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 (C2H in the present case) to methanimine (CH2NH) leads to reaction channels open also in the harsh conditions of the interstellar medium. Three possible isomers can be formed in the CH2NH + C2H 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 × 10−10 cm3 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] ∼ 1.5, which is only slightly underestimated with respect to the observational datum.