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

A. Bergantini, M.J. Abplanalp, P. Pokhilko, A.I. Krylov, C.N. Shingledecker, E. Herbst, and R.I. Kaiser
Proc. Nat. Acad. Sci., submitted (2017)

The origin of homochirality in molecular biology is likely linked to racemic mixtures of chiral, organic molecules synthesized in the interstellar medium (ISM), after which one enantiomer was preferentially destroyed. If a primordial enantiomeric excess existed in molecular clouds, this excess could have been propagated to its descendants such as solar systems and planet(esimal)s thus contributing to the contemporary homochirality of life on Earth. This homochirality necessitates an intimate understanding of the fundamental mechanisms how racemic mixtures of complex organic molecules (COMs) are synthesized in the ISM. Here we reveal 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 via an exotic, cosmic ray initiated non-equilibrium chemistry 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 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.

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