A co-crystal between benzene and ethane: a potential evaporite material for Saturn's moon Titan.

Helen E Maynard-Casely,Robert Hodyss,Tuan Hoang Vu,Morgan L Cable,Martin Rahm

doi:10.1107/s2052252516002815

Abstract

Using synchrotron X-ray powder diffraction, the structure of a co-crystal between benzene and ethane formed in situ at cryogenic conditions has been determined, and validated using dispersion-corrected density functional theory calculations. The structure comprises a lattice of benzene molecules hosting ethane molecules within channels. Similarity between the intermolecular interactions found in the co-crystal and in pure benzene indicate that the C-H⋯π network of benzene is maintained in the co-crystal, however, this expands to accommodate the guest ethane molecules. The co-crystal has a 3:1 benzene:ethane stoichiometry and is described in the space group [Formula: see text] with a = 15.977 (1) Å and c = 5.581 (1) Å at 90 K, with a density of 1.067 g cm(-3). The conditions under which this co-crystal forms identify it is a potential that forms from evaporation of Saturn's moon Titan's lakes, an evaporite material.

Highlights

Crystallographic studies of benzene have a history of moving scientific understanding significantly forward
Further investigation indicated that these peaks persisted after the co-crystal melted in the first run but that they were not observed during the second run of the experiment
The first potential ‘cryogenic mineral’ to be identified where its intermolecular interactions are not dominated by hydrogen bonding

Summary

Introduction

Crystallographic studies of benzene have a history of moving scientific understanding significantly forward. Kathleen Lonsdale’s pioneering work on the structure of hexamethylbenzene showed the community that the benzene molecule was flat (Lonsdale, 1929), a study which, at least in part, laid the groundwork for molecular crystallography as we know it today. The surface temperature on Titan is 91–95 K, and at these cryogenic temperatures the fluids that drive the cycle are small hydrocarbon molecules (Stofan et al, 2007) such as methane and ethane, as well as dissolved dinitrogen. There are a number of other small molecular species observed in the atmosphere, which are hypothesized to be present at Titan’s surface. These include organic molecules such as hydrogen cyanide, acetylene, ethylene, acetonitrile and benzene, formed photochemically from CH4 and N2 in the

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