  We investigate the relative efficiencies of low-temperature chemical
reactions in the Interstellar medium (ISM) with H$^-$ anion reacting in the gas
phase with cyanopolyyne neutral molecules, leading to the formation of anionic
C$_x$N$^-$ linear chains of different length and of H$_2$. All the reactions
turn out to be without barriers, highly exothermic reactions which provide a
chemical route to the formation of anionic chains of the same length . Some of
the anions have been observed in the dark molecular clouds and in the diffuse
interstellar envelopes.Quantum calculations are carried for the corresponding
reactive potential energy surfaces (RPESs) for all the odd-numbered members of
the series (x=1, 3, 5, 7). We employ the Minimum Energy paths (MEPs) to obtain
the relevant Transition State (TS) configurations and use the latter within the
Variational Transition State ( VTS) model to obtain the chemical rates. The
present results indicate that, at typical temperatures around 100 K, a set of
significantly larger rate values exists for x=3 and x=5, while are smaller for
CN$^-$ and C$_7$N$^-$. At those temperatures, however, all the rates turn out
to be larger than the estimates in the current literature for the Radiative
Electron Attachment (REA) rates, thus indicating the greater importance of the
present chemical path with respect to REA processes at those temperatures. The
physical reasons for our findings are discussed in detail and linked with the
existing observational findings.
