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.

Existence of Acetaldehyde Clathrate Hydrate and Its Dissociation Leading to Cubic Ice under Ultrahigh Vacuum and Cryogenic Conditions.

Acetaldehyde in a dilute aqueous solution gets hydrated to produce a geminal diol under atmospheric conditions. The acetaldehyde-water ice system under high pressure also converts to a geminal diol, and therefore, its stable clathrate hydrate (CH) phase, which in most systems forms at high pressures, is unknown. In the present study, we showed that acetaldehyde CH exists in ultrahigh vacuum (10-10 mbar) under cryogenic conditions (below 140 K) and continues to exist at 115 K for periods well over 1 day. Decomposition of acetaldehyde CH at 130-135 K produces water ice in its cubic crystalline form. The mechanism and kinetics involved in the process have also been studied. Reflection absorption infrared spectroscopy and temperature-programmed desorption mass spectrometry were utilized to confirm the CH formation. Our study establishes the possibility of a stable CH phase for acetaldehyde in interstellar and cometary environments.

Dissociative reactions of [Au25(SR)18]- at copper oxide nanoparticles and formation of aggregated nanostructures.

Reactions between nanoclusters (NCs) have been studied widely in the recent past, but such processes between NCs and metal-oxide nanoparticles (NPs), belonging to two different size ranges, have not been explored earlier. For the first time, we demonstrate the spontaneous reactions between an atomically precise NC, [Au25(PET)18]- (PET = 2-phenylethanethiolate), and polydispersed copper oxide nanoparticles with an average diameter of 50 nm under ambient conditions. These interparticle reactions result in the formation of alloy NCs and copper-doped NC fragments, which assemble to form nanospheres at the end of the reaction. High-resolution electrospray ionization mass spectrometry (ESI MS), transmission electron microscopy (HR-TEM), electron tomography, and X-ray photoelectron spectroscopy (XPS) studies were performed to understand the structures formed. The results from our study show that interparticle reactions can be extended to a range of chemical systems, leading to diverse alloy NCs and self-assembled colloidal superstructures.

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.

Rapid crystallization of amorphous solid water by porosity induction.

Rapid crystallization of amorphous solid water (ASW) is often associated with crystallization that initiates at random nucleation sites in the bulk and expands in all directions. In this work, by preparing sandwich films of acetonitrile (ACN) and ASW in the form of ACN@ASW and ASW@ACN in an ultrahigh vacuum (UHV), we demonstrate a new method for rapid crystallization of ASW via ACN diffusion-desorption induced porosity in the ASW matrix even in the window of 128-134 K, well below the normal crystallization temperature of 155 K. By placing an HDO (5% D2O in H2O) probe layer in ASW, we found that when ACN diffuses and desorbs through ASW, it induces ASW crystallization where the crystal grows both from the top and from the bottom simultaneously into the bulk. Crystallization kinetics and activation energy (Ea) for the formation of crystalline ice (CI) were evaluated using the Avrami equation and were compared with the previous reports. The evaluated Ea was ∼53 kJ mol-1, close to the Ea of crystal growth (47-56 kJ mol-1) and it suggested the absence of a nucleation process and supported rapid crystallization. Such occurrence of CI due to diffusion of ACN suggests a possible mechanism for the former's existence in many astrophysical environments.

Desorption-induced evolution of cubic and hexagonal ices in an ultrahigh vacuum and cryogenic temperatures.

Reflection absorption infrared spectroscopic investigations of multilayer films of acetonitrile (ACN) and water in an ultrahigh vacuum under isothermal conditions showed the emergence of cubic (ice Ic) and hexagonal (ice Ih) ices depending on the composition of the film. The experiments were conducted with a mixed film of 300 monolayers in thickness and the ACN : H2O monolayer ratios were varied from 1 : 5 to 5 : 1. Mixed films were deposited at 10 K and warmed to 130-135 K, where ACN desorbed subsequently and IR spectral evolution was monitored continuously. While the emergence of ice Ic at 130 K has been reported, the occurrence of ice Ih at this temperature was seen for the first time. Detailed investigations showed that ice Ih can form at 125 K as well. Crystallization kinetics and activation energy (Ea) for the emergence of ice Ih were evaluated using the Avrami equation.

Formation of Cubic Ice via Clathrate Hydrate, Prepared in Ultrahigh Vacuum under Cryogenic Conditions.

Cubic ice (ice Ic) is a crystalline phase of solid water, which exists in the earth's atmosphere and extraterrestrial environments. We provide experimental evidence that dissociation of acetone clathrate hydrate (CH) makes ice Ic in ultrahigh vacuum (UHV) at 130-135 K. In this process, we find that crystallization of ice Ic occurs below its normal crystallization temperature. Time-dependent reflection absorption infrared spectroscopy (RAIRS) and reflection high-energy electron diffraction (RHEED) were utilized to confirm the formation of ice Ic. Associated crystallization kinetics and activation energy (Ea) for the process were evaluated. We suggest that enhanced mobility or diffusion of water molecules during acetone hydrate dissociation enabled crystallization. Moreover, this finding implied that CHs might exist in extreme low-pressure environments present in comets. These hydrates, subjected to prolonged thermal annealing, transform into ice Ic. This unique process of crystallization hints at a possible mechanistic route for the formation of ice Ic in comets.