In-depth exploration of catalytic sites on amorphous solid water: I. The astrosynthesis of aminomethanol

Chemical processes taking place on ice-grain mantles are pivotal to the complex chemistry of interstellar environments. In this study, we conducted a comprehensive analysis of the catalytic effects of an amorphous solid water (ASW) surface on the reaction between ammonia (NH3) and formaldehyde (H2CO) to form aminomethanol (NH2CH2OH) using density functional theory. We identified potential catalytic sites based on the binding energy distribution of NH3 and H2CO reactants, on a set-of-clusters surface model composed of 22 water molecules and found a total of 14 reaction paths. Our results indicate that the catalytic sites can be categorized into four groups, depending on the interactions of the carbonyl oxygen and the amino group with the ice surface in the reactant complex. A detailed analysis of the reaction mechanism using Intrinsic Reaction Coordinate and reaction force analysis, revealed three distinct chemical events for this reaction: formation of the C-N bond, breaking of the N-H bond, and formation of the O-H hydroxyl bond. Depending on the type of catalytic site, these events can occur within a single, concerted, albeit asynchronous, step, or can be isolated in a step-wise mechanism, with the lowest overall transition state energy observed at 1.3 kcal mol-1. A key requirement for the low-energy mechanism is the presence of a pair of dangling OH bonds on the surface, found at 5% of the potential catalytic sites on an ASW porous surface.

A statistical and machine learning approach to the study of astrochemistry

In order to obtain a good understanding of astrochemistry, it is crucial to better understand the key parameters that govern grain-surface chemistry. For many chemical networks, these crucial parameters are the binding energies of the species. However, there exists much disagreement regarding these values in the literature. In this work, a Bayesian inference approach is taken to estimate these values. It is found that this is difficult to do in the absence of enough data. The Massive Optimised Parameter Estimation and Data (MOPED) compression algorithm is then used to help determine which species should be prioritised for future detections in order to better constrain the values of binding energies. Finally, an interpretable machine learning approach is taken in order to better understand the non-linear relationship between binding energies and the final abundances of specific species of interest.

Theoretical Distribution of the Ammonia Binding Energy at Interstellar Icy Grains: A New Computational Framework

The binding energies (BE) of molecules on the interstellar grains are crucial in the chemical evolution of the interstellar medium (ISM). Both temperature-programmed desorption (TPD) laboratory experiments and quantum chemistry computations have often provided, so far, only single values of the BE for each molecule. This is a severe limitation, as the ices enveloping the grain mantles are structurally amorphous, giving rise to a manifold of possible adsorption sites, each with different BEs. However, the amorphous ice nature prevents the knowledge of structural details, hindering the development of a common accepted atomistic icy model. In this work, we propose a computational framework that closely mimics the formation of the interstellar grain mantle through a water by water accretion. On that grain, an unbiased random (but well reproducible) positioning of the studied molecule is then carried out. Here we present the test case of NH₃, a ubiquitous species in the molecular ISM. We provide the BE distribution computed by a hierarchy approach, using the semiempirical xTB-GFN2 as a low-level method to describe the whole icy cluster in combination with the B97D3 DFT functional as a high-level method on the local zone of the NH₃ interaction. The final ZPE-corrected BE is computed at the ONIOM(DLPNO-CCSD(T)//B97D3:xTB-GFN2) level, ensuring the best cost/accuracy ratio. The main peak of the predicted NH₃ BE distribution is in agreement with experimental TPD and computed data in the literature. A second broad peak at very low BE values is also present, which has never been detected before. It may provide the solution to a longstanding puzzle about the presence of gaseous NH₃ also observed in cold ISM objects.

Energy Redistribution Following CO2 Formation on Cold Amorphous Solid Water

The formation of molecules in and on amorphous solid water (ASW) as it occurs in interstellar space releases appreciable amounts of energy that need to be dissipated to the environment. Here, energy transfer between CO₂ formed within and on the surface of amorphous solid water (ASW) and the surrounding water is studied. Following CO(¹Σ⁺) + O(¹D) recombination the average translational and internal energy of the water molecules increases on the ∼10 ps time scale by 15–25% depending on whether the reaction takes place on the surface or in an internal cavity of ASW. Due to tight coupling between CO₂ and the surrounding water molecules the internal energy exhibits a peak at early times which is present for recombination on the surface but absent for the process inside ASW. Energy transfer to the water molecules is characterized by a rapid ∼10 ps and a considerably slower ∼1 ns component. Within 50 ps a mostly uniform temperature increase of the ASW across the entire surface is found. The results suggest that energy transfer between a molecule formed on and within ASW is efficient and helps to stabilize the reaction products generated.

O2 formation in cold environments

The diffusional dynamics of atomic oxygen in and on amorphous solid water (ASW) to form molecular oxygen is characterized. Reactive molecular dynamics simulations to study bond breaking and bond formation show that vibrational relaxation of the highly excited diatomic occurs on the 10 ns to 100 ns time scale. The relaxation process is highly nonexponential and can be characterized by a stretched exponential decay reminiscent of the dynamics of glasses. The stretched exponents range from β = 0.15 for relaxation on the surface to β = 0.21 for the dynamics in bulk. It is also found that coupling of the O2 relaxation to the internal water modes occurs which speeds up the vibrational relaxation by a factor of 4. Extrapolation of the stretched exponential decay to 1 μs yields a final vibrational quantum number v = 2 for O2(X3Σ-g), consistent with experimental results from photolysis of SO2 on ASW at 193 nm which find v ≤ 3. Desorption energies of water from the surface range from 1.5 to 2.0 kcal mol-1 compared with 1.8 kcal mol-1 found from experiment, depending on whether the water molecules are flexible or not.