1,3-Butadiene on Titan: Crystal Structure, Thermal Expansivity, and Raman Signatures

Titan
is one of the most Earth-like objects in the Solar
System,
hosting a nitrogen-rich atmosphere and an impressive inventory of
organics ranging from simple to complex. 1,3-Butadiene has been suggested
to be one of the photochemical products that could be present on Titan’s
surface. As the simplest conjugated hydrocarbon, it is expected to
partake in many chemical processes that influence Titan’s geology
and evolution. However, the crystal structure and thermal expansivity
of 1,3-butadiene, two of its most fundamental properties, have remained
largely unknown for over eight decades, especially at cryogenic conditions
often encountered on planetary bodies. In this work, a series of powder
X-ray diffraction and micro-Raman experiments have been conducted
in order to elucidate the crystal structure and thermal behavior of
solid 1,3-butadiene between 85 and 165 K. Specifically, a single crystalline
phase (space group P21/c) is observed in the temperature range of study in which the butadiene
molecules form layers of zigzag chains that are stabilized by an intricate
network of intermolecular C–H···π interactions.
This monoclinic structure is shown to exhibit significant anisotropic
thermal expansion along the three directions with the majority occurring
along the crystallographic b-axis. The calculated
bulk density of 1,3-butadiene (0.877 g/cm3 at 150 K) is
considerably lower than previously realized. These results help enhance
our understanding of the putative molecular solids that make up Titan’s
surface and could hold important implications for physical as well
as mechanical processes occurring on this icy satellite.