Formation of Anionic C, N-bearing Chains in the Interstellar Medium via Reactions of H- with HCxN for Odd-valued x from 1 to 7

We investigate the relative efficiencies of low-temperature chemical reactions in the interstellar medium with H- anion reacting in the gas phase with cyanopolyyne neutral molecules, leading to the formation of anionic CxN- linear chains of different lengths and of H-2. All the reactions turn out to be without barriers, highly exothermic reactions that 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 out for the corresponding reactive potential energy surfaces for all the odd-numbered members of the series (x = 1, 3, 5, 7). We employ the minimum energy paths to obtain the relevant transition state configurations and use the latter within the variational transition state 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 the rate values are smaller for CN- and C7N-. 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.