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

Abstract

The Saturnian moon Titan has a thick, organic-rich atmosphere, and condensed phases of small organic molecules are anticipated to be stable on its surface. Of particular importance are crystalline phases of organics, known as cryominerals, which can play important roles in surface chemistry and geological processes on Titan. Many of these cryominerals could exhibit rich phase behavior, especially multicomponent cryominerals whose component molecules have multiple solid phases. One such cryomineral is the acetylene:ammonia (1:1) co-crystal, and here we use density functional theory-based ab initio molecular dynamics simulations to quantify its structure and dynamics at Titan conditions. We show that the acetylene:ammonia (1:1) co-crystal is a plastic co-crystal (or rotator phase) at Titan conditions because the ammonia molecules are orientationally disordered. Moreover, the ammonia molecules within this co-crystal rotate on picosecond time scales, and this rotation is accompanied by the breakage and reformation of hydrogen bonds between the ammonia hydrogens and the π-system of acetylene. The robustness of our predictions is supported by comparing the predictions of two density functional approximations at different levels of theory, as well as through the prediction of infrared and Raman spectra that agree well with experimental measurements. We anticipate that these results will aid in understanding geochemistry on the surface of Titan.

Neutron Vibrational Spectroscopic Study of the Acetonitrile:Acetylene (1:2) Cocrystal Relevant to Titan, Saturn’s Moon

“…19 Based on the colossal red shift of 22.5 meV (181 cm -1 ) observed in the acetonitrile:acetylene (1:2) cocrystal, we hypothesized that this solid may exhibit plastic crystal behavior. In fact, during the final stages of the preparation of this paper, molecular dynamics simulations were published, 38 further confirming the plastic nature of the cocrystal.…”

On the Formation of Vinylamine (C2H3NH2) in Interstellar Ice Analogs

“…Those simple prebiotic molecules may also sublime to be incorporated into planetoids, asteroids, and comets in the star-forming regions, which can be eventually delivered to early Earth through meteoritic parent bodies such as the Murchison meteorite, supporting the hypothesis of an exogenous origin and delivery of biologically important molecules to early Earth (Shimoyama & Ogasawara 2002). In addition, acetylene and ammonia molecules, along with acetylene-ammonia cocrystals, have been proposed in Titan's atmosphere and on Titan's surface by the Cassini-Huygens mission (Fulchignoni et al 2005;Waite et al 2005;Coustenis et al 2007;Niemann et al 2010;Yelle et al 2010;Singh et al 2016;Cable et al 2018;Thakur & Remsing 2023). The formation of vinylamine through the exposure of acetylene and ammonia ice mixtures to GCR proxies suggests that vinylamine can form on the Titan's surface by GCR-triggering chemistry and may even be involved in a chain of complex chemical reactions leading eventually to biomolecules, for example, amino acids and nucleobases.…”

Neutron Vibrational Spectroscopic Study of the Acetylene: Ammonia (1:1) Cocrystal Relevant to Titan, Saturn’s Moon

“…Molecular crystals, especially those comprised of small organic molecules, often feature properties such as colossal and anisotropic thermal expansion, , plasticity (rotor phases), − and similar macroscopic properties that originate from their unique crystal packing and bonding. Changes in intermolecular interactions within the crystal packing can have major implications on the physical properties.…”

“…Nonetheless, presumably due to the geometrical simplicity of the molecule, this structure also does not exhibit plasticity. Interestingly, the 1:1 cocrystal shows pronounced dynamic disorder and plasticity. − The cocrystal has hydrogen bonding interactions between the ammonia nitrogen center and the acetylene C–H bond. The NH···N hydrogen bonding interaction is stronger in the cocrystal than in the bare ammonia structure demonstrated by its shorter intermolecular C–H···N distance of 2.538(2) Å.…”

“…The ammonia molecules within this packing rotate on picosecond time scales. 63,64 The dramatic change in the hydrogen bonding environment in the The vibrational spectra were collected in tandem with diffraction patterns upon heating up from the crash-cooled sample at 5 K. Exact ranges for assigned peaks can be found in Table 1. single-component solids vs 1:1 cocrystal is also expected to be manifested in the NVS spectra with pronounced shifts in the energy of the phonon and vibrational modes.…”