Potential energy surfaces and branching ratio of the dissociative recombination reaction HCNH++e−: An ab initio molecular orbital study

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Abstract
Two-dimensional potential energy surfaces for the dissociative recombination reaction, HCNH++e− → (HCNH)* → HNC/HCN+H, have been calculated by the CASSCF-MRSDCI ab initio molecular orbital method. Near the HCNH+ ground state surface, which represents the upper limit of neutral HCNH Rydberg state surfaces, only two dissociative valence states of HCNH are located. One is the 2Σ+ state resulting in H–C bond dissociation to produce HNC, and the other is the 2Σ+ state resulting in H–N bond dissociation to produce HCN. The seam of intersection of these two potential surfaces almost bisects the zero-point vibrational wave function of the Rydberg-limit state of the neutral HCNH species, indicating that, through descending Rydberg ladder, transition from the Rydberg-limit state to each dissociative surface occurs with almost the same probability. Thus, branching ratio to HNC and HCN is predicted to be of the order of one or slightly higher, explaining the thermochemically unrealistic interstellar HNC/HCN abundance ratio.