Methanimine as a key precursor of imines in the interstellar medium: the
  case of propargylimine
  A gas-phase formation route is proposed for the recently detected
propargylimine molecule. In analogy to other imines, such as cyanomethanimine,
the addition of a reactive radical (C$_2$H in the present case) to methanimine
(CH$_2$NH}) leads to reaction channels open also in the harsh conditions of the
interstellar medium. Three possible isomers can be formed in the C$H_2$NH +
C$_2$H reaction: Z- and E-propargylimine (Z-,E-PGIM) as well as
N-ethynyl-methanimine (N-EMIM). For both PGIM species, the computed global rate
coefficient is nearly constant in the 20-300 K temperature range, and of the
order of 2-3 $\times$ 10$^{-10}$ cm$^3$ molecule$^{-1}$ s$^{-1}$, while that
for N-EMIM is about two orders of magnitude smaller. Assuming equal destruction
rates for the two isomers, these results imply an abundance ratio for PGIM of
[Z]/[E] $\sim$ 1.5, which is only slightly underestimated with respect to the
observational datum.
