Possible Role of Metal-Ions in the Chemistry of Prochirality and the Origin of Chirality in the Interstellar Medium

Abstract

Propylene oxide (c-C 3 H 6 O) is the first chiral molecule detected in the interstellar medium (ISM). Its recent detection has opened up newer avenues to be explored in interstellar chemistry. On earth, metal-ions play a preeminent role in the chemistry of chirality and prochirality. Taking a cue from this idea, we computationally investigate the possible role of metal-ions in (a) the origin of chirality in the ISM and (b) advancing interstellar prochiral chemistry; noting that prochirality is a severely understudied concept in astrochemistry. The gas-phase reactions of ethylene, and propylene with OOH radical to lead to the formation of ethylene oxide and propylene oxide respectively were noted to be excellent model reactions, and have been chosen to examine the role of the metal-ions. The interstellar metal-ions being studied (viz. Al + , Mg + , and Mg 2+ ) are considered in three different forms�as bare metal-ions, or as hydrated metal-ions bound to one molecule of water, and as part of a neutral molecule. Carefully calibrated electronic structure calculations (CCSD(T) and DFT) were used to obtain the relevant saddle points (minimum energy structures and transition states). One key conclusion from this work is that, metal-ions, regardless of what form they are in, lead to the barrierless formation of the epoxides, thereby facilitating the generation of interstellar chirality. The key chemical, and astrochemical facets involved in the chemistry of metal-ions in the circumstellar envelope of IRC+10216 are also discussed herein in detail. This work further recommends the detection of four molecules in the ISM, namely, aziridine, methyl aziridine, 2-amino-3-hydroxy propane, and 2-hydroxy-3-amino propane. Lastly, a foremost point to emerge out of this study and related contemporary works probing the chemistry of metal-ions in the ISM is that metal-ions may act as the glue connecting interstellar gas-phase chemistry with the surface chemistry occurring on ice-grains, and could aid in the unravelling of complex and new aspects in astrochemistry.

Possible Roles of Transition Metal Cations in the Formation of Interstellar Benzene via Catalytic Acetylene Cyclotrimerization

“…Under low-density conditions, the intramolecular redistribution of vibrational energies derived from the exothermic energy could play an important role in releasing the transition metal cation. Furthermore, one of the most reasonable pathways to obtain the final product fragments is the dissociative electron attachment: (benzene-TM + ) + e − → benzene + TM [46,47]. For example, the reaction involving a spin transition, ( 1 benzene-1 Ti) → 1 benzene + 3 Ti, is only a 0.5 kcal/mol endothermic reaction (see Table S1).…”

“…The metal-molecule complex therefore enables the dissociation to the benzene and neutral metal fragments. Alternative pathways are photodissociation by irradiation of the cosmic rays and collisional dissociation by encountering other systems in the dense clouds [46,47].…”