# The Case of H$_2$C$_3$O Isomers, Revisited: Solving the Mystery of the Missing Propadienone

## TL;DR

This study uses quantum chemical calculations to explain why the propadienone isomer of H$_2$C$_3$O remains undetected in space, revealing that kinetic factors, not just stability, govern interstellar molecule abundances.

## Contribution

The paper demonstrates that barrierless destruction of propadienone explains its absence, emphasizing the role of kinetics over thermodynamics in astrochemical isomer distributions.

## Key findings

- Propadienone destruction is barrierless and exothermic.
- Radical CH$_2$CHCO reacts with H to form detectable propenal.
- Kinetic effects explain the non-detection of certain isomers.

## Abstract

To date, two isomers of H$_2$C$_3$O have been detected, namely, propynal (HCCCHO) and cylclopropenone (c-H$_2$C$_3$O). A third, propadienone (CH$_2$CCO), 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 of 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 \textit{ab initio} quantum chemical calculations of the reaction between H and (a) HC$_3$O, (b) H$_2$C$_3$O (both propynal and propadienone), and (c) CH$_2$CHCO. 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 non-detection of CH$_2$CCO is further suggested by our finding that the product of this process, the radical CH$_2$CHCO, can subsequently react barrierlessly with H to form propenal (CH$_2$CHCHO) which has, in fact, been detected in regions where the other two H$_2$C$_3$O 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.