Formation and Stability of the Propionitrile:Acetylene Co-Crystal Under Titan-Relevant Conditions

Propionitrile (also known as ethyl cyanide, CH₃CH₂CN) and acetylene (C₂H₂) are two organic molecules that have been detected in Titan's atmosphere. Over time, they may interact with each other as they are transported to Titan's surface. We sought to determine if any reactions or associations such as co-crystal formation might occur between the two molecules. Using micro-Raman spectroscopy, we characterized band shifts, new bands, and morphological changes, which are characteristic of co-crystal formation. We found that the propionitrile:acetylene co-crystal forms within minutes at 90 K and is stable from 90 to 160 K. A cryogenic powder X-ray diffraction study confirms co-crystal formation at 90 K and indexes to a monoclinic unit cell, P2₁/a. A thermal expansion study between 90 and 140 K indicates that the co-crystal exhibits anisotropic thermal expansion, with a limited change in the b axis over the temperature range. This information gives insight into the preferred form of propionitrile:acetylene and the nature of these molecular interactions under Titan-relevant conditions. We discuss broader implications of the propionitrile:acetylene co-crystal's participation in forming Titan's geologic features such as the karstic, labyrinth terrain. Additionally, co-crystals that include acetylene as a coformer may provide a source of energy for acetylenotrophs to harness, should putative life exist on Titan's surface or in the subsurface. The Dragonfly mission to Titan will explore the nature and distribution of Titan's organics at the surface; thus, characterizing these organics in the laboratory before surface operations will inform the likely phases Dragonfly may encounter and support data analysis and interpretation of this exciting mission.