  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.
