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

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.