Isomers in Interstellar Environments (I): The Case of Z- and E-Cyanomethanimine

TL;DR

This study investigates the chemistry and abundance ratios of Z- and E-cyanomethanimine in interstellar environments using quantum chemical calculations and astrochemical modeling, providing insights into their formation and destruction mechanisms.

Contribution

It combines ab initio calculations with astrochemical models to explain the observed Z/E ratio of cyanomethanimine in space, offering a new rule-of-thumb for isomer abundance estimation.

Findings

Reaction barriers for H + Z/E-HNCHCN are similar, with rate coefficients around 10^{-17} cm^3 s^{-1}.

Model predictions of the Z/E ratio (~3) align with observations, slightly lower than previous estimates.

Differences in ion-molecule destruction rates due to dipole moments influence the Z/E ratio.

Abstract

In this work, we present the results of our investigation into the chemistry of Z- and E-cyanomethanimine (HNCHCN), both of which are possible precursors to the nucleobase adenine. Ab initio quantum chemical calculations for a number of reactions with atomic hydrogen were carried out. We find that the reaction H + Z/E-HNCHCN leading both to H-addition as well as H$_2$-abstraction proceed via similar short-range barriers with bimolecular rate coefficients on the order of $\sim10^{-17}$ cm$^{3}$ s$^{-1}$. These results were then incorporated into astrochemical models and used in simulations of the giant molecular cloud G+0.693. The calculated abundances obtained from these models were compared with previous observational data and found to be in good agreement, with a predicted [Z/E] ratio of $\sim3$ - somewhat smaller than the previously derived value of $6.1\pm2.4$. We find that the [Z/E] ratio in our simulations is due mostly to ion-molecule destruction rates driven by the different permanent dipoles of the two conformers. Based on these results, we propose a general rule-of-thumb for estimating the abundances of isomers in interstellar environments.