Reply to Choukroun et al.: IR and TPD data suggest the formation of clathrate hydrates in laboratory experiments simulating ISM

Partitioning photochemically formed CO2 into clathrate hydrate under interstellar conditions

Clathrate hydrates (CHs), host-guest compounds of water forming hydrogen-bonded cages around guest molecules, are now known to exist under interstellar conditions. Experimental evidence demonstrated that prolonged thermal treatment of a solid mixture of water and CO2/CH4 produces CHs at 10-30 K under simulated interstellar conditions. However, in the current study, we show that CO2 produced photochemically by vacuum ultraviolet irradiation of H2O-CO mixtures at 10 K and ∼10-10 mbar, gets partitioned into its CH phase and a matrix phase embedded in amorphous ice. The process occurring under simulated interstellar conditions was studied at different temperatures and H2O-CO compositions. The formation of CO2 CH and other photoproducts was confirmed using reflection absorption infrared spectroscopy. The UV-induced photodesorption event of CO2 may provide the mobility required for the formation of CHs, while photoproducts like methanol can stabilize such CH structures. Our study suggests that new species originating during such energetic processing in ice matrices may form CH, potentially altering the chemical composition of astrophysical environments.

Formation and Transformation of Clathrate Hydrates under Interstellar Conditions

ConspectusContinuing efforts by many research groups have led to the discovery of ∼240 species in the interstellar medium (ISM). Observatory- and laboratory-based astrochemical experiments have led to the discovery of these species, including several complex organic molecules (COMs). Interstellar molecular clouds, consisting of water-rich icy grains, have been recognized as the primordial sources of COMs even at extremely low temperatures (∼10 K). Therefore, it is paramount to understand the chemical processes of this region, which may contribute to the chemical evolution and formation of new planetary systems and the origin of life.This Account discusses our effort to discover clathrate hydrates (CHs) of several molecules and their structural varieties, transformations, and kinetics in a simulated interstellar environment. CHs are nonstochiometric crystalline host-guest complexes in which water molecules form cages of different sizes to entrap guest molecules. CHs are abundant on earth and require moderate temperatures and high pressures for their formation. Our focus has been to form CHs at extremely low pressure and temperature as in the ISM, although their existence under such conditions has been a long-standing question since water and guest molecules (CH4, CO2, CO, etc.) exist in space. In multiple studies conducted at ∼10-10 mbar, we showed that CH4, CO2, and C2H6 hydrates could be formed at 30, 10, and 60 K, respectively. Well-defined IR spectroscopic features supported by quantum chemical simulations and temperature-programmed desorption mass spectrometric analyses confirmed the existence of the 512 (for CH4 and CO2) and 51262 (for C2H6) CH cages. Mild thermal activation for long periods under ultrahigh vacuum (UHV) allowed efficient molecular diffusion, which is crucial for forming CHs. We also explored the formation of THF hydrate (a promoter/stabilizer for binary CHs), and a spontaneous method was found for its formation under UHV. In a subsequent study, we observed a binary THF-CO2 hydrate and its thermal processing at 130 K leading to the transportation of CO2 from the hydrate cages to the matrix of amorphous water. The findings imply that such systems possess a dynamic setting that facilitates the movement of molecules, potentially accounting for the chemical changes observed in the ISM. Furthermore, an intriguing fundamental phenomenon is the consequences of these CHs and their dynamics. We showed that preformed acetone and formaldehyde hydrates dissociate to form cubic (Ic) and hexagonal (Ih) ices at 130-135 K, respectively. These unique processes could be the mechanistic routes for the formation of various ices in astrophysical environments.Other than adding a new entry, namely, CHs, to the list of species found in ISM, its existence opens new directions to astrochemistry, observational astronomy, and astrobiology. Our work provides a molecular-level understanding of the formation pathways of CHs and their transformation to crystalline ices, which sheds light on the chemical evolution of simple molecules to COMs in ISM. Furthermore, CHs can be potential candidates for studies involving radiation, ionization, and electron impact to initiate chemical transformations between the host and guest species and may be critical in understanding the origin of life.

Induced Migration of CO2 from Hydrate Cages to Amorphous Solid Water under Ultrahigh Vacuum and Cryogenic Conditions

Restricted migration of reactive species limits chemical transformations within interstellar and cometary ices. We report the migration of CO₂ from clathrate hydrate (CH) cages to amorphous solid water (ASW) in the presence of tetrahydrofuran (THF) under ultrahigh vacuum (UHV) and cryogenic conditions. Thermal annealing of sequentially deposited CO₂ and H₂O ice, CO₂@H₂O, to 90 K resulted in the partitioning of CO₂ in 5¹² and 5¹²6² CH cages (CO₂@5¹², CO₂@5¹²6²). However, upon preparing a composite ice film composed of CO₂@5¹², CO₂@5¹²6² and THF distributed in the water matrix at 90 K, and annealing the mixture for 6 h at 130 K produced mixed CO₂-THF CH, where THF occupied the 5¹²6⁴ cages (THF@5¹²6⁴) exclusively while CO₂ in 5¹²6² cages (CO₂@5¹²6²) got transferred to the ASW matrix and CO₂ in the 5¹² cages (CO₂@5¹²) remained as is. This cage-matrix exchange may create a more conducive environment for chemical transformations in interstellar environments.