CH = N-N = CHCH₃ by a radical combination reaction of CH₃CH = N? was shown in this work to be inconsistent with other experiments where the CH₃CH = N? radical is thought to form but where no CH₃CH = N-N = CHCH₃ was detected. The importance of the role of H atom abstraction reactions was demonstrated and an alternative pathway for CH₃CH = N-N = CHCH₃ formation involving nucleophilic reaction between N₂H₄ and CH₃CH = NH is advanced.