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 di ssociative recombination reaction of HCNH+, we have examined the potential energy surface for the dissociation of the transient HC3NH following the re combination between HC3NH+ and electrons. We find that HC3NH+ gains 151 kca l mol(-1) energy by the neutralization; this energy is high enough to isome rize 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 detecte d in the dark cloud TMC-1. The unobserved species HCNCC is energetically th e least stable. Since interstellar HC3N molecules are synthesized predomina ntly via neutral-neutral reactions, the HC3NH+ + e(-) process serves mainly to produce the metastable isomers of HC3N. We have incorporated these reac tion products, as well as other relevant ion-molecule and neutral-neutral r eactions, into our new standard model of dense interstellar clouds. Our mod el results show that the observed abundances in TMC-1 of HC3N, its two dete cted metastable isomers, and the related species C3H, C3N, and HC3NH+ can, in the main, be reproduced.