Molecular Structure and Rotational Dynamics in the Acetonitrile:Acetylene (1:2) Plastic Co-Crystal at Titan Conditions

TL;DR

This study uses ab initio molecular dynamics to analyze the structure and rotational dynamics of a cryomineral co-crystal relevant to Titan's surface, revealing persistent plastic phase behavior at cryogenic temperatures.

Contribution

First detailed molecular simulation of acetonitrile:acetylene cryomineral, showing its plastic phase and rotational disorder at Titan-like conditions.

Findings

Acetonitrile:acetylene co-crystal exhibits a plastic phase.

Rotational disorder persists up to 30 K.

Implications for Titan surface chemistry understanding.

Abstract

The surface of Saturn's moon Titan is coated with small molecule organic solids termed cryominerals. Cryominerals play an analogous role to minerals on Earth in Titan's surface geology and geochemistry. To develop a predictive understanding of Titan's surface geochemistry, we need to characterize the structure and dynamics of cryominerals at the molecular scale. We use ab initio molecular dynamics simulations to quantify the structure and dynamics of the acetonitrile:acetylene (1:2) co-crystal at Titan surface conditions. We suggest that acetonitrile:acetylene is in a plastic phase, in which acetonitrile molecules are dynamically disordered about the N-C-C axis on sub-picosecond timescales, and that this rotational, plastic disorder persists at least to temperatures of 30 K. We anticipate that many cryominerals may have plastic phases at or near Titan surface conditions, and understanding this disorder will be crucial to predicting chemistry on Titan's surface.