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

Abstract

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