Propargylimine in the laboratory and in space: millimetre-wave
  spectroscopy and first detection in the ISM
  Small imines containing up to three carbon atoms are present in the
interstellar medium. As alkynyl compounds are abundant in this medium,
propargylimine thus represents a promising candidate for a new interstellar
detection. The goal of the present work is to perform a comprehensive
laboratory investigation of the rotational spectrum of propargylimine in its
ground vibrational state in order to obtain a highly precise set of
rest-frequencies and to search it in space. The rotational spectra of $E$ and
$Z$ geometrical isomers of propargylimine have been recorded in laboratory in
the 83-500 GHz frequency interval. The measurements have been performed using a
source-modulation millimetre-wave spectrometer equipped with a pyrolysis system
for the production of unstable species. High-level ab initio calculations were
performed to assist the analysis and to obtain reliable estimates for an
extended set of spectroscopic quantities. The improved spectral data allow us
to perform a successful search for this new imine in the quiescent
G+0.693-0.027 molecular cloud. Eighteen lines of $Z$-propargylimine have been
detected at level $>2.5 \sigma$, resulting in a column density estimate of $N =
(0.24\pm 0.02)\times 10^{14}$ cm$^{-2}$. An upper limit was retrieved for the
higher-energy $E$ isomer, which was not detected in the data. The fractional
abundance (w.r.t. H$_2$) derived for $Z$-propargylimine is $1.8\times
10^{-10}$. We discuss the possible formation routes by comparing the derived
abundance with those measured in the source for possible chemical precursors.
