1 Introduction

HCN, HNC, and HCNH molecules are important in the chemical evolution process in interstellar molecular clouds because these molecules are considered as initial species in the synthesis of amino acid and protein (Suzuki et al. 1992). Study of the formation mechanism would provide knowledge of early stages of chemical evolution. HCN and HNC molecules have been considered to be formed by electron capture of HCNH⁺ as following reactions (Talbi et al. 1996),

$$\begin{eqnarray*}\rm HCNH^+ + e^- \to [HCNH]^* &\to& \rm HNC + H \\ &\to&\rm HCN + H \end{eqnarray*}$$

where [HCNH]* is the neutral HCNH molecule with 1 $^2\Sigma^+$, 2 $^2\Sigma^+$, and 1$^2\Pi$ electronic states or Rydberg states. The interaction between HCNH⁺ and electrons are composed of an attractive Coulomb force, so that a large cross-section is expected. This means that electron capture of HCNH⁺ proceeds efficiently. Also, HCNH⁺ is known to be a stable ion that does not react with large number of hydrogen molecules existing in interstellar molecular clouds. Hence, HCNH⁺ exists as a long-lived molecular ion in the clouds and is a major candidate in the formation of HCN and HCN molecules.

The HCN/HNC ratios in molecular clouds are often observed to be 1. From ab-initio molecular orbital (MO) calculations of the Franck-Condon (FC) region, this ratio was supported theoretically (Shiba et al. 1998; Herbst 1979; Talbi & Ellinger 1998, 1996). It is suggested that the H dissociation from HCNH occurs mainly on the 1 $^2\Sigma^+$ and 2 $^2\Sigma^+$ state surfaces. Thus, the hydrogen dissociations from HCNH at 1 $^2\Sigma^+$ and 2 $^2\Sigma^+$ states were well understood theoretically. However, the decomposition of HCNH on the ground state potential energy surface (X²A' in Cs symmetry or X$^2\Pi$ in $C_{\infty\rm v}$ symmetry) is not clearly understood.

In the present study, ab-initio MO calculations are carried out for the hydrogen dissociation reactions from HCNH on the ground state potential energy surface (PES). Also, the Rice-Ramsperger-Kassel-Marcus (RRKM) theory is applied to the reaction system in order to obtain the branching ratio of HCN and HNC. This PES corresponds to that of a hydrogen exchange reaction $\rm H + CNH \to HCN + H$, which is also important in the formation of the HCN molecule. This work could provide important information for the origin of amino acids and protein in the Universe.