Methanimine as a key precursor of imines in the interstellar medium: the case of propargylimine

TL;DR

This study proposes a gas-phase formation pathway for propargylimine in the interstellar medium, highlighting methanimine's role as a precursor, supported by computational rate coefficients and abundance ratio predictions consistent with observations.

Contribution

It introduces a new formation mechanism for propargylimine involving methanimine and reactive radicals, supported by quantum chemical calculations and astrochemical modeling.

Findings

Reaction rate coefficients are nearly constant between 20-300 K.

Predicted abundance ratio [Z]/[E] is approximately 1.5.

The proposed pathway aligns with observational data.

Abstract

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.