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

Abstract

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 P2 1 /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/cm 3 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.

Experimental Characterization of the Pyridine:Acetylene Co-crystal and Implications for Titan’s Surface

“…The former, slightly shorter N···H contact, can be found to reside mostly along the a and c axes (which lie in the plane formed by X1 and X2), thereby leading to the smaller thermal expansion in these directions relative to X3 (which coincides with the b axis). A similar anisotropic thermal expansion behavior has been observed with other putative Titan materials (e.g., 1,3-butadiene, which also adopts a monoclinic structure) . The volume of the pyridine–acetylene unit cell expands by ∼2.5% (Figure ), which is on par with previously characterized co-crystals …”

“…A similar anisotropic thermal expansion behavior has been observed with other putative Titan materials (e.g., 1,3-butadiene, which also adopts a monoclinic structure). 48 The volume of the pyridine–acetylene unit cell expands by ∼2.5% ( Figure 8 ), which is on par with previously characterized co-crystals. 57 …”

Propyne: Determination of Physical Properties and Unit Cell Parameters under Titan-Relevant Conditions

“…While the quality of the powder diffraction data is insufficient to yield a viable crystal structure, the temperature series does indicate significant anisotropy in thermal expansion, with the majority occurring along the b axis. The unit cell volume is found to expand by ∼4% over the temperature range between 85 and 160 K. Such anisotropic behavior and magnitude of expansion are on par with the behavior of other small organic solids that have been characterized under Titan-relevant conditions (e.g., butadiene, 34 benzene 36 ).…”

“…The general position in the Pbca space group has a multiplicity of 8–hence pursuing a structure solution with one molecule would result in an unlikely density of 441.05 kg m –3 . With two molecules each at a general position, the resultant density is 882.10 kg m –3 , on par with the behavior of other small organic solids (877.0 kg m –3 for butadiene at 90 K 34 and 830 kg m –3 for propane at 30 K 35 ). A previously published propyne study 26 measured a density of 866 kg m –3 at 80 K, a value in very good agreement to that reported here (determined at 90 K).…”

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

“…Infrared (IR) and Raman spectroscopy are complementary vibrational techniques that are well-suited for studying Titan cryominerals because of the sensitivity of molecular vibrations to their environment. ,,− To provide molecular-scale interpretation of experimental spectra and to validate the accuracy of our models through comparison with experiments, we computed both the IR and Raman spectra of the acetylene:ammonia (1:1) co-crystal. The IR absorption spectra were computed from the Fourier transform of the dipole derivative autocorrelation function defined according to A ( ω ) ∝ prefix∫ ⟨ μ̇ false( 0 false) · μ̇ false( t false) ⟩ nobreak0em0.25em⁡ normale − i ω t nobreak0em0.25em⁡ normald t where μ( t ) refers to the instantaneous dipole moment and μ̇ ( t ) is its time derivative.…”