Knowledge Bank

Two dimensional potential energy surfaces for the dissociative recombination reaction. $HCNH^{+} + e^{-} \rightarrow (HCNH)- \rightarrow HCN/HNC + $H, have teen calculated by the CASSCF-MRSDCI/cc-pVQZ {ab initio} molecular orbital method. Near the $HCNH+$ ground state surface, which also represents the upper limit of the neutral HCNH Rydbergs state surface, only two dissociative valence states of The HCNH are located: One is the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma +$ state resulting in the H-C bond seission yielding HNC and the other the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma +$ state resulting in the H-N bond session yielding HCN. The seam of intersection of these two surfaces almost bisects the zero-point vibrational wave function of the Rydberg-limit state of the neutral HCNH species, indicating that, while descending Rydberg ladder, transition to each dissociative surface occurs with almost equal probability. Thus, the branching ratio [HNC]/[HCN] is predicted to be nearly one or slightly more, explaining the thermochemically unrealistic interstellar |HNC]/[HCN] ratio of 4.4 observed toward L134N.