The Role of State-of-the-Art Quantum-Chemical Calculations in
  Astrochemistry: Formation Route and Spectroscopy of Ethanimine as a
  Paradigmatic Case
  The gas-phase formation and spectroscopic characteristics of ethanimine have
been re-investigated as a paradigmatic case illustrating the accuracy of
state-of-the-art quantum-chemical (QC) methodologies in the field of
astrochemistry. According to our computations, the reaction between the
amidogen, NH, and ethyl, C$_2$H$_5$, radicals is very fast, close to the
gas-kinetics limit. Although the main reaction channel under conditions typical
of the interstellar medium leads to methanimine and the methyl radical, the
predicted amount of the two E,Z stereoisomers of ethanimine is around 10%.
State-of-the-art QC and kinetic models lead to a [E-CH$_3$CHNH]/[Z-CH$_3$CHNH]
ratio of ca. 1.4, slightly higher than the previous computations, but still far
from the value determined from astronomical observations (ca. 3). An accurate
computational characterization of the molecular structure, energetics, and
spectroscopic properties of the E and Z isomers of ethanimine combined with
millimeter-wave measurements up to 300 GHz, allows for predicting the
rotational spectrum of both isomers up to 500 GHz, thus opening the way toward
new astronomical observations.
