Decomposition dynamics of interstellar HCNH: Ab-initio MO and RRKM studies

Ab-initio MO calculations have been carried out for hydrogen-dissociation reactions HCNH, i.e. (I) and (II), in order to elucidate the branching ratio of HCN/HNC on the ground state potential energy surface. The calculations showed that the transition state for reaction I is lower in energy than that of reaction II. The “bare” barrier heights for channel I and II were calculated to be 33.5–34.8 kcal/mol and 38.5–40.7 kcal/mol, respectively. The energy difference between transition states I and II was calculated to be 3.7–6.9 kcal/mol, meaning that reaction I preferentially occurs in the threshold energy region. Rice-Ramsperger-Kassel-Marcus (RRKM) theory including the tunneling effect indicated that reaction I is more favorable than reaction II at lower energy region, if tunneling effects are included in the rate calculations. On the other hand, the higher energy region above ca. 50 kcal/mol, channel II became dominant. The branching ratio (HCN/HNC) was calculated to be 0.3 at eV, which corresponds to the electron affinity of HCNH+. The mechanism of the reaction is discussed on the basis of theoretical results.