A molecular orbital study of the HC₃NH⁺ + e^- dissociative recombination and its role in the production of cyanoacetylene isomers in interstellar clouds

The dissociative recombination reaction between HC₃NH⁺ and electrons is an important process in the chemistry of interstellar clouds. Motivated by the evidence that HCN and its metastable isomer HNC are both formed via the dissociative recombination reaction of HCNH⁺, we have examined the potential energy surface for the dissociation of the transient HC₃NH following the recombination between HC₃NH⁺ and electrons. We find that HC₃NH⁺ gains 151 kcal mol^-1 energy by the neutralization; this energy is high enough to isomerize HC₃NH to HCCNCH, as well as to dissociate a hydrogen atom from either end of the molecule. The present calculation suggests, therefore, that the metastable linear and near-linear isomers of HC₃N, namely, HNC₃, HCCNC, and HCNCC, can be formed via the dissociative recombination between HC₃NH⁺ and electrons. Two of these isomers, HNC₃ and HCCNC, have already been detected in the dark cloud TMC-1. The unobserved species HCNCC is energetically the least stable. Since interstellar HC₃N molecules are synthesized predominantly via neutral-neutral reactions, the HC₃NH⁺ + e^- process serves mainly to produce the metastable isomers of HC₃N. We have incorporated these reaction products, as well as other relevant ion-molecule and neutral-neutral reactions, into our new standard model of dense interstellar clouds. Our model results show that the observed abundances in TMC-1 of HC₃N, its two detected metastable isomers, and the related species C₃H, C₃N, and HC₃NH⁺ can, in the main, be reproduced.