POTENTIAL-ENERGY SURFACES AND BRANCHING RATIO OF THE DISSOCIATIVE RECOMBINATION REACTION HCNH-) - AN AB-INITIO MOLECULAR-ORBITAL STUDY(+E()

Two-dimensional potential energy surfaces for the dissociative recombi nation reaction, HCNH+ +e(-) --> (HCNH) --> HNC/HCN+H, have been calc ulated by the CASSCF-MRSDCI ab initio molecular orbital method. Near t he HCNH+ ground state surface, which represents the upper limit of neu tral HCNH Rydberg state surfaces, only two dissociative valence states of HCNH are located. One is the (2) Sigma(+) state resulting in H-C b ond dissociation to produce HNC, and the other is the (2) Sigma(+) sta te resulting in H-N bond dissociation to produce HCN. The seam of inte rsection of these two potential surfaces almost bisects the zero-point vibrational wave function of the Rydberg-limit state of the neutral H CNH species, indicating that, through descending Rydberg ladder, trans ition from the Rydberg-limit state to each dissociative surface occurs with almost the same probability. Thus, branching ratio to HNC and HC N is predicted to be of the order of one or slightly higher, explainin g the thermochemically unrealistic interstellar HNC/HCN abundance rati o. (C) 1998 American Institute of Physics.