Laboratory studies on the reactions of formation of nitriles and oxygenated compounds of relevance to the atmosphere of Titan

Accurate modeling of planetary atmospheres requires the knowledge of a series of chemical parameters, possibly determined in laboratory experiments. In particular, for gas-phase bimolecular reactions the kinetic rate constants and product branching ratios are desired. In our laboratory we have undertaken a systematic investigation of some bimolecular reactions, which are believed to form key steps in the models, with the aim of characterizing the reaction mechanism and product branching ratio. We have employed the crossed molecular beam scattering method with mass-spectrometric detection to study reactions involving atomic nitrogen in the first electronically excited state, N(2D), and ground state atomic oxygen, O(3P), with species relatively abundant in the atmospheres of Titan (as well as Mars and Triton). Results have been obtained for the reactions of N(2D) with H2 and the hydrocarbons CH4, C2H2 and C2H4, which are amongst the most abundant minor components of the atmosphere of Titan. In the cases of the reactions with hydrocarbons, molecular products containing a novel CN bond are formed, thus suggesting possible routes towards the production of gas-phase nitriles. Interestingly, most of the observed nitriles are either cyclic or unsaturated and could, therefore, be responsible for the building-up of the nitrogen-rich atmospheric aerosols of Titan. We have also investigated the reactions between O(3P) and unsaturated hydrocarbons, which might play a role in the conversion of oxygen to CO. Finally, we have extended the same approach to the study of radical-radical reactions, namely O(3P)+CH3 and C3H5. Molecular products other than those already considered in the modeling of planetary atmospheres have been identified and new routes of formation of CO can be envisaged