Estudo teórico da formação de moléculas de importância prebiótica no meio interestelar

Abstract

In astrochemistry the molecule glycolaldehyde or 2-hydroxyethanal plays a crucial role. Besides being the simplest sugar ever detected in the interstellar medium, its significance lies in its ability to synthesize other biologically relevant molecules, such as ribose, a key component in genetic material. However, its formation process remains undefined as current proposals fail to explain its detected abundance. Thus, initially, the best formation route for glycolaldehyde in the gas phase was defined, where three distinct routes for its formation were tested. The formaldehyde molecule is the primary candidate as a precursor, and based on this, a radical formation route and a neutral one were analyzed. The best-found formation route was the radical pathway, through two elementary reactions. Initially, the HCO radical reacts with formaldehyde, generating an intermediate that subsequently reacts with a hydrogen radical, resulting in glycolaldehyde. This reaction exhibited an energy barrier of 27 kJ mol−1 , with relative energy to the final reagents of -455 kJ mol−1 . This route was then established for subsequent studies. As it is proposed that organic molecules in interstellar space are catalyzed or have their formation facilitated on the surface of astronomical ice or grain surfaces, it was then assessed whether amorphous ice surfaces reduce the energy barrier and favor the formation of glycolaldehyde as the main route of acquisition in the interstellar medium. For this purpose, two distinct methodologies were tested, both validated with experimental data on adsorption energy. In periodic calculations with amorphous ice surfaces, an average reduction of 49% in the energy barrier was obtained using DFT B3LYP, employing the Eley-Rideal mechanism. Meanwhile, in amorphous ice clusters, the average reduction ranged between 105% to 129% using DFT ωB97X-D3 and 48% to 54% using Coupled Cluster at the Langmuir-Hinshelwood mechanism. It was also evaluated whether the same mechanism could be shared for the formation of methyl formate, which proved to be inefficient both in ice and in stellar dust grains. Therefore, this study proposes a new, effective, and promising formation route for glycolaldehyde with the essential involvement of astrochemical ice.