Nitrile regio-synthesis by Ni centers on a siliceous surface: implications in prebiotic chemistry.


 Comets are a source of prebiotic molecules that likely enriched the early Earth during the Late Heavy Bombardment period. Laboratory experiments that replicate cometary conditions may facilitate understanding of the chemical reactions and supplement observational studies of these icy bodies. Prebiotic compounds, such as formic acid and formaldehyde, have been observed in comets. Furthermore, these compounds can easily be formed in experimental models using a variety of gas combinations and energy sources. We conducted experimental cometary simulations using radiation chemistry tools to obtain insight into the possible fate of formic acid and formaldehyde. The main results suggest a redundant system, signifying that the irradiation of formic acid forms formaldehyde molecules and vice versa. This phenomenon ensures the permanence of prebiotic molecules in high-radiation environments. Additionally, the potential role of forsterite and graphite was explored in cometary simulations. Our experimental results show the differential formation of aldehydes and other carbonyl-containing compounds dependent on the mineral phase present.

/pdf/radiation-induced-reactions-in-comet-analogues-2invhuz0.pdf

Radiation-induced reactions in comet analogues

ABSTRACT In this work, by means of quantum chemistry (Density Functional Theory (DFT), PW6B95/def2-TZVPP; DLPNO-CCSD(T)/CBS), HCN polymerization [(HCN)1 − 4] initiated and catalysed by a siloxyl radical (Si-O•) on a model silica surface is analysed. Linear HCN polymers (pHCN) are obtained by a radical initiated mechanism at a SiO• site and are characterized by a -(HC-N)- skeleton due to radical localization on the terminal N atom and radical attack on the C centre. NC heterocycles are formed by cyclization of the linear SiO-(HCN)3 − 4 and are always thermodynamically preferred over their linear counterparts, acting as thermodynamic sinks. Of particular interest to the astrochemistry community is the formation of the N-heterocycle 1,3,5-triazine that can be released into the gas phase at relatively low T (ΔG† = 23.3 kcal/mol). Full hydrogenation of SiO-(HCN•) follows two reaction channels with products: (a) SiO-CH3 + •NH2 or (b) amino-methanol + Si•, though characterized by slow kinetics. Nucleophilic addition of H2O to the electron-rich SiO-(HCN•) shows an unfavourable thermodynamics as well as a high-activation energy. The cleavage of the linear (HCN)1−4 from the SiO• site also shows a high thermodynamic energy penalty (ΔG≥82.0 kcal/mol). As a consequence, the silicate surface will be passivated by a chemically active ‘pHCN brush’ modifying the surface physico-chemical properties. The prospect of surface-catalysed HCN polymers exhibiting a high degree of chemical reactivity and proposed avenues for the formation of 1,3,5-triazine and amino-methanol opens exciting new chemical pathways to Complex Organic Matter formation in astrochemistry. 

OUP accepted manuscript

Abstract Forsterite is a primary constituent of interstellar dust and planetary systems. It is believed to originate from the outflows of oxygen-rich stars and undergo further processing within the interstellar medium (ISM) through the action of cosmic rays and shocks. Under these harsh conditions, point defects may form, such as MgO Schottky vacancies. These vacancies can then undergo atom reconstruction as part of a chemical process to maintain the system’s crystalline structure. Polycyclic Aromatic Hydrocarbons (PAHs) are ubiquitously observed interstellar molecules and are thought to form through gas phase reactions akin to sooting flames. However, their role and impact on dust stability remain unknown. In this study, we employ an atomistic Artificial Intelligence (AI)-based method, surrogate machine learning trained directly by density functional theory (DFT). Specifically, we utilize Global Optimization with First-principle Energy Expressions (GOFEE) to predict possible reconstructions of MgO vacancies on a crystalline forsterite (010) surface as an important component of interstellar dust and planetary systems. We identify nine possible reconstructions involving the formation of unbound Si and O atoms. We investigate their energy stability and find that the reconstruction of Si-O atoms stabilizes the vacancy by about 0.54 eV. Additionally, if PAHs bind with the unbound O and Si atoms of the vacancy, the vacancy is stabilized by approximately 1.76 eV. We demonstrate that PAHs, along with the reconstruction of unbound atoms on the dust surface, affect the stability of the dust, which might open up avenues for diverse chemistry. 

The role of point defect reconstructions and Polycyclic Aromatic Hydrocarbons (PAHs) in silicate dust preservation

Unidentified infrared emission features in mid-infrared spectrum of comet 21P/Giacobini-Zinner

Catalytic conversion of methanol to larger organic molecules over crystalline forsterite: Laboratory study and astrophysical implications

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Catalytic conversion of methanol to larger organic molecules over crystalline forsterite: Laboratory study and astrophysical implications