Formation of anionic C,N-bearing chains in the Interstellar medium via reactions of H$^{-}$ with HC$_x$N for odd-valued x from 1 to 7

TL;DR

This study explores barrierless, exothermic reactions of H$^-$ with cyanopolyynes in the interstellar medium, revealing efficient formation pathways for anionic chains of various lengths, especially for x=3 and 5, at typical ISM temperatures.

Contribution

It provides quantum chemical calculations and reaction rate estimates for the formation of C$_x$N$^-$ chains via H$^-$ reactions, highlighting their significance over radiative electron attachment in space.

Findings

Reaction rates are higher for x=3 and 5 at 100 K.

Reactions are barrierless and highly exothermic.

The formation pathway is more efficient than REA at typical ISM temperatures.

Abstract

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.