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.
Highlights
Over 210 different species (McGuire 2018) have been detected in the interstellar and circumstellar media (ISM)
Summary and concluding remarks Assuming that the detection of propylene oxide is not purely accidental, this report addresses the question of whether some chiral molecules have a probability of being detected in the ISM
To this end we tried to pinpoint the specificities of propylene oxide that would have made it suitable to radio detection, which means its presence in the gas phase of molecular clouds
Summary
Over 210 different species (McGuire 2018) have been detected in the interstellar and circumstellar media (ISM). This would be compatible with an abundance in the cold media that is too low to be detected with the present instruments At this point we note that microwave/millimetric spectra used to probe the presence and relative abundances of gas-phase species cannot provide the number of molecules effectively formed, but can only count those molecules that escape trapping on the surface of the icy grains. Whether it is a free flyer or not a free flyer, the key parameter is the adsorption energy on water ice. In other words, it can be conjectured that not detecting a molecule does not mean that it is not present in the ISM. The adsorption energies of these species in the range 6–11 kcal mol−1 are all below the binding energy of H2O on solid water (13– 14 kcal mol−1), which is consistent with the fact that these species are free flyers
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