Presentation #509.03 in the session Titan Craters, Chemistry and Exploration.
Titan has a plethora of hydrocarbons and nitriles in the atmosphere and on the surface that interact with each other. For example, acetylene (C2H2) is a primary photochemical product in the atmosphere  and has been identified there and on the surface [2,3]. Pyridine (C5H5N), a simple nitrogen heterocycle, has not been directly detected at Titan. However, in the presence of HCN (which has been routinely observed in Titan’s atmosphere [e.g., 4]), acetylene polymerization may produce N-heterocycles [5,6]. Interestingly, when acetylene and pyridine interact under specific temperatures and molar ratios, a co-crystal can form . Co-crystals are stable structures held together by weak interactions (e.g., H-bonding) . Here, we present experimental results on the pyridine:acetylene co-crystal to add to the body of knowledge on this expanding field of Titan cryominerals.
The sample was analyzed via micro-Raman spectroscopy and powder X-ray Diffraction (XRD). The pyridine:acetylene co-crystal forms within minutes at 150 K and is stable from 80-190 K. The co-crystal is identified in Raman spectra by ~20 cm-1 blue shifts of the acetylene C≡C stretch (ν2), and ~14 cm-1 blue shifts of the pyridine in plane ring bend (ν1 and ν12). The most distinctive co-crystal feature is a new band at ~1966 cm-1. Additionally, the co-crystal is stable and detectable after being exposed to liquid ethane, indicating stability during an ethane flooding event on Titan. XRD results confirm the monoclinic co-crystal structure at 11° 2θ. Lattice parameters were derived at each temperature point by performing Pawley refinements, which indicate thermal expansion along the b and c axes of the structure. Lack of thermal expansion on the a axis is likely due to relatively stable N•••H-C interactions. Microscopic images showed changes to sample morphology upon co-crystal formation, such as decreased albedo and an irregularly undulated, maze-like texture.
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This research was supported by an appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by Oak Ridge Associated Universities under contract with NASA. This work was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.