Ab Initio Direct Trajectory Simulation with Nonadiabatic Transitions of the Dissociative Recombination Reaction HCNH⁺+e⁻→ HNC/HCN + H

Theoretical Elucidation of the Unusually High [HNC]/[HCN] Abundance Ratio in Interstellar Space: Two‐dimensional and Two‐State Quantum Wave Packet Dynamics Study on the Branching Ratio of the Dissociative Recombination Reaction HCNH++e−→ HNC/HCN + H

“…At t ¼ 10 fs 90% of the wave packet has been transferred from 2 2 AE + to 1 2 AE + , and at t ¼ 80 fs only 1% of the wave packet remains on the 2 2 AE + PES. In previous ab initio direct trajectory simulations (Taketsugu et al 2004), the surface hopping from 2 2 AE + to 1 2 AE + occurred within 5 fs in 34% of trajectories, and this rate increased to 76% at t ¼ 10 fs. In both quantum and semiclassical simulations, the initial transition from 2 2 AE + to 1 2 AE + states occurs very quickly, indicating the strong coupling of these two adiabatic states in the Franck-Condon region just below the equilibrium structure of the cationic ground state.…”

“…The final branching ratios are listed in Table 1: 0.77 for (0, 0), 1.32 for (0, 1), 0.79 for (1, 0), 1.13 for (0, 2), 0.96 for (1, 1), and 1.00 for (2, 0). It is noted that in ab initio direct trajectory simulations where zeropoint vibrational energies were given to the respective vibrational modes in the initial states, the branching ratio of [HNC]/ [HCN ] was evaluated as1.14 (Taketsugu et al 2004). We should also pay attention to the radiative relaxation study of the highly excited H/C/N system (Barger et al 2003), in which the radiative branching ratio of HNC/HCN was shown to be on the order of 1 when highly excited delocalized states above the barrier to isomerization are included in the initial states.…”

“…Due to this restriction, HCNH bending motions and isomerization reactions between HNC and HCN have not been permitted. According to ab initio direct trajectory simulations (Taketsugu et al 2004), actually the molecule begins to bend in the nonadiabatic region between 1 2 Å and 1 2 AE + , and the isomerization reactions between HNC and HCN occur in three cases among 50 trajectories during the dissociation processes. This amount is small but not negligible, and thus we plan to investigate the effects of bending motions on the branching ratios by performing four-dimensional three-state quantum wave packet simulations.…”

“…Talbi and coworkers, however, proposed the direct mechanism based on the results of their ab initio MO calculations (Talbi & Ellinger 1998;Hickman et al 2005). Very recently, we carried out ab initio direct trajectory simulations (Taketsugu et al 2004) for reaction (1), taking into account nonadiabatic transitions among adiabatic electronic states, and showed that among 50 trajectories, 24 lead to HNC þ H, 21 lead to HCN þ H, three lead to CN þ H þ H, and two remain in the region of HCNH. This result also supports that HNC and HCN are generated with almost the same amount from reaction (1).…”

The interstellar gas-phase chemistry of HCN and HNC

“…There are various theoretical studies of the DR reaction of HCNH + leading to similar results despite disagreement for the mechanism which involves either a direct formation pathway (Talbi & Ellinger 1998, Hickman et al 2005 or an indirect process (Shiba et al 1998, Taketsugu et al 2004, Ishii et al 2006 as these authors found no possibility for the direct process. However all studies of the direct and indirect processes lead to similar branching ratios for HCN and HNC formation.…”