Experimental and computational studies on the reactivity of methanimine radical cation (H$_2$CNH$^{+\cdot}$) and its isomer aminomethylene (HCNH$_2^{+\cdot}$) with C$_2$H$_2$

TL;DR

This study combines experimental and computational methods to investigate how methanimine and aminomethylene radical cations react with ethyne, revealing reaction pathways and astrochemical relevance.

Contribution

It provides new insights into the reaction mechanisms and pathways of methanimine and aminomethylene radical cations with ethyne, supported by both experimental data and ab initio calculations.

Findings

Major reaction channel involves H atom elimination forming [C3NH4]+

Aminomethylene reacts via barrierless pathways to multiple [C3NH4]+ isomers

Methanimine's reaction pathway uniquely leads to protonated vinyl cyanide, relevant in astrochemistry

Abstract

Experimental and theoretical studies are presented on the reactivity of the radical cation isomers methanimine and aminomethylene with ethyne. Selective isomer generation is performed via dissociative photoionization of suitable neutral precursors and via direct photoionization of methanimine. Reactive cross sections and product branching ratios are measured as a function of photon and collision energies. Results are discussed in light of ab initio calculations of reaction mechanisms. The major channels, for both isomers, are due to H atom elimination from covalently bound adducts to give [C3NH4]+. Theoretical calculations show that while for the reaction of aminomethylene with acetylene any of the three lowest energy [C3NH4]+ isomers can form via barrierless and exothermic pathways, for the methanimine reagent the only barrierless pathway is the one leading to the production of protonated vinyl cyanide (CH2CHCNH+), a prototypical branched nitrile species that has been proposed as a likely intermediate in star forming regions and in the atmosphere of Titan. The astrochemical implications of the results are briefly addressed.