A Molecular Orbital Study of the HC_3NH^++e^- Dissociative Recombination and Its Role in the Production of Cyanoacetylene Isomers in Interstellar Clouds

Astronomy and Astrophysics – Astronomy

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Subject Headings: Ism: Molecules, Molecular Processes

Scientific paper

The dissociative recombination reaction between HC_3NH^+ 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_3NH following the recombination between HC_3NH^+ and electrons. We find that HC_3NH^+ gains 151 kcal mol^-1 energy by the neutralization; this energy is high enough to isomerize HC_3NH 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_3N, namely, HNC_3, HCCNC, and HCNCC, can be formed via the dissociative recombination between HC_3NH^+ and electrons. Two of these isomers, HNC_3 and HCCNC, have already been detected in the dark cloud TMC-1. The unobserved species HCNCC is energetically the least stable. Since interstellar HC_3N molecules are synthesized predominantly via neutral-neutral reactions, the HC_3NH^++e^- process serves mainly to produce the metastable isomers of HC_3N. 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 HC_3N, its two detected metastable isomers, and the related species C_3H, C_3N, and HC_3NH^+ can, in the main, be reproduced.

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