Neutron Vibrational\nSpectroscopic Study of the Acetonitrile:Acetylene\n(1:2) Cocrystal Relevant to Titan, Saturn’s Moon

Saturn’s moon Titan features a surface composed\nof various\norganic solids with pronounced compositional and structural diversity.\nOn top of the icy core, the surface experiences a temperature of ≈93\nK and pressure of ≈1.45 atm. Under these conditions, most small\norganic molecules exist as solids and form Titanean minerals. Acetonitrile\nand acetylene are two of these molecules, which can form single-component\nmolecular solids and also a 1:2 binary cocrystal. Here, we present\na combined neutron vibrational spectroscopic study, neutron powder\ndiffraction study, and theoretical modeling of the cocrystal and corresponding\nsingle-phase solids. This combined study resulted in insightful spectra–structure–property\ncorrelations for the cocrystal and the molecular solids. Furthermore,\nwe observed quenching of the high-temperature form of acetonitrile\nin the presence of the cocrystal, which supports the possibility of\nthe existence of metastable solids as minerals on Titan. The results\npresented in this study further the knowledge of the putative structure\nand composition of the surface of Titan and, at the same time, contribute\nto a better understanding of the fundamental thermodynamic properties\nof two of the smallest organic molecules on Earth and in the Universe.