Abstract: In this paper,thermal isomerization of HCN to isocyanide HNC has been studied. The topological properties of the structures of some points on IRC path in the vicinity of transition state have been analyzed. It is found that the transition state for HCN→HNC isomerization reaction is a bond switching point on IRC path in view of its topological properties. It is a point wherc the "old" chemical bond H—C begins to break and a "new" bond between H atom and the bond critical point of CN begins to form and this state lasts for a period about ΔS=0.3 (from S=0.00 to S=0.30) on the IRC path. The changes of other properties along the IRC reaction path has also been discussed.
Together with the reverse reaction, it is realized that the changing of topological properties of structures in the vicinity of transition state on IRC path is different for exothermal and endothermal reactions.

Key words: Topological analysis, IRC, transition state, HNC, HCN