  Recent astronomical observations of both isomers E and Z of imines such as
cyanomethanimine, ethanimine and 2-propyn-1-imine, have revealed that the
abundances in the ISM of these isomers differ by factors of ~3-10. Several
theories have been proposed to explain the observed behavior, but none of them
successfully explains the [E]/[Z] ratios. In this work we present a detailed
study of the kinetics of the one-step E-Z isomerization reactions of
cyanomethanimine, ethanimine and 2-propyn-1-imine under interstellar conditions
(in the 10-400 K temperature range). This reaction was previously thought to be
non-viable in the ISM due to its associated high-energy barrier (about 13,000
K). In this Letter, we show that considering the multidimensional small
curvature tunneling approximation, the tunneling effect enables the
isomerization even at low temperatures. This is due to the fact that the
representative tunneling energy lies in the vibrational ground state of the
least stable isomer up to approximately 150 K, making the reaction constants of
the isomerization from the least stable to the most stable isomer basically
constant. The predicted [E]/[Z] ratios are almost the same as those reported
from the astronomical observations for all imines observed. This study
demonstrates that the [E]/[Z] ratio of imines in the ISM strongly depends on
their relative stability.
