How Quantum are Titan's Minerals? Exploring Structure, Dynamics, and Nuclear Quantum Effects in Organic Co-Crystals.

Titan – Saturn’s largest moon and perhaps the most ‘Earth-Like’ planetary body within our solar system is reported to possess an arsenal of organic molecules which can condense onto Titan’s surface in the form of stable surface liquids and organic co-crystals, examples of which include the acetylene-ammonia and benzene-ethane co-crystals. Such an array of organic minerals are important to Titan’s surface geology and could even promote a prebiotic chemical evolution by facilitating a variety of bulk and surface phenomena though their rich phase behavior. We investigate the presence of dynamically disordered phases within acetylene-ammonia co-crystals using density functional theory-based ab-initio molecular dynamics simulations. By quantifying molecular rotational dynamics within acetylene-ammonia co-crystals, we show evidence of the rotator phase of ammonia molecules possessing a discrete rotational symmetry. Given the cryogenic surface conditions of Titan (~90 K ), quantum  effects of the nuclei could further impact the dynamics within these co-crystals. Therefore, we also investigate the role of nuclear quantum effects on the rotator phase using ab-initio path integral molecular dynamics simulations. We find that nuclear quantum effects significantly accelerate the rotational diffusion of ammonia molecules within the acetylene-ammonia co-crystal. We rationalize the observed orientational diffusion in terms of transient breakage and reformation dynamics of N-H^…π hydrogen bonds within the crystal. We discuss our findings within the broader context of understanding fundamental physical processes occurring within organic co-crystals beyond Titan’s surface, which could have significant implications for pharmaceuticals, solid-state electrolytes, and organic semiconductors.