2019ApJ...878...80S

To date, two isomers of H₂C₃O have been detected, namely, propynal (HCCCHO) and cylclopropenone (c-H₂C₃O). A third, propadienone (CH₂CCO), has thus far eluded observers despite the fact that it is the lowest in energy of the three. This previously noted result is in contradiction to the minimum energy principle, which posits that the abundances of isomers in interstellar environments can be predicted based on their relative stabilities and suggests, rather, the importance of kinetic over thermodynamic effects in explaining the role of such species. Here, we report results of ab initio quantum chemical calculations of the reaction between H and (a) HC₃O, (b) H₂C₃O (both propynal and propadienone), and (c) CH₂CHCO. We have found that, among all possible reactions between atomic hydrogen and either propadienone or propynal, only the destruction of propadienone is barrierless and exothermic. That this destruction pathway is indeed behind the nondetection of CH₂CCO is further suggested by our finding that the product of this process, the radical CH₂CHCO, can subsequently react barrierlessly with H to form propenal (CH₂CHCHO) which has, in fact, been detected in regions where the other two H₂C₃O isomers are observed. Thus, these results not only shed light on a previously unresolved astrochemical mystery, but also further highlight the importance of kinetics in understanding the abundances of interstellar molecules.