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