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

The surface of Titan, Saturn’s icy moon, is believed
to
be composed of various molecular minerals with a great diversity in
structure and composition. Under the surface conditions, 93 K and
1.45 atm, most small molecules solidify and form minerals, including
acetylene and ammonia. These two compounds can not only form single-component
solids but also a 1:1 binary cocrystal that exhibits intriguing rotor
phase behavior. This cocrystal is a putative mineral on Titan and
other planetary bodies such as comets. In addition, the structure
of the cocrystal is relevant to fundamental science as it can help
better understand the emergence of rotor phases. Here, we present
a detailed vibrational neutron spectroscopic study supported by a
neutron powder diffraction study on the cocrystal and the single-phase
solids. The experimentally observed spectral bands were assigned based
on theoretical calculations. The established spectra–properties
correlations for the cocrystal corroborate the observed properties.
To the best of our knowledge, this study presents the first example
of the application of neutron vibrational spectroscopy in studying
Titan-relevant organic minerals.