2016ApJ...830....2G

Using quantum chemical methods, we investigate the possible outcomes of \rmH^– reactions with acetylene and diacetylene molecules. We find both reactions to be exothermic reactions without barriers, yielding stable anions of the corresponding polyynes: \rmC₂\rmH^– and \rmC₄\rmH^–. We show in this work that the computed chemical rates in the case of the formation of the \rmC₄\rmH^– anion would be larger than those existing for the direct radiative electron attachment (REA) process, the main mechanism generally suggested for their formation. In the case of the \rmC₂\rmH^– anion, however, the present chemical rates of formation at low T are even lower than those known for its REA process, both mechanisms being inefficient for its formation under astrochemical conditions. The present results are discussed in view of their consequences on the issue of the possible presence of such anions in the ISM environments. They clearly indicate the present chemical route to \rmC₂\rmH^– formation to be inefficient at the expected temperatures of a dark molecular cloud, whereas this is found not to be the case for the \rmC₄\rmH^–, in line with the available experimental findings.