The quest of chirality in the interstellar medium

I. Lessons of propylene oxide detection

¹ Sorbonne Université, CNRS, Laboratoire de Chimie Théorique, LCT, 75005 Paris, France
e-mail: yves.ellinger@sorbonne-universite.fr
² Univ. Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR UMR6226, 35000 Rennes, France
e-mail: jean-claude.guillemin@ensc-rennes.fr

Received: 11 October 2019
Accepted: 29 November 2019

Abstract

Context. All but one complex organic molecule (COM) detected so far in the interstellar medium (ISM) are achiral; propylene oxide (c-C₂H₃O)-CH₃ is the only exception to this. Finding other chiral species is a priority for astrobiology to progress in the understanding of the emergence of life. Whatever the conditions of their formation, i.e., gas phase or grain chemistry, the detection relies on rotational spectra. This means that, if adsorbed after formation in the gas phase or directly formed on the icy grains, these COMs must escape in the gas phase as free flyers to be detectable.

Aims. Learning the lesson drawn from the only observation of a chiral compound and considering the structural constraints imposed to a molecule to be chiral, we look at what species could satisfy these conditions and be potential targets for a radio astronomy search in the ISM gas phase.

Methods. This question was addressed by combining two complementary approaches that rely on density functional theory. The structure, energetics, and spectroscopic parameters of each potential candidate were determined using molecular calculations. The propensity for a molecule to remain trapped on the ice coating of the grains was evaluated by numerical simulations making use of a solid state periodic model.

Results. Replacing the -CH₃ group on rigid propylene oxide by -CN, -CCH, -NH₂, -OH, or -HCO gives oxirane daughter molecules whose adsorption energies divide into two classes: below and above the adsorption energy of H₂O on solid water-ice ~13.5 kcal mol⁻¹.

Conclusions. The best chiral candidate would be a rigid molecule for an easier determination of its radio spectra. This molecule would be composed of a central carbon linked to one hydrogen and three different chemical groups as simple as possible. If not the most stable isomer, this candidate should be as close as possible on the energy scale, possess a significant dipole moment, and be less strongly attached to the ice than H₂O itself.