Ion-molecule reactions producing HC₃NH⁺ in interstellar space: Forbiddenness of the reaction between cyclic C₃H⁺₃ and the N atom

The reaction between cyclic C₃H⁺₃ and the N atom is thought to be a key element in the synthesis of the interstellar molecule HC₃N, which is the most abundant species in the cyanopolyyne series in dark clouds. We have examined the potential energy surfaces for the reaction between cyclic C₃H⁺₃ and the N atom using ab initio quantum mechanical methods in order to confirm whether this reaction is feasible in interstellar space. The potential energy surface between cyclic C₃H⁺₃ and the ground-state N atom is, however, predicted to be repulsive. The reaction between cyclic C₃H⁺₃ and the N atom ground state (⁴S) is concluded not to produce HC₃NH⁺ in interstellar space, in addition to the fact that this reaction is spin forbidden. The alternative pathway to produce HC₃NH⁺ is the reaction between HCCH⁺ and HCN. This reaction is found to be very critical for the production of HC₃NH⁺ and the H atom because this process requires several reaction steps in a complicated unimolecular rearrangement and must go through an energy barrier which is very close to the energy of the reactants. The other possible way to produce HC₃NH⁺ is shown to be the reaction between HCCH⁺ and HNC. The theoretical potential energy surface suggests that the HCCNCH⁺ isomer of HC₃NH⁺ cannot be formed from HCCH⁺ + HCN because of the endothermicity and the high energy barrier for this process. The present study supports the view that the ion-molecule reactions are not a significant source for the production of the HC₃NH⁺, and therefore of the HC₃N molecule in interstellar space. The probable pathways to form HC₃NH⁺ are the reaction HCCH⁺ + HNC and the protonation of HC₃N, produced mainly via neutral-neutral reactions.