  Many molecular species can presumably still be observed in space if they are
adequately characterized chemically. In this paper, we suggest that this could
be the case of the neutral (C$_4$N$^0$) and anion (C$_4$N$^-$)
cyanopropynylidene chains, which were not yet identified in space although both
the neutral (C$_3$N$^0$ and C$_5$N$^0$) and anion (C$_3$N$^-$} and C$_5$N$^-$)
neighboring members of the homologous series were observed. Extensive data
obtained from quantum chemical calculations using density functional theory
(DFT), coupled cluster (CC), and quadratic configuration interaction (QCI)
methods for all charge and spin states of interest for space science (doublet
and quartet neutrals, triplet and singlet anions, and singlet and triplet
cations) are reported: e.g., bond metric and natural bond order data,
enthalpies of formation, dissociation and reaction energies, spin gaps,
rotational constants, vibrational properties, dipole and quadrupole momenta,
electron attachment energies ($EA$) and ionization potentials ($IP$). The fact
that (not only for C$_4$N but also for C$_2$N and C$_6$N) the quantum chemical
methods utilized here are able to excellently reproduce the experimental $EA$
value -- which is often a challenge for theory -- is particularly encouraging,
since this indicates that theoretical estimates of chemical reactivity indices
(which are key input parameters for modeling astrochemical evolution) can be
trusted. The presently calculated enthalpies of formation and dissociation
energies do not substantiate any reason to assume that C$_4$N is absent in
space. To further support this idea, we analyze potential chemical pathways of
formation of both C$_4$N$^0$ and C$_4$N$^-$, which include association and
exchange reactions.
