Browsing by Author "Bovolenta, Giulia"

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    Chemistry on interstellar ices: binding energy distributions and synthesis of prebiotic molecules.
    (Universidad de Concepción, 2023) Bovolenta, Giulia; Vogt Geisse, Stefan; Bovino, Stefano
    Ice mantles covering dust grains are important drivers of surface chemistry in the interstellar medium. This thesis aimed to establish a comprehensive framework for accurately determining parameters, such as binding energies and transition state barriers, essential for understanding the formation of prebiotic species in that environment. The focus on binding energies is motivated by their role in dictating desorption rates from ice mantle surfaces, while transition state barriers are essential for calculating reaction rates between adjacent molecules. The research explored the impact of an amorphous solid water (ASW) surface on these parameters, constructing accurate ASW ice models and employing high level electronic structure theory methods. Various ASW ice models, including a set of amorphized water clusters and a large periodic ASW surface, were generated using ab initio molecular dynamics and Machine-Learning Potentials techniques, and characterized in terms of dangling-H atoms, as they constitute important catalytic sites, where adsorption and reaction preferentially occur. The DFT model chemistries used for binding energy evaluation and reactivity studies, were carefully benchmarked using small water clusters with respect to a CCSD(T)/CBS reference value, in order to obtain highly accurate energy values for the proposed ASW models. To automate the computation of binding energies on realistic ASW models, a Binding Energy Evaluation Platform (BEEP) was build, driven by the idea that binding energies on amorphous surfaces are best described by distributions. This multi-binding approach paired with accurate determination of binding energies, resulted to have a significant impact on astrophysical observables, particularly on the position of snow-lines in protoplanetary disks. Additionally, the analysis of binding modes of reactants in a Strecker synthesis of glycine, revealed that not all adsorption motives facilitate the necessary reactive encounters. Therefore, a multi-binding approach constitutes a more comprehensive starting point for the study of reactivity on ASW surfaces. The second main objective of the thesis was to investigate the role of ASW in reactions associated to the Strecker synthesis of glycine using the multi-binding approach as a starting point. Accordingly, each reaction pathway was explored using DFT methods in a varied set of reactive sites with different morphological characteristics, as offered by realistic ASW ice models. Transition states of the different reaction paths were optimized and characterized together with a detailed analysis of the reaction mechanisms by means of obtaining a Intrinsic Reaction Coordinate (IRC) profile. The results indicate that, depending on the nature of the catalytic site, significant variations of the reaction mechanisms occur. Notably, water-assisted proton transfer reactions shifted from concerted to step-wise, displaying lower energy barriers, which might provide viable synthetic routes at extremely low temperature (<10 K). The research concluded that a viable route exists under interstellar conditions for the first stage of Strecker’s synthesis, the nucleophilic addition of ammonia and formaldehyde yielding the product aminomethanol. Such result, coupled with the analysis of the binding energy values calculated for the species, suggests that the aminomethanol is present on the surface of interstellar ice mantles. Therefore, the fact that the species has not been observed in the interstellar medium, might be related to difficulties in its spectroscopic identification. However, the second stage of the synthesis, the dehydration of the aminomethanol, posed challenges, impacting the validity of the Strecker synthesis as a viable pathway for amino acid formation in extremely cold interstellar regions.
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