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Description
The dissociative recombination reaction between HC_3_NH^+^ 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 HC_3_NH following the recombination between HC_3_NH^+^ and electrons. We find that HC_3_NH^+^ gains 151 kcal mol^-1^ energy by the neutralization; this energy is high enough to isomerize HC_3_NH 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 HC_3_N, namely, HNC_3_, HCCNC, and HCNCC, can be formed via the dissociative recombination between HC_3_NH^+^ 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 HC_3_N molecules are synthesized predominantly via neutral-neutral reactions, the HC_3_NH^+^ + e^-^ process serves mainly to produce the metastable isomers of HC_3_N.
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