A Combined Experimental and Theoretical Study on the Formation of Interstellar Propylene Oxide (CH₃CHCH₂O)—A Chiral Molecule

Abstract

This work reveals via a combined experimental, computational, and astrochemical modeling study that racemic propylene oxide (c-C3H6O)—the first chiral molecule detected outside Earth toward the high-mass star-forming region Sagittarius B2(N)—can be synthesized by non-equilibrium reactions initiated by the effects of secondary electrons generated in the track of cosmic rays interacting with ice-coated interstellar grains through excited-state and spin-forbidden reaction pathways operating within low-temperature interstellar ices at 10 K. Our findings confront traditional hypotheses that thermal chemistries followed by processing of interstellar grains dictate the formation of complex organic molecules (COMs) in molecular clouds. Instead, we reveal a hitherto poorly quantified reaction class involving excited-state and spin-forbidden chemistry leading to racemic mixtures of COMs inside interstellar ices prior to their sublimation in star-forming regions. This fundamental production mechanism is of essential consequence in aiding our understanding of the origin and evolution of chiral molecules in the universe.

Chemical and radiative transfer modeling of propylene oxide

“…Recently, Bergantini et al (2018) carried out a combined theoretical and experimental work to study the formation of interstellar PrO. They found that the galactic cosmic ray induced ice phase chemistry with the supra-thermal oxygen (O * or O( 1 D)) may lead to the significant amount of PrO (R17 of Table 1).…”

“…where, G is the radiolysis yield per 100eV, S e is the electronic stopping cross section, Φ ism is the interstellar cosmic ray flux and ζ is the cosmic ray ionization rate. Here, we have used same G value as used in Bergantini et al (2018) (2.23, 0.7 and 0.8 for CO 2 , H 2 O and CO respectively), φ ism = 10 cm −2 s −1 following Abplanalp et al (2016). We have used ζ = 1.3 × 10 −17 s −1 instead of 1.3 × 10 −16 used in Bergantini et al (2018).…”

“…Here, we have used same G value as used in Bergantini et al (2018) (2.23, 0.7 and 0.8 for CO 2 , H 2 O and CO respectively), φ ism = 10 cm −2 s −1 following Abplanalp et al (2016). We have used ζ = 1.3 × 10 −17 s −1 instead of 1.3 × 10 −16 used in Bergantini et al (2018). Though Abplanalp et al (2016) mentioned that S e is the electron stopping cross section for electrons, Bergantini et al (2018) used S e as the electron stopping cross section for protons from the PSTAR programme.…”

“…We have used ζ = 1.3 × 10 −17 s −1 instead of 1.3 × 10 −16 used in Bergantini et al (2018). Though Abplanalp et al (2016) mentioned that S e is the electron stopping cross section for electrons, Bergantini et al (2018) used S e as the electron stopping cross section for protons from the PSTAR programme. Here, we have used ESTAR programme as used by Abplanalp et al (2016) For the consideration of the supra-thermal oxygen (O( 1 D)) in our network, we have considered some trivial approximations.…”

“…The radiative transfer models have been carried out to identify the most active transition of PrO in the millimeter(mm) wave regions. Absorption Hickson et al (2016), b (http://kinetics.nist.gov/kinetics/), c This work, d Ward & Price (2011), e Bergantini et al (2018) features of PrO and integral absorption coefficients obtained with high-level quantum chemical methods could be handy for its future detection by the high-resolution Stratospheric Observatory for Infrared Astronomy (SOFIA) and the most awaited JWST. The remainder of this paper is as follows: Section 2 describes the reaction pathways considered in our modeling.…”