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The Saturnian moon Titan has a thick, organic-rich atmosphere, and condensedphases of small organic molecules are anticipated to be stable on its surface.Of particular importance are crystalline phases of organics, known ascryominerals, which can play important roles in surface chemistry andgeological processes on Titan. Many of these cryominerals could exhibit richphase behavior, especially multicomponent cryominerals whose componentmolecules have multiple solid phases. One such cryomineral is theacetylene:ammonia (1:1) co-crystal, and here we use density functionaltheory-based ab initio molecular dynamics simulations to quantify its structureand dynamics at Titan conditions. We show that the acetylene:ammonia (1:1)co-crystal is a plastic co-crystal (or rotator phase) at Titan conditionsbecause the ammonia molecules are orientationally disordered. Moreover, theammonia molecules within this co-crystal rotate on picosecond timescales, andthis rotation is accompanied by the breakage and reformation of hydrogen bondsbetween the ammonia hydrogens and the {\pi}-system of acetylene. The robustnessof our predictions is supported by comparing the predictions of two densityfunctional approximations at different levels of theory, as well as through theprediction of infrared and Raman spectra that agree well with experimentalmeasurements. We anticipate that these results will aid in understandinggeochemistry on the surface of Titan.

Molecular Structure, Dynamics, and Vibrational Spectroscopy of the Acetylene:Ammonia (1:1) Plastic Co-Crystal at Titan Conditions